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Title: Geological Observations on South America
Author: Darwin, Charles
Language: English
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Geological Observations on South America
By Charles Darwin
CONTENTS
EDITORIAL NOTE.
DETAILED TABLE OF CONTENTS.
GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA
CRITICAL INTRODUCTION.
CHAPTER I. ON THE ELEVATION OF THE EASTERN COAST OF SOUTH
AMERICA.
CHAPTER II. ON THE ELEVATION OF THE WESTERN COAST OF SOUTH
AMERICA.
CHAPTER III. ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS
SUPERFICIAL DEPOSITS.
CHAPTER IV. ON THE FORMATIONS OF THE PAMPAS.
CHAPTER V. ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND
CHILE.
CHAPTER VI. PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND
FOLIATION.
CHAPTER VII. CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.
CHAPTER VIII. NORTHERN CHILE. CONCLUSION.
EDITORIAL NOTE.
Although in some respects more technical in their subjects and
style than Darwin's "Journal," the books here reprinted will
never lose their value and interest for the originality of the
observations they contain. Many parts of them are admirably
adapted for giving an insight into problems regarding the
structure and changes of the earth's surface, and in fact they
form a charming introduction to physical geology and physiography
in their application to special domains. The books themselves
cannot be obtained for many times the price of the present
volume, and both the general reader, who desires to know more of
Darwin's work, and the student of geology, who naturally wishes
to know how a master mind reasoned on most important geological
subjects, will be glad of the opportunity of possessing them in a
convenient and cheap form.
The three introductions, which my friend Professor Judd has
kindly furnished, give critical and historical information which
makes this edition of special value.
G.T.B.
DETAILED TABLE OF CONTENTS.
CRITICAL INTRODUCTION.
I.—ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.
Upraised shells of La Plata.—Bahia Blanca, Sand-dunes and
Pumice-pebbles.- -Step-formed plains of Patagonia, with upraised
shells.—Terrace-bounded valley of Santa Cruz, formerly a
sea-strait.—Upraised shells of Tierra del Fuego.—Length and
breadth of the elevated area.—Equability of the movements, as
shown by the similar heights of the plains.—Slowness of the
elevatory process.—Mode of formation of the step-formed
plains.—Summary.- -Great shingle formation of Patagonia; its
extent, origin, and distribution.—Formation of sea-cliffs.
II.—ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA.
Chonos Archipelago.—Chiloe, recent and gradual elevation of,
traditions of the inhabitants on this subject.—Concepcion,
earthquake and elevation of.- -VALPARAISO, great elevation of,
upraised shells, earth or marine origin, gradual rise of the land
within the historical period.—COQUIMBO, elevation of, in recent
times; terraces of marine origin, their inclination, their
escarpments not horizontal.—Guasco, gravel terraces
of.—Copiapo.—PERU.— Upraised shells of Cobija, Iquique, and
Arica.—Lima, shell-beds and sea- beach on San Lorenzo.—Human
remains, fossil earthenware, earthquake debacle, recent
subsidence.—On the decay of upraised shells.—General summary.
III.—ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL
DEPOSITS.
Basin-like plains of Chile; their drainage, their marine
origin.—Marks of sea-action on the eastern flanks of the
Cordillera.—Sloping terrace-like fringes of stratified shingle
within the valleys of the Cordillera; their marine
origin.—Boulders in the valley of Cachapual.—Horizontal elevation
of the Cordillera.—Formation of valleys.—Boulders moved by
earthquake- waves.—Saline superficial deposits.—Bed of nitrate of
soda at Iquique.— Saline incrustations.—Salt-lakes of La Plata
and Patagonia; purity of the salt; its origin.
IV.—ON THE FORMATIONS OF THE PAMPAS.
Mineralogical constitution.—Microscopical structure.—Buenos
Ayres, shells embedded in tosca-rock.—Buenos Ayres to the
Colorado.—S. Ventana.—Bahia Blanca; M. Hermoso, bones and
infusoria of; P. Alta, shells, bones, and infusoria of;
co-existence of the recent shells and extinct mammifers.— Buenos
Ayres to St. Fe.—Skeletons of Mastodon.—Infusoria.—Inferior
marine tertiary strata, their age.—Horse's tooth. BANDA
ORIENTAL.— Superficial Pampean formation.—Inferior tertiary
strata, variation of, connected with volcanic action;
Macrauchenia Patachonica at S. Julian in Patagonia, age of,
subsequent to living mollusca and to the erratic block period.
SUMMARY.—Area of Pampean formation.—Theories of origin.—Source of
sediment.—Estuary origin.—Contemporaneous with existing
mollusca.— Relations to underlying tertiary strata. Ancient
deposit of estuary origin.—Elevation and successive deposition of
the Pampean formation.— Number and state of the remains of
mammifers; their habitation, food, extinction, and
range.—Conclusion.—Supplement on the thickness of the Pampean
formation.—Localities in Pampas at which mammiferous remains have
been found.
V.—ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.
Rio Negro.—S. Josef.—Port Desire, white pumiceous mudstone with
infusoria.—Port S. Julian.—Santa Cruz, basaltic lava of.—P.
Gallegos.— Eastern Tierra del Fuego; leaves of extinct
beech-trees.—Summary on the Patagonian tertiary
formations.—Tertiary formations of the Western Coast.- -Chonos
and Chiloe groups, volcanic rocks of.—Concepcion.—Navidad.—
Coquimbo.—Summary.—Age of the tertiary formations.—Lines of
elevation.— Silicified wood.—Comparative ranges of the extinct
and living mollusca on the West Coast of S. America.—Climate of
the tertiary period.—On the causes of the absence of recent
conchiferous deposits on the coasts of South America.—On the
contemporaneous deposition and preservation of sedimentary
formations.
VI.—PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION.
Brazil, Bahia, gneiss with disjointed metamorphosed dikes.—Strike
of foliation.—Rio de Janeiro, gneiss-granite, embedded fragment
in, decomposition of.—La Plata, metamorphic and old volcanic
rocks of.—S. Ventana.—Claystone porphyry formation of Patagonia;
singular metamorphic rocks; pseudo-dikes.—Falkland Islands,
palaeozoic fossils of.—Tierra del Fuego, clay-slate formation,
cretaceous fossils of; cleavage and foliation; form of
land.—Chonos Archipelago, mica-schists, foliation disturbed by
granitic axis; dikes.—Chiloe.—Concepcion, dikes, successive
formation of.—Central and Northern Chile.—Concluding remarks on
cleavage and foliation.—Their close analogy and similar
origin.—Stratification of metamorphic schists.—Foliation of
intrusive rocks.—Relation of cleavage and foliation to the lines
of tension during metamorphosis.
VII.—CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.
Central Chile.—Basal formations of the Cordillera.—Origin of the
porphyritic clay-stone conglomerate.—Andesite.—Volcanic
rocks.—Section of the Cordillera by the Peuquenes or Portillo
Pass.—Great gypseous formation.—Peuquenes line; thickness of
strata, fossils of.—Portillo line.—Conglomerate, orthitic
granite, mica-schist, volcanic rocks of.— Concluding remarks on
the denudation and elevation of the Portillo line.— Section by
the Cumbre, or Uspallata Pass.—Porphyries.—Gypseous strata.—
Section near the Puente del Inca; fossils of.—Great
subsidence.—Intrusive porphyries.—Plain of Uspallata.—Section of
the Uspallata chain.— Structure and nature of the
strata.—Silicified vertical trees.—Great subsidence.—Granitic
rocks of axis.—Concluding remarks on the Uspallata range; origin
subsequent to that of the main Cordillera; two periods of
subsidence; comparison with the Portillo chain.
VIII.—NORTHERN CHILE.—CONCLUSION.
A Section from Illapel to Combarbala; gypseous formation with
silicified wood.—Panuncillo.—Coquimbo; mines of Arqueros; section
up valley; fossils.—Guasco, fossils of.—Copiapo, section up
valley; Las Amolanas, silicified wood.—Conglomerates, nature of
former land, fossils, thickness of strata, great
subsidence.—Valley of Despoblado, fossils, tufaceous deposit,
complicated dislocations of.—Relations between ancient orifices
of eruption and subsequent axes of injection.—Iquique, Peru,
fossils of, salt-deposits.—Metalliferous veins.—Summary on the
porphyritic conglomerate and gypseous formations.—Great
subsidence with partial elevations during the cretaceo-oolitic
period.—On the elevation and structure of the
Cordillera.—Recapitulation on the tertiary series.— Relation
between movements of subsidence and volcanic action.—Pampean
formation.—Recent elevatory movements.—Long-continued volcanic
action in the Cordillera.—Conclusion.
GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA
CRITICAL INTRODUCTION.
Of the remarkable "trilogy" constituted by Darwin's writings
which deal with the geology of the "Beagle," the member which has
perhaps attracted least attention, up to the present time is that
which treats of the geology of South America. The actual writing
of this book appears to have occupied Darwin a shorter period
than either of the other volumes of the series; his diary records
that the work was accomplished within ten months, namely, between
July 1844 and April 1845; but the book was not actually issued
till late in the year following, the preface bearing the date
"September 1846." Altogether, as Darwin informs us in his
"Autobiography," the geological books "consumed four and a half
years' steady work," most of the remainder of the ten years that
elapsed between the return of the "Beagle," and the completion of
his geological books being, it is sad to relate, "lost through
illness!"
Concerning the "Geological Observations on South America," Darwin
wrote to his friend Lyell, as follows:—"My volume will be about
240 pages, dreadfully dull, yet much condensed. I think whenever
you have time to look through it, you will think the collection
of facts on the elevation of the land and on the formation of
terraces pretty good."
"Much condensed" is the verdict that everyone must endorse, on
rising from the perusal of this remarkable book; but by no means
"dull." The three and a half years from April 1832 to September
1835, were spent by Darwin in South America, and were devoted to
continuous scientific work; the problems he dealt with were
either purely geological or those which constitute the borderland
between the geological and biological sciences. It is impossible
to read the journal which he kept during this time without being
impressed by the conviction that it contains all the germs of
thought which afterwards developed into the "Origin of Species."
But it is equally evident that after his return to England,
biological speculations gradually began to exercise a more
exclusive sway over Darwin's mind, and tended to dispossess
geology, which during the actual period of the voyage certainly
engrossed most of his time and attention. The wonderful series of
observations made during those three and a half years in South
America could scarcely be done justice to, in the 240 pages
devoted to their exposition. That he executed the work of
preparing the book on South America in somewhat the manner of a
task, is shown by many references in his letters. Writing to Sir
Joseph Hooker in 1845, he says, "I hope this next summer to
finish my South American Geology, then to get out a little
Zoology, and HURRAH FOR MY SPECIES WORK!"
It would seem that the feeling of disappointment, which Darwin so
often experienced in comparing a book when completed, with the
observations and speculations which had inspired it, was more
keenly felt in the case of his volume on South America than any
other. To one friend he writes, "I have of late been slaving
extra hard, to the great discomfiture of wretched digestive
organs, at South America, and thank all the fates, I have done
three-fourths of it. Writing plain English grows with me more and
more difficult, and never attainable. As for your pretending that
you will read anything so dull as my pure geological
descriptions, lay not such a flattering unction on my soul, for
it is incredible." To another friend he writes, "You do not know
what you threaten when you propose to read it—it is purely
geological. I said to my brother, 'You will of course read it,'
and his answer was, 'Upon my life, I would sooner even buy it.'"
In spite of these disparaging remarks, however, we are strongly
inclined to believe that this book, despised by its author, and
neglected by his contemporaries, will in the end be admitted to
be one of Darwin's chief titles to fame. It is, perhaps, an
unfortunate circumstance that the great success which he attained
in biology by the publication of the "Origin of Species" has, to
some extent, overshadowed the fact that Darwin's claims as a
geologist, are of the very highest order. It is not too much to
say that, had Darwin not been a geologist, the "Origin of
Species" could never have been written by him. But apart from
those geological questions, which have an important bearing on
biological thought and speculation, such as the proofs of
imperfection in the geological record, the relations of the later
tertiary faunas to the recent ones in the same areas, and the
apparent intermingling of types belonging to distant geological
epochs, when we study the palaeontology of remote
districts,—there are other purely geological problems, upon which
the contributions made by Darwin are of the very highest value. I
believe that the verdict of the historians of science will be
that if Darwin had not taken a foremost place among the
biologists of this century, his position as a geologist would
have been an almost equally commanding one.
But in the case of Darwin's principal geological work—that
relating to the origin of the crystalline schists,—geologists
were not at the time prepared to receive his revolutionary
teachings. The influence of powerful authority was long
exercised, indeed, to stifle his teaching, and only now, when
this unfortunate opposition has disappeared, is the true nature
and importance of Darwin's purely geological work beginning to be
recognised.
The two first chapters of the "Geological Observations on South
America," deal with the proofs which exist of great, but
frequently interrupted, movements of elevation during very recent
geological times. In connection with this subject, Darwin's
particular attention was directed to the relations between the
great earthquakes of South America—of some of which he had
impressive experience—and the permanent changes of elevation
which were taking place. He was much struck by the rapidity with
which the evidence of such great earth movements is frequently
obliterated; and especially with the remarkable way in which the
action of rain-water, percolating through deposits on the earth's
surface, removes all traces of shells and other calcareous
organisms. It was these considerations which were the parents of
the generalisation that a palaeontological record can only be
preserved during those periods in which long-continued slow
subsidence is going on. This in turn, led to the still wider and
more suggestive conclusion that the geological record as a whole
is, and never can be more than, a series of more or less isolated
fragments. The recognition of this important fact constitutes the
keystone to any theory of evolution which seeks to find a basis
in the actual study of the types of life that have formerly
inhabited our globe.
In his third chapter, Darwin gives a number of interesting facts,
collected during his visits to the plains and valleys of Chili,
which bear on the question of the origin of saliferous
deposits—the accumulation of salt, gypsum, and nitrate of soda.
This is a problem that has excited much discussion among
geologists, and which, in spite of many valuable observations,
still remains to a great extent very obscure. Among the important
considerations insisted upon by Darwin is that relating to the
absence of marine shells in beds associated with such deposits.
He justly argues that if the strata were formed in shallow
waters, and then exposed by upheaval to subaerial action, all
shells and other calcareous organisms would be removed by
solution.
Following Lyell's method, Darwin proceeds from the study of
deposits now being accumulated on the earth's surface, to those
which have been formed during the more recent periods of the
geological history.
His account of the great Pampean formation, with its wonderful
mammalian remains—Mastodon, Toxodon, Scelidotherium,
Macrauchenia, Megatherium, Megalonyx, Mylodon, and Glyptodon—this
full of interest. His discovery of the remains of a true Equus
afforded a remarkable confirmation of the fact- -already made out
in North America—that species of horse had existed and become
extinct in the New World, before their introduction by the
Spaniards in the sixteenth century. Fully perceiving the
importance of the microscope in studying the nature and origin of
such deposits as those of the Pampas, Darwin submitted many of
his specimens both to Dr. Carpenter in this country, and to
Professor Ehrenberg in Berlin. Many very important notes on the
microscopic organisms contained in the formation will be found
scattered through the chapter.
Darwin's study of the older tertiary formations, with their
abundant shells, and their relics of vegetable life buried under
great sheets of basalt, led him to consider carefully the
question of climate during these earlier periods. In opposition
to prevalent views on this subject, Darwin points out that his
observations are opposed to the conclusion that a higher
temperature prevailed universally over the globe during early
geological periods. He argues that "the causes which gave to the
older tertiary productions of the quite temperate zones of Europe
a tropical character, WERE OF A LOCAL CHARACTER AND DID NOT
AFFECT THE WHOLE GLOBE." In this, as in many similar instances,
we see the beneficial influence of extensive travel in freeing
Darwin's mind from prevailing prejudices. It was this widening of
experience which rendered him so especially qualified to deal
with the great problem of the origin of species, and in doing so
to emancipate himself from ideas which were received with
unquestioning faith by geologists whose studies had been
circumscribed within the limits of Western Europe.
In the Cordilleras of Northern and Central Chili, Darwin, when
studying still older formations, clearly recognised that they
contain an admixture of the forms of life, which in Europe are
distinctive of the Cretaceous and Jurassic periods respectively.
He was thus led to conclude that the classification of geological
periods, which fairly well expresses the facts that had been
discovered in the areas where the science was first studied, is
no longer capable of being applied when we come to the study of
widely distant regions. This important conclusion led up to the
further generalisation that each great geological period has
exhibited a geographical distribution of the forms of animal and
vegetable life, comparable to that which prevails in the existing
fauna and flora. To those who are familiar with the extent to
which the doctrine of universal formations has affected
geological thought and speculation, both long before and since
the time that Darwin wrote, the importance of this new standpoint
to which he was able to attain will be sufficiently apparent.
Like the idea of the extreme imperfection of the Geological
Record, the doctrine of LOCAL geological formations is found
permeating and moulding all the palaeontological reasonings of
his great work.
In one of Darwin's letters, written while he was in South
America, there is a passage we have already quoted, in which he
expresses his inability to decide between the rival claims upon
his attention of "the old crystalline group of rocks," and "the
softer fossiliferous beds" respectively. The sixth chapter of the
work before us, entitled "Plutonic and Metamorphic Rocks—Cleavage
and Foliation," contains a brief summary of a series of
observations and reasonings upon these crystalline rocks, which
are, we believe, calculated to effect a revolution in geological
science, and— though their value and importance have long been
overlooked—are likely to entitle Darwin in the future to a
position among geologists, scarcely, if at all, inferior to that
which he already occupies among biologists.
Darwin's studies of the great rock-masses of the Andes convinced
him of the close relations between the granitic or Plutonic
rocks, and those which were undoubtedly poured forth as lavas.
Upon his return, he set to work, with the aid of Professor
Miller, to make a careful study of the minerals composing the
granites and those which occur in the lavas, and he was able to
show that in all essential respects they are identical. He was
further able to prove that there is a complete gradation between
the highly crystalline or granitic rock-masses, and those
containing more or less glassy matter between their crystals,
which constitute ordinary lavas. The importance of this
conclusion will be realised when we remember that it was then the
common creed of geologists—and still continues to be so on the
Continent—that all highly crystalline rocks are of great
geological antiquity, and that the igneous ejections which have
taken place since the beginning of the tertiary periods differ
essentially, in their composition, their structure, and their
mode of occurrence, from those which have made their appearance
at earlier periods of the world's history.
Very completely have the conclusions of Darwin upon these
subjects been justified by recent researches. In England, the
United States, and Italy, examples of the gradual passage of
rocks of truly granitic structure into ordinary lavas have been
described, and the reality of the transition has been
demonstrated by the most careful studies with the microscope.
Recent researches carried on in South America by Professor
Stelzner, have also shown the existence of a class of highly
crystalline rocks—the "Andengranites"—which combine in themselves
many of the characteristics which were once thought to be
distinctive of the so-called Plutonic and volcanic rocks. No one
familiar with recent geological literature—even in Germany and
France, where the old views concerning the distinction of igneous
products of different ages have been most stoutly maintained—can
fail to recognise the fact that the principles contended for by
Darwin bid fair at no distant period to win universal acceptance
among geologists all over the globe.
Still more important are the conclusions at which Darwin arrived
with respect to the origin of the schists and gneisses which
cover so large an area in South America.
Carefully noting, by the aid of his compass and clinometer, at
every point which he visited, the direction and amount of
inclination of the parallel divisions in these rocks, he was led
to a very important generalisation— namely, that over very wide
areas the direction (strike) of the planes of cleavage in slates,
and of foliation in schists and gneisses, remained constant,
though the amount of their inclination (dip) often varied within
wide limits. Further than this it appeared that there was always
a close correspondence between the strike of the cleavage and
foliation and the direction of the great axes along which
elevation had taken place in the district.
In Tierra del Fuego, Darwin found striking evidence that the
cleavage intersecting great masses of slate-rocks was quite
independent of their original stratification, and could often,
indeed, be seen cutting across it at right angles. He was also
able to verify Sedgwick's observation that, in some slates,
glossy surfaces on the planes of cleavage arise from the
development of new minerals, chlorite, epidote or mica, and that
in this way a complete graduation from slates to true schists may
be traced.
Darwin further showed that in highly schistose rocks, the folia
bend around and encircle any foreign bodies in the mass, and that
in some cases they exhibit the most tortuous forms and
complicated puckerings. He clearly saw that in all cases the
forces by which these striking phenomena must have been produced
were persistent over wide areas, and were connected with the
great movements by which the rocks had been upheaved and folded.
That the distinct folia of quartz, feldspar, mica, and other
minerals composing the metamorphic schists could not have been
separately deposited as sediment was strongly insisted upon by
Darwin; and in doing so he opposed the view generally prevalent
among geologists at that time. He was thus driven to the
conclusion that foliation, like cleavage, is not an original, but
a superinduced structure in rock-masses, and that it is the
result of re-crystallisation, under the controlling influence of
great pressure, of the materials of which the rock was composed.
In studying the lavas of Ascension, as we have already seen,
Darwin was led to recognise the circumstance that, when igneous
rocks are subjected to great differential movements during the
period of their consolidation, they acquire a foliated structure,
closely analogous to that of the crystalline schists. Like his
predecessor in this field of inquiry, Mr. Poulett Scrope, Charles
Darwin seems to have been greatly impressed by these facts, and
he argued from them that the rocks exhibiting the foliated
structure must have been in a state of plasticity, like that of a
cooling mass of lava. At that time the suggestive experiments of
Tresca, Daubree, and others, showing that solid masses under the
influence of enormous pressure become actually plastic, had not
been published. Had Darwin been aware of these facts he would
have seen that it was not necessary to assume a state of
imperfect solidity in rock-masses in order to account for their
having yielded to pressure and tension, and, in doing so,
acquiring the new characters which distinguish the crystalline
schists.
The views put forward by Darwin on the origin of the crystalline
schists found an able advocate in Mr. Daniel Sharpe, who in 1852
and 1854 published two papers, dealing with the geology of the
Scottish Highlands and of the Alps respectively, in which he
showed that the principles arrived at by Darwin when studying the
South American rocks afford a complete explanation of the
structure of the two districts in question.
But, on the other hand, the conclusions of Darwin and Sharpe were
met with the strongest opposition by Sir Roderick Murchison and
Dr. A. Geikie, who in 1861 read a paper before the Geological
Society "On the Coincidence between Stratification and Foliation
in the Crystalline Rocks of the Scottish Highlands," in which
they insisted that their observations in Scotland tended to
entirely disprove the conclusions of Darwin that foliation in
rocks is a secondary structure, and entirely independent of the
original stratification of the rock-masses.
Now it is a most significant circumstance that, no sooner did the
officers of the Geological Survey commence the careful and
detailed study of the Scottish Highlands than they found
themselves compelled to make a formal retraction of the views
which had been put forward by Murchison and Geikie in opposition
to the conclusions of Darwin. The officers of the Geological
Survey have completely abandoned the view that the foliation of
the Highland rocks has been determined by their original
stratification, and admit that the structure is the result of the
profound movements to which the rocks have been subjected. The
same conclusions have recently been supported by observations
made in many different districts—among which we may especially
refer to those of Dr. H. Reusch in Norway, and those of Dr. J.
Lehmann in Saxony. At the present time the arguments so clearly
stated by Darwin in the work before us, have, after enduring
opposition or neglect for a whole generation, begun to "triumph
all along the line," and we may look forward confidently to the
near future, when his claim to be regarded as one of the greatest
of geological discoverers shall be fully vindicated.
JOHN W. JUDD.
CHAPTER I. ON THE ELEVATION OF THE EASTERN COAST OF SOUTH
AMERICA.
Upraised shells of La Plata. Bahia Blanca, Sand-dunes and
Pumice-pebbles. Step-formed plains of Patagonia, with upraised
Shells. Terrace-bounded Valley of Santa Cruz, formerly a Sea-strait.
Upraised shells of Tierra del Fuego. Length and breadth of the
elevated area. Equability of the movements, as shown by the similar
heights of the plains. Slowness of the elevatory process. Mode of
formation of the step-formed plains. Summary. Great Shingle Formation
of Patagonia; its extent, origin, and distribution. Formation of
sea-cliffs.
In the following Volume, which treats of the geology of South
America, and almost exclusively of the parts southward of the
Tropic of Capricorn, I have arranged the chapters according to
the age of the deposits, occasionally departing from this order,
for the sake of geographical simplicity.
The elevation of the land within the recent period, and the
modifications of its surface through the action of the sea (to
which subjects I paid particular attention) will be first
discussed; I will then pass on to the tertiary deposits, and
afterwards to the older rocks. Only those districts and sections
will be described in detail which appear to me to deserve some
particular attention; and I will, at the end of each chapter,
give a summary of the results. We will commence with the proofs
of the upheaval of the eastern coast of the continent, from the
Rio Plata southward; and, in the Second Chapter, follow up the
same subject along the shores of Chile and Peru.
On the northern bank of the great estuary of the Rio Plata, near
Maldonado, I found at the head of a lake, sometimes brackish but
generally containing fresh water, a bed of muddy clay, six feet
in thickness, with numerous shells of species still existing in
the Plata, namely, the Azara labiata, d'Orbigny, fragments of
Mytilus eduliformis, d'Orbigny, Paludestrina Isabellei,
d'Orbigny, and the Solen Caribaeus, Lam., which last was embedded
vertically in the position in which it had lived. These shells
lie at the height of only two feet above the lake, nor would they
have been worth mentioning, except in connection with analogous
facts.
At Monte Video, I noticed near the town, and along the base of
the mount, beds of a living Mytilus, raised some feet above the
surface of the Plata: in a similar bed, at a height from thirteen
to sixteen feet, M. Isabelle collected eight species, which,
according to M. d'Orbigny, now live at the mouth of the estuary.
("Voyage dans l'Amerique Merid.: Part. Geolog." page 21.) At
Colonia del Sacramiento, further westward, I observed at the
height of about fifteen feet above the river, there of quite
fresh water, a small bed of the same Mytilus, which lives in
brackish water at Monte Video. Near the mouth of Uruguay, and for
at least thirty-five miles northward, there are at intervals
large sandy tracts, extending several miles from the banks of the
river, but not raised much above its level, abounding with small
bivalves, which occur in such numbers that at the Agraciado they
are sifted and burnt for lime. Those which I examined near the A.
S. Juan were much worn: they consisted of Mactra Isabellei,
d'Orbigny, mingled with few of Venus sinuosa, Lam., both
inhabiting, as I am informed by M. d'Orbigny, brackish water at
the mouth of the Plata, nearly or quite as salt as the open sea.
The loose sand, in which these shells are packed, is heaped into
low, straight, long lines of dunes, like those left by the sea at
the head of many bays. M. d'Orbigny has described an analogous
phenomenon on a greater scale, near San Pedro on the river
Parana, where he found widely extended beds and hillocks of sand,
with vast numbers of the Azara labiata, at the height of nearly
100 feet (English) above the surface of that river. (Ibid page
43.) The Azara inhabits brackish water, and is not known to be
found nearer to San Pedro than Buenos Ayres, distant above a
hundred miles in a straight line. Nearer Buenos Ayres, on the
road from that place to San Isidro, there are extensive beds, as
I am informed by Sir Woodbine Parish, of the Azara labiata, lying
at about forty feet above the level of the river, and distant
between two and three miles from it. ("Buenos Ayres" etc. by Sir
Woodbine Parish page 168.) These shells are always found on the
highest banks in the district: they are embedded in a stratified
earthy mass, precisely like that of the great Pampean deposit
hereafter to be described. In one collection of these shells,
there were some valves of the Venus sinuosa, Lam., the same
species found with the Mactra on the banks of the Uruguay. South
of Buenos Ayres, near Ensenada, there are other beds of the
Azara, some of which seem to have been embedded in yellowish,
calcareous, semi-crystalline matter; and Sir W. Parish has given
me from the banks of the Arroyo del Tristan, situated in this
same neighbourhood, at the distance of about a league from the
Plata, a specimen of a pale- reddish, calcereo-argillaceous stone
(precisely like parts of the Pampean deposit the importance of
which fact will be referred to in a succeeding chapter),
abounding with shells of an Azara, much worn, but which in
general form and appearance closely resemble, and are probably
identical with, the A. labiata. Besides these shells, cellular,
highly crystalline rock, formed of the casts of small bivalves,
is found near Ensenada; and likewise beds of sea-shells, which
from their appearance appear to have lain on the surface. Sir W.
Parish has given me some of these shells, and M. d'Orbigny
pronounces them to be:—
1. Buccinanops globulosum, d'Orbigny.
2. Olivancillaria auricularia, d'Orbigny.
3. Venus flexuosa, Lam.
4. Cytheraea (imperfect).
5. Mactra Isabellei, d'Orbigny.
6. Ostrea pulchella, d'Orbigny.
Besides these, Sir W. Parish procured ("Buenos Ayres" etc. by Sir
W. Parish page 168.) (as named by Mr. G.B. Sowerby) the following
shells:—
7. Voluta colocynthis.
8. Voluta angulata.
9. Buccinum (not spec.?).
All these species (with, perhaps, the exception of the last) are
recent, and live on the South American coast. These shell-beds
extend from one league to six leagues from the Plata, and must
lie many feet above its level. I heard, also, of beds of shells
on the Somborombon, and on the Rio Salado, at which latter place,
as M. d'Orbigny informs me, the Mactra Isabellei and Venus
sinuosa are found.
During the elevation of the Provinces of La Plata, the waters of
the ancient estuary have but little affected (with the exception
of the sand- hills on the banks of the Parana and Uruguay) the
outline of the land. M. Parchappe, however, has described groups
of sand dunes scattered over the wide extent of the Pampas
southward of Buenos Ayres (D'Orbigny "Voyage Geolog." page 44.),
which M. d'Orbigny attributes with much probability to the action
of the sea, before the plains were raised above its level.
(Before proceeding to the districts southward of La Plata, it may
be worth while just to state, that there is some evidence that
the coast of Brazil has participated in a small amount of
elevation. Mr. Burchell informs me, that he collected at Santos
(latitude 24 degrees S.) oyster-shells, apparently recent, some
miles from the shore, and quite above the tidal action. Westward
of Rio de Janeiro, Captain Elliot is asserted (see Harlan "Med.
and Phys. Res." page 35 and Dr. Meigs in "Transactions of the
American Philosophical Society"), to have found human bones,
encrusted with sea-shells, between fifteen and twenty feet above
the level of the sea. Between Rio de Janeiro and Cape Frio I
crossed sandy tracts abounding with sea-shells, at a distance of
a league from the coast; but whether these tracts have been
formed by upheaval, or through the mere accumulation of drift
sand, I am not prepared to assert. At Bahia (latitude 13 degrees
S.), in some parts near the coast, there are traces of sea-action
at the height of about twenty feet above its present level; there
are also, in many parts, remnants of beds of sandstone and
conglomerate with numerous recent shells, raised a little above
the sea-level. I may add, that at the head of Bahia Bay there is
a formation, about forty feet in thickness, containing tertiary
shells apparently of fresh-water origin, now washed by the sea
and encrusted with Balini; this appears to indicate a small
amount of subsidence subsequent to its deposition. At Pernambuco
(latitude 8 degrees S.), in the alluvial or tertiary cliffs,
surrounding the low land on which the city stands, I looked in
vain for organic remains, or other evidence of changes in level.)
SOUTHWARD OF THE PLATA.
The coast as far as Bahia Blanca (in latitude 39 degrees S.) is
formed either of a horizontal range of cliffs, or of immense
accumulations of sand-dunes. Within Bahia Blanca, a small piece
of tableland, about twenty feet above high-water mark, called
Punta Alta, is formed of strata of cemented gravel and of red
earthy mud, abounding with shells (with others lying loose on the
surface), and the bones of extinct mammifers. These shells,
twenty in number, together with a Balanus and two corals, are all
recent species, still inhabiting the neighbouring seas. They will
be enumerated in the Fourth Chapter, when describing the Pampean
formation; five of them are identical with the upraised ones from
near Buenos Ayres. The northern shore of Bahia Blanca is, in main
part, formed of immense sand-dunes, resting on gravel with recent
shells, and ranging in lines parallel to the shore. These ranges
are separated from each other by flat spaces, composed of stiff
impure red clay, in which, at the distance of about two miles
from the coast, I found by digging a few minute fragments of
sea-shells. The sand-dunes extend several miles inland, and stand
on a plain, which slopes up to a height of between one hundred
and two hundred feet. Numerous, small, well-rounded pebbles of
pumice lie scattered both on the plain and sand-hillocks: at
Monte Hermoso, on the flat summit of a cliff, I found many of
them at a height of 120 feet (angular measurement) above the
level of the sea. These pumice pebbles, no doubt, were originally
brought down from the Cordillera by the rivers which cross the
continent, in the same way as the river Negro anciently brought
down, and still brings down, pumice, and as the river Chupat
brings down scoriae: when once delivered at the mouth of a river,
they would naturally have travelled along the coasts, and been
cast up during the elevation of the land, at different heights.
The origin of the argillaceous flats, which separate the parallel
ranges of sand-dunes, seems due to the tides here having a
tendency (as I believe they have on most shoal, protected coasts)
to throw up a bar parallel to the shore, and at some distance
from it; this bar gradually becomes larger, affording a base for
the accumulation of sand- dunes, and the shallow space within
then becomes silted up with mud. The repetition of this process,
without any elevation of the land, would form a level plain
traversed by parallel lines of sand-hillocks; during a slow
elevation of the land, the hillocks would rest on a gently
inclined surface, like that on the northern shore of Bahia
Blanca. I did not observe any shells in this neighbourhood at a
greater height than twenty feet; and therefore the age of the
sea-drifted pebbles of pumice, now standing at the height of 120
feet, must remain uncertain.
The main plain surrounding Bahia Blanca I estimated at from two
hundred to three hundred feet; it insensibly rises towards the
distant Sierra Ventana. There are in this neighbourhood some
other and lower plains, but they do not abut one at the foot of
the other, in the manner hereafter to be described, so
characteristic of Patagonia. The plain on which the settlement
stands is crossed by many low sand-dunes, abounding with the
minute shells of the Paludestrina australis, d'Orbigny, which now
lives in the bay. This low plain is bounded to the south, at the
Cabeza del Buey, by the cliff-formed margin of a wide plain of
the Pampean formation, which I estimated at sixty feet in height.
On the summit of this cliff there is a range of high sand-dunes
extending several miles in an east and west line.
Southward of Bahia Blanca, the river Colorado flows between two
plains, apparently from thirty to forty feet in height. Of these
plains, the southern one slopes up to the foot of the great
sandstone plateau of the Rio Negro; and the northern one against
an escarpment of the Pampean deposit; so that the Colorado flows
in a valley fifty miles in width, between the upper escarpments.
I state this, because on the low plain at the foot of the
northern escarpment, I crossed an immense accumulation of high
sand-dunes, estimated by the Gauchos at no less than eight miles
in breadth. These dunes range westward from the coast, which is
twenty miles distant, to far inland, in lines parallel to the
valley; they are separated from each other by argillaceous flats,
precisely like those on the northern shore of Bahia Blanca. At
present there is no source whence this immense accumulation of
sand could proceed; but if, as I believe, the upper escarpments
once formed the shores of an estuary, in that case the sandstone
formation of the river Negro would have afforded an inexhaustible
supply of sand, which would naturally have accumulated on the
northern shore, as on every part of the coast open to the south
winds between Bahia Blanca and Buenos Ayres.
At San Blas (40 degrees 40' S.) a little south of the mouth of
the Colorado, M. d'Orbigny found fourteen species of existing
shells (six of them identical with those from Bahia Blanca),
embedded in their natural positions. ("Voyage" etc. page 54.)
From the zone of depth which these shells are known to inhabit,
they must have been uplifted thirty-two feet. He also found, at
from fifteen to twenty feet above this bed, the remains of an
ancient beach.
Ten miles southward, but 120 miles to the west, at Port S.
Antonio, the Officers employed on the Survey assured me that they
saw many old sea- shells strewed on the surface of the ground,
similar to those found on other parts of the coast of Patagonia.
At San Josef, ninety miles south in nearly the same longitude, I
found, above the gravel, which caps an old tertiary formation, an
irregular bed and hillock of sand, several feet in thickness,
abounding with shells of Patella deaurita, Mytilus Magellanicus,
the latter retaining much of its colour; Fusus Magellanicus (and
a variety of the same), and a large Balanus (probably B. Tulipa),
all now found on this coast: I estimated this bed at from eighty
to one hundred feet above the level of the sea. To the westward
of this bay, there is a plain estimated at between two hundred
and three hundred feet in height: this plain seems, from many
measurements, to be a continuation of the sandstone platform of
the river Negro. The next place southward, where I landed, was at
Port Desire, 340 miles distant; but from the intermediate
districts I received, through the kindness of the Officers of the
Survey, especially from Lieutenant Stokes and Mr. King, many
specimens and sketches, quite sufficient to show the general
uniformity of the whole line of coast. I may here state, that the
whole of Patagonia consists of a tertiary formation, resting on
and sometimes surrounding hills of porphyry and quartz: the
surface is worn into many wide valleys and into level step-formed
plains, rising one above another, all capped by irregular beds of
gravel, chiefly composed of porphyritic rocks. This gravel
formation will be separately described at the end of the chapter.
My object in giving the following measurements of the plains, as
taken by the Officers of the Survey, is, as will hereafter be
seen, to show the remarkable equability of the recent elevatory
movements. Round the southern parts of Nuevo Gulf, as far as the
River Chupat (seventy miles southward of San Josef), there appear
to be several plains, of which the best defined are here
represented.
(In the following Diagrams: 1. Baseline is Level of sea. 2. Scale
is 1/20 of inch to 100 feet vertical. 3. Height is shown in feet
thus: An. M. always stands for angular or trigonometrical
measurement. Ba. M. always stands for barometrical measurement.
Est. always stands for estimation by the Officers of the Survey.
DIAGRAM 1. SECTION OF STEP-FORMED PLAINS SOUTH OF NUEVO GULF.
From East (sea level) to West (high): Terrace 1. 80 Est. Terrace
2. 200-220 An. M. Terrace 3. 350 An. M.)
The upper plain is here well defined (called Table Hills); its
edge forms a cliff or line of escarpment many miles in length,
projecting over a lower plain. The lowest plain corresponds with
that at San Josef with the recent shells on its surface. Between
this lowest and the uppermost plain, there is probably more than
one step-formed terrace: several measurements show the existence
of the intermediate one of the height given in Diagram 1.
(DIAGRAM 2. SECTION OF PLAINS IN THE BAY OF ST. GEORGE.
From East (sea level) to West (high): Terrace 1. 250 An. M.
Terrace 2. 330 An. M. Terrace 3. 580 An. M. Terraces 4, 5 and 6
not measured. Terrace 7. 1,200 Est.)
Near the north headland of the great Bay of St. George (100 miles
south of the Chupat), two well-marked plains of 250 and 330 feet
were measured: these are said to sweep round a great part of the
Bay. At its south headland, 120 miles distant from the north
headland, the 250 feet plain was again measured. In the middle of
the bay, a higher plain was found at two neighbouring places
(Tilli Roads and C. Marques) to be 580 feet in height. Above this
plain, towards the interior, Mr. Stokes informs me that there
were several other step-formed plains, the highest of which was
estimated at 1,200 feet, and was seen ranging at apparently the
same height for 150 miles northward. All these plains have been
worn into great valleys and much denuded. The section in Diagram
3 is illustrative of the general structure of the great Bay of
St. George. At the south headland of the Bay of St. George (near
C. Three Points) the 250 plain is very extensive.
(DIAGRAM 3. SECTION OF PLAINS AT PORT DESIRE.
From East (sea level) to West (high): Terrace 1. 100 Est. Terrace
2. 245-255 Ba. M. Shells on surface. Terrace 3. 330 Ba. M. Shells
on surface. Terrace 4. Not measured.)
At Port Desire (forty miles southward) I made several
measurements with the barometer of a plain, which extends along
the north side of the port and along the open coast, and which
varies from 245 to 255 feet in height: this plain abuts against
the foot of a higher plain of 330 feet, which extends also far
northward along the coast, and likewise into the interior. In the
distance a higher inland platform was seen, of which I do not
know the height. In three separate places, I observed the cliff
of the 245-255 feet plain, fringed by a terrace or narrow plain
estimated at about one hundred feet in height. These plains are
represented in the section Diagram 3.
In many places, even at the distance of three and four miles from
the coast, I found on the gravel-capped surface of the 245-255
feet, and of the 330 feet plain, shells of Mytilus Magellanicus,
M. edulis, Patella deaurita, and another Patella, too much worn
to be identified, but apparently similar to one found abundantly
adhering to the leaves of the kelp. These species are the
commonest now living on this coast. The shells all appeared very
old; the blue of the mussels was much faded; and only traces of
colour could be perceived in the Patellas, of which the outer
surfaces were scaling off. They lay scattered on the smooth
surface of the gravel, but abounded most in certain patches,
especially at the heads of the smaller valleys: they generally
contained sand in their insides; and I presume that they have
been washed by alluvial action out of thin sandy layers, traces
of which may sometimes be seen covering the gravel. The several
plains have very level surfaces; but all are scooped out by
numerous broad, winding, flat-bottomed valleys, in which, judging
from the bushes, streams never flow. These remarks on the state
of the shells, and on the nature of the plains, apply to the
following cases, so need not be repeated.
(DIAGRAM 4. SECTION OF PLAINS AT PORT S. JULIAN.
From East (sea level) to West (high): Terrace 1. Shells on
surface. 90 Est. Terrace 2. 430 An. M. Terrace 3. 560 An. M.
Terrace 4. 950 An. M.)
Southward of Port Desire, the plains have been greatly denuded,
with only small pieces of tableland marking their former
extension. But opposite Bird Island, two considerable step-formed
plains were measured, and found respectively to be 350 and 590
feet in height. This latter plain extends along the coast close
to Port St. Julian (110 miles south of Port Desire); see Diagram
4.
The lowest plain was estimated at ninety feet: it is remarkable
from the usual gravel-bed being deeply worn into hollows, which
are filled up with, as well as the general surface covered by,
sandy and reddish earthy matter: in one of the hollows thus
filled up, the skeleton of the Macrauchenia Patachonica, as will
hereafter be described, was embedded. On the surface and in the
upper parts of this earthy mass, there were numerous shells of
Mytilus Magellanicus and M. edulis, Patella deaurita, and
fragments of other species. This plain is tolerably level, but
not extensive; it forms a promontory seven or eight miles long,
and three or four wide. The upper plains in Diagram 4 were
measured by the Officers of the Survey; they were all capped by
thick beds of gravel, and were all more or less denuded; the 950
plain consists merely of separate, truncated, gravel-capped
hills, two of which, by measurement, were found to differ only
three feet. The 430 feet plain extends, apparently with hardly a
break, to near the northern entrance of the Rio Santa Cruz (fifty
miles to the south); but it was there found to be only 330 feet
in height.
(DIAGRAM 5. SECTION OF PLAINS AT THE MOUTH OF THE RIO SANTA CRUZ.
From East (sea level) to West (high): Terrace 1. (sloping) 355
Ba. M. Shells on surface. 463 Ba. M. Terrace 2. 710 An. M.
Terrace 3. 840 An. M.)
On the southern side of the mouth of the Santa Cruz we have
Diagram 5, which I am able to give with more detail than in the
foregoing cases.
The plain marked 355 feet (as ascertained by the barometer and by
angular measurement) is a continuation of the above-mentioned 330
feet plain: it extends in a N.W. direction along the southern
shores of the estuary. It is capped by gravel, which in most
parts is covered by a thin bed of sandy earth, and is scooped out
by many flat-bottomed valleys. It appears to the eye quite level,
but in proceeding in a S.S.W. course, towards an escarpment
distant about six miles, and likewise ranging across the country
in a N.W. line, it was found to rise at first insensibly, and
then for the last half-mile, sensibly, close up to the base of
the escarpment: at this point it was 463 feet in height, showing
a rise of 108 feet in the six miles. On this 355-463 feet plain,
I found several shells of Mytilus Magellanicus and of a Mytilus,
which Mr. Sowerby informs me is yet unnamed, though well-known as
recent on this coast; Patella deaurita; Fusus, I believe,
Magellanicus, but the specimen has been lost; and at the distance
of four miles from the coast, at the height of about four hundred
feet, there were fragments of the same Patella and of a Voluta
(apparently V. ancilla) partially embedded in the superficial
sandy earth. All these shells had the same ancient appearance
with those from the foregoing localities. As the tides along this
part of the coast rise at the Syzygal period forty feet, and
therefore form a well-marked beach-line, I particularly looked
out for ridges in crossing this plain, which, as we have seen,
rises 108 feet in about six miles, but I could not see any traces
of such. The next highest plain is 710 feet above the sea; it is
very narrow, but level, and is capped with gravel; it abuts to
the foot of the 840 feet plain. This summit-plain extends as far
as the eye can range, both inland along the southern side of the
valley of the Santa Cruz, and southward along the Atlantic.
THE VALLEY OF THE R. SANTA CRUZ.
This valley runs in an east and west direction to the Cordillera,
a distance of about one hundred and sixty miles. It cuts through
the great Patagonian tertiary formation, including, in the upper
half of the valley, immense streams of basaltic lava, which as
well as the softer beds, are capped by gravel; and this gravel,
high up the river, is associated with a vast boulder formation.
(I have described this formation in a paper in the "Geological
Transactions" volume 6 page 415.) In ascending the valley, the
plain which at the mouth on the southern side is 355 feet high,
is seen to trend towards the corresponding plain on the northern
side, so that their escarpments appear like the shores of a
former estuary, larger than the existing one: the escarpments,
also, of the 840 feet summit-plain (with a corresponding northern
one, which is met with some way up the valley), appear like the
shores of a still larger estuary. Farther up the valley, the
sides are bounded throughout its entire length by level,
gravel-capped terraces, rising above each other in steps. The
width between the upper escarpments is on an average between
seven and ten miles; in one spot, however, where cutting through
the basaltic lava, it was only one mile and a half. Between the
escarpments of the second highest terrace the average width is
about four or five miles. The bottom of the valley, at the
distance of 110 miles from its mouth, begins sensibly to expand,
and soon forms a considerable plain, 440 feet above the level of
the sea, through which the river flows in a gut from twenty to
forty feet in depth. I here found, at a point 140 miles from the
Atlantic, and seventy miles from the nearest creek of the
Pacific, at the height of 410 feet, a very old and worn shell of
Patella deaurita. Lower down the valley, 105 miles from the
Atlantic (longitude 71 degrees W.), and at an elevation of about
300 feet, I also found, in the bed of the river, two much worn
and broken shells of the Voluta ancilla, still retaining traces
of their colours; and one of the Patella deaurita. It appeared
that these shells had been washed from the banks into the river;
considering the distance from the sea, the desert and absolutely
unfrequented character of the country, and the very ancient
appearance of the shells (exactly like those found on the plains
nearer the coast), there is, I think, no cause to suspect that
they could have been brought here by Indians.
The plain at the head of the valley is tolerably level, but
water-worn, and with many sand-dunes on it like those on a
sea-coast. At the highest point to which we ascended, it was
sixteen miles wide in a north and south line; and forty-five
miles in length in an east and west line. It is bordered by the
escarpments, one above the other, of two plains, which diverge as
they approach the Cordillera, and consequently resemble, at two
levels, the shores of great bays facing the mountains; and these
mountains are breached in front of the lower plain by a
remarkable gap. The valley, therefore, of the Santa Cruz consists
of a straight broad cut, about ninety miles in length, bordered
by gravel-capped terraces and plains, the escarpments of which at
both ends diverge or expand, one over the other, after the manner
of the shores of great bays. Bearing in mind this peculiar form
of the land—the sand-dunes on the plain at the head of the
valley—the gap in the Cordillera, in front of it—the presence in
two places of very ancient shells of existing species—and lastly,
the circumstance of the 355-453 feet plain, with the numerous
marine remains on its surface, sweeping from the Atlantic coast,
far up the valley, I think we must admit, that within the recent
period, the course of the Santa Cruz formed a sea-strait
intersecting the continent. At this period, the southern part of
South America consisted of an archipelago of islands 360 miles in
a north and south line. We shall presently see, that two other
straits also, since closed, then cut through Tierra del Fuego; I
may add, that one of them must at that time have expanded at the
foot of the Cordillera into a great bay (now Otway Water) like
that which formerly covered the 440 feet plain at the head of the
Santa Cruz.
(DIAGRAM 6. NORTH AND SOUTH SECTION ACROSS THE TERRACES BOUNDING
THE VALLEY OF THE RIVER SANTA CRUZ, HIGH UP ITS COURSE.
The height of each terrace, above the level of the river
(furthest to nearest to the river) in feet:
A, north and south: 1,122 B, north and south: 869 C, north and
south: 639 D, north: not measured. D, north? (suggest south): 185
E: 20 Bed of River.
Vertical scale 1/20 of inch to 100 feet; but terrace E, being
only twenty feet above the river, has necessarily been raised.
The horizontal distances much contracted; the distance from the
edge of A North to A South being on an average from seven to ten
miles.) I have said that the valley in its whole course is
bordered by gravel- capped plains. The section (Diagram 6),
supposed to be drawn in a north and south line across the valley,
can scarcely be considered as more than illustrative; for during
our hurried ascent it was impossible to measure all the plains at
any one place. At a point nearly midway between the Cordillera
and the Atlantic, I found the plain (A north) 1,122 feet above
the river; all the lower plains on this side were here united
into one great broken cliff: at a point sixteen miles lower down
the stream, I found by measurement and estimation that B (north)
was 869 above the river: very near to where A (north) was
measured, C (north) was 639 above the same level: the terrace D
(north) was nowhere measured: the lowest E (north) was in many
places about twenty feet above the river. These plains or
terraces were best developed where the valley was widest; the
whole five, like gigantic steps, occurred together only at a few
points. The lower terraces are less continuous than the higher
ones, and appear to be entirely lost in the upper third of the
valley. Terrace C (south), however was traced continuously for a
great distance. The terrace B (north), at a point fifty- five
miles from the mouth of the river, was four miles in width;
higher up the valley this terrace (or at least the second highest
one, for I could not always trace it continuously) was about
eight miles wide. This second plain was generally wider than the
lower ones—as indeed follows from the valley from A (north) to A
(south) being generally nearly double the width of from B (north)
to B (south). Low down the valley, the summit-plain A (south) is
continuous with the 840 feet plain on the coast, but it is soon
lost or unites with the escarpment of B (south). The
corresponding plain A (north), on the north side of the valley,
appears to range continuously from the Cordillera to the head of
the present estuary of the Santa Cruz, where it trends northward
towards Port St. Julian. Near the Cordillera the summit-plain on
both sides of the valley is between 3,200 and 3,300 feet in
height; at 100 miles from the Atlantic, it is 1,416 feet, and on
the coast 840 feet, all above the sea-beach; so that in a
distance of 100 miles the plain rises 576 feet, and much more
rapidly near to the Cordillera. The lower terraces B and C also
appear to rise as they run up the valley; thus D (north),
measured at two points twenty-four miles apart, was found to have
risen 185 feet. From several reasons I suspect, that this gradual
inclination of the plains up the valley, has been chiefly caused
by the elevation of the continent in mass, having been the
greater the nearer to the Cordillera.
All the terraces are capped with well-rounded gravel, which rests
either on the denuded and sometimes furrowed surface of the soft
tertiary deposits, or on the basaltic lava. The difference in
height between some of the lower steps or terraces seems to be
entirely owing to a difference in the thickness of the capping
gravel. Furrows and inequalities in the gravel, where such occur,
are filled up and smoothed over with sandy earth. The pebbles,
especially on the higher plains, are often whitewashed, and even
cemented together by a white aluminous substance, and I
occasionally found this to be the case with the gravel on the
terrace D. I could not perceive any trace of a similar deposition
on the pebbles now thrown up by the river, and therefore I do not
think that terrace D was river-formed. As the terrace E generally
stands about twenty feet above the bed of the river, my first
impression was to doubt whether even this lowest one could have
been so formed; but it should always be borne in mind, that the
horizontal upheaval of a district, by increasing the total
descent of the streams, will always tend to increase, first near
the sea-coast and then further and further up the valley, their
corroding and deepening powers: so that an alluvial plain, formed
almost on a level with a stream, will, after an elevation of this
kind, in time be cut through, and left standing at a height never
again to be reached by the water. With respect to the three upper
terraces of the Santa Cruz, I think there can be no doubt, that
they were modelled by the sea, when the valley was occupied by a
strait, in the same manner (hereafter to be discussed) as the
greater step-formed, shell- strewed plains along the coast of
Patagonia.
To return to the shores of the Atlantic: the 840 feet plain, at
the mouth of the Santa Cruz, is seen extending horizontally far
to the south; and I am informed by the Officers of the Survey,
that bending round the head of Coy Inlet (sixty-five miles
southward), it trends inland. Outliers of apparently the same
height are seen forty miles farther south, inland of the river
Gallegos; and a plain comes down to Cape Gregory (thirty-five
miles southward), in the Strait of Magellan, which was estimated
at between eight hundred and one thousand feet in height, and
which, rising towards the interior, is capped by the boulder
formation. South of the Strait of Magellan, there are large
outlying masses of apparently the same great tableland, extending
at intervals along the eastern coast of Tierra del Fuego: at two
places here, 110 miles a part, this plain was found to be 950 and
970 feet in height.
From Coy Inlet, where the high summit-plain trends inland, a
plain estimated at 350 feet in height, extends for forty miles to
the river Gallegos. From this point to the Strait of Magellan,
and on each side of that Strait, the country has been much
denuded and is less level. It consists chiefly of the boulder
formation, which rises to a height of between one hundred and
fifty and two hundred and fifty feet, and is often capped by beds
of gravel. At N.S. Gracia, on the north side of the Inner Narrows
of the Strait of Magellan, I found on the summit of a cliff, 160
feet in height, shells of existing Patellae and Mytili, scattered
on the surface and partially embedded in earth. On the eastern
coast, also, of Tierra del Fuego, in latitude 53 degrees 20'
south, I found many Mytili on some level land, estimated at 200
feet in height. Anterior to the elevation attested by these
shells, it is evident by the present form of the land, and by the
distribution of the great erratic boulders on the surface, that
two sea-channels connected the Strait of Magellan both with
Sebastian Bay and with Otway Water. ("Geological Transactions"
volume 6 page 419.)
CONCLUDING REMARKS ON THE RECENT ELEVATION OF THE SOUTH-EASTERN
COASTS OF AMERICA, AND ON THE ACTION OF THE SEA ON THE LAND.
Upraised shells of species, still existing as the commonest kinds
in the adjoining sea, occur, as we have seen, at heights of
between a few feet and 410 feet, at intervals from latitude 33
degrees 40' to 53 degrees 20' south. This is a distance of 1,180
geographical miles—about equal from London to the North Cape of
Sweden. As the boulder formation extends with nearly the same
height 150 miles south of 53 degrees 20', the most southern point
where I landed and found upraised shells; and as the level Pampas
ranges many hundred miles northward of the point, where M.
d'Orbigny found at the height of 100 feet beds of the Azara, the
space in a north and south line, which has been uplifted within
the recent period, must have been much above the 1,180 miles. By
the term "recent," I refer only to that period within which the
now living mollusca were called into existence; for it will be
seen in the Fourth Chapter, that both at Bahia Blanca and P. S.
Julian, the mammiferous quadrupeds which co-existed with these
shells belong to extinct species. I have said that the upraised
shells were found only at intervals on this line of coast, but
this in all probability may be attributed to my not having landed
at the intermediate points; for wherever I did land, with the
exception of the river Negro, shells were found: moreover, the
shells are strewed on plains or terraces, which, as we shall
immediately see, extend for great distances with a uniform
height. I ascended the higher plains only in a few places, owing
to the distance at which their escarpments generally range from
the coast, so that I am far from knowing that 410 feet is the
maximum of elevation of these upraised remains. The shells are
those now most abundant in a living state in the adjoining sea.
(Captain King "Voyages of 'Adventure' and 'Beagle'" volume 1
pages 6 and 133.) All of them have an ancient appearance; but
some, especially the mussels, although lying fully exposed to the
weather, retain to a considerable extent their colours: this
circumstance appears at first surprising, but it is now known
that the colouring principle of the Mytilus is so enduring, that
it is preserved when the shell itself is completely
disintegrated. (See Mr. Lyell "Proofs of a Gradual Rising in
Sweden" in the "Philosophical Transactions" 1835 page 1. See also
Mr. Smith of Jordan Hill in the "Edinburgh New Philosophical
Journal" volume 25 page 393.) Most of the shells are broken; I
nowhere found two valves united; the fragments are not rounded,
at least in none of the specimens which I brought home.
With respect to the breadth of the upraised area in an east and
west line, we know from the shells found at the Inner Narrows of
the Strait of Magellan, that the entire width of the plain,
although there very narrow, has been elevated. It is probable
that in this southernmost part of the continent, the movement has
extended under the sea far eastward; for at the Falkland Islands,
though I could not find any shells, the bones of whales have been
noticed by several competent observers, lying on the land at a
considerable distance from the sea, and at the height of some
hundred feet above it. ("Voyages of the 'Adventure' and 'Beagle'"
volume 2 page 227. And Bougainville's "Voyage" tome 1 page 112.)
Moreover, we know that in Tierra del Fuego the boulder formation
has been uplifted within the recent period, and a similar
formation occurs on the north-western shores (Byron Sound) of
these islands. (I owe this fact to the kindness of Captain
Sulivan, R.N., a highly competent observer. I mention it more
especially, as in my Paper (page 427) on the Boulder Formation, I
have, after having examined the northern and middle parts of the
eastern island, said that the formation was here wholly absent.)
The distance from this point to the Cordillera of Tierra del
Fuego, is 360 miles, which we may take as the probable width of
the recently upraised area. In the latitude of the R. Santa Cruz,
we know from the shells found at the mouth and head, and in the
middle of the valley, that the entire width (about 160 miles) of
the surface eastward of the Cordillera has been upraised. From
the slope of the plains, as shown by the course of the rivers,
for several degrees northward of the Santa Cruz, it is probable
that the elevation attested by the shells on the coast has
likewise extended to the Cordillera. When, however, we look as
far northward as the provinces of La Plata, this conclusion would
be very hazardous; not only is the distance from Maldonado (where
I found upraised shells) to the Cordillera great, namely, 760
miles, but at the head of the estuary of the Plata, a N.N.E. and
S.S.W. range of tertiary volcanic rocks has been observed (This
volcanic formation will be described in Chapter IV. It is not
improbable that the height of the upraised shells at the head of
the estuary of the Plata, being greater than at Bahia Blanca or
at San Blas, may be owing to the upheaval of these latter places
having been connected with the distant line of the Cordillera,
whilst that of the provinces of La Plata was in connection with
the adjoining tertiary volcanic axis.), which may well indicate
an axis of elevation quite distinct from that of the Andes.
Moreover, in the centre of the Pampas in the chain of Cordova,
severe earthquakes have been felt (See Sir W. Parish's work on
"La Plata" page 242. For a notice of an earthquake which drained
a lake near Cordova, see also Temple's "Travels in Peru." Sir W.
Parish informs me, that a town between Salta and Tucuman (north
of Cordova) was formerly utterly overthrown by an earthquake.);
whereas at Mendoza, at the eastern foot of the Cordillera, only
gentle oscillations, transmitted from the shores of the Pacific,
have ever been experienced. Hence the elevation of the Pampas may
be due to several distinct axes of movement; and we cannot judge,
from the upraised shells round the estuary of the Plata, of the
breadth of the area uplifted within the recent period.
Not only has the above specified long range of coast been
elevated within the recent period, but I think it may be safely
inferred from the similarity in height of the gravel-capped
plains at distant points, that there has been a remarkable degree
of equability in the elevatory process. I may premise, that when
I measured the plains, it was simply to ascertain the heights at
which shells occurred; afterwards, comparing these measurements
with some of those made during the Survey, I was struck with
their uniformity, and accordingly tabulated all those which
represented the summit-edges of plains. The extension of the 330
to 355 feet plain is very striking, being found over a space of
500 geographical miles in a north and south line. A table (Table
1) of the measurements is given below. The angular measurements
and all the estimations (in feet) are by the Officers of the
Survey; the barometrical ones by myself:—
TABLE 1.
Gallegos River to Coy Inlet (partly angular partly estimation)
350 South Side of Santa Cruz (angular and barometric) 355 North
Side of Santa Cruz (angular and barometric) 330 Bird Island,
plain opposite to (angular) 350 Port Desire, plain extending far
along coast (barometric) 330 St. George's Bay, north promontory
(angular) 330 Table Land, south of New Bay (angular) 350
A plain, varying from 245 to 255 feet, seems to extend with much
uniformity from Port Desire to the north of St. George's Bay, a
distance of 170 miles; and some approximate measurements (in
feet), also given in Table 2 below, indicate the much greater
extension of 780 miles:—
TABLE 2.
Coy Inlet, south of (partly angular and partly estimation) 200 to
300 Port Desire (barometric) 245 to 255 C. Blanco (angular) 250
North Promontory of St. George's Bay (angular) 250 South of New
Bay (angular) 200 to 220 North of S. Josef (estimation) 200 to
300 Plain of Rio Negro (angular) 200 to 220 Bahia Blanca
(estimation) 200 to 300
The extension, moreover, of the 560 to 580, and of the 80 to 100
feet, plains is remarkable, though somewhat less obvious than in
the former cases. Bearing in mind that I have not picked these
measurements out of a series, but have used all those which
represented the edges of plains, I think it scarcely possible
that these coincidences in height should be accidental. We must
therefore conclude that the action, whatever it may have been, by
which these plains have been modelled into their present forms,
has been singularly uniform.
These plains or great terraces, of which three and four often
rise like steps one behind the other, are formed by the
denudation of the old Patagonian tertiary beds, and by the
deposition on their surfaces of a mass of well-rounded gravel,
varying, near the coast, from ten to thirty-five feet in
thickness, but increasing in thickness towards the interior. The
gravel is often capped by a thin irregular bed of sandy earth.
The plains slope up, though seldom sensibly to the eye, from the
summit edge of one escarpment to the foot of the next highest
one. Within a distance of 150 miles, between Santa Cruz to Port
Desire, where the plains are particularly well developed, there
are at least seven stages or steps, one above the other. On the
three lower ones, namely, those of 100 feet, 250 feet, and 350
feet in height, existing littoral shells are abundantly strewed,
either on the surface, or partially embedded in the superficial
sandy earth. By whatever action these three lower plains have
been modelled, so undoubtedly have all the higher ones, up to a
height of 950 feet at S. Julian, and of 1,200 feet (by
estimation) along St. George's Bay. I think it will not be
disputed, considering the presence of the upraised marine shells,
that the sea has been the active power during stages of some kind
in the elevatory process.
We will now briefly consider this subject: if we look at the
existing coast-line, the evidence of the great denuding power of
the sea is very distinct; for, from Cape St. Diego, in latitude
54 degrees 30' to the mouth of the Rio Negro, in latitude 31
degrees (a length of more than eight hundred miles), the shore is
formed, with singularly few exceptions, of bold and naked cliffs:
in many places the cliffs are high; thus, south of the Santa
Cruz, they are between eight and nine hundred feet in height,
with their horizontal strata abruptly cut off, showing the
immense mass of matter which has been removed. Nearly this whole
line of coast consists of a series of greater or lesser curves,
the horns of which, and likewise certain straight projecting
portions, are formed of hard rocks; hence the concave parts are
evidently the effect and the measure of the denuding action on
the softer strata. At the foot of all the cliffs, the sea shoals
very gradually far outwards; and the bottom, for a space of some
miles, everywhere consists of gravel. I carefully examined the
bed of the sea off the Santa Cruz, and found that its inclination
was exactly the same, both in amount and in its peculiar
curvature, with that of the 355 feet plain at this same place.
If, therefore, the coast, with the bed of the adjoining sea, were
now suddenly elevated one or two hundred feet, an inland line of
cliffs, that is an escarpment, would be formed, with a
gravel-capped plain at its foot gently sloping to the sea, and
having an inclination like that of the existing 355 feet plain.
From the denuding tendency of the sea, this newly formed plain
would in time be eaten back into a cliff: and repetitions of this
elevatory and denuding process would produce a series of
gravel-capped sloping terraces, rising one above another, like
those fronting the shores of Patagonia.
The chief difficulty (for there are other inconsiderable ones) on
this view, is the fact,—as far as I can trust two continuous
lines of soundings carefully taken between Santa Cruz and the
Falkland Islands, and several scattered observations on this and
other coasts,—that the pebbles at the bottom of the sea QUICKLY
and REGULARLY decrease in size with the increasing depth and
distance from the shore, whereas in the gravel on the sloping
plains, no such decrease in size was perceptible.
Table 3 below gives the average result of many soundings off the
Santa Cruz:— TABLE 3.
Under two miles from the shore, many of the pebbles were of large
size, mingled with some small ones.
Column 1. Distance in miles from the shore.
Column 2. Depth in fathoms.
Column 3. Size of Pebbles.
I particularly attended to the size of the pebbles on the 355
feet Santa Cruz plain, and I noticed that on the summit-edge of
the present sea cliffs many were as large as half a man's head;
and in crossing from these cliffs to the foot of the next highest
escarpment, a distance of six miles, I could not observe any
increase in their size. We shall presently see that the theory of
a slow and almost insensible rise of the land, will explain all
the facts connected with the gravel-capped terraces, better than
the theory of sudden elevations of from one to two hundred feet.
M. d'Orbigny has argued, from the upraised shells at San Blas
being embedded in the positions in which they lived, and from the
valves of the Azara labiata high on the banks of the Parana being
united and unrolled, that the elevation of Northern Patagonia and
of La Plata must have been sudden; for he thinks, if it had been
gradual, these shells would all have been rolled on successive
beach-lines. But in PROTECTED bays, such as in that of Bahia
Blanca, wherever the sea is accumulating extensive mud-banks, or
where the winds quietly heap up sand-dunes, beds of shells might
assuredly be preserved buried in the positions in which they had
lived, even whilst the land retained the same level; any, the
smallest, amount of elevation would directly aid in their
preservation. I saw a multitude of spots in Bahia Blanca where
this might have been effected; and at Maldonado it almost
certainly has been effected. In speaking of the elevation of the
land having been slow, I do not wish to exclude the small starts
which accompany earthquakes, as on the coast of Chile; and by
such movements beds of shells might easily be uplifted, even in
positions exposed to a heavy surf, without undergoing any
attrition: for instance, in 1835, a rocky flat off the island of
Santa Maria was at one blow upheaved above high-water mark, and
was left covered with gaping and putrefying mussel-shells, still
attached to the bed on which they had lived. If M. d'Orbigny had
been aware of the many long parallel lines of sand-hillocks, with
infinitely numerous shells of the Mactra and Venus, at a low
level near the Uruguay; if he had seen at Bahia Blanca the
immense sand-dunes, with water-worn pebbles of pumice, ranging in
parallel lines, one behind the other, up a height of at least 120
feet; if he had seen the sand-dunes, with the countless
Paludestrinas, on the low plain near the Fort at this place, and
that long line on the edge of the cliff, sixty feet higher up; if
he had crossed that long and great belt of parallel sand-dunes,
eight miles in width, standing at the height of from forty to
fifty feet above the Colorado, where sand could not now
collect,—I cannot believe he would have thought that the
elevation of this great district had been sudden. Certainly the
sand-dunes (especially when abounding with shells), which stand
in ranges at so many different levels, must all have required
long time for their accumulation; and hence I do not doubt that
the last 100 feet of elevation of La Plata and Northern Patagonia
has been exceedingly slow.
If we extend this conclusion to Central and Southern Patagonia,
the inclination of the successively rising gravel-capped plains
can be explained quite as well, as by the more obvious view
already given of a few comparatively great and sudden elevations;
in either case we must admit long periods of rest, during which
the sea ate deeply into the land. Let us suppose the present
coast to rise at a nearly equable, slow rate, yet sufficiently
quick to prevent the waves quite removing each part as soon as
brought up; in this case every portion of the present bed of the
sea will successively form a beach-line, and from being exposed
to a like action will be similarly affected. It cannot matter to
what height the tides rise, even if to forty feet as at Santa
Cruz, for they will act with equal force and in like manner on
each successive line. Hence there is no difficulty in the fact of
the 355 feet plain at Santa Cruz sloping up 108 feet to the foot
of the next highest escarpment, and yet having no marks of any
one particular beach-line on it; for the whole surface on this
view has been a beach. I cannot pretend to follow out the precise
action of the tidal-waves during a rise of the land, slow, yet
sufficiently quick to prevent or check denudation: but if it be
analogous to what takes place on protected parts of the present
coast, where gravel is now accumulating in large quantities, an
inclined surface, thickly capped by well-rounded pebbles of about
the same size, would be ultimately left. (On the eastern side of
Chiloe, which island we shall see in the next chapter is now
rising, I observed that all the beaches and extensive tidal-flats
were formed of shingle.) On the gravel now accumulating, the
waves, aided by the wind, sometimes throw up a thin covering of
sand, together with the common coast-shells. Shells thus cast up
by gales, would, during an elevatory period, never again be
touched by the sea. Hence, on this view of a slow and gradual
rising of the land, interrupted by periods of rest and
denudation, we can understand the pebbles being of about the same
size over the entire width of the step-like plains,—the
occasional thin covering of sandy earth,—and the presence of
broken, unrolled fragments of those shells, which now live
exclusively near the coast.
A SUMMARY OF RESULTS.
It may be concluded that the coast on this side of the continent,
for a space of at least 1,180 miles, has been elevated to a
height of 100 feet in La Plata, and of 400 feet in Southern
Patagonia, within the period of existing shells, but not of
existing mammifers. That in La Plata the elevation has been very
slowly effected: that in Patagonia the movement may have been by
considerable starts, but much more probably slow and quiet. In
either case, there have been long intervening periods of
comparative rest, during which the sea corroded deeply, as it is
still corroding, into the land. (I say COMPARATIVE and not
ABSOLUTE rest, because the sea acts, as we have seen, with great
denuding power on this whole line of coast; and therefore, during
an elevation of the land, if excessively slow (and of course
during a subsidence of the land), it is quite possible that lines
of cliff might be formed.) That the periods of denudation and
elevation were contemporaneous and equable over great spaces of
coast, as shown by the equable heights of the plains; that there
have been at least eight periods of denudation, and that the
land, up to a height of from 950 to 1,200 feet, has been
similarly modelled and affected: that the area elevated, in the
southernmost part of the continent, extended in breadth to the
Cordillera, and probably seaward to the Falkland Islands; that
northward, in La Plata, the breadth is unknown, there having been
probably more than one axis of elevation; and finally, that,
anterior to the elevation attested by these upraised shells, the
land was divided by a Strait where the River Santa Cruz now
flows, and that further southward there were other sea-straits,
since closed. I may add, that at Santa Cruz, in latitude 50
degrees S., the plains have been uplifted at least 1,400 feet,
since the period when gigantic boulders were transported between
sixty and seventy miles from their parent rock, on floating
icebergs.
Lastly, considering the great upward movements which this long
line of coast has undergone, and the proximity of its southern
half to the volcanic axis of the Cordillera, it is highly
remarkable that in the many fine sections exposed in the Pampean,
Patagonian tertiary, and Boulder formations, I nowhere observed
the smallest fault or abrupt curvature in the strata.
GRAVEL FORMATION OF PATAGONIA.
I will here describe in more detail than has been as yet
incidentally done, the nature, origin, and extent of the great
shingle covering of Patagonia: but I do not mean to affirm that
all of this shingle, especially that on the higher plains,
belongs to the recent period. A thin bed of sandy earth, with
small pebbles of various porphyries and of quartz, covering a low
plain on the north side of the Rio Colorado, is the extreme
northern limit of this formation. These little pebbles have
probably been derived from the denudation of a more regular bed
of gravel, capping the old tertiary sandstone plateau of the Rio
Negro. The gravel-bed near the Rio Negro is, on an average, about
ten or twelve feet in thickness; and the pebbles are larger than
on the northern side of the Colorado, being from one or two
inches in diameter, and composed chiefly of rather dark-tinted
porphyries. Amongst them I here first noticed a variety often to
be referred to, namely, a peculiar gallstone-yellow siliceous
porphyry, frequently, but not invariably, containing grains of
quartz. The pebbles are embedded in a white, gritty, calcareous
matrix, very like mortar, sometimes merely coating with a
whitewash the separate stones, and sometimes forming the greater
part of the mass. In one place I saw in the gravel concretionary
nodules (not rounded) of crystallised gypsum, some as large as a
man's head. I traced this bed for forty-five miles inland, and
was assured that it extended far into the interior. As the
surface of the calcareo- argillaceous plain of Pampean formation,
on the northern side of the wide valley of the Colorado, stands
at about the same height with the mortar- like cemented gravel
capping the sandstone on the southern side, it is probable,
considering the apparent equability of the subterranean movements
along this side of America, that this gravel of the Rio Negro and
the upper beds of the Pampean formation northward of the
Colorado, are of nearly contemporaneous origin, and that the
calcareous matter has been derived from the same source.
Southward of the Rio Negro, the cliffs along the great bay of S.
Antonio are capped with gravel: at San Josef, I found that the
pebbles closely resembled those on the plain of the Rio Negro,
but that they were not cemented by calcareous matter. Between San
Josef and Port Desire, I was assured by the Officers of the
Survey that the whole face of the country is coated with gravel.
At Port Desire and over a space of twenty-five miles inland, on
the three step-formed plains and in the valleys, I everywhere
passed over gravel which, where thickest, was between thirty and
forty feet. Here, as in other parts of Patagonia, the gravel, or
its sandy covering, was, as we have seen, often strewed with
recent marine shells. The sandy covering sometimes fills up
furrows in the gravel, as does the gravel in the underlying
tertiary formations. The pebbles are frequently whitewashed and
even cemented together by a peculiar, white, friable, aluminous,
fusible substance, which I believe is decomposed feldspar. At
Port Desire, the gravel rested sometimes on the basal formation
of porphyry, and sometimes on the upper or the lower denuded
tertiary strata. It is remarkable that most of the porphyritic
pebbles differ from those varieties of porphyry which occur here
abundantly in situ. The peculiar gallstone-yellow variety was
common, but less numerous than at Port S. Julian, where it formed
nearly one-third of the mass of the gravel; the remaining part
there consisting of pale grey and greenish porphyries with many
crystals of feldspar. At Port S. Julian, I ascended one of the
flat- topped hills, the denuded remnant of the highest plain, and
found it, at the height of 950 feet, capped with the usual bed of
gravel.
Near the mouth of the Santa Cruz, the bed of gravel on the 355
feet plain is from twenty to about thirty-five feet in thickness.
The pebbles vary from minute ones to the size of a hen's egg, and
even to that of half a man's head; they consist of paler
varieties of porphyry than those found further northward, and
there are fewer of the gallstone-yellow kind; pebbles of compact
black clay-slate were here first observed. The gravel, as we have
seen, covers the step-formed plains at the mouth, head, and on
the sides of the great valley of the Santa Cruz. At a distance of
110 miles from the coast, the plain has risen to the height of
1,416 feet above the sea; and the gravel, with the associated
great boulder formation, has attained a thickness of 212 feet.
The plain, apparently with its usual gravel covering, slopes up
to the foot of the Cordillera to the height of between 3,200 and
3,300 feet. In ascending the valley, the gravel gradually becomes
entirely altered in character: high up, we have pebbles of
crystalline feldspathic rocks, compact clay-slate, quartzose
schists, and pale-coloured porphyries; these rocks, judging both
from the gigantic boulders in the surface and from some small
pebbles embedded beneath 700 feet in thickness of the old
tertiary strata, are the prevailing kinds in this part of the
Cordillera; pebbles of basalt from the neighbouring streams of
basaltic lava are also numerous; there are few or none of the
reddish or of the gallstone-yellow porphyries so common near the
coast. Hence the pebbles on the 350 feet plain at the mouth of
the Santa Cruz cannot have been derived (with the exception of
those of compact clay- slate, which, however, may equally well
have come from the south) from the Cordillera in this latitude;
but probably, in chief part, from farther north.
Southward of the Santa Cruz, the gravel may be seen continuously
capping the great 840 feet plain: at the Rio Gallegos, where this
plain is succeeded by a lower one, there is, as I am informed by
Captain Sulivan, an irregular covering of gravel from ten to
twelve feet in thickness over the whole country. The district on
each side of the Strait of Magellan is covered up either with
gravel or the boulder formation: it was interesting to observe
the marked difference between the perfectly rounded state of the
pebbles in the great shingle formation of Patagonia, and the more
or less angular fragments in the boulder formation. The pebbles
and fragments near the Strait of Magellan nearly all belong to
rocks known to occur in Fuegia. I was therefore much surprised in
dredging south of the Strait to find, in latitude 54 degrees 10'
south, many pebbles of the gallstone-yellow siliceous porphyry; I
procured others from a great depth off Staten Island, and others
were brought me from the western extremity of the Falkland
Islands. (At my request, Mr. Kent collected for me a bag of
pebbles from the beach of White Rock harbour, in the northern
part of the sound, between the two Falkland Islands. Out of these
well-rounded pebbles, varying in size from a walnut to a hen's
egg, with some larger, thirty-eight evidently belonged to the
rocks of these islands; twenty-six were similar to the pebbles of
porphyry found on the Patagonian plains, which rocks do not exist
in situ in the Falklands; one pebble belonged to the peculiar
yellow siliceous porphyry; thirty were of doubtful origin.) The
distribution of the pebbles of this peculiar porphyry, which I
venture to affirm is not found in situ either in Fuegia, the
Falkland Islands, or on the coast of Patagonia, is very
remarkable, for they are found over a space of 840 miles in a
north and south line, and at the Falklands, 300 miles eastward of
the coast of Patagonia. Their occurrence in Fuegia and the
Falklands may, however, perhaps be due to the same ice-agency by
which the boulders have been there transported.
We have seen that porphyritic pebbles of a small size are first
met with on the northern side of the Rio Colorado, the bed
becoming well developed near the Rio Negro: from this latter
point I have every reason to believe that the gravel extends
uninterruptedly over the plains and valleys of Patagonia for at
least 630 nautical miles southward to the Rio Gallegos. From the
slope of the plains, from the nature of the pebbles, from their
extension at the Rio Negro far into the interior, and at the
Santa Cruz close up to the Cordillera, I think it highly probable
that the whole breadth of Patagonia is thus covered. If so, the
average width of the bed must be about two hundred miles. Near
the coast the gravel is generally from ten to thirty feet in
thickness; and as in the valley of Santa Cruz it attains, at some
distance from the Cordillera, a thickness of 214 feet, we may, I
think, safely assume its average thickness over the whole area of
630 by 200 miles, at fifty feet!
The transportal and origin of this vast bed of pebbles is an
interesting problem. From the manner in which they cap the
step-formed plains, worn by the sea within the period of existing
shells, their deposition, at least on the plains up to a height
of 400 feet, must have been a recent geological event. From the
form of the continent, we may feel sure that they have come from
the westward, probably, in chief part from the Cordillera, but,
perhaps, partly from unknown rocky ridges in the central
districts of Patagonia. That the pebbles have not been
transported by rivers, from the interior towards the coast, we
may conclude from the fewness and smallness of the streams of
Patagonia: moreover, in the case of the one great and rapid river
of Santa Cruz, we have good evidence that its transporting power
is very trifling. This river is from two to three hundred yards
in width, about seventeen feet deep in its middle, and runs with
a singular degree of uniformity five knots an hour, with no lakes
and scarcely any still reaches: nevertheless, to give one
instance of its small transporting power, upon careful
examination, pebbles of compact basalt could not be found in the
bed of the river at a greater distance than ten miles below the
point where the stream rushes over the debris of the great
basaltic cliffs forming its shore: fragments of the CELLULAR
varieties have been washed down twice or thrice as far. That the
pebbles in Central and Northern Patagonia have not been
transported by ice-agency, as seems to have been the case to a
considerable extent farther south, and likewise in the northern
hemisphere, we may conclude, from the absence of all angular
fragments in the gravel, and from the complete contrast in many
other respects between the shingle and neighbouring boulder
formation.
Looking to the gravel on any one of the step-formed plains, I
cannot doubt, from the several reasons assigned in this chapter,
that it has been spread out and leveled by the long-continued
action of the sea, probably during the slow rise of the land. The
smooth and perfectly rounded condition of the innumerable pebbles
alone would prove long-continued action. But how the whole mass
of shingle on the coast-plains has been transported from the
mountains of the interior, is another and more difficult
question. The following considerations, however, show that the
sea by its ordinary action has considerable power in distributing
pebbles. Table 3 above shows how very uniformly and gradually the
pebbles decrease in size with the gradually seaward increasing
depth and distance. (I may mention, that at the distance of 150
miles from the Patagonian shore I carefully examined the minute
rounded particles in the sand, and found them to be fusible like
the porphyries of the great shingle bed. I could even distinguish
particles of the gallstone-yellow porphyry. It was interesting to
notice how gradually the particles of white quartz increased, as
we approached the Falkland Islands, which are thus constituted.
In the whole line of soundings between these islands and the
coast of Patagonia dead or living organic remains were most rare.
On the relations between the depth of water and the nature of the
bottom, see Martin White on "Soundings in the Channel" pages 4,
6, 175; also Captain Beechey's "Voyage to the Pacific" chapter
18.) A series of this kind irresistibly leads to the conclusion,
that the sea has the power of sifting and distributing the loose
matter on its bottom. According to Martin White, the bed of the
British Channel is disturbed during gales at depths of
sixty-three and sixty-seven fathoms, and at thirty fathoms,
shingle and fragments of shells are often deposited, afterwards
to be carried away again. ("Soundings in the Channel" pages 4,
166. M. Siau states ("Edinburgh New Philosophical Journal" volume
31 page 246), that he found the sediment, at a depth of 188
metres, arranged in ripples of different degrees of fineness.
There are some excellent discussions on this and allied subjects
in Sir H. De la Beche's "Theoretical Researches.") Groundswells,
which are believed to be caused by distant gales, seem especially
to affect the bottom: at such times, according to Sir R.
Schomburgk, the sea to a great distance round the West Indian
Islands, at depths from five to fifteen fathoms, becomes
discoloured, and even the anchors of vessels have been moved.
("Journal of Royal Geographical Society" volume 5 page 25. It
appears from Mr. Scott Russell's investigations (see Mr.
Murchison's "Anniversary Address Geological Society" 1843 page
40), that in waves of translation the motion of the particles of
water is nearly as great at the bottom as at the top.) There are,
however, some difficulties in understanding how the sea can
transport pebbles lying at the bottom, for, from experiments
instituted on the power of running water, it would appear that
the currents of the sea have not sufficient velocity to move
stones of even moderate size: moreover, I have repeatedly found
in the most exposed situations that the pebbles which lie at the
bottom are encrusted with full-grown living corallines, furnished
with the most delicate, yet unbroken spines: for instance, in ten
fathoms water off the mouth of the Santa Cruz, many pebbles,
under half an inch in diameter, were thus coated with Flustracean
zoophytes. (A pebble, one and a half inch square and half an inch
thick, was given me, dredged up from twenty-seven fathoms depth
off the western end of the Falkland Islands, where the sea is
remarkably stormy, and subject to violent tides. This pebble was
encrusted on all sides by a delicate living coralline. I have
seen many pebbles from depths between forty and seventy fathoms
thus encrusted; one from the latter depth off Cape Horn.) Hence
we must conclude that these pebbles are not often violently
disturbed: it should, however, be borne in mind that the growth
of corallines is rapid. The view, propounded by Professor
Playfair, will, I believe, explain this apparent
difficulty,—namely, that from the undulations of the sea TENDING
to lift up and down pebbles or other loose bodies at the bottom,
such are liable, when thus quite or partially raised, to be moved
even by a very small force, a little onwards. We can thus
understand how oceanic or tidal currents of no great strength, or
that recoil movement of the bottom-water near the land, called by
sailors the "undertow" (which I presume must extend out seaward
as far as the BREAKING waves impel the surface-water towards the
beach), may gain the power during storms of sifting and
distributing pebbles even of considerable size, and yet without
so violently disturbing them as to injure the encrusting
corallines. (I may take this opportunity of remarking on a
singular, but very common character in the form of the bottom, in
the creeks which deeply penetrate the western shores of Tierra
del Fuego; namely, that they are almost invariably much shallower
close to the open sea at their mouths than inland. Thus, Cook, in
entering Christmas Sound, first had soundings in thirty-seven
fathoms, then in fifty, then in sixty, and a little farther in no
bottom with 170 fathoms. The sealers are so familiar with this
fact, that they always look out for anchorage near the entrances
of the creeks. See, also, on this subject, the "Voyages of the
'Adventure' and 'Beagle'" volume 1 page 375 and "Appendix" page
313. This Shoalness of the sea- channels near their entrances
probably results from the quantity of sediment formed by the wear
and tear of the outer rocks exposed to the full force of the open
sea. I have no doubt that many lakes, for instance in Scotland,
which are very deep within, and are separated from the sea
apparently only by a tract of detritus, were originally
sea-channels with banks of this nature near their mouths, which
have since been upheaved.)
The sea acts in another and distinct manner in the distribution
of pebbles, namely by the waves on the beach. Mr. Palmer, in his
excellent memoir on this subject, has shown that vast masses of
shingle travel with surprising quickness along lines of coast,
according to the direction with which the waves break on the
beach and that this is determined by the prevailing direction of
the winds. ("Philosophical Transactions" 1834 page 576.) This
agency must be powerful in mingling together and disseminating
pebbles derived from different sources: we may, perhaps, thus
understand the wide distribution of the gallstone-yellow
porphyry; and likewise, perhaps, the great difference in the
nature of the pebbles at the mouth of the Santa Cruz from those
in the same latitude at the head of the valley.
I will not pretend to assign to these several and complicated
agencies their shares in the distribution of the Patagonian
shingle: but from the several considerations given in this
chapter, and I may add, from the frequency of a capping of gravel
on tertiary deposits in all parts of the world, as I have myself
observed and seen stated in the works of various authors, I
cannot doubt that the power of widely dispersing gravel is an
ordinary contingent on the action of the sea; and that even in
the case of the great Patagonian shingle-bed we have no occasion
to call in the aid of debacles. I at one time imagined that
perhaps an immense accumulation of shingle had originally been
collected at the foot of the Cordillera; and that this
accumulation, when upraised above the level of the sea, had been
eaten into and partially spread out (as off the present line of
coast); and that the newly-spread out bed had in its turn been
upraised, eaten into, and re-spread out; and so onwards, until
the shingle, which was first accumulated in great thickness at
the foot of the Cordillera, had reached in thinner beds its
present extension. By whatever means the gravel formation of
Patagonia may have been distributed, the vastness of its area,
its thickness, its superficial position, its recent origin, and
the great degree of similarity in the nature of its pebbles, all
appear to me well deserving the attention of geologists, in
relation to the origin of the widely-spread beds of conglomerate
belonging to past epochs.
FORMATION OF CLIFFS.
(DIAGRAM 7.—SECTION OF COAST-CLIFFS AND BOTTOM OF SEA, OFF THE
ISLAND OF ST. HELENA.
Height in feet above sea level.
Depths in fathoms.
Vertical and horizontal scale, two inches to a nautical mile. The
point marked 1,600 feet is at the foot of High Knoll; point
marked 510 feet is on the edge of Ladder Hill. The strata consist
of basaltic streams.
A Section left to right:
Height at the foot of High Knoll: 1,600 at top of strata.
Height on the edge of Ladder Hill: 510 at top of strata.
Bottom at coast rocky only to a depth of five or six fathoms.
30 fathoms: bottom mud and sand.
100 fathoms sloping more sharply to 250 fathoms.)
When viewing the sea-worn cliffs of Patagonia, in some parts
between eight hundred and nine hundred feet in height, and formed
of horizontal tertiary strata, which must once have extended far
seaward—or again, when viewing the lofty cliffs round many
volcanic islands, in which the gentle inclination of the
lava-streams indicates the former extension of the land, a
difficulty often occurred to me, namely, how the strata could
possibly have been removed by the action of the sea at a
considerable depth beneath its surface. The section in Diagram 7,
which represents the general form of the land on the northern and
leeward side of St. Helena (taken from Mr. Seale's large model
and various measurements), and of the bottom of the adjoining sea
(taken chiefly from Captain Austin's survey and some old charts),
will show the nature of this difficulty.
If, as seems probable, the basaltic streams were originally
prolonged with nearly their present inclination, they must, as
shown by the dotted line in the section, once have extended at
least to a point, now covered by the sea to a depth of nearly
thirty fathoms: but I have every reason to believe they extended
considerably further, for the inclination of the streams is less
near the coast than further inland. It should also be observed,
that other sections on the coast of this island would have given
far more striking results, but I had not the exact measurements;
thus, on the windward side, the cliffs are about two thousand
feet in height and the cut-off lava streams very gently inclined,
and the bottom of the sea has nearly a similar slope all round
the island. How, then, has all the hard basaltic rock, which once
extended beneath the surface of the sea, been worn away?
According to Captain Austin, the bottom is uneven and rocky only
to that very small distance from the beach within which the depth
is from five to six fathoms; outside this line, to a depth of
about one hundred fathoms, the bottom is smooth, gently inclined,
and formed of mud and sand; outside the one hundred fathoms, it
plunges suddenly into unfathomable depths, as is so very commonly
the case on all coasts where sediment is accumulating. At greater
depths than the five or six fathoms, it seems impossible, under
existing circumstances, that the sea can both have worn away hard
rock, in parts to a thickness of at least 150 feet, and have
deposited a smooth bed of fine sediment. Now, if we had any
reason to suppose that St. Helena had, during a long period, gone
on slowly subsiding, every difficulty would be removed: for
looking at the diagram, and imagining a fresh amount of
subsidence, we can see that the waves would then act on the
coast-cliffs with fresh and unimpaired vigour, whilst the rocky
ledge near the beach would be carried down to that depth, at
which sand and mud would be deposited on its bare and uneven
surface: after the formation near the shore of a new rocky shoal,
fresh subsidence would carry it down and allow it to be smoothly
covered up. But in the case of the many cliff-bounded islands,
for instance in some of the Canary Islands and of Madeira, round
which the inclination of the strata shows that the land once
extended far into the depths of the sea, where there is no
apparent means of hard rock being worn away—are we to suppose
that all these islands have slowly subsided? Madeira, I may
remark, has, according to Mr. Smith of Jordan Hill, subsided. Are
we to extend this conclusion to the high, cliff- bound,
horizontally stratified shores of Patagonia, off which, though
the water is not deep even at the distance of several miles, yet
the smooth bottom of pebbles gradually decreasing in size with
the increasing depth, and derived from a foreign source, seem to
declare that the sea is now a depositing and not a corroding
agent? I am much inclined to suspect, that we shall hereafter
find in all such cases, that the land with the adjoining bed of
the sea has in truth subsided: the time will, I believe, come,
when geologists will consider it as improbable, that the land
should have retained the same level during a whole geological
period, as that the atmosphere should have remained absolutely
calm during an entire season.
CHAPTER II. ON THE ELEVATION OF THE WESTERN COAST OF SOUTH
AMERICA.
Chonos Archipelago. Chiloe, recent and gradual elevation of,
traditions of the inhabitants on this subject. Concepcion, earthquake
and elevation of. VALPARAISO, great elevation of, upraised shells,
earth of marine origin, gradual rise of the land within the
historical period. COQUIMBO, elevation of, in recent times; terraces
of marine origin, their inclination, their escarpments not
horizontal. Guasco, gravel terraces of. Copiapo. PERU. Upraised
shells of Cobija, Iquique, and Arica. Lima, shell-beds and sea-beach
on San Lorenzo, human remains, fossil earthenware, earthquake
debacle, recent subsidence. On the decay of upraised shells. General
summary.
Commencing at the south and proceeding northward, the first place
at which I landed, was at Cape Tres Montes, in latitude 46
degrees 35'. Here, on the shores of Christmas Cove, I observed in
several places a beach of pebbles with recent shells, about
twenty feet above high-water mark. Southward of Tres Montes
(between latitude 47 and 48 degrees), Byron remarks, "We thought
it very strange, that upon the summits of the highest hills were
found beds of shells, a foot or two thick." ("Narrative of the
Loss of the 'Wager'.") In the Chonos Archipelago, the island of
Lemus (latitude 44 degrees 30') was, according to M. Coste,
suddenly elevated eight feet, during the earthquake of 1829: he
adds, "Des roches jadis toujours couvertes par la mer, restant
aujourd'hui constamment decouvertes." ("Comptes Rendus" October
1838 page 706.) In other parts of this archipelago, I observed
two terraces of gravel, abutting to the foot of each other: at
Lowe's Harbour (43 degrees 48'), under a great mass of the
boulder formation, about three hundred feet in thickness, I found
a layer of sand, with numerous comminuted fragments of
sea-shells, having a fresh aspect, but too small to be
identified.
THE ISLAND OF CHILOE.
The evidence of recent elevation is here more satisfactory. The
bay of San Carlos is in most parts bounded by precipitous cliffs
from about ten to forty feet in height, their bases being
separated from the present line of tidal action by a talus, a few
feet in height, covered with vegetation. In one sheltered creek
(west of P. Arena), instead of a loose talus, there was a bare
sloping bank of tertiary mudstone, perforated, above the line of
the highest tides, by numerous shells of a Pholas now common in
the harbour. The upper extremities of these shells, standing
upright in their holes with grass growing out of them, were
abraded about a quarter of an inch, to the same level with the
surrounding worn strata. In other parts, I observed (as at
Pudeto) a great beach, formed of comminuted shells, twenty feet
above the present shore. In other parts again, there were small
caves worn into the foot of the low cliffs, and protected from
the waves by the talus with its vegetation: one such cave, which
I examined, had its mouth about twenty feet, and its bottom,
which was filled with sand containing fragments of shells and
legs of crabs, from eight to ten feet above high-water mark. From
these several facts, and from the appearance of the upraised
shells, I inferred that the elevation had been quite recent; and
on inquiring from Mr. Williams, the Portmaster, he told me he was
convinced that the land had risen, or the sea fallen, four feet
within the last four years. During this period, there had been
one severe earthquake, but no particular change of level was then
observed; from the habits of the people who all keep boats in the
protected creeks, it is absolutely impossible that a rise of four
feet could have taken place suddenly and been unperceived. Mr.
Williams believes that the change has been quite gradual. Without
the elevatory movement continues at a quick rate, there can be no
doubt that the sea will soon destroy the talus of earth at the
foot of the cliffs round the bay, and will then reach its former
lateral extension, but not of course its former level: some of
the inhabitants assured me that one such talus, with a footpath
on it, was even already sensibly decreasing in width.
I received several accounts of beds of shells, existing at
considerable heights in the inland parts of Chiloe; and to one of
these, near Catiman, I was guided by a countryman. Here, on the
south side of the peninsula of Lacuy, there was an immense bed of
the Venus costellata and of an oyster, lying on the summit-edge
of a piece of tableland, 350 feet (by the barometer) above the
level of the sea. The shells were closely packed together,
embedded in and covered by a very black, damp, peaty mould, two
or three feet in thickness, out of which a forest of great trees
was growing. Considering the nature and dampness of this peaty
soil, it is surprising that the fine ridges on the outside of the
Venus are perfectly preserved, though all the shells have a
blackened appearance. I did not doubt that the black soil, which
when dry, cakes hard, was entirely of terrestrial origin, but on
examining it under the microscope, I found many very minute
rounded fragments of shells, amongst which I could distinguish
bits of Serpulae and mussels. The Venus costellata, and the
Ostrea (O. edulis, according to Captain King) are now the
commonest shells in the adjoining bays. In a bed of shells, a few
feet below the 350 feet bed, I found a horn of the little Cervus
humilis, which now inhabits Chiloe.
The eastern or inland side of Chiloe, with its many adjacent
islets, consists of tertiary and boulder deposits, worn into
irregular plains capped by gravel. Near Castro, and for ten miles
southward, and on the islet of Lemuy, I found the surface of the
ground to a height of between twenty and thirty feet above
high-water mark, and in several places apparently up to fifty
feet, thickly coated by much comminuted shells, chiefly of the
Venus costellata and Mytilus Chiloensis; the species now most
abundant on this line of coast. As the inhabitants carry immense
numbers of these shells inland, the continuity of the bed at the
same height was often the only means of recognising its natural
origin. Near Castro, on each side of the creek and rivulet of the
Gamboa, three distinct terraces are seen: the lowest was
estimated at about one hundred and fifty feet in height, and the
highest at about five hundred feet, with the country irregularly
rising behind it; obscure traces, also, of these same terraces
could be seen along other parts of the coast. There can be no
doubt that their three escarpments record pauses in the elevation
of the island. I may remark that several promontories have the
word Huapi, which signifies in the Indian tongue, island,
appended to them, such as Huapilinao, Huapilacuy, Caucahuapi,
etc.; and these, according to Indian traditions, once existed as
islands. In the same manner the term Pulo in Sumatra is appended
to the names of promontories, traditionally said to have been
islands (Marsden's "Sumatra" page 31.); in Sumatra, as in Chiloe,
there are upraised recent shells. The Bay of Carelmapu, on the
mainland north of Chiloe, according to Aguerros, was in 1643 a
good harbour ("Descripcion Hist. de la Provincia de Chiloe" page
78. From the account given by the old Spanish writers, it would
appear that several other harbours, between this point and
Concepcion, were formerly much deeper than they now are.); it is
now quite useless, except for boats.
VALDIVIA.
I did not observe here any distinct proofs of recent elevation;
but in a bed of very soft sandstone, forming a fringe-like plain,
about sixty feet in height, round the hills of mica-slate, there
are shells of Mytilus, Crepidula, Solen, Novaculina, and
Cytheraea, too imperfect to be specifically recognised. At
Imperial, seventy miles north of Valdivia, Aguerros states that
there are large beds of shells, at a considerable distance from
the coast, which are burnt for lime. (Ibid page 25.) The island
of Mocha, lying a little north of Imperial, was uplifted two
feet, during the earthquake of 1835. ("Voyages of 'Adventure' and
'Beagle'" volume 2 page 415.)
CONCEPCION.
I cannot add anything to the excellent account by Captain Fitzroy
of the elevation of the land at this place, which accompanied the
earthquake of 1835. (Ibid volume 2 page 412 et seq. In volume 5
page 601 of the "Geological Transactions" I have given an account
of the remarkable volcanic phenomena, which accompanied this
earthquake. These phenomena appear to me to prove that the
action, by which large tracts of land are uplifted, and by which
volcanic eruptions are produced, is in every respect identical.)
I will only recall to the recollection of geologists, that the
southern end of the island of St. Mary was uplifted eight feet,
the central part nine, and the northern end ten feet; and the
whole island more than the surrounding districts. Great beds of
mussels, patellae, and chitons still adhering to the rocks were
upraised above high-water mark; and some acres of a rocky flat,
which was formerly always covered by the sea, was left standing
dry, and exhaled an offensive smell, from the many attached and
putrefying shells. It appears from the researches of Captain
Fitzroy that both the island of St. Mary and Concepcion (which
was uplifted only four or five feet) in the course of some weeks
subsided, and lost part of their first elevation. I will only add
as a lesson of caution, that round the sandy shores of the great
Bay of Concepcion, it was most difficult, owing to the
obliterating effects of the great accompanying wave, to recognise
any distinct evidence of this considerable upheaval; one spot
must be excepted, where there was a detached rock which before
the earthquake had always been covered by the sea, but afterwards
was left uncovered.
On the island of Quiriquina (in the Bay of Concepcion), I found,
at an estimated height of four hundred feet, extensive layers of
shells, mostly comminuted, but some perfectly preserved and
closely packed in black vegetable mould; they consisted of
Concholepas, Fissurella, Mytilus, Trochus, and Balanus. Some of
these layers of shells rested on a thick bed of bright-red, dry,
friable earth, capping the surface of the tertiary sandstone, and
extending, as I observed whilst sailing along the coast, for 150
miles southward: at Valparaiso, we shall presently see that a
similar red earthy mass, though quite like terrestrial mould, is
really in chief part of recent marine origin. On the flanks of
this island of Quiriquina, at a less height than the 400 feet,
there were spaces several feet square, thickly strewed with
fragments of similar shells. During a subsequent visit of the
"Beagle" to Concepcion, Mr. Kent, the assistant-surgeon, was so
kind as to make for me some measurements with the barometer: he
found many marine remains along the shores of the whole bay, at a
height of about twenty feet; and from the hill of Sentinella
behind Talcahuano, at the height of 160 feet, he collected
numerous shells, packed together close beneath the surface in
black earth, consisting of two species of Mytilus, two of
Crepidula, one of Concholepas, of Fissurella, Venus, Mactra,
Turbo, Monoceros, and the Balanus psittacus. These shells were
bleached, and within some of the Balani other Balani were
growing, showing that they must have long lain dead in the sea.
The above species I compared with living ones from the bay, and
found them identical; but having since lost the specimens, I
cannot give their names: this is of little importance, as Mr.
Broderip has examined a similar collection, made during Captain
Beechey's expedition, and ascertained that they consisted of ten
recent species, associated with fragments of Echini, crabs, and
Flustrae; some of these remains were estimated by Lieutenant
Belcher to lie at the height of nearly a thousand feet above the
level of the sea. ("Zoology of Captain Beechey's Voyage" page
162.) In some places round the bay, Mr. Kent observed that there
were beds formed exclusively of the Mytilus Chiloensis: this
species now lives in parts never uncovered by the tides. At
considerable heights, Mr. Kent found only a few shells; but from
the summit of one hill, 625 feet high, he brought me specimens of
the Concholepas, Mytilus Chiloensis, and a Turbo. These shells
were softer and more brittle than those from the height of 164
feet; and these latter had obviously a much more ancient
appearance than the same species from the height of only twenty
feet.
COAST NORTH OF CONCEPCION.
The first point examined was at the mouth of the Rapel (160 miles
north of Concepcion and sixty miles south of Valparaiso), where I
observed a few shells at the height of 100 feet, and some
barnacles adhering to the rocks three or four feet above the
highest tides: M. Gay found here recent shells at the distance of
two leagues from the shore. ("Annales des Scienc. Nat." Avril
1833.) Inland there are some wide, gravel-capped plains,
intersected by many broad, flat-bottomed valleys (now carrying
insignificant streamlets), with their sides cut into successive
wall-like escarpments, rising one above another, and in many
places, according to M. Gay, worn into caves. The one cave (C.
del Obispo) which I examined, resembled those formed on many
sea-coasts, with its bottom filled with shingle. These inland
plains, instead of sloping towards the coast, are inclined in an
opposite direction towards the Cordillera, like the successively
rising terraces on the inland or eastern side of Chiloe: some
points of granite, which project through the plains near the
coast, no doubt once formed a chain of outlying islands, on the
inland shores of which the plains were accumulated. At Bucalemu,
a few miles northward of the Rapel, I observed at the foot, and
on the summit-edge of a plain, ten miles from the coast, many
recent shells, mostly comminuted, but some perfect. There were,
also, many at the bottom of the great valley of the Maypu. At San
Antonio, shells are said to be collected and burnt for lime. At
the bottom of a great ravine (Quebrada Onda, on the road to Casa
Blanca), at the distance of several miles from the coast, I
noticed a considerable bed, composed exclusively of Mesodesma
donaciforme, Desh., lying on a bed of muddy sand: this shell now
lives associated together in great numbers, on tidal-flats on the
coast of Chile.
VALPARAISO.
During two successive years I carefully examined, part of the
time in company with Mr. Alison, into all the facts connected
with the recent elevation of this neighbourhood. In very many
parts a beach of broken shells, about fourteen or fifteen feet
above high-water mark, may be observed; and at this level the
coast-rocks, where precipitous, are corroded in a band. At one
spot, Mr. Alison, by removing some birds' dung, found at this
same level barnacles adhering to the rocks. For several miles
southward of the bay, almost every flat little headland, between
the heights of 60 and 230 feet (measured by the barometer), is
smoothly coated by a thick mass of comminuted shells, of the same
species, and apparently in the same proportional numbers with
those existing in the adjoining sea. The Concholepas is much the
most abundant, and the best preserved shell; but I extracted
perfectly preserved specimens of the Fissurella biradiata, a
Trochus and Balanus (both well-known, but according to Mr.
Sowerby yet unnamed) and parts of the Mytilus Chiloensis. Most of
these shells, as well as an encrusting Nullipora, partially
retain their colour; but they are brittle, and often stained red
from the underlying brecciated mass of primary rocks; some are
packed together, either in black or reddish moulds; some lie
loose on the bare rocky surfaces. The total number of these
shells is immense; they are less numerous, though still far from
rare, up a height of 1,000 feet above the sea. On the summit of a
hill, measured 557 feet, there was a small horizontal band of
comminuted shells, of which MANY consisted (and likewise from
lesser heights) of very young and small specimens of the still
living Concholepas, Trochus, Patellae, Crepidulae, and of Mytilus
Magellanicus (?) (Mr. Cuming informs me that he does not think
this species identical with, though closely resembling, the true
M. Magellanicus of the southern and eastern coast of South
America; it lives abundantly on the coast of Chile.): several of
these shells were under a quarter of an inch in their greatest
diameter. My attention was called to this circumstance by a
native fisherman, whom I took to look at these shell-beds; and he
ridiculed the notion of such small shells having been brought up
for food; nor could some of the species have adhered when alive
to other larger shells. On another hill, some miles distant, and
648 feet high, I found shells of the Concholepas and Trochus,
perfect, though very old, with fragments of Mytilus Chiloensis,
all embedded in reddish-brown mould: I also found these same
species, with fragments of an Echinus and of Balanus psittacus,
on a hill 1,000 feet high. Above this height, shells became very
rare, though on a hill 1,300 feet high (Measured by the
barometer: the highest point in the range behind Valparaiso I
found to be 1,626 feet above the level of the sea.), I collected
the Concholepas, Trochus, Fissurella, and a Patella. At these
greater heights the shells are almost invariably embedded in
mould, and sometimes are exposed only by tearing up bushes. These
shells obviously had a very much more ancient appearance than
those from the lesser heights; the apices of the Trochi were
often worn down; the little holes made by burrowing animals were
greatly enlarged; and the Concholepas was often perforated quite
through, owing to the inner plates of shell having scaled off.
Many of these shells, as I have said, were packed in, and were
quite filled with, blackish or reddish-brown earth, resting on
the granitic detritus. I did not doubt until lately that this
mould was of purely terrestrial origin, when with a microscope
examining some of it from the inside of a Concholepas from the
height of about one hundred feet, I found that it was in
considerable part composed of minute fragments of the spines,
mouth- bones, and shells of Echini, and of minute fragments, of
chiefly very young Patellae, Mytili, and other species. I found
similar microscopical fragments in earth filling up the central
orifices of some large Fissurellae. This earth when crushed emits
a sickly smell, precisely like that from garden-mould mixed with
guano. The earth accidentally preserved within the shells, from
the greater heights, has the same general appearance, but it is a
little redder; it emits the same smell when rubbed, but I was
unable to detect with certainty any marine remains in it. This
earth resembles in general appearance, as before remarked, that
capping the rocks of Quiriquina in the Bay of Concepcion, on
which beds of sea-shells lay. I have, also, shown that the black,
peaty soil, in which the shells at the height of 350 feet at
Chiloe were packed, contained many minute fragments of marine
animals. These facts appear to me interesting, as they show that
soils, which would naturally be considered of purely terrestrial
nature, may owe their origin in chief part to the sea.
Being well aware from what I have seen at Chiloe and in Tierra
del Fuego, that vast quantities of shells are carried, during
successive ages, far inland, where the inhabitants chiefly
subsist on these productions, I am bound to state that at greater
heights than 557 feet, where the number of very young and small
shells proved that they had not been carried up for food, the
only evidence of the shells having been naturally left by the
sea, consists in their invariable and uniform appearance of
extreme antiquity—in the distance of some of the places from the
coast, in others being inaccessible from the nearest part of the
beach, and in the absence of fresh water for men to drink—in the
shells NOT LYING IN HEAPS,—and, lastly, in the close similarity
of the soil in which they are embedded, to that which lower down
can be unequivocally shown to be in great part formed from the
debris of the sea animals. (In the "Proceedings of the Geological
Society" volume 2 page 446, I have given a brief account of the
upraised shells on the coast of Chile, and have there stated that
the proofs of elevation are not satisfactory above the height of
230 feet. I had at that time unfortunately overlooked a separate
page written during my second visit to Valparaiso, describing the
shells now in my possession from the 557 feet hill; I had not
then unpacked my collections, and had not reconsidered the
obvious appearance of greater antiquity of the shells from the
greater heights, nor had I at that time discovered the marine
origin of the earth in which many of the shells are packed.
Considering these facts, I do not now feel a shadow of doubt that
the shells, at the height of 1,300 feet, have been upraised by
natural causes into their present position.)
With respect to the position in which the shells lie, I was
repeatedly struck here, at Concepcion, and at other places, with
the frequency of their occurrence on the summits and edges either
of separate hills, or of little flat headlands often terminating
precipitously over the sea. The several above-enumerated species
of mollusca, which are found strewed on the surface of the land
from a few feet above the level of the sea up to the height of
1,300 feet, all now live either on the beach, or at only a few
fathoms' depth: Mr. Edmondston, in a letter to Professor E.
Forbes, states that in dredging in the Bay of Valparaiso, he
found the common species of Concholepas, Fissurella, Trochus,
Monoceros, Chitons, etc., living in abundance from the beach to a
depth of seven fathoms; and dead shells occurred only a few
fathoms deeper. The common Turritella cingulata was dredged up
living at even from ten to fifteen fathoms; but this is a species
which I did not find here amongst the upraised shells.
Considering this fact of the species being all littoral or
sub-littoral, considering their occurrence at various heights,
their vast numbers, and their generally comminuted state, there
can be little doubt that they were left on successive beach-lines
during a gradual elevation of the land. The presence, however, of
so many whole and perfectly preserved shells appears at first a
difficulty on this view, considering that the coast is exposed to
the full force of an open ocean: but we may suppose, either that
these shells were thrown during gales on flat ledges of rock just
above the level of high-water mark, and that during the elevation
of the land they are never again touched by the waves, or, that
during earthquakes, such as those of 1822, 1835, and 1837, rocky
reefs covered with marine-animals were it one blow uplifted above
the future reach of the sea. This latter explanation is, perhaps,
the most probable one with respect to the beds at Concepcion
entirely composed of the Mytilus Chiloensis, a species which
lives below the lowest tides; and likewise with respect to the
great beds occurring both north and south of Valparaiso, of the
Mesodesma donaciforme,—a shell which, as I am informed by Mr.
Cuming, inhabits sandbanks at the level of the lowest tides. But
even in the case of shells having the habits of this Mytilus and
Mesodesma, beds of them, wherever the sea gently throws up sand
or mud, and thus protects its own accumulations, might be
upraised by the slowest movement, and yet remain undisturbed by
the waves of each new beach-line.
It is worthy of remark, that nowhere near Valparaiso above the
height of twenty feet, or rarely of fifty feet, I saw any lines
of erosion on the solid rocks, or any beds of pebbles; this, I
believe, may be accounted for by the disintegrating tendency of
most of the rocks in this neighbourhood. Nor is the land here
modelled into terraces: Mr. Alison, however, informs me, that on
both sides of one narrow ravine, at the height of 300 feet above
the sea, he found a succession of rather indistinct step-formed
beaches, composed of broken shells, which together covered a
space of about eighty feet vertical.
I can add nothing to the accounts already published of the
elevation of the land at Valparaiso, which accompanied the
earthquake of 1822 (Dr. Meyen "Reise um Erde" Th. 1 s. 221, found
in 1831 seaweed and other bodies still adhering to some rocks
which during the shock of 1822 were lifted above the sea.): but I
heard it confidently asserted, that a sentinel on duty,
immediately after the shock, saw a part of a fort, which
previously was not within the line of his vision, and this would
indicate that the uplifting was not horizontal: it would even
appear from some facts collected by Mr. Alison, that only the
eastern half of the bay was then elevated. Through the kindness
of this same gentleman, I am able to give an interesting account
of the changes of level, which have supervened here within
historical periods: about the year 1680 a long sea-wall (or
Prefil) was built, of which only a few fragments now remain; up
to the year 1817, the sea often broke over it, and washed the
houses on the opposite side of the road (where the prison now
stands); and even in 1819, Mr. J. Martin remembers walking at the
foot of this wall, and being often obliged to climb over it to
escape the waves. There now stands (1834) on the seaward side of
this wall, and between it and the beach, in one part a single row
of houses, and in another part two rows with a street between
them. This great extension of the beach in so short a time cannot
be attributed simply to the accumulation of detritus; for a
resident engineer measured for me the height between the lowest
part of the wall visible, and the present beach-line at
spring-tides, and the difference was eleven feet six inches. The
church of S. Augustin is believed to have been built in 1614, and
there is a tradition that the sea formerly flowed very near it;
by levelling, its foundations were found to stand nineteen feet
six inches above the highest beach-line; so that we see in a
period of 220 years, the elevation cannot have been as much as
nineteen feet six inches. From the facts given with respect to
the sea-wall, and from the testimony of the elder inhabitants, it
appears certain that the change in level began to be manifest
about the year 1817. The only sudden elevation of which there is
any record occurred in 1822, and this seems to have been less
than three feet. Since that year, I was assured by several
competent observers, that part of an old wreck, which is firmly
embedded near the beach, has sensibly emerged; hence here, as at
Chiloe, a slow rise of the land appears to be now in progress. It
seems highly probable that the rocks which are corroded in a band
at the height of fourteen feet above the sea were acted on during
the period, when by tradition the base of S. Augustin church, now
nineteen feet six inches above the highest water-mark, was
occasionally washed by the waves.
VALPARAISO TO COQUIMBO.
For the first seventy-five miles north of Valparaiso I followed
the coast- road, and throughout this space I observed innumerable
masses of upraised shells. About Quintero there are immense
accumulations (worked for lime) of the Mesodesma donaciforme,
packed in sandy earth; they abound chiefly about fifteen feet
above high-water, but shells are here found, according to Mr.
Miers, to a height of 500 feet, and at a distance of three
leagues from the coast ("Travels in Chile" volume 1 pages 395,
458. I received several similar accounts from the inhabitants,
and was assured that there are many shells on the plain of Casa
Blanca, between Valparaiso and Santiago, at the height of 800
feet.): I here noticed barnacles adhering to the rocks three or
four feet above the highest tides. In the neighbourhood of
Plazilla and Catapilco, at heights of between two hundred and
three hundred feet, the number of comminuted shells, with some
perfect ones, especially of the Mesodesma, packed in layers, was
truly immense: the land at Plazilla had evidently existed as a
bay, with abrupt rocky masses rising out of it, precisely like
the islets in the broken bays now indenting this coast. On both
sides of the rivers Ligua, Longotomo, Guachen, and Quilimari,
there are plains of gravel about two hundred feet in height, in
many parts absolutely covered with shells. Close to Conchalee, a
gravel-plain is fronted by a lower and similar plain about sixty
feet in height, and this again is separated from the beach by a
wide tract of low land: the surfaces of all three plains or
terraces were strewed with vast numbers of the Concholepas,
Mesodesma, an existing Venus, and other still existing littoral
shells. The two upper terraces closely resemble in miniature the
plains of Patagonia; and like them are furrowed by dry,
flat-bottomed, winding valleys. Northward of this place I turned
inward; and therefore found no more shells: but the valleys of
Chuapa, Illapel, and Limari, are bounded by gravel-capped plains,
often including a lower terrace within. These plains send
bay-like arms between and into the surrounding hills; and they
are continuously united with other extensive gravel-capped
plains, separating the coast mountain-ranges from the Cordillera.
COQUIMBO.
A narrow fringe-like plain, gently inclined towards the sea, here
extends for eleven miles along the coast, with arms stretching up
between the coast-mountains, and likewise up the valley of
Coquimbo: at its southern extremity it is directly connected with
the plain of Limari, out of which hills abruptly rise like
islets, and other hills project like headlands on a coast. The
surface of the fringe-like plain appears level, but differs
insensibly in height, and greatly in composition, in different
parts.
At the mouth of the valley of Coquimbo, the surface consists
wholly of gravel, and stands from 300 to 350 feet above the level
of the sea, being about one hundred feet higher than in other
parts. In these other and lower parts the superficial beds
consist of calcareous matter, and rest on ancient tertiary
deposits hereafter to be described. The uppermost calcareous
layer is cream-coloured, compact, smooth-fractured, sub-
stalactiform, and contains some sand, earthy matter, and recent
shells. It lies on, and sends wedge-like veins into, a much more
friable, calcareous, tuff-like variety; and both rest on a mass
about twenty feet in thickness, formed of fragments of recent
shells, with a few whole ones, and with small pebbles firmly
cemented together. (In many respects this upper hard, and the
underlying more friable, varieties, resemble the great
superficial beds at King George's Sound in Australia, which I
have described in my "Geological Observations on Volcanic
Islands." There could be little doubt that the upper layers there
have been hardened by the action of rain on the friable,
calcareous matter, and that the whole mass has originated in the
decay of minutely comminuted sea-shells and corals.) This latter
rock is called by the inhabitants losa, and is used for building:
in many parts it is divided into strata, which dip at an angle of
ten degrees seaward, and appear as if they had originally been
heaped in successive layers (as may be seen on coral-reefs) on a
steep beach. This stone is remarkable from being in parts
entirely formed of empty, pellucid capsules or cells of
calcareous matter, of the size of small seeds: a series of
specimens unequivocally showed that all these capsules once
contained minute rounded fragments of shells which have since
been gradually dissolved by water percolating through the mass.
(I have incidentally described this rock in the above work on
Volcanic Islands.)
The shells embedded in the calcareous beds forming the surface of
this fringe-like plain, at the height of from 200 to 250 feet
above the sea, consist of:—
1. Venus opaca. 2. Mulinia Byronensis. 3. Pecten purpuratus. 4.
Mesodesma donaciforme. 5. Turritella cingulata. 6. Monoceros
costatum. 7. Concholepas Peruviana. 8. Trochus (common Valparaiso
species). 9. Calyptraea Byronensis.
Although these species are all recent, and are all found in the
neighbouring sea, yet I was particularly struck with the
difference in the proportional numbers of the several species,
and of those now cast up on the present beach. I found only one
specimen of the Concholepas, and the Pecten was very rare, though
both these shells are now the commonest kinds, with the
exception, perhaps, of the Calyptraea radians, of which I did not
find one in the calcareous beds. I will not pretend to determine
how far this difference in the proportional numbers depends on
the age of the deposit, and how far on the difference in nature
between the present sandy beaches and the calcareous bottom, on
which the embedded shells must have lived.
(DIAGRAM 8.—SECTION OF PLAIN OF COQUIMBO.
A Section through Plain B-B and Ravine A.
Surface of plain 252 feet above sea.
A. Stratified sand, with recent shells in same proportions as on
the beach, half filling up a ravine.
B. Surface of plain, with scattered shells in nearly same
proportions as on the beach.
C. Upper calcareous bed, and D. Lower calcareous sandy bed
(Losa), both with recent shells, but not in same proportions as
on the beach.
E. Upper ferrugino-sandy old tertiary stratum, and F. Lower old
tertiary stratum, both with all, or nearly all, extinct shells.)
On the bare surface of the calcareous plain, or in a thin
covering of sand, there were lying, at a height from 200 to 252
feet, many recent shells, which had a much fresher appearance
than the embedded ones: fragments of the Concholepas, and of the
common Mytilus, still retaining a tinge of its colour, were
numerous, and altogether there was manifestly a closer approach
in proportional numbers to those now lying on the beach. In a
mass of stratified, slightly agglutinated sand, which in some
places covers up the lower half of the seaward escarpment of the
plain, the included shells appeared to be in exactly the same
proportional numbers with those on the beach. On one side of a
steep-sided ravine, cutting through the plain behind Herradura
Bay, I observed a narrow strip of stratified sand, containing
similar shells in similar proportional numbers; a section of the
ravine is represented in Diagram 8, which serves also to show the
general composition of the plain. I mention this case of the
ravine chiefly because without the evidence of the marine shells
in the sand, any one would have supposed that it had been
hollowed out by simple alluvial action.
The escarpment of the fringe-like plain, which stretches for
eleven miles along the coast, is in some parts fronted by two or
three narrow, step- formed terraces, one of which at Herradura
Bay expands into a small plain. Its surface was there formed of
gravel, cemented together by calcareous matter; and out of it I
extracted the following recent shells, which are in a more
perfect condition than those from the upper plain:—
1. Calyptraea radians. 2. Turritella cingulata. 3. Oliva
Peruviana. 4. Murex labiosus, var. 5. Nassa (identical with a
living species). 6. Solen Dombeiana. 7. Pecten purpuratus. 8.
Venus Chilensis. 9. Amphidesma rugulosum. The small irregular
wrinkles of the posterior part of this shell are rather stronger
than in the recent specimens of this species from Coquimbo. (G.B.
Sowerby.) 10. Balanus (identical with living species).
On the syenitic ridge, which forms the southern boundary of
Herradura Bay and Plain, I found the Concholepas and Turritella
cingulata (mostly in fragments), at the height of 242 feet above
the sea. I could not have told that these shells had not formerly
been brought up by man, if I had not found one very small mass of
them cemented together in a friable calcareous tuff. I mention
this fact more particularly, because I carefully looked, in many
apparently favourable spots, at lesser heights on the side of
this ridge, and could not find even the smallest fragment of a
shell. This is only one instance out of many, proving that the
absence of sea-shells on the surface, though in many respects
inexplicable, is an argument of very little weight in opposition
to other evidence on the recent elevation of the land. The
highest point in this neighbourhood at which I found upraised
shells of existing species was on an inland calcareous plain, at
the height of 252 feet above the sea.
It would appear from Mr. Caldcleugh's researches, that a rise has
taken place here within the last century and a half ("Proceedings
of the Geological Society" volume 2 page 446.); and as no sudden
change of level has been observed during the not very severe
earthquakes, which have occasionally occurred here, the rising
has probably been slow, like that now, or quite lately, in
progress at Chiloe and at Valparaiso: there are three well-known
rocks, called the Pelicans, which in 1710, according to Feuillee,
were a fleur d'eau, but now are said to stand twelve feet above
low-water mark: the spring-tides rise here only five feet. There
is another rock, now nine feet above high-water mark, which in
the time of Frezier and Feuillee rose only five or six feet out
of water. Mr. Caldcleugh, I may add, also shows (and I received
similar accounts) that there has been a considerable decrease in
the soundings during the last twelve years in the Bays of
Coquimbo, Concepcion, Valparaiso, and Guasco; but as in these
cases it is nearly impossible to distinguish between the
accumulation of sediment and the upheavement of the bottom, I
have not entered into any details.
VALLEY OF COQUIMBO.
(FIGURE 9. EAST AND WEST SECTION THROUGH THE TERRACES AT
COQUIMBO, WHERE THEY DEBOUCH FROM THE VALLEY, AND FRONT THE SEA.
Vertical scale 1/10 of inch to 100 feet: horizontal scale much
contracted.
Height of terrace in feet from east (high) to west (low): Terrace
F. 364 Terrace E. 302 Terrace D. shown dotted, height not given.
Terrace C. 120 Terrace B. 70 Terrace A. 25 sloping down to level
of sea at Town of Coquimbo.)
The narrow coast-plain sends, as before stated, an arm, or more
correctly a fringe, on both sides, but chiefly on the southern
side, several miles up the valley. These fringes are worn into
steps or terraces, which present a most remarkable appearance,
and have been compared (though not very correctly) by Captain
Basil Hall, to the parallel roads of Glen Roy in Scotland: their
origin has been ably discussed by Mr. Lyell. ("Principles of
Geology" 1st edition volume 3 page 131.) The first section which
I will give (Figure 9), is not drawn across the valley, but in an
east and west line at its mouth, where the step-formed terraces
debouch and present their very gently inclined surfaces towards
the Pacific.
The bottom plain (A) is about a mile in width, and rises quite
insensibly from the beach to a height of twenty-five feet at the
foot of the next plain; it is sandy, and abundantly strewed with
shells.
Plain or terrace B is of small extent, and is almost concealed by
the houses of the town, as is likewise the escarpment of terrace
C. On both sides of a ravine, two miles south of the town, there
are two little terraces, one above the other, evidently
corresponding with B and C; and on them marine remains of the
species already enumerated were plentiful. Terrace E is very
narrow, but quite distinct and level; a little southward of the
town there were traces of a terrace D intermediate between E and
C. Terrace F is part of the fringe-like plain, which stretches
for the eleven miles along the coast; it is here composed of
shingle, and is 100 feet higher than where composed of calcareous
matter. This greater height is obviously due to the quantity of
shingle, which at some former period has been brought down the
great valley of Coquimbo.
Considering the many shells strewed over the terraces A, B, and
C, and a few miles southward on the calcareous plain, which is
continuously united with the upper step-like plain F, there
cannot, I apprehend, be any doubt, that these six terraces have
been formed by the action of the sea; and that their five
escarpments mark so many periods of comparative rest in the
elevatory movement, during which the sea wore into the land. The
elevation between these periods may have been sudden and on AN
AVERAGE not more than seventy-two feet each time, or it may have
been gradual and insensibly slow. From the shells on the three
lower terraces, and on the upper one, and I may add on the three
gravel-capped terraces at Conchalee, being all littoral and
sub-littoral species, and from the analogical facts given at
Valparaiso, and lastly from the evidence of a slow rising lately
or still in progress here, it appears to me far more probable
that the movement has been slow. The existence of these
successive escarpments, or old cliff- lines, is in another
respect highly instructive, for they show periods of comparative
rest in the elevatory movement, and of denudation, which would
never even have been suspected from a close examination of many
miles of coast southward of Coquimbo.
(FIGURE 10. NORTH AND SOUTH SECTION ACROSS THE VALLEY OF
COQUIMBO.
From north F (high) through E?, D, C, B, A (low), B?, C, D?, E, F
(high).
Vertical scale 1/10 of inch to 100 feet: horizontal scale much
contracted.
Terraces marked with ? do not occur on that side of the valley,
and are introduced only to make the diagram more intelligible. A
river and bottom- plain of valley C, E, and F, on the south side
of valley, are respectively, 197, 377, and 420 feet above the
level of the sea.
AA. The bottom of the valley, believed to be 100 feet above the
sea: it is continuously united with the lowest plain A of Figure
9.
B. This terrace higher up the valley expands considerably;
seaward it is soon lost, its escarpment being united with that of
C: it is not developed at all on the south side of the valley.
C. This terrace, like the last, is considerably expanded higher
up the valley. These two terraces apparently correspond with B
and C of Figure 9.
D is not well developed in the line of this section; but seaward
it expands into a plain: it is not present on the south side of
the valley; but it is met with, as stated under the former
section, a little south of the town.
E is well developed on the south side, but absent on the north
side of the valley: though not continuously united with E of
Figure 9, it apparently corresponds with it.
F. This is the surface-plain, and is continuously united with
that which stretches like a fringe along the coast. In ascending
the valley it gradually becomes narrower, and is at last, at the
distance of about ten miles from the sea, reduced to a row of
flat-topped patches on the sides of the mountains. None of the
lower terraces extend so far up the valley.)
We come now to the terraces on the opposite sides of the east and
west valley of Coquimbo: the section in Figure 10 is taken in a
north and south line across the valley at a point about three
miles from the sea. The valley measured from the edges of the
escarpments of the upper plain FF is about a mile in width; but
from the bases of the bounding mountains it is from three to four
miles wide. The terraces marked with an interrogative do not
exist on that side of the valley, but are introduced merely to
render the diagram more intelligible.
These five terraces are formed of shingle and sand; three of
them, as marked by Captain B. Hall (namely, B, C, and F), are
much more conspicuous than the others. From the marine remains
copiously strewed at the mouth of the valley on the lower
terraces, and southward of the town on the upper one, they are,
as before remarked, undoubtedly of marine origin; but within the
valley, and this fact well deserves notice, at a distance of from
only a mile and a half to three or four miles from the sea, I
could not find even a fragment of a shell.
ON THE INCLINATION OF THE TERRACES OF COQUIMBO, AND ON THE UPPER
AND BASAL EDGES OF THEIR ESCARPMENTS NOT BEING HORIZONTAL.
The surfaces of these terraces slope in a slight degree, as shown
by the sections in Figures 9 and 10 taken conjointly, both
towards the centre of the valley, and seawards towards its mouth.
This double or diagonal inclination, which is not the same in the
several terraces, is, as we shall immediately see, of simple
explanation. There are, however, some other points which at first
appear by no means obvious,—namely, first, that each terrace,
taken in its whole breadth from the summit-edge of one escarpment
to the base of that above it, and followed up the valley, is not
horizontal; nor have the several terraces, when followed up the
valley, all the same inclination; thus I found the terraces C, E,
and F, measured at a point about two miles from the mouth of the
valley, stood severally between fifty-six to seventy-seven feet
higher than at the mouth. Again, if we look to any one line of
cliff or escarpment, neither its summit-edge nor its base is
horizontal. On the theory of the terraces having been formed
during a slow and equable rise of the land, with as many
intervals of rest as there are escarpments, it appears at first
very surprising that horizontal lines of some kind should not
have been left on the land.
The direction of the diagonal inclination in the different
terraces being different,—in some being directed more towards the
middle of the valley, in others more towards its mouth,—naturally
follows on the view of each terrace, being an accumulation of
successive beach-lines round bays, which must have been of
different forms and sizes when the land stood at different
levels: for if we look to the actual beach of a narrow creek, its
slope is directed towards the middle; whereas, in an open bay, or
slight concavity on a coast, the slope is towards the mouth, that
is, almost directly seaward; hence as a bay alters in form and
size, so will the direction of the inclination of its successive
beaches become changed.
(FIGURE 11. DIAGRAM OF A BAY IN A DISTRICT WHICH HAS BEGUN SLOWLY
RISING)
If it were possible to trace any one of the many beach-lines,
composing each sloping terrace, it would of course be horizontal;
but the only lines of demarcation are the summit and basal edges
of the escarpments. Now the summit-edge of one of these
escarpments marks the furthest line or point to which the sea has
cut into a mass of gravel sloping seaward; and as the sea will
generally have greater power at the mouth than at the protected
head of the bay, so will the escarpment at the mouth be cut
deeper into the land, and its summit-edge be higher; consequently
it will not be horizontal. With respect to the basal or lower
edges of the escarpments, from picturing in one's mind ancient
bays ENTIRELY surrounded at successive periods by cliff-formed
shores, one's first impression is that they at least necessarily
must be horizontal, if the elevation has been horizontal. But
here is a fallacy: for after the sea has, during a cessation of
the elevation, worn cliffs all round the shores of a bay, when
the movement recommences, and especially if it recommences
slowly, it might well happen that, at the exposed mouth of the
bay, the waves might continue for some time wearing into the
land, whilst in the protected and upper parts successive
beach-lines might be accumulating in a sloping surface or terrace
at the foot of the cliffs which had been lately reached: hence,
supposing the whole line of escarpment to be finally uplifted
above the reach of the sea, its basal line or foot near the mouth
will run at a lower level than in the upper and protected parts
of the bay; consequently this basal line will not be horizontal.
And it has already been shown that the summit-edges of each
escarpment will generally be higher near the mouth (from the
seaward sloping land being there most exposed and cut into) than
near the head of the bay; therefore the total height of the
escarpments will be greatest near the mouth; and further up the
old bay or valley they will on both sides generally thin out and
die away: I have observed this thinning out of the successive
escarpment at other places besides Coquimbo; and for a long time
I was quite unable to understand its meaning. The rude diagram in
Figure 11 will perhaps render what I mean more intelligible; it
represents a bay in a district which has begun slowly rising.
Before the movement commenced, it is supposed that the waves had
been enabled to eat into the land and form cliffs, as far up, but
with gradually diminishing power, as the points AA: after the
movement had commenced and gone on for a little time, the sea is
supposed still to have retained the power, at the exposed mouth
of the bay, of cutting down and into the land as it slowly
emerged; but in the upper parts of the bay it is supposed soon to
have lost this power, owing to the more protected situation and
to the quantity of detritus brought down by the river;
consequently low land was there accumulated. As this low land was
formed during a slow elevatory movement, its surface will gently
slope upwards from the beach on all sides. Now, let us imagine
the bay, not to make the diagram more complicated, suddenly
converted into a valley: the basal line of the cliffs will of
course be horizontal, as far as the beach is now seen extending
in the diagram; but in the upper part of the valley, this line
will be higher, the level of the district having been raised
whilst the low land was accumulating at the foot of the inland
cliffs. If, instead of the bay in the diagram being suddenly
converted into a valley, we suppose with much more probability it
to be upraised slowly, then the waves in the upper parts of the
bay will continue very gradually to fail to reach the cliffs,
which are now in the diagram represented as washed by the sea,
and which, consequently, will be left standing higher and higher
above its level; whilst at the still exposed mouth, it might well
happen that the waves might be enabled to cut deeper and deeper,
both down and into the cliffs, as the land slowly rose.
The greater or lesser destroying power of the waves at the mouths
of successive bays, comparatively with this same power in their
upper and protected parts, will vary as the bays become changed
in form and size, and therefore at different levels, at their
mouths and heads, more or less of the surfaces between the
escarpments (that is, the accumulated beach-lines or terraces)
will be left undestroyed: from what has gone before we can see
that, according as the elevatory movements after each cessation
recommence more or less slowly, according to the amount of
detritus delivered by the river at the heads of the successive
bays, and according to the degree of protection afforded by their
altered forms, so will a greater or less extent of terrace be
accumulated in the upper part, to which there will be no surface
at a corresponding level at the mouth: hence we can perceive why
no one terrace, taken in its whole breadth and followed up the
valley, is horizontal, though each separate beach-line must have
been so; and why the inclination of the several terraces, both
transversely, and longitudinally up the valley, is not alike.
I have entered into this case in some detail, for I was long
perplexed (and others have felt the same difficulty) in
understanding how, on the idea of an equable elevation with the
sea at intervals eating into the land, it came that neither the
terraces nor the upper nor lower edges of the escarpments were
horizontal. Along lines of coast, even of great lengths, such as
that of Patagonia, if they are nearly uniformly exposed, the
corroding power of the waves will be checked and conquered by the
elevatory movement, as often as it recommences, at about the same
period; and hence the terraces, or accumulated beach-lines, will
commence being formed at nearly the same levels: at each
succeeding period of rest, they will, also, be eaten into at
nearly the same rate, and consequently there will be a much
closer coincidence in their levels and inclinations, than in the
terraces and escarpments formed round bays with their different
parts very differently exposed to the action of the sea. It is
only where the waves are enabled, after a long lapse of time,
slowly to corrode hard rocks, or to throw up, owing to the supply
of sediment being small and to the surface being steeply
inclined, a narrow beach or mound, that we can expect, as at Glen
Roy in Scotland ("Philosophical Transactions" 1839 page 39.), a
distinct line marking an old sea-level, and which will be
strictly horizontal, if the subsequent elevatory movements have
been so: for in these cases no discernible effects will be
produced, except during the long intervening periods of rest;
whereas in the case of step-formed coasts, such as those
described in this and the preceding chapter, the terraces
themselves are accumulated during the slow elevatory process, the
accumulation commencing sooner in protected than in exposed
situations, and sooner where there is copious supply of detritus
than where there is little; on the other hand, the steps or
escarpments are formed during the stationary periods, and are
more deeply cut down and into the coast-land in exposed than in
protected situations;—the cutting action, moreover, being
prolonged in the most exposed parts, both during the beginning
and ending, if slow, of the upward movement.
Although in the foregoing discussion I have assumed the elevation
to have been horizontal, it may be suspected, from the
considerable seaward slope of the terraces, both up the valley of
S. Cruz and up that of Coquimbo, that the rising has been greater
inland than nearer the coast. There is reason to believe (Mr.
Place in the "Quarterly Journal of Science" 1824 volume 17 page
42.), from the effects produced on the water-course of a mill
during the earthquake of 1822 in Chile, that the upheaval one
mile inland was nearly double, namely, between five and seven
feet, to what it was on the Pacific. We know, also, from the
admirable researches of M. Bravais, that in Scandinavia the
ancient sea-beaches gently slope from the interior
mountain-ranges towards the coast, and that they are not parallel
one to the other ("Voyages de la Comm. du Nord" etc. also
"Comptes Rendus" October 1842.), showing that the proportional
difference in the amount of elevation on the coast and in the
interior, varied at different periods.
COQUIMBO TO GUASCO.
In this distance of ninety miles, I found in almost every part
marine shells up to a height of apparently from two hundred to
three hundred feet. The desert plain near Choros is thus covered;
it is bounded by the escarpment of a higher plain, consisting of
pale-coloured, earthy, calcareous stone, like that of Coquimbo,
with the same recent shells embedded in it. In the valley of
Chaneral, a similar bed occurs in which, differently from that of
Coquimbo, I observed many shells of the Concholepas: near Guasco
the same calcareous bed is likewise met with.
In the valley of Guasco, the step-formed terraces of gravel are
displaced in a more striking manner than at any other point. I
followed the valley for thirty-seven miles (as reckoned by the
inhabitants) from the coast to Ballenar; in nearly the whole of
this distance, five grand terraces, running at corresponding
heights on both sides of the broad valley, are more conspicuous
than the three best-developed ones at Coquimbo. They give to the
landscape the most singular and formal aspect; and when the
clouds hung low, hiding the neighbouring mountains, the valley
resembled in the most striking manner that of Santa Cruz. The
whole thickness of these terraces or plains seems composed of
gravel, rather firmly aggregated together, with occasional
parting seams of clay: the pebbles on the upper plain are often
whitewashed with an aluminous substance, as in Patagonia. Near
the coast I observed many sea-shells on the lower plains. At
Freyrina (twelve miles up the valley), there are six terraces
beside the bottom- surface of the valley: the two lower ones are
here only from two hundred to three hundred yards in width, but
higher up the valley they expand into plains; the third terrace
is generally narrow; the fourth I saw only in one place, but
there it was distinct for the length of a mile; the fifth is very
broad; the sixth is the summit-plain, which expands inland into a
great basin. Not having a barometer with me, I did not ascertain
the height of these plains, but they appeared considerably higher
than those at Coquimbo. Their width varies much, sometimes being
very broad, and sometimes contracting into mere fringes of
separate flat-topped projections, and then quite disappearing: at
the one spot, where the fourth terrace was visible, the whole six
terraces were cut off for a short space by one single bold
escarpment. Near Ballenar (thirty-seven miles from the mouth of
the river), the valley between the summit-edges of the highest
escarpments is several miles in width, and the five terraces on
both sides are broadly developed: the highest cannot be less than
six hundred feet above the bed of the river, which itself must, I
conceive, be some hundred feet above the sea.
A north and south section across the valley in this part is
represented in Figure 12.
(FIGURE 12. NORTH AND SOUTH SECTION ACROSS THE VALLEY OF GUASCO,
AND OF A PLAIN NORTH OF IT.
From left (north, high) to right (south, high) through plains B
and A and the River of Guasco at the Town of Ballenar.)
On the northern side of the valley the summit-plain of gravel, A,
has two escarpments, one facing the valley, and the other a great
basin-like plain, B, which stretches for several leagues
northward. This narrow plain, A, with the double escarpment,
evidently once formed a spit or promontory of gravel, projecting
into and dividing two great bays, and subsequently was worn on
both sides into steep cliffs. Whether the several escarpments in
this valley were formed during the same stationary periods with
those of Coquimbo, I will not pretend to conjecture; but if so
the intervening and subsequent elevatory movements must have been
here much more energetic, for these plains certainly stand at a
much higher level than do those of Coquimbo.
COPIAPO.
From Guasco to Copiapo, I followed the road near the foot of the
Cordillera, and therefore saw no upraised remains. At the mouth,
however, of the valley of Copiapo there is a plain, estimated by
Meyen ("Reise um die Erde" th. 1 s. 372 et seq.) between fifty
and seventy feet in height, of which the upper part consists
chiefly of gravel, abounding with recent shells, chiefly of the
Concholepas, Venus Dombeyi, and Calyptraea trochiformis. A little
inland, on a plain estimated by myself at nearly three hundred
feet, the upper stratum was formed of broken shells and sand
cemented by white calcareous matter, and abounding with embedded
recent shells, of which the Mulinia Byronensis and Pecten
purpuratus were the most numerous. The lower plain stretches for
some miles southward, and for an unknown distance northward, but
not far up the valley; its seaward face, according to Meyen, is
worn into caves above the level of the present beach. The valley
of Copiapo is much less steeply inclined and less direct in its
course than any other valley which I saw in Chile; and its bottom
does not generally consist of gravel: there are no step-formed
terraces in it, except at one spot near the mouth of the great
lateral valley of the Despoblado where there are only two, one
above the other: lower down the valley, in one place I observed
that the solid rock had been cut into the shape of a beach, and
was smoothed over with shingle.
Northward of Copiapo, in latitude 26 degrees S., the old voyager
Wafer found immense numbers of sea-shells some miles from the
coast. (Burnett's "Collection of Voyages" volume 4 page 193.) At
Cobija (latitude 22 degrees 34') M. d'Orbigny observed beds of
gravel and broken shells, containing ten species of recent
shells; he also found, on projecting points of porphyry, at a
height of 300 feet, shells of Concholepas, Chiton, Calyptraea,
Fissurella, and Patella, still attached to the spots on which
they had lived. M. d'Orbigny argues from this fact, that the
elevation must have been great and sudden ("Voyage, Part Geolog."
page 94. M. d'Orbigny (page 98), in summing up, says: "S'il est
certain (as he believes) que tous les terrains en pente, compris
entre la mer et les montagnes sont l'ancien rivage de la mer, on
doit supposer, pour l'ensemble, un exhaussement que ce ne serait
pas moindre de deux cent metres; il faudrait supposer encore que
ce soulevement n'a point ete graduel;...mais qu'il resulterait
d'une seule et meme cause fortuite," etc. Now, on this view, when
the sea was forming the beach at the foot of the mountains, many
shells of Concholepas, Chiton, Calyptraea, Fissurella, and
Patella (which are known to live close to the beach), were
attached to rocks at a depth of 300 feet, and at a depth of 600
feet several of these same shells were accumulating in great
numbers in horizontal beds. From what I have myself seen in
dredging, I believe this to be improbable in the highest degree,
if not impossible; and I think everyone who has read Professor E.
Forbes's excellent researches on the subject, will without
hesitation agree in this conclusion.): to me it appears far more
probable that the movement was gradual, with small starts as
during the earthquakes of 1822 and 1835, by which whole beds of
shells attached to the rocks were lifted above the subsequent
reach of the waves. M. d'Orbigny also found rolled pebbles
extending up the mountain to a height of at least six hundred
feet. At Iquique (latitude 20 degrees 12' S.), in a great
accumulation of sand, at a height estimated between one hundred
and fifty and two hundred feet, I observed many large sea-shells
which I thought could not have been blown up by the wind to that
height. Mr. J.H. Blake has lately described these shells: he
states that "inland toward the mountains they form a compact
uniform bed, scarcely a trace of the original shells being
discernible; but as we approach the shore, the forms become
gradually more distinct till we meet with the living shells on
the coast." ("Silliman's American Journal of Science" volume 44
page 2.) This interesting observation, showing by the gradual
decay of the shells how slowly and gradually the coast must have
been uplifted, we shall presently see fully confirmed at Lima. At
Arica (latitude 18 degrees 28'), M. d'Orbigny found a great range
of sand-dunes, fourteen leagues in length, stretching towards
Tacna, including recent shells and bones of Cetacea, and reaching
up to a height of 300 feet above the sea. ("Voyage" etc. page
101.) Lieutenant Freyer has given some more precise facts: he
states (In a letter to Mr. Lyell "Geological Proceedings" volume
2 page 179.) that the Morro of Arica is about four hundred feet
high; it is worn into obscure terraces, on the bare rock of which
he found Balini and Milleporae adhering. At the height of between
twenty and thirty feet the shells and corals were in a quite
fresh state, but at fifty feet they were much abraded; there
were, however, traces of organic remains at greater heights. On
the road from Tacna to Arequipa, between Loquimbo and Moquegua,
Mr. M. Hamilton found numerous recent sea shells in sand, at a
considerable distance from the sea. ("Edinburgh New Philosophical
Journal" volume 30 page 155.)
LIMA.
Northward of Arica, I know nothing of the coast for about a space
of five degrees of latitude; but near Callao, the port of Lima,
there is abundant and very curious evidence of the elevation of
the land. The island of San Lorenzo is upwards of one thousand
feet high; the basset edges of the strata composing the lower
part are worn into three obscure, narrow, sloping steps or
ledges, which can be seen only when standing on them: they
probably resemble those described by Lieutenant Freyer at Arica.
The surface of the lower ledge, which extends from a low cliff
overhanging the sea to the foot of the next upper escarpment, is
covered by an enormous accumulation of recent shells. (M.
Chevalier, in the "Voyage of the 'Bonite'" observed these shells;
but his specimens were lost.—"L'Institut" 1838 page 151.) The bed
is level, and in some parts more than two feet in thickness; I
traced it over a space of one mile in length, and heard of it in
other places: the uppermost part is eighty-five feet by the
barometer above high-water mark. The shells are packed together,
but not stratified: they are mingled with earth and stones, and
are generally covered by a few inches of detritus; they rest on a
mass of nearly angular fragments of the underlying sandstone,
sometimes cemented together by common salt. I collected eighteen
species of shells of all ages and sizes. Several of the univalves
had evidently long lain dead at the bottom of the sea, for their
INSIDES were incrusted with Balani and Serpulae. All, according
to Mr. G.B. Sowerby, are recent species: they consist of:—
1. Mytilus Magellanicus: same as that found at Valparaiso, and
there stated to be probably distinct from the true M.
Magellanicus of the east coast.
2. Venus costellata, Sowerby "Zoological Proceedings."
3. Pecten purpuratus, Lam.
4. Chama, probably echinulata, Brod.
5. Calyptraea Byronensis, Gray.
6. Calyptraea radians (Trochus, Lam.)
7. Fissurella affinis, Gray.
8. Fissurella biradiata, Trembly.
9. Purpura chocolatta, Duclos.
10. Purpura Peruviana, Gray.
11. Purpura labiata, Gray.
12. Purpura buxea (Murex, Brod.).
13. Concholepas Peruviana.
14. Nassa, related to reticulata.
15. Triton rudis, Brod.
16. Trochus, not yet described, but well-known and very common.
17 and 18. Balanus, two species, both common on the coast.
These upraised shells appear to be nearly in the same
proportional numbers- -with the exception of the Crepidulae being
more numerous—with those on the existing beach. The state of
preservation of the different species differed much; but most of
them were much corroded, brittle, and bleached: the upper and
lower surfaces of the Concholepas had generally quite scaled off:
some of the Trochi and Fissurellae still partially retain their
colours. It is remarkable that these shells, taken all together,
have fully as ancient an appearance, although the extremely arid
climate appears highly favourable for their preservation, as
those from 1,300 feet at Valparaiso, and certainly a more ancient
appearance than those from five to six hundred feet from
Valparaiso and Concepcion; at which places I have seen grass and
other vegetables actually growing out of the shells. Many of the
univalves here at San Lorenzo were filled with, and united
together by, pure salt, probably left by the evaporation of the
sea-spray, as the land slowly emerged. (The underlying sandstone
contains true layers of salt; so that the salt may possibly have
come from the beds in the higher parts of the island; but I think
more probably from the sea-spray. It is generally asserted that
rain never falls on the coast of Peru; but this is not quite
accurate; for, on several days, during our visit, the so-called
Peruvian dew fell in sufficient quantity to make the streets
muddy, and it would certainly have washed so deliquescent a
substance as salt into the soil. I state this because M.
d'Orbigny, in discussing an analogous subject, supposes that I
had forgotten that it never rains on this whole line of coast.
See Ulloa's "Voyage" volume 2 English Translation page 67 for an
account of the muddy streets of Lima, and on the continuance of
the mists during the whole winter. Rain, also, falls at rare
intervals even in the driest districts, as, for instance, during
forty days, in 1726, at Chocope (7 degrees 46'); this rain
entirely ruined ("Ulloa" etc. page 18) the mud houses of the
inhabitants.) On the highest parts of the ledge, small fragments
of the shells were mingled with, and evidently in process of
reduction into, a yellowish-white, soft, calcareous powder,
tasting strongly of salt, and in some places as fine as prepared
medicinal chalk.
FOSSIL-REMAINS OF HUMAN ART.
In the midst of these shells on San Lorenzo, I found light
corallines, the horny ovule-cases of Mollusca, roots of seaweed
(Mr. Smith of Jordan Hill found pieces of seaweed in an upraised
pleistocene deposit in Scotland. See his admirable Paper in the
"Edinburgh New Philosophical Journal" volume 25 page 384.), bones
of birds, the heads of Indian corn and other vegetable matter, a
piece of woven rushes, and another of nearly decayed COTTON
string. I extracted these remains by digging a hole, on a level
spot; and they had all indisputably been embedded with the
shells. I compared the plaited rush, the COTTON string, and
Indian corn, at the house of an antiquary, with similar objects,
taken from the Huacas or burial-grounds of the ancient Peruvians,
and they were undistinguishable; it should be observed that the
Peruvians used string only of cotton. The small quantity of sand
or gravel with the shells, the absence of large stones, the width
and thickness of the bed, and the time requisite for a ledge to
be cut into the sandstone, all show that these remains were not
thrown high up by an earthquake-wave: on the other hand, these
facts, together with the number of dead shells, and of floating
objects, both marine and terrestrial, both natural and human,
render it almost certain that they were accumulated on a true
beach, since upraised eighty-five feet, and upraised this much
since INDIAN MAN INHABITED PERU. The elevation may have been,
either by several small sudden starts, or quite gradual; in this
latter case the unrolled shells having been thrown up during
gales beyond the reach of the waves which afterwards broke on the
slowly emerging land. I have made these remarks, chiefly because
I was at first surprised at the complete difference in nature,
between this broad, smooth, upraised bed of shells, and the
present shingle-beach at the foot of the low sandstone-cliffs;
but a beach formed, when the sea is cutting into the land, as is
shown now to be the case by the low bare sandstone-cliffs, ought
not to be compared with a beach accumulated on a gently inclined
rocky surface, at a period when the sea (probably owing to the
elevatory movement in process) was not able to eat into the land.
With respect to the mass of nearly angular, salt- cemented
fragments of sandstone, which lie under the shells, and which are
so unlike the materials of an ordinary sea-beach; I think it
probable after having seen the remarkable effects of the
earthquake of 1835 (I have described this in my "Journal of
Researches" page 303 2nd edition.), in absolutely shattering as
if by gunpowder the SURFACE of the primary rocks near Concepcion,
that a smooth bare surface of stone was left by the sea covered
by the shelly mass, and that afterwards when upraised, it was
superficially shattered by the severe shocks so often experienced
here.
The very low land surrounding the town of Callao, is to the south
joined by an obscure escarpment to a higher plain (south of Bella
Vista), which stretches along the coast for a length of about
eight miles. This plain appears to the eye quite level; but the
sea-cliffs show that its height varies (as far as I could
estimate) from seventy to one hundred and twenty feet. It is
composed of thin, sometimes waving, beds of clay, often of bright
red and yellow colours, of layers of impure sand, and in one part
with a great stratified mass of granitic pebbles. These beds are
capped by a remarkable mass, varying from two to six feet in
thickness, of reddish loam or mud, containing many scattered and
broken fragments of recent marine shells, sometimes though rarely
single large round pebble, more frequently short irregular layers
of fine gravel, and very many pieces of red coarse earthenware,
which from their curvatures must once have formed parts of large
vessels. The earthenware is of Indian manufacture; and I found
exactly similar pieces accidentally included within the bricks,
of which the neighbouring ancient Peruvian burial-mounds are
built. These fragments abounded in such numbers in certain spots,
that it appeared as if waggon-loads of earthenware had been
smashed to pieces. The broken sea- shells and pottery are strewed
both on the surface, and throughout the whole thickness of this
upper loamy mass. I found them wherever I examined the cliffs,
for a space of between two and three miles, and for half a mile
inland; and there can be little doubt that this same bed extends
with a smooth surface several miles further over the entire
plain. Besides the little included irregular layers of small
pebbles, there are occasionally very obscure traces of
stratification.
At one of the highest parts of the cliff, estimated 120 feet
above the sea, where a little ravine came down, there were two
sections, at right angles to each other, of the floor of a shed
or building. In both sections or faces, two rows, one over the
other, of large round stones could be distinctly seen; they were
packed close together on an artificial layer of sand two inches
thick, which had been placed on the natural clay-beds; the round
stones were covered by three feet in thickness of the loam with
broken sea-shells and pottery. Hence, before this widely
spread-out bed of loam was deposited, it is certain that the
plain was inhabited; and it is probable, from the broken vessels
being so much more abundant in certain spots than in others, and
from the underlying clay being fitted for their manufacture, that
the kilns stood here.
The smoothness and wide extent of the plain, the bulk of matter
deposited, and the obscure traces of stratification seem to
indicate that the loam was deposited under water; on the other
hand, the presence of sea-shells, their broken state, the pebbles
of various sizes, and the artificial floor of round stones,
almost prove that it must have originated in a rush of water from
the sea over the land. The height of the plain, namely, 120 feet,
renders it improbable that an earthquake-wave, vast as some have
here been, could have broken over the surface at its present
level; but when the land stood eighty-five feet lower, at the
period when the shells were thrown up on the ledge at S. Lorenzo,
and when as we know man inhabited this district, such an event
might well have occurred; and if we may further suppose, that the
plain was at that time converted into a temporary lake, as
actually occurred, during the earthquakes of 1713 and 1746, in
the case of the low land round Callao owing to its being
encircled by a high shingle-beach, all the appearances above
described will be perfectly explained. I must add, that at a
lower level near the point where the present low land round
Callao joins the higher plain, there are appearances of two
distinct deposits both apparently formed by debacles: in the
upper one, a horse's tooth and a dog's jaw were embedded; so that
both must have been formed after the settlement of the Spaniards:
according to Acosta, the earthquake-wave of 1586 rose eighty-four
feet.
The inhabitants of Callao do not believe, as far as I could
ascertain, that any change in level is now in progress. The great
fragments of brickwork, which it is asserted can be seen at the
bottom of the sea, and which have been adduced as a proof of a
late subsidence, are, as I am informed by Mr. Gill, a resident
engineer, loose fragments; this is probable, for I found on the
beach, and not near the remains of any building, masses of
brickwork, three and four feet square, which had been washed into
their present places, and smoothed over with shingle during the
earthquake of 1746. The spit of land, on which the ruins of OLD
Callao stand, is so extremely low and narrow, that it is
improbable in the highest degree that a town should have been
founded on it in its present state; and I have lately heard that
M. Tschudi has come to the conclusion, from a comparison of old
with modern charts, that the coast both south and north of Callao
has subsided. (I am indebted for this fact to Dr. E. Dieffenbach.
I may add that there is a tradition, that the islands of San
Lorenzo and Fronton were once joined, and that the channel
between San Lorenzo and the mainland, now above two miles in
width, was so narrow that cattle used to swim over.) I have shown
that the island of San Lorenzo has been upraised eighty-five feet
since the Peruvians inhabited this country; and whatever may have
been the amount of recent subsidence, by so much more must the
elevation have exceeded the eighty-five feet. In several places
in this neighbourhood, marks of sea-action have been observed:
Ulloa gives a detailed account of such appearances at a point
five leagues northward of Callao: Mr. Cruikshank found near Lima
successive lines of sea-cliffs, with rounded blocks at their
bases, at a height of 700 feet above the present level of the
sea. ("Observaciones sobre el Clima del Lima" par Dr. H. Unanue
page 4.—Ulloa's "Voyage" volume 2 English Translation page
97.—For Mr. Cruikshank's observations, see Mr. Lyell's
"Principles of Geology" 1st edition volume 3 page 130.) ON THE
DECAY OF UPRAISED SEA-SHELLS.
I have stated that many of the shells on the lower inclined ledge
or terrace of San Lorenzo are corroded in a peculiar manner, and
that they have a much more ancient appearance than the same
species at considerably greater heights on the coast of Chile. I
have, also, stated that these shells in the upper part of the
ledge, at the height of eighty-five feet above the sea, are
falling, and in some parts are quite changed into a fine, soft,
saline, calcareous powder. The finest part of this powder has
been analysed for me, at the request of Sir H. De la Beche, by
the kindness of Mr. Trenham Reeks of the Museum of Economic
Geology; it consists of carbonate of lime in abundance, of
sulphate and muriate of lime, and of muriate and sulphate of
soda. The carbonate of lime is obviously derived from the shells;
and common salt is so abundant in parts of the bed, that, as
before remarked, the univalves are often filled with it. The
sulphate of lime may have been derived, as has probably the
common salt, from the evaporation of the sea-spray, during the
emergence of the land; for sulphate of lime is now copiously
deposited from the spray on the shores of Ascension. (See
"Volcanic Islands" etc. by the Author.) The other saline bodies
may perhaps have been partially thus derived, but chiefly, as I
conclude from the following facts, through a different means.
On most parts of the second ledge or old sea-beach, at a height
of 170 feet, there is a layer of white powder of variable
thickness, as much in some parts as two inches, lying on the
angular, salt-cemented fragments of sandstone and under about
four inches of earth, which powder, from its close resemblance in
nature to the upper and most decayed parts of the shelly mass, I
can hardly doubt originally existed as a bed of shells, now much
collapsed and quite disintegrated. I could not discover with the
microscope a trace of organic structure in it; but its chemical
constituents, according to Mr. Reeks, are the same as in the
powder extracted from amongst the decaying shells on the lower
ledge, with the marked exception that the carbonate of lime is
present in only very small quantity. On the third and highest
ledge, I observed some of this powder in a similar position, and
likewise occasionally in small patches at considerably greater
heights near the summit of the island. At Iquique, where the
whole face of the country is covered by a highly saliferous
alluvium, and where the climate is extremely dry, we have seen
that, according to Mr. Blake, the shells which are perfect near
the beach become, in ascending, gradually less and less perfect,
until scarcely a trace of their original structure can be
discovered. It is known that carbonate of lime and common salt
left in a mass together, and slightly moistened, partially
decompose each other (I am informed by Dr. Kane, through Mr.
Reeks, that a manufactory was established on this principle in
France, but failed from the small quantity of carbonate of soda
produced. Sprengel "Gardeners' Chronicle" 1845 page 157, states,
that salt and carbonate of lime are liable to mutual
decomposition in the soil. Sir H. De la Beche informs me, that
calcareous rocks washed by the spray of the sea, are often
corroded in a peculiar manner; see also on this latter subject
"Gardeners' Chronicle" page 675 1844.): now we have at San
Lorenzo and at Iquique, in the shells and salt packed together,
and occasionally moistened by the so- called Peruvian dew, the
proper elements for this action. We can thus understand the
peculiar corroded appearance of the shells on San Lorenzo, and
the great decrease of quantity in the carbonate of lime in the
powder on the upper ledge. There is, however, a great difficulty
on this view, for the resultant salts should be carbonate of soda
and muriate of lime; the latter is present, but not the carbonate
of soda. Hence I am led to the perhaps unauthorised conjecture
(which I shall hereafter have to refer to) that the carbonate of
soda, by some unexplained means, becomes converted into a
sulphate.
If the above remarks be just, we are led to the very unexpected
conclusion, that a dry climate, by leaving the salt from the
sea-spray undissolved, is much less favourable to the
preservation of upraised shells than a humid climate. However
this may be, it is interesting to know the manner in which masses
of shells, gradually upraised above the sea-level, decay and
finally disappear.
A SUMMARY ON THE RECENT ELEVATION OF THE WEST COAST OF SOUTH
AMERICA.
We have seen that upraised marine remains occur at intervals, and
in some parts almost continuously, from latitude 45 degrees 35'
to 12 degrees S., along the shores of the Pacific. This is a
distance, in a north and south line, of 2,075 geographical miles.
From Byron's observations, the elevation has no doubt extended
sixty miles further south; and from the similarity in the form of
the country near Lima, it has probably extended many leagues
further north. (I may take this opportunity of stating that in a
MS. in the Geological Society by Mr. Weaver, it is stated that
beds of oysters and other recent shells are found thirty feet
above the level of the sea, in many parts of Tampico, in the Gulf
of Mexico.) Along this great line of coast, besides the organic
remains, there are in very many parts, marks of erosion, caves,
ancient beaches, sand-dunes, and successive terraces of gravel,
all above the present level of the sea. From the steepness of the
land on this side of the continent, shells have rarely been found
at greater distances inland than from two to three leagues; but
the marks of sea-action are evident farther from the coast; for
instance, in the valley of Guasco, at a distance of between
thirty and forty miles. Judging from the upraised shells alone,
the elevation in Chiloe has been 350 feet, at Concepcion
certainly 625 feet; and by estimation 1,000 feet; at Valparaiso
1,300 feet; at Coquimbo 252 feet; northward of this place,
sea-shells have not, I believe, been found above 300 feet; and at
Lima they were falling into decay (hastened probably by the salt)
at 85 feet. Not only has this amount of elevation taken place
within the period of existing Mollusca and Cirripedes; but their
proportional numbers in the neighbouring sea have in most cases
remained the same. Near Lima, however, a small change in this
respect between the living and the upraised was observed: at
Coquimbo this was more evident, all the shells being existing
species, but with those embedded in the uppermost calcareous
plain not approximating so closely in proportional numbers, as do
those that lie loose on its surface at the height of 252 feet,
and still less closely than those which are strewed on the lower
plains, which latter are identical in proportional numbers with
those now cast up on the beach. From this circumstance, and from
not finding, upon careful examination, near Coquimbo any shells
at a greater height than 252 feet, I believe that the recent
elevation there has been much less than at Valparaiso, where it
has been 1,300 feet, and I may add, than at Concepcion. This
considerable inequality in the amount of elevation at Coquimbo
and Valparaiso, places only 200 miles apart, is not improbable,
considering, first, the difference in the force and number of the
shocks now yearly affecting different parts of this coast; and,
secondly, the fact of single areas, such as that of the province
of Concepcion, having been uplifted very unequally during the
same earthquake. It would, in most cases, be very hazardous to
infer an inequality of elevation, from shells being found on the
surface or in superficial beds at different heights; for we do
not know on what their rate of decay depends; and at Coquimbo one
instance out of many has been given, of a promontory, which, from
the occurrence of one very small collection of lime-cemented
shells, has indisputably been elevated 242 feet, and yet on
which, not even a fragment of shell could be found on careful
examination between this height and the beach, although many
sites appeared very favourable for the preservation of organic
remains: the absence, also, of shells on the gravel-terraces a
short distance up the valley of Coquimbo, though abundant on the
corresponding terraces at its mouth, should be borne in mind.
There are other epochs, besides that of the existence of recent
Mollusca, by which to judge of the changes of level on this
coast. At Lima, as we have just seen, the elevation has been at
least eighty-five feet, within the Indo-human period; and since
the arrival of the Spaniards in 1530, there has apparently been a
sinking of the surface. At Valparaiso, in the course of 220
years, the rise must have been less than nineteen feet; but it
has been as much as from ten to eleven feet in the seventeen
years subsequently to 1817, and of this rise only a part can be
attributed to the earthquake of 1822, the remainder having been
insensible and apparently still, in 1834, in progress. At Chiloe
the elevation has been gradual, and about four feet during four
years. At Coquimbo, also, it has been gradual, and in the course
of 150 years has amounted to several feet. The sudden small
upheavals, accompanied by earthquakes, as in 1822 at Valparaiso,
in 1835 at Concepcion, and in 1837 in the Chonos Archipelago, are
familiar to most geologists, but the gradual rising of the coast
of Chile has been hardly noticed; it is, however, very important,
as connecting together these two orders of events.
The rise of Lima, having been eighty-five feet within the period
of man, is the more surprising if we refer to the eastern coast
of the continent, for at Port S. Julian, in Patagonia, there is
good evidence (as we shall hereafter see) that when the land
stood ninety feet lower, the Macrauchenia, a mammiferous beast,
was alive; and at Bahia Blanca, when it stood only a few feet
lower than it now does, many gigantic quadrupeds ranged over the
adjoining country. But the coast of Patagonia is some way distant
from the Cordillera, and the movement at Bahia Blanca is perhaps
noways connected with this great range, but rather with the
tertiary volcanic rocks of Banda Oriental, and therefore the
elevation at these places may have been infinitely slower than on
the coast of Peru. All such speculations, however, must be vague,
for as we know with certainty that the elevation of the whole
coast of Patagonia has been interrupted by many and long pauses,
who will pretend to say that, in such cases, many and long
periods of subsidence may not also have been intercalated?
In many parts of the coast of Chile and Peru there are marks of
the action of the sea at successive heights on the land, showing
that the elevation has been interrupted by periods of comparative
rest in the upward movement, and of denudation in the action of
the sea. These are plainest at Chiloe, where, in a height of
about five hundred feet, there are three escarpments,—at
Coquimbo, where in a height of 364 feet, there are five,— at
Guasco, where there are six, of which five may perhaps correspond
with those at Coquimbo, but if so, the subsequent and intervening
elevatory movements have been here much more energetic,—at Lima,
where, in a height of about 250 feet there are three terraces,
and others, as it is asserted, at considerably greater heights.
The almost entire absence of ancient marks of sea-action at
defined levels along considerable spaces of coast, as near
Valparaiso and Concepcion, is highly instructive, for as it is
improbable that the elevation at these places alone should have
been continuous, we must attribute the absence of such marks to
the nature and form of the coast-rocks. Seeing over how many
hundred miles of the coast of Patagonia, and on how many places
on the shores of the Pacific, the elevatory process has been
interrupted by periods of comparative rest, we may conclude,
conjointly with the evidence drawn from other quarters of the
world, that the elevation of the land is generally an
intermittent action. From the quantity of matter removed in the
formation of the escarpments, especially of those of Patagonia,
it appears that the periods of rest in the movement, and of
denudation of the land, have generally been very long. In
Patagonia, we have seen that the elevation has been equable, and
the periods of denudation synchronous over very wide spaces of
coast; on the shores of the Pacific, owing to the terraces
chiefly occurring in the valleys, we have not equal means of
judging on this point; and the very different heights of the
upraised shells at Coquimbo, Valparaiso, and Concepcion seem
directly opposed to such a conclusion.
Whether on this side of the continent the elevation, between the
periods of comparative rest when the escarpments were formed, has
been by small sudden starts, such as those accompanying recent
earthquakes, or, as is most probable, by such starts conjointly
with a gradual upward movement, or by great and sudden upheavals,
I have no direct evidence. But as on the eastern coast, I was led
to think, from the analogy of the last hundred feet of elevation
in La Plata, and from the nearly equal size of the pebbles over
the entire width of the terraces, and from the upraised shells
being all littoral species, that the elevation had been gradual;
so do I on this western coast, from the analogy of the movements
now in progress, and from the vast numbers of shells now living
exclusively on or close to the beach, which are strewed over the
whole surface of the land up to very considerable heights,
conclude, that the movement here also has been slow and gradual,
aided probably by small occasional starts. We know at least that
at Coquimbo, where five escarpments occur in a height of 364
feet, the successive elevations, if they have been sudden, cannot
have been very great. It has, I think, been shown that the
occasional preservation of shells, unrolled and unbroken, is not
improbable even during a quite gradual rising of the land; and
their preservation, if the movement has been aided by small
starts, is quite conformable with what actually takes place
during recent earthquakes.
Judging from the present action of the sea, along the shores of
the Pacific, on the deposits of its own accumulation, the present
time seems in most places to be one of comparative rest in the
elevatory movement, and of denudation of the land. Undoubtedly
this is the case along the whole great length of Patagonia. At
Chiloe, however, we have seen that a narrow sloping fringe,
covered with vegetation, separates the present sea-beach from a
line of low cliffs, which the waves lately reached; here, then,
the land is gaining in breadth and height, and the present period
is not one of rest in the elevation and of contingent denudation;
but if the rising be not prolonged at a quick rate, there is
every probability that the sea will soon regain its former
horizontal limits. I observed similar low sloping fringes on
several parts of the coast, both northward of Valparaiso and near
Coquimbo; but at this latter place, from the change in form which
the coast has undergone since the old escarpments were worn, it
may be doubted whether the sea, acting for any length of time at
its present level, would eat into the land; for it now rather
tends to throw up great masses of sand. It is from facts such as
these that I have generally used the term COMPARATIVE rest, as
applied to the elevation of the land; the rest or cessation in
the movement being comparative both with what has preceded it and
followed it, and with the sea's power of corrosion at each spot
and at each level. Near Lima, the cliff-formed shores of San
Lorenzo, and on the mainland south of Callao, show that the sea
is gaining on the land; and as we have here some evidence that
its surface has lately subsided or is still sinking, the periods
of comparative rest in the elevation and of contingent
denudation, may probably in many cases include periods of
subsidence. It is only, as was shown in detail when discussing
the terraces of Coquimbo, when the sea with difficulty and after
a long lapse of time has either corroded a narrow ledge into
solid rock, or has heaped up on a steep surface a NARROW mound of
detritus, that we can confidently assert that the land at that
level and at that period long remained absolutely stationary. In
the case of terraces formed of gravel or sand, although the
elevation may have been strictly horizontal, it may well happen
that no one level beach-line may be traceable, and that neither
the terraces themselves nor the summit nor basal edges of their
escarpments may be horizontal.
Finally, comparing the extent of the elevated area, as deduced
from the upraised recent organic remains, on the two sides of the
continent, we have seen that on the Atlantic, shells have been
found at intervals from Eastern Tierra del Fuego for 1,180 miles
northward, and on the Pacific for a space of 2,075 miles. For a
length of 775 miles, they occur in the same latitudes on both
sides of the continent. Without taking this circumstance into
consideration, it is probable from the reasons assigned in the
last chapter, that the entire breadth of the continent in Central
Patagonia has been uplifted in mass; but from other reasons there
given, it would be hazardous to extend this conclusion to La
Plata. From the continent being narrow in the southern-most parts
of Patagonia, and from the shells found at the Inner Narrows of
the Strait of Magellan, and likewise far up the valley of the
Santa Cruz, it is probable that the southern part of the western
coast, which was not visited by me, has been elevated within the
period of recent Mollusca: if so, the shores of the Pacific have
been continuously, recently, and in a geological sense
synchronously upraised, from Lima for a length of 2,480 nautical
miles southward,—a distance equal to that from the Red Sea to the
North Cape of Scandinavia!
CHAPTER III. ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS
SUPERFICIAL DEPOSITS.
Basin-like plains of Chile; their drainage, their marine origin.
Marks of sea-action on the eastern flanks of the Cordillera. Sloping
terrace-like fringes of stratified shingle within the valleys of the
Cordillera; their marine origin. Boulders in the valley of Cachapual.
Horizontal elevation of the Cordillera. Formation of valleys.
Boulders moved by earthquake-waves. Saline superficial deposits. Bed
of nitrate of soda at Iquique. Saline incrustations. Salt-lakes of La
Plata and Patagonia; purity of the salt; its origin.
The space between the Cordillera and the coast of Chile is on a
rude average from eighty to above one hundred miles in width; it
is formed, either of an almost continuous mass of mountains, or
more commonly of several nearly parallel ranges, separated by
plains; in the more southern parts of this province the mountains
are quite subordinate to the plains; in the northern part the
mountains predominate.
The basin-like plains at the foot of the Cordillera are in
several respects remarkable; that on which the capital of Chile
stands is fifteen miles in width, in an east and west line, and
of much greater length in a north and south line; it stands 1,750
feet above the sea; its surface appears smooth, but really falls
and rises in wide gentle undulations, the hollows corresponding
with the main valleys of the Cordillera: the striking manner in
which it abruptly comes up to the foot of this great range has
been remarked by every author since the time of Molina. (This
plain is partially separated into two basins by a range of hills;
the southern half, according to Meyen ("Reise um Erde" Th. 1 s.
274), falls in height, by an abrupt step, of between fifteen and
twenty feet.) Near the Cordillera it is composed of a stratified
mass of pebbles of all sizes, occasionally including rounded
boulders: near its western boundary, it consists of reddish sandy
clay, containing some pebbles and numerous fragments of pumice,
and sometimes passes into pure sand or into volcanic ashes. At
Podaguel, on this western side of the plain, beds of sand are
capped by a calcareous tuff, the uppermost layers being generally
hard and substalagmitic, and the lower ones white and friable,
both together precisely resembling the beds at Coquimbo, which
contain recent marine shells. Abrupt, but rounded, hummocks of
rock rise out of this plain: those of Sta. Lucia and S. Cristoval
are formed of greenstone-porphyry almost entirely denuded of its
original covering of porphyritic claystone breccia; on their
summits, many fragments of rock (some of them kinds not found in
situ) are coated and united together by a white, friable,
calcareous tuff, like that found at Podaguel. When this matter
was deposited on the summit of S. Cristoval, the water must have
stood 946 feet above the surface of the surrounding plain. (Or
2,690 feet above the sea, as measured barometrically by Mr. Eck.
This tuff appears to the eye nearly pure; but when placed in acid
it leaves a considerable residue of sand and broken crystals,
apparently of feldspar. Dr. Meyen ("Reise" Th. 1 s. 269) says he
found a similar substance on the neighbouring hill of Dominico
(and I found it also on the Cerro Blanco), and he attributes it
to the weathering of the stone. In some places which I examined,
its bulk put this view of its origin quite out of the question;
and I should much doubt whether the decomposition of a porphyry
would, in any case, leave a crust chiefly composed of carbonate
of lime. The white crust, which is commonly seen on weathered
feldspathic rocks, does not appear to contain any free carbonate
of lime.)
To the south this basin-like plain contracts, and rising scarcely
perceptibly with a smooth surface, passes through a remarkable
level gap in the mountains, forming a true land-strait, and
called the Angostura. It then immediately expands into a second
basin-formed plain: this again to the south contracts into
another land-strait, and expands into a third basin, which,
however, falls suddenly in level about forty feet. This third
basin, to the south, likewise contracts into a strait, and then
again opens into the great plain of San Fernando, stretching so
far south that the snowy peaks of the distant Cordillera are seen
rising above its horizon as above the sea. These plains, near the
Cordillera, are generally formed of a thick stratified mass of
shingle (The plain of San Fernando has, according to MM. Meyen
and Gay "Reise" etc. Th. 1 ss. 295 and 298, near the Cordillera,
an upper step-formed plain of clay, on the surface of which they
found numerous blocks of rocks, from two to three feet long,
either lying single or piled in heaps, but all arranged in nearly
straight lines.); in other parts, of a red sandy clay, often with
an admixture of pumiceous matter. Although these basins are
connected together like a necklace, in a north and south line, by
smooth land-straits, the streams which drain them do not all flow
north and south, but mostly westward, through breaches worn in
the bounding mountains; and in the case of the second basin, or
that of Rancagua, there are two distinct breaches. Each basin,
moreover, is not drained singly; thus, to give the most striking
instance, but not the only one, in proceeding southward over the
plain of Rancagua, we first find the water flowing northward to
and through the northern land-strait; then, without crossing any
marked ridge or watershed, we see it flowing south-westward
towards the northern one of the two breaches in the western
mountainous boundary; and lastly, again without any ridge, it
flows towards the southern breach in these same mountains. Hence
the surface of this one basin-like plain, appearing to the eye so
level, has been modelled with great nicety, so that the drainage,
without any conspicuous watersheds, is directed towards three
openings in the encircling mountains. ((It appears from Captain
Herbert's account of the Diluvium of the Himalaya, "Gleanings of
Science" Calcutta volume 2 page 164, that precisely similar
remarks apply to the drainage of the plains or valleys between
those great mountains.) The streams flowing from the southern
basin-like plains, after passing through the breaches to the
west, unite and form the river Rapel, which enters the Pacific
near Navidad. I followed the southernmost branch of this river,
and found that the basin or plain of San Fernando is continuously
and smoothly united with those plains, which were described in
the Second Chapter, as being worn near the coast into successive
cave-eaten escarpments, and still nearer to the coast, as being
strewed with upraised recent marine remains.
I might have given descriptions of numerous other plains of the
same general form, some at the foot of the Cordillera, some near
the coast, and some halfway between these points. I will allude
only to one other, namely, the plain of Uspallata, lying on the
eastern or opposite side of the Cordillera, between that great
range and the parallel lower range of Uspallata. According to
Miers, its surface is 6,000 feet above the level of the sea: it
is from ten to fifteen miles in width, and is said to extend with
an unbroken surface for 180 miles northwards: it is drained by
two rivers passing through breaches in the mountains to the east.
On the banks of the River Mendoza it is seen to be composed of a
great accumulation of stratified shingle, estimated at 400 feet
in thickness. In general appearance, and in numerous points of
structure, this plain closely resembles those of Chile.
The origin and manner of formation of the thick beds of gravel,
sandy clay, volcanic detritus, and calcareous tuff, composing
these basin-like plains, is very important; because, as we shall
presently show, they send arms or fringes far up the main valleys
of the Cordillera. Many of the inhabitants believe that these
plains were once occupied by lakes, suddenly drained; but I
conceive that the number of the separate breaches at nearly the
same level in the mountains surrounding them quite precludes this
idea. Had not such distinguished naturalists as MM. Meyen and Gay
stated their belief that these deposits were left by great
debacles rushing down from the Cordillera, I should not have
noticed a view, which appears to me from many reasons improbable
in the highest degree—namely, from the vast accumulation of
WELL-ROUNDED PEBBLES—their frequent stratification with layers of
sand—the overlying beds of calcareous tuff—this same substance
coating and uniting the fragments of rock on the hummocks in the
plain of Santiago—and lastly even from the worn, rounded, and
much denuded state of these hummocks, and of the headlands which
project from the surrounding mountains. On the other hand, these
several circumstances, as well as the continuous union of the
basins at the foot of the Cordillera, with the great plain of the
Rio Rapel which still retains the marks of sea-action at various
levels, and their general similarity in form and composition with
the many plains near the coast, which are either similarly marked
or are strewed with upraised marine remains, fully convince me
that the mountains bounding these basin-plains were breached,
their islet-like projecting rocks worn, and the loose stratified
detritus forming their now level surfaces deposited, by the sea,
as the land slowly emerged. It is hardly possible to state too
strongly the perfect resemblance in outline between these
basin-like, long, and narrow plains of Chile (especially when in
the early morning the mists hanging low represented water), and
the creeks and fiords now intersecting the southern and western
shores of the continent. We can on this view of the sea, when the
land stood lower, having long and tranquilly occupied the spaces
between the mountain-ranges, understand how the boundaries of the
separate basins were breached in more than one place; for we see
that this is the general character of the inland bays and
channels of Tierra del Fuego; we there, also, see in the sawing
action of the tides, which flow with great force in the cross
channels, a power sufficient to keep the breaches open as the
land emerged. We can further see that the waves would naturally
leave the smooth bottom of each great bay or channel, as it
became slowly converted into land, gently inclined to as many
points as there were mouths, through which the sea finally
retreated, thus forming so many watersheds, without any marked
ridges, on a nearly level surface. The absence of marine remains
in these high inland plains cannot be properly adduced as an
objection to their marine origin: for we may conclude, from
shells not being found in the great shingle beds of Patagonia,
though copiously strewed on their surfaces, and from many other
analogous facts, that such deposits are eminently unfavourable
for the embedment of such remains; and with respect to shells not
being found strewed on the surface of these basin-like plains, it
was shown in the last chapter that remains thus exposed in time
decay and disappear.
(FIGURE 13. SECTION OF THE PLAIN AT THE EASTERN FOOT OF THE
CHILEAN CORDILLERA.
From Cordillera (left) through Talus-plain and Level surface,
2,700 feet above sea, to Gravel terraces (right).)
I observed some appearances on the plains at the eastern and
opposite foot of the Cordillera which are worth notice, as
showing that the sea there long acted at nearly the same level as
on the basin-plains of Chile. The mountains on this eastern side
are exceedingly abrupt; they rise out of a smooth, talus-like,
very gentle, slope, from five to ten miles in width (as
represented in Figure 13), entirely composed of perfectly rounded
pebbles, often white-washed with an aluminous substance like
decomposed feldspar. This sloping plain or talus blends into a
perfectly flat space a few miles in width, composed of reddish
impure clay, with small calcareous concretions as in the Pampean
deposit,—of fine white sand with small pebbles in layers,—and of
the above-mentioned white aluminous earth, all interstratified
together. This flat space runs as far as Mendoza, thirty miles
northward, and stands probably at about the same height, namely,
2,700 feet (Pentland and Miers) above the sea. To the east it is
bounded by an escarpment, eighty feet in height, running for many
miles north and south, and composed of perfectly round pebbles,
and loose, white-washed, or embedded in the aluminous earth:
behind this escarpment there is a second and similar one of
gravel. Northward of Mendoza, these escarpments become broken and
quite obliterated; and it does not appear that they ever enclosed
a lake-like area: I conclude, therefore, that they were formed by
the sea, when it reached the foot of the Cordillera, like the
similar escarpments occurring at so many points on the coasts of
Chile and Patagonia.
The talus-like plain slopes up with a smooth surface into the
great dry valleys of the Cordillera. On each hand of the Portillo
valley, the mountains are formed of red granite, mica-slate, and
basalt, which all have suffered a truly astonishing amount of
denudation; the gravel in the valley, as well as on the
talus-like plain in front of it, is composed of these rocks; but
at the mouth of the valley, in the middle (height probably about
three thousand five hundred feet above the sea), a few small
isolated hillocks of several varieties of porphyry project, round
which, on all sides, smooth and often white-washed pebbles of
these same porphyries, to the exclusion of all others, extend to
a circumscribed distance. Now, it is difficult to conceive any
other agency, except the quiet and long-continued action of the
sea on these hillocks, which could have rounded and whitewashed
the fragments of porphyry, and caused them to radiate from such
small and quite insignificant centres, in the midst of that vast
stream of stones which has descended from the main Cordillera.
SLOPING TERRACES OF GRAVEL IN THE VALLEYS OF THE CORDILLERA.
(FIGURE 14. GROUND-PLAN OF A BIFURCATING VALLEY IN THE
CORDILLERA, bordered by smooth, sloping gravel-fringes (AA), worn
along the course of the river into cliffs.)
All the main valleys on both flanks of the Chilean Cordillera
have formerly had, or still have, their bottoms filled up to a
considerable thickness by a mass of rudely stratified shingle. In
Central Chile the greater part of this mass has been removed by
the torrents; cliff-bounded fringes, more or less continuous,
being left at corresponding heights on both sides of the valleys.
These fringes, or as they may be called terraces, have a smooth
surface, and as the valleys rise, they gently rise with them:
hence they are easily irrigated, and afford great facilities for
the construction of the roads. From their uniformity, they give a
remarkable character to the scenery of these grand, wild, broken
valleys. In width, the fringes vary much, sometimes being only
broad enough for the roads, and sometimes expanding into narrow
plains. Their surfaces, besides gently rising up the valley, are
slightly inclined towards its centre in such a manner as to show
that the whole bottom must once have been filled up with a smooth
and slightly concave mass, as still are the dry unfurrowed
valleys of Northern Chile. Where two valleys unite into one,
these terraces are particularly well exhibited, as is represented
in Figure 14. The thickness of the gravel forming these fringes,
on a rude average, may be said to vary from thirty to sixty or
eighty feet; but near the mouths of the valleys it was in several
places from two to three hundred feet. The amount of matter
removed by the torrents has been immense; yet in the lower parts
of the valleys the terraces have seldom been entirely worn away
on either side, nor has the solid underlying rock been reached:
higher up the valleys, the terraces have frequently been removed
on one or the other side, and sometimes on both sides; but in
this latter case they reappear after a short interval on the
line, which they would have held had they been unbroken. Where
the solid rock has been reached, it has been cut into deep and
narrow gorges. Still higher up the valleys, the terraces
gradually become more and more broken, narrower, and less thick,
until, at a height of from seven to nine thousand feet, they
become lost, and blended with the piles of fallen detritus.
I carefully examined in many places the state of the gravel, and
almost everywhere found the pebbles equally and perfectly
rounded, occasionally with great blocks of rock, and generally
distinctly stratified, often with parting seams of sand. The
pebbles were sometimes coated with a white aluminous, and less
frequently with a calcareous, crust. At great heights up the
valleys the pebbles become less rounded; and as the terraces
become obliterated, the whole mass passes into the nature of
ordinary detritus. I was repeatedly struck with the great
difference between this detritus high up the valleys, and the
gravel of the terraces low down, namely, in the greater number of
the quite angular fragments in the detritus,—in the unequal
degree to which the other fragments have been rounded,—in the
quantity of associated earth,—in the absence of
stratification,—and in the irregularity of the upper surfaces.
This difference was likewise well shown at points low down the
valleys, where precipitous ravines, cutting through mountains of
highly coloured rock, have thrown down wide, fan- shaped
accumulations of detritus on the terraces: in such cases, the
line of separation between the detritus and the terrace could be
pointed out to within an inch or two; the detritus consisting
entirely of angular and only partially rounded fragments of the
adjoining coloured rocks; the stratified shingle (as I
ascertained by close inspection, especially in one case, in the
valley of the River Mendoza) containing only a small proportion
of these fragments, and those few well rounded.
I particularly attended to the appearance of the terraces where
the valleys made abrupt and considerable bends, but I could
perceive no difference in their structure: they followed the
bends with their usual nearly equable inclination. I observed,
also, in several valleys, that wherever large blocks of any rock
became numerous, either on the surface of the terrace or embedded
in it, this rock soon appeared higher up in situ: thus I have
noticed blocks of porphyry, of andesitic syenite, of porphyry and
of syenite, alternately becoming numerous, and in each case
succeeded by mountains thus constituted. There is, however, one
remarkable exception to this rule; for along the valley of the
Cachapual, M. Gay found numerous large blocks of white granite,
which does not occur in the neighbourhood. I observed these
blocks, as well as others of andesitic syenite (not occurring
here in situ), near the baths of Cauquenes at a height of between
two and three hundred feet above the river, and therefore quite
above the terrace or fringe which borders that river; some miles
up the valleys there were other blocks at about the same height.
I also noticed, at a less height, just above the terrace, blocks
of porphyries (apparently not found in the immediately impending
mountains), arranged in rude lines, as on a sea-beach. All these
blocks were rounded, and though large, not gigantic, like the
true erratic boulders of Patagonia and Fuegia. M. Gay states that
the granite does not occur in situ within a distance of twenty
leagues ("Annales des Science Nat. " 1 series tome 28. M. Gay, as
I was informed, penetrated the Cordillera by the great oblique
valley of Los Cupressos, and not by the most direct line.); I
suspect, for several reasons, that it will ultimately be found at
a much less distance, though certainly not in the immediate
neighbourhood. The boulders found by MM. Meyen and Gay on the
upper plain of San Fernando (mentioned in a previous note)
probably belong to this same class of phenomena.
These fringes of stratified gravel occur along all the great
valleys of the Cordillera, as well as along their main branches;
they are strikingly developed in the valleys of the Maypu,
Mendoza, Aconcagua, Cachapual, and according to Meyen, in the
Tinguirica. ("Reise" etc. Th. 1 s. 302.) In the valleys, however,
of Northern Chile, and in some on the eastern flank of the
Cordillera, as in the Portillo Valley, where streams have never
flowed, or are quite insignificant in volume, the presence of a
mass of stratified gravel can be inferred only from the smooth
slightly concave form of the bottom. One naturally seeks for some
explanation of so general and striking a phenomenon; that the
matter forming the fringes along the valleys, or still filling up
their entire beds, has not fallen from the adjoining mountains
like common detritus, is evident from the complete contrast in
every respect between the gravel and the piles of detritus,
whether seen high up the valleys on their sides, or low down in
front of the more precipitous ravines; that the matter has not
been deposited by debacles, even if we could believe in debacles
having rushed down EVERY valley, and all their branches, eastward
and westward from the central pinnacles of the Cordillera, we
must admit from the following reasons,—from the distinct
stratification of the mass,—its smooth upper surface,—the
well-rounded and sometimes encrusted state of the pebbles, so
different from the loose debris on the mountains,—and especially
from the terraces preserving their uniform inclination round the
most abrupt bends. To suppose that as the land now stands, the
rivers deposited the shingle along the course of every valley,
and all their main branches, appears to me preposterous, seeing
that these same rivers not only are now removing and have removed
much of this deposit, but are everywhere tending to cut deep and
narrow gorges in the hard underlying rocks.
I have stated that these fringes of gravel, the origin of which
are inexplicable on the notion of debacles or of ordinary
alluvial action, are directly continuous with the
similarly-composed basin-like plains at the foot of the
Cordillera, which, from the several reasons before assigned, I
cannot doubt were modelled by the agency of the sea. Now if we
suppose that the sea formerly occupied the valleys of the Chilean
Cordillera, in precisely the same manner as it now does in the
more southern parts of the continent, where deep winding creeks
penetrate into the very heart of, and in the case of Obstruction
Sound quite through, this great range; and if we suppose that the
mountains were upraised in the same slow manner as the eastern
and western coasts have been upraised within the recent period,
then the origin and formation of these sloping, terrace-like
fringes of gravel can be simply explained. For every part of the
bottom of each valley will, on this view, have long stood at the
head of a sea creek, into which the then existing torrents will
have delivered fragments of rocks, where, by the action of the
tides, they will have been rolled, sometimes encrusted, rudely
stratified, and the whole surface levelled by the blending
together of the successive beach lines. (Sloping terraces of
precisely similar structure have been described by me
"Philosophical Transactions" 1839 page 58, in the valleys of
Lochaber in Scotland, where, at higher levels, the parallel roads
of Glen Roy show the marks of the long and quiet residence of the
sea. I have no doubt that these sloping terraces would have been
present in the valleys of most of the European ranges, had not
every trace of them, and all wrecks of sea-action, been swept
away by the glaciers which have since occupied them. I have shown
that this is the case with the mountains ("London and Edinburgh
Philosophical Journal" volume 21 page 187) of North Wales.) As
the land rose, the torrents in every valley will have tended to
have removed the matter which just before had been arrested on,
or near, the beach-lines; the torrents, also, having continued to
gain in force by the continued elevation increasing their total
descent from their sources to the sea. This slow rising of the
Cordillera, which explains so well the otherwise inexplicable
origin and structure of the terraces, judging from all known
analogies, will probably have been interrupted by many periods of
rest; but we ought not to expect to find any evidence of these
periods in the structure of the gravel- terraces: for, as the
waves at the heads of deep creeks have little erosive power, so
the only effect of the sea having long remained at the same level
will be that the upper parts of the creeks will have become
filled up at such periods to the level of the water with gravel
and sand; and that afterwards the rivers will have thrown down on
the filled-up parts a talus of similar matter, of which the
inclination (as at the head of a partially filled-up lake) will
have been determined by the supply of detritus, and the force of
the stream. (I have attempted to explain this process in a more
detailed manner, in a letter to Mr. Maclaren, published in the
"Edinburgh New Philosophical Journal" volume 35 page 288.) Hence,
after the final conversion of the creeks into valleys, almost the
only difference in the terraces at those points at which the sea
stood long, will be a somewhat more gentle inclination, with
river-worn instead of sea-worn detritus on the surface.
I know of only one difficulty on the foregoing view, namely, the
far- transported blocks of rock high on the sides of the valley
of the Cachapual: I will not attempt any explanation of this
phenomenon, but I may state my belief that a mountain-ridge near
the Baths of Cauquenes has been upraised long subsequently to all
the other ranges in the neighbourhood, and that when this was
effected the whole face of the country must have been greatly
altered. In the course of ages, moreover, in this and other
valleys, events may have occurred like, but even on a grander
scale than, that described by Molina, when a slip during the
earthquake of 1762 banked up for ten days the great River Lontue,
which then bursting its barrier "inundated the whole country,"
and doubtless transported many great fragments of rock.
("Compendio de la Hist." etc. etc. tome 1 page 30. M. Brongniart,
in his report on M. Gay's labours "Annales des Sciences" 1833,
considers that the boulders in the Cachapual belong to the same
class with the erratic boulders of Europe. As the blocks which I
saw are not gigantic, and especially as they are not angular, and
as they have not been transported fairly across low spaces or
wide valleys, I am unwilling to class them with those which, both
in the northern and southern hemisphere "Geological Transactions"
volume 6 page 415, have been transported by ice. It is to be
hoped that when M. Gay's long-continued and admirable labours in
Chile are published, more light will be thrown on this subject.
However, the boulders may have been primarily transported; the
final position of those of porphyry, which have been described as
arranged at the foot of the mountain in rude lines, I cannot
doubt, has been due to the action of waves on a beach. The valley
of the Cachapual, in the part where the boulders occur, bursts
through the high ridge of Cauquenes, which runs parallel to, but
at some distance from, the Cordillera. This ridge has been
subjected to excessive violence; trachytic lava has burst from
it, and hot springs yet flow at its base. Seeing the enormous
amount of denudation of solid rock in the upper and much broader
parts of this valley where it enters the Cordillera, and seeing
to what extent the ridge of Cauquenes now protects the great
range, I could not help believing (as alluded to in the text)
that this ridge with its trachytic eruptions had been thrown up
at a much later period than the Cordillera. If this has been the
case, the boulders, after having been transported to a low level
by the torrents (which exhibit in every valley proofs of their
power of moving great fragments), may have been raised up to
their present height, with the land on which they rested.)
Finally, notwithstanding this one case of difficulty, I cannot
entertain any doubt, that these terrace-like fringes, which are
continuously united with the basin-shaped plains at the foot of
the Cordillera, have been formed by the arrestment of river-borne
detritus at successive levels, in the same manner as we see now
taking place at the heads of all those many, deep, winding fiords
intersecting the southern coasts. To my mind, this has been one
of the most important conclusions to which my observations on the
geology of South America have led me; for we thus learn that one
of the grandest and most symmetrical mountain-chains in the
world, with its several parallel lines, has been together
uplifted in mass between seven and nine thousand feet, in the
same gradual manner as have the eastern and western coasts within
the recent period. (I do not wish to affirm that all the lines
have been uplifted quite equally; slight differences in the
elevation would leave no perceptible effect on the terraces. It
may, however, be inferred, perhaps with one exception, that since
the period when the sea occupied these valleys, the several
ranges have not been dislocated by GREAT and ABRUPT faults or
upheavals; for if such had occurred, the terraces of gravel at
these points would not have been continuous. The one exception is
at the lower end of a plain in the Valle del Yeso (a branch of
the Maypu), where, at a great height, the terraces and valley
appear to have been broken through by a line of upheaval, of
which the evidence is plain in the adjoining mountains; this
dislocation, perhaps, occurred AFTER THE ELEVATION of this part
of the valley above the level of the sea. The valley here is
almost blocked up by a pile about one thousand feet in thickness,
formed, as far as I could judge, from three sides, entirely, or
at least in chief part, of gravel and detritus. On the south
side, the river has cut quite through this mass; on the northern
side, and on the very summit, deep ravines, parallel to the line
of the valley, are worn, as if the drainage from the valley above
had passed by these two lines before following its present
course.)
FORMATION OF VALLEYS.
The bulk of solid rock which has been removed in the lower parts
of the valleys of the Cordillera has been enormous. It is only by
reflecting on such cases as that of the gravel beds of Patagonia,
covering so many thousand square leagues of surface, and which,
if heaped into a ridge, would form a mountain-range almost equal
to the Cordillera, that the amount of denudation becomes
credible. The valleys within this range often follow anticlinal
but rarely synclinal lines; that is, the strata on the two sides
more often dip from the line of valley than towards it. On the
flanks of the range, the valleys most frequently run neither
along anticlinal nor synclinal axes, but along lines of flexure
or faults: that is, the strata on both sides dip in the same
direction, but with different, though often only slightly
different, inclinations. As most of the nearly parallel ridges
which together form the Cordillera run approximately north and
south, the east and west valleys cross them in zig-zag lines,
bursting through the points where the strata have been least
inclined. No doubt the greater part of the denudation was
affected at the periods when tidal- creeks occupied the valleys,
and when the outer flanks of the mountains were exposed to the
full force of an open ocean. I have already alluded to the power
of the tidal action in the channels connecting great bays; and I
may here mention that one of the surveying vessels in a channel
of this kind, though under sail, was whirled round and round by
the force of the current. We shall hereafter see, that of the two
main ridges forming the Chilean Cordillera, the eastern and
loftiest one owes the greater part of its ANGULAR upheaval to a
period subsequent to the elevation of the western ridge; and it
is likewise probable that many of the other parallel ridges have
been angularly upheaved at different periods; consequently many
parts of the surfaces of these mountains must formerly have been
exposed to the full force of the waves, which, if the Cordillera
were now sunk into the sea, would be protected by parallel chains
of islands. The torrents in the valleys certainly have great
power in wearing the rocks; as could be told by the dull rattling
sound of the many fragments night and day hurrying downwards; and
as was attested by the vast size of certain fragments, which I
was assured had been carried onwards during floods; yet we have
seen in the lower parts of the valleys, that the torrents have
seldom removed all the sea-checked shingle forming the terraces,
and have had time since the last elevation in mass only to cut in
the underlying rocks, gorges, deep and narrow, but quite
insignificant in dimensions compared with the entire width and
depth of the valleys.
Along the shores of the Pacific, I never ceased during my many
and long excursions to feel astonished at seeing every valley,
ravine, and even little inequality of surface, both in the hard
granitic and soft tertiary districts, retaining the exact
outline, which they had when the sea left their surfaces coated
with organic remains. When these remains shall have decayed,
there will be scarcely any difference in appearance between this
line of coast-land and most other countries, which we are
accustomed to believe have assumed their present features chiefly
through the agency of the weather and fresh-water streams. In the
old granitic districts, no doubt it would be rash to attribute
all the modifications of outline exclusively to the sea-action;
for who can say how often this lately submerged coast may not
previously have existed as land, worn by running streams and
washed by rain? This source of doubt, however, does not apply to
the districts superficially formed of the modern tertiary
deposits. The valleys worn by the sea, through the softer
formations, both on the Atlantic and Pacific sides of the
continent, are generally broad, winding, and flat-bottomed: the
only district of this nature now penetrated by arms of the sea,
is the island of Chiloe.
Finally, the conclusion at which I have arrived, with respect to
the relative powers of rain and sea water on the land, is, that
the latter is far the most efficient agent, and that its chief
tendency is to widen the valleys; whilst torrents and rivers tend
to deepen them, and to remove the wreck of the sea's destroying
action. As the waves have more power, the more open and exposed
the space may be, so will they always tend to widen more and more
the mouths of valleys compared with their upper parts: hence,
doubtless, it is, that most valleys expand at their mouths,—that
part, at which the rivers flowing in them, generally have the
least wearing power.
When reflecting on the action of the sea on the land at former
levels, the effect of the great waves, which generally accompany
earthquakes, must not be overlooked: few years pass without a
severe earthquake occurring on some part of the west coast of
South America; and the waves thus caused have great power. At
Concepcion, after the shock of 1835, I saw large slabs of
sandstone, one of which was six feet long, three in breadth, and
two in thickness, thrown high up on the beach; and from the
nature of the marine animals still adhering to it, it must have
been torn up from a considerable depth. On the other hand, at
Callao, the recoil-wave of the earthquake of 1746 carried great
masses of brickwork, between three and four feet square, some way
out seaward. During the course of ages, the effect thus produced
at each successive level, cannot have been small; and in some of
the tertiary deposits on this line of coast, I observed great
boulders of granite and other neighbouring rocks, embedded in
fine sedimentary layers, the transportal of which, except by the
means of earthquake-waves, always appeared to me inexplicable.
SUPERFICIAL SALINE DEPOSITS.
This subject may be here conveniently treated of: I will begin
with the most interesting case, namely, the superficial saline
beds near Iquique in Peru. The porphyritic mountains on the coast
rise abruptly to a height of between one thousand nine hundred
and three thousand feet: between their summits and an inland
plain, on which the celebrated deposit of nitrate of soda lies,
there is a high undulatory district, covered by a remarkable
superficial saliferous crust, chiefly composed of common salt,
either in white, hard, opaque nodules, or mingled with sand, in
this latter case forming a compact sandstone. This saliferous
superficial crust extends from the edge of the coast-escarpment,
over the whole face of the country; but never attains, as I am
assured by Mr. Bollaert (long resident here) any great thickness.
Although a very slight shower falls only at intervals of many
years, yet small funnel-shaped cavities show that the salt has
been in some parts dissolved. (It is singular how slowly,
according to the observations of M. Cordier on the salt-mountain
of Cardona in Spain "Ann. des Mines, Translation of Geolog. Mem."
by De la Beche page 60, salt is dissolved, where the amount of
rain is supposed to be as much as 31.4 of an inch in the year. It
is calculated that only five feet in thickness is dissolved in
the course of a century.) In several places I saw large patches
of sand, quite moist, owing to the quantity of muriate of lime
(as ascertained by Mr. T. Reeks) contained in them. From the
compact salt- cemented sand being either red, purplish, or
yellow, according to the colour of the rocky strata on which it
rested, I imagined that this substance had probably been derived
through common alluvial action from the layers of salt which
occur interstratified in the surrounding mountains ("Journal of
Researches" page 444 first edition.): but from the interesting
details given by M. d'Orbigny, and from finding on a fresh
examination of this agglomerated sand, that it is not irregularly
cemented, but consists of thin layers of sand of different tints
of colour, alternating with excessively fine parallel layers of
salt, I conclude that it is not of alluvial origin. M. d'Orbigny
observed analogous saline beds extending from Cobija for five
degrees of latitude northward, and at heights varying from six
hundred to nine hundred feet ("Voyage" etc. page 102. M.
d'Orbigny found this deposit intersected, in many places, by deep
ravines, in which there was no salt. Streams must once, though
historically unknown, have flowed in them; and M. d'Orbigny
argues from the presence of undissolved salt over the whole
surrounding country, that the streams must have arisen from rain
or snow having fallen, not in the adjoining country, but on the
now arid Cordillera. I may remark, that from having observed
ruins of Indian buildings in absolutely sterile parts of the
Chilian Cordillera ("Journal" 2nd edition page 357), I am led to
believe that the climate, at a time when Indian man inhabited
this part of the continent, was in some slight degree more humid
than it is at present.): from finding recent sea- shells strewed
on these saliferous beds, and under them, great well-rounded
blocks, exactly like those on the existing beach, he believes
that the salt, which is invariably superficial, has been left by
the evaporation of the sea-water. This same conclusion must, I
now believe, be extended to the superficial saliferous beds of
Iquique, though they stand about three thousand feet above the
level of the sea.
Associated with the salt in the superficial beds, there are
numerous, thin, horizontal layers of impure, dirty-white,
friable, gypseous and calcareous tuffs. The gypseous beds are
very remarkable, from abounding with, so as sometimes to be
almost composed of, irregular concretions, from the size of an
egg to that of a man's head, of very hard, compact, heavy gypsum,
in the form of anhydrite. This gypsum contains some foreign
particles of stone; it is stained, judging from its action with
borax, with iron, and it exhales a strong aluminous odour. The
surfaces of the concretions are marked by sharp, radiating, or
bifurcating ridges, as if they had been (but not really)
corroded: internally they are penetrated by branching veins (like
those of calcareous spar in the septaria of the London clay) of
pure white anhydrite. These veins might naturally have been
thought to have been formed by subsequent infiltration, had not
each little embedded fragment of rock been likewise edged in a
very remarkable manner by a narrow border of the same white
anhydrite: this shows that the veins must have been formed by a
process of segregation, and not of infiltration. Some of the
little included and CRACKED fragments of foreign rock are
penetrated by the anhydrite, and portions have evidently been
thus mechanically displaced: at St. Helena, I observed that
calcareous matter, deposited by rain water, also had the power to
separate small fragments of rock from the larger masses.
("Volcanic Islands" etc. page 87.) I believe the superficial
gypseous deposit is widely extended: I received specimens of it
from Pisagua, forty miles north of Iquique, and likewise from
Arica, where it coats a layer of pure salt. M. d'Orbigny found at
Cobija a bed of clay, lying above a mass of upraised recent
shells, which was saturated with sulphate of soda, and included
thin layers of fibrous gypsum. ("Voyage Geolog." etc. page 95.)
These widely extended, superficial, beds of salt and gypsum,
appear to me an interesting geological phenomenon, which could be
presented only under a very dry climate.
The plain or basin, on the borders of which the famous bed of
nitrate of soda lies, is situated at the distance of about thirty
miles from the sea, being separated from it by the saliferous
district just described. It stands at a height of 3,300 feet; its
surface is level, and some leagues in width; it extends forty
miles northward, and has a total length (as I was informed by Mr.
Belford Wilson, the Consul-General at Lima) of 420 miles. In a
well near the works, thirty-six yards in depth, sand, earth, and
a little gravel were found: in another well, near Almonte, fifty
yards deep, the whole consisted, according to Mr. Blake, of clay,
including a layer of sand two feet thick, which rested on fine
gravel, and this on coarse gravel, with large rounded fragments
of rock. (See an admirable paper "Geological and Miscellaneous
Notices of Tarapaca" in "Silliman's American Journal" volume 44
page 1.) In many parts of this now utterly desert plain, rushes
and large prostrate trees in a hardened state, apparently
Mimosas, are found buried, at a depth from three to six feet;
according to Mr. Blake, they have all fallen to the south-west.
The bed of nitrate of soda is said to extend for forty to fifty
leagues along the western margin of the plain, but is not found
in its central parts: it is from two to three feet in thickness,
and is so hard that it is generally blasted with gunpowder; it
slopes gently upwards from the edge of the plain to between ten
and thirty feet above its level. It rests on sand in which, it is
said, vegetable remains and broken shells have been found; shells
have also been found, according to Mr. Blake, both on and in the
nitrate of soda. It is covered by a superficial mass of sand,
containing nodules of common salt, and, as I was assured by a
miner, much soft gypseous matter, precisely like that in the
superficial crust already described: certainly this crust, with
its characteristic concretions of anhydrite, comes close down to
the edge of the plain.
The nitrate of soda varies in purity in different parts, and
often contains nodules of common salt. According to Mr. Blake,
the proportion of nitrate of soda varies from 20 to 75 per cent.
An analysis by Mr. A. Hayes, of an average specimen, gave:—
Nitrate of Soda.... 64.98 Sulphate of Soda.... 3.00 Chloride of Soda...
28.69 Iodic Salts......... 0.63 Shells and Marl..... 2.60 99.90
The "mother-water" at some of the refineries is very rich in
iodic salts, and is supposed to contain much muriate of lime.
("Literary Gazette" 1841 page 475.) In an unrefined specimen
brought home by myself, Mr. T. Reeks has ascertained that the
muriate of lime is very abundant. With respect to the origin of
this saline mass, from the manner in which the gently inclined,
compact bed follows for so many miles the sinuous margin of the
plain, there can be no doubt that it was deposited from a sheet
of water: from the fragments of embedded shells, from the
abundant iodic salts, from the superficial saliferous crust
occurring at a higher level and being probably of marine origin,
and from the plain resembling in form those of Chile and that of
Uspallata, there can be little doubt that this sheet of water
was, at least originally, connected with the sea. (From an
official document, shown me by Mr. Belford Wilson, it appears
that the first export of nitrate of soda to Europe was in July
1830, on French account, in a British ship:—
In year, the entire export was in Quintals.
1830............................ 17,300
1831............................ 40,885
1832............................ 51,400
1833............................ 91,335 1834...........................
149,538 The Spanish quintal nearly equals 100 English pounds.)
THIN, SUPERFICIAL, SALINE INCRUSTATIONS.
These saline incrustations are common in many parts of America:
Humboldt met with them on the tableland of Mexico, and the Jesuit
Falkner and other authors state that they occur at intervals over
the vast plains extending from the mouth of the Plata to Rioja
and Catamarca. (Azara "Travels" volume 1 page 55, considers that
the Parana is the eastern boundary of the saliferous region; but
I heard of "salitrales" in the Province of Entre Rios.) Hence it
is that during droughts, most of the streams in the Pampas are
saline. I nowhere met with these incrustations so abundantly as
near Bahia Blanca: square miles of the mud-flats, which near that
place are raised only a few feet above the sea, just enough to
protect them from being overflowed, appear, after dry weather,
whiter than the ground after the thickest hoar-frost. After rain
the salts disappear, and every puddle of water becomes highly
saline; as the surface dries, the capillary action draws the
moisture up pieces of broken earth, dead sticks, and tufts of
grass, where the salt effloresces. The incrustation, where
thickest, does not exceed a quarter of an inch. M. Parchappe has
analysed it (M. d'Orbigny "Voyage" etc. Part. Hist. tome 1 page
664.); and finds that the specimens collected at the extreme head
of the low plain, near the River Manuello, consist of 93 per cent
of sulphate of soda, and 7 of common salt; whilst the specimens
taken close to the coast contain only 63 per cent of the
sulphate, and 37 of the muriate of soda. This remarkable fact,
together with our knowledge that the whole of this low muddy
plain has been covered by the sea within the recent period, must
lead to the suspicion that the common salt, by some unknown
process, becomes in time changed into the sulphate. Friable,
calcareous matter is here abundant, and the case of the apparent
double decomposition of the shells and salt on San Lorenzo,
should not be forgotten.
The saline incrustations, near Bahia Blanca, are not confined to,
though most abundant on, the low muddy flats; for I noticed some
on a calcareous plain between thirty and forty feet above the
sea, and even a little occurs in still higher valleys. Low
alluvial tracts in the valleys of the Rivers Negro and Colorado
are also encrusted, and in the latter valley such spaces appeared
to be occasionally overflowed by the river. I observed saline
incrustations in some of the valleys of Southern Patagonia. At
Port Desire a low, flat, muddy valley was thickly incrusted by
salts, which on analysis by Mr. T. Reeks, are found to consist of
a mixture of sulphate and muriate of soda, with carbonate of lime
and earthy matter. On the western side of the continent, the
southern coasts are much too humid for this phenomenon; but in
Northern Chile I again met with similar incrustations. On the
hardened mud, in parts of the broad, flat-bottomed valley of
Copiapo, the saline matter encrusts the ground to the thickness
of some inches: specimens, sent by Mr. Bingley to Apothecaries'
Hall for analysis, were said to consist of carbonate and sulphate
of soda. Much sulphate of soda is found in the desert of Atacama.
In all parts of South America, the saline incrustations occur
most frequently on low damp surfaces of mud, where the climate is
rather dry; and these low surfaces have, in almost every case,
been upraised above the level of the sea, within the recent
period.
SALT-LAKES OF PATAGONIA AND LA PLATA.
Salinas, or natural salt-lakes, occur in various formations on
the eastern side of the continent,—in the argillaceo-calcareous
deposit of the Pampas, in the sandstone of the Rio Negro, where
they are very numerous, in the pumiceous and other beds of the
Patagonian tertiary formation, and in small primary districts in
the midst of this latter formation. Port S. Julian is the most
southerly point (latitude 49 degrees to 50 degrees) at which
salinas are known to occur. (According to Azara "Travels" volume
1 page 56, there are salt-lakes as far north as Chaco (latitude
25 degrees), on the banks of the Vermejo. The salt-lakes of
Siberia appear (Pallas "Travels" English Translation volume 1
page 284) to occur in very similar depressions to those of
Patagonia.) The depressions, in which these salt-lakes lie, are
from a few feet to sixty metres, as asserted by M. d'Orbigny,
below the surface of the surrounding plains ("Voyage Geolog."
page 63.); and, according to this same author, near the Rio Negro
they all trend, either in the N.E. and S.W. or in E. and W.
lines, coincident with the general slope of the plain. These
depressions in the plain generally have one side lower than the
others, but there are no outlets for drainage. Under a less dry
climate, an outlet would soon have been formed, and the salt
washed away. The salinas occur at different elevations above the
sea; they are often several leagues in diameter; they are
generally very shallow, but there is a deep one in a quartz-rock
formation near C. Blanco. In the wet season, the whole, or a
part, of the salt is dissolved, being redeposited during the
succeeding dry season. At this period the appearance of the
snow-white expanse of salt crystallised in great cubes, is very
striking. In a large salina, northward of the Rio Negro, the salt
at the bottom, during the whole year, is between two and three
feet in thickness.
The salt rests almost always on a thick bed of black muddy sand,
which is fetid, probably from the decay of the burrowing worms
inhabiting it. (Professor Ehrenberg examined some of this muddy
sand, but was unable to find in it any infusoria.) In a salina,
situated about fifteen miles above the town of El Carmen on the
Rio Negro, and three or four miles from the banks of that river,
I observed that this black mud rested on gravel with a calcareous
matrix, similar to that spread over the whole surrounding plains:
at Port S. Julian the mud, also, rested on the gravel: hence the
depressions must have been formed anteriorly to, or
contemporaneously with, the spreading out of the gravel. I was
informed that one small salina occurs in an alluvial plain within
the valley of the Rio Negro, and therefore its origin must be
subsequent to the excavation of that valley. When I visited the
salina, fifteen miles above the town, the salt was beginning to
crystallise, and on the muddy bottom there were lying many
crystals, generally placed crossways of sulphate of soda (as
ascertained by Mr. Reeks), and embedded in the mud, numerous
crystals of sulphate of lime, from one to three inches in length:
M. d'Orbigny states that some of these crystals are acicular and
more than even nine inches in length ("Voyage Geolog." page 64.);
others are macled and of great purity: those I found all
contained some sand in their centres. As the black and fetid sand
overlies the gravel, and that overlies the regular tertiary
strata, I think there can be no doubt that these remarkable
crystals of sulphate of lime have been deposited from the waters
of the lake. The inhabitants call the crystals of selenite, the
padre del sal, and those of the sulphate of soda, the madre del
sal; they assured me that both are found under the same
circumstances in several of the neighbouring salinas; and that
the sulphate of soda is annually dissolved, and is always
crystallised before the common salt on the muddy bottom. (This is
what might have been expected; for M. Ballard asserts "Acad. des
Sciences" October 7, 1844, that sulphate of soda is precipitated
from solution more readily from water containing muriate of soda
in excess, than from pure water.) The association of gypsum and
salt in this case, as well as in the superficial deposits of
Iquique, appears to me interesting, considering how generally
these substances are associated in the older stratified
formations.
Mr. Reeks has analysed for me some of the salt from the salina
near the Rio Negro; he finds it composed entirely of chloride of
sodium, with the exception of 0.26 of sulphate of lime and of
0.22 of earthy matter: there are no traces of iodic salts. Some
salt from the salina Chiquitos, in the Pampean formation, is
equally pure. It is a singular fact, that the salt from these
salinas does not serve so well for preserving meat, as sea-salt
from the Cape de Verde Islands; and a merchant at Buenos Ayres
told me that he considered it as 50 per cent less valuable. The
purity of the Patagonian salt, or absence from it of those other
saline bodies found in all sea- water, is the only assignable
cause for this inferiority; a conclusion which is supported by
the fact lately ascertained, that those salts answer best for
preserving cheese which contain most of the deliquescent
chlorides. ("Horticultural and Agricultural Gazette" 1845 page
93.) (It would probably well answer for the merchants of Buenos
Ayres (considering the great consumption there of salt for
preserving meat) to import the deliquescent chlorides to mix with
the salt from the salinas: I may call attention to the fact, that
at Iquique, a large quantity of muriate of lime, left in the
MOTHER-WATER during the refinement of the nitrate of soda, is
annually thrown away.)
With respect to the origin of the salt in the salinas, the
foregoing analysis seems opposed to the view entertained by M.
d'Orbigny and others, and which seems so probable considering the
recent elevation of this line of coast, namely, that it is due to
the evaporation of sea-water and to the drainage from the
surrounding strata impregnated with sea-salt. I was informed (I
know not whether accurately) that on the northern side of the
salina on the Rio Negro, there is a small brine spring which
flows at all times of the year: if this be so, the salt in this
case at least, probably is of subterranean origin. It at first
appears very singular that fresh water can often be procured in
wells, and is sometimes found in small lakes, quite close to
these salinas. (Sir W. Parish states "Buenos Ayres" etc. pages
122 and 170, that this is the case near the great salinas
westward of the S. Ventana. I have seen similar statements in an
ancient MS. Journal lately published by S. Angelis. At Iquique,
where the surface is so thickly encrusted with saline matter, I
tasted water only slightly brackish, procured in a well
thirty-six yards deep; but here one feels less surprise at its
presence, as pure water might percolate under ground from the not
very distant Cordillera.) I am not aware that this fact bears
particularly on the origin of the salt; but perhaps it is rather
opposed to the view of the salt having been washed out of the
surrounding superficial strata, but not to its having been the
residue of sea-water, left in depressions as the land was slowly
elevated.
CHAPTER IV. ON THE FORMATIONS OF THE PAMPAS.
Mineralogical constitution. Microscopical structure. Buenos Ayres,
shells embedded in tosca-rock. Buenos Ayres to the Colorado. San
Ventana. Bahia Blanca; M. Hermoso, bones and infusoria of; P. Alta,
shells, bones, and infusoria of; co-existence of the recent shells
and extinct mammifers. Buenos Ayres to Santa Fe. Skeletons of
Mastodon. Infusoria. Inferior marine tertiary strata, their age.
Horse's tooth. BANDA ORIENTAL. Superficial Pampean formation.
Inferior tertiary strata, variation of, connected with volcanic
action; Macrauchenia Patachonica at San Julian in Patagonia, age of,
subsequent to living mollusca and to the erratic block period.
SUMMARY. Area of Pampean formation. Theories of origin. Source of
sediment. Estuary origin. Contemporaneous with existing mollusca.
Relations to underlying tertiary strata. Ancient deposit of estuary
origin. Elevation and successive deposition of the Pampean formation.
Number and state of the remains of mammifers; their habitation, food,
extinction, and range. Conclusion. Localities in Pampas at which
mammiferous remains have been found.
The Pampean formation is highly interesting from its vast extent,
its disputed origin, and from the number of extinct gigantic
mammifers embedded in it. It has upon the whole a very uniform
character: consisting of a more or less dull reddish, slightly
indurated, argillaceous earth or mud, often, but not always,
including in horizontal lines concretions of marl, and frequently
passing into a compact marly rock. The mud, wherever I examined
it, even close to the concretions, did not contain any carbonate
of lime. The concretions are generally nodular, sometimes rough
externally, sometimes stalactiformed; they are of a compact
structure, but often penetrated (as well as the mud) by hair-like
serpentine cavities, and occasionally with irregular fissures in
their centres, lined with minute crystals of carbonate of lime;
they are of white, brown, or pale pinkish tints, often marked by
black dendritic manganese or iron; they are either darker or
lighter tinted than the surrounding mass; they contain much
carbonate of lime, but exhale a strong aluminous odour, and
leave, when dissolved in acids, a large but varying residue, of
which the greater part consists of sand. These concretions often
unite into irregular strata; and over very large tracts of
country, the entire mass consists of a hard, but generally
cavernous marly rock: some of the varieties might be called
calcareous tuffs.
Dr. Carpenter has kindly examined under the microscope, sliced
and polished specimens of these concretions, and of the solid
marl-rock, collected in various places between the Colorado and
Santa Fe Bajada. In the greater number, Dr. Carpenter finds that
the whole substance presents a tolerably uniform amorphous
character, but with traces of incipient crystalline
metamorphosis; in other specimens he finds microscopically minute
rounded concretions of an amorphous substance (resembling in size
those in oolitic rocks, but not having a concentric structure),
united by a cement which is often crystalline. In some, Dr.
Carpenter can perceive distinct traces of shells, corals,
Polythalamia, and rarely of spongoid bodies. For the sake of
comparison, I sent Dr. Carpenter specimens of the calcareous
rock, formed chiefly of fragments of recent shells, from Coquimbo
in Chile: in one of these specimens, Dr. Carpenter finds, besides
the larger fragments, microscopical particles of shells, and a
varying quantity of opaque amorphous matter; in another specimen
from the same bed, he finds the whole composed of the amorphous
matter, with layers showing indications of an incipient
crystalline metamorphosis: hence these latter specimens, both in
external appearance and in microscopical structure, closely
resemble those of the Pampas. Dr. Carpenter informs me that it is
well known that chemical precipitation throws down carbonate of
lime in the opaque amorphous state; and he is inclined to believe
that the long-continued attrition of a calcareous body in a state
of crystalline or semi-crystalline aggregation (as, for instance,
in the ordinary shells of Mollusca, which, when sliced, are
transparent) may yield the same result. From the intimate
relations between all the Coquimbo specimens, I can hardly doubt
that the amorphous carbonate of lime in them has resulted from
the attrition and decay of the larger fragments of shell: whether
the amorphous matter in the marly rocks of the Pampas has
likewise thus originated, it would be hazardous to conjecture.
For convenience' sake, I will call the marly rock by the name
given to it by the inhabitants, namely, Tosca-rock; and the
reddish argillaceous earth, Pampean mud. This latter substance, I
may mention, has been examined for me by Professor Ehrenberg, and
the result of his examination will be given under the proper
localities.
I will commence my descriptions at a central spot, namely, at
Buenos Ayres, and thence proceed first southward to the extreme
limit of the deposit, and afterwards northward. The plain on
which Buenos Ayres stands is from thirty to forty feet in height.
The Pampean mud is here of a rather pale colour, and includes
small nearly white nodules, and other irregular strata of an
unusually arenaceous variety of tosca-rock. In a well at the
depth of seventy feet, according to Ignatio Nunez, much
tosca-rock was met with, and at several points, at one hundred
feet deep, beds of sand have been found. I have already given a
list of the recent marine and estuary shells found in many parts
on the surface near Buenos Ayres, as far as three or four leagues
from the Plata. Specimens from near Ensenada, given me by Sir W.
Parish, where the rock is quarried just beneath the surface of
the plain, consist of broken bivalves, cemented by and converted
into white crystalline carbonate of lime. I have already alluded,
in the first chapter, to a specimen (also given me by Sir W.
Parish) from the A. del Tristan, in which shells, resembling in
every respect the Azara labiata, d'Orbigny, as far as their worn
condition permits of comparison, are embedded in a reddish,
softish, somewhat arenaceous marly rock: after careful
comparison, with the aid of a microscope and acids, I can
perceive no difference between the basis of this rock and the
specimens collected by me in many parts of the Pampas. I have
also stated, on the authority of Sir W. Parish, that northward of
Buenos Ayres, on the highest parts of the plain, about forty feet
above the Plata, and two or three miles from it, numerous shells
of the Azara labiata (and I believe of Venus sinuosa) occur
embedded in a stratified earthy mass, including small marly
concretions, and said to be precisely like the great Pampean
deposit. Hence we may conclude that the mud of the Pampas
continued to be deposited to within the period of this existing
estuary shell. Although this formation is of such immense extent,
I know of no other instance of the presence of shells in it.
BUENOS AYRES TO THE RIO COLORADO.
With the exception of a few metamorphic ridges, the country
between these two points, a distance of 400 geographical miles,
belongs to the Pampean formation, and in the southern part is
generally formed of the harder and more calcareous varieties. I
will briefly describe my route: about twenty- five miles S.S.W.
of the capital, in a well forty yards in depth, the upper part,
and, as I was assured, the entire thickness, was formed of dark
red Pampean mud without concretions. North of the River Salado,
there are many lakes; and on the banks of one (near the Guardia)
there was a little cliff similarly composed, but including many
nodular and stalactiform concretions: I found here a large piece
of tessellated armour, like that of the Glyptodon, and many
fragments of bones. The cliffs on the Salado consist of
pale-coloured Pampean mud, including and passing into great
masses of tosca-rock: here a skeleton of the Megatherium and the
bones of other extinct quadrupeds (see the list at the end of
this chapter) were found. Large quantities of crystallised gypsum
(of which specimens were given me) occur in the cliffs of this
river; and likewise (as I was assured by Mr. Lumb) in the Pampean
mud on the River Chuelo, seven leagues from Buenos Ayres: I
mention this because M. d'Orbigny lays some stress on the
supposed absence of this mineral in the Pampean formation.
Southward of the Salado the country is low and swampy, with
tosca-rock appearing at long intervals at the surface. On the
banks, however, of the Tapalguen (sixty miles south of the
Salado) there is a large extent of tosca-rock, some highly
compact and even semi-crystalline, overlying pale Pampean mud
with the usual concretions. Thirty miles further south, the small
quartz-ridge of Tapalguen is fringed on its northern and southern
flank, by little, narrow, flat-topped hills of tosca-rock, which
stand higher than the surrounding plain. Between this ridge and
the Sierra of Guitru-gueyu, a distance of sixty miles, the
country is swampy, with the tosca-rock appearing only in four or
five spots: this sierra, precisely like that of Tapalguen, is
bordered by horizontal, often cliff-bounded, little hills of
tosca-rock, higher than the surrounding plain. Here, also, a new
appearance was presented in some extensive and level banks of
alluvium or detritus of the neighbouring metamorphic rocks; but I
neglected to observe whether it was stratified or not. Between
Guitru-gueyu and the Sierra Ventana, I crossed a dry plain of
tosca-rock higher than the country hitherto passed over, and with
small pieces of denuded tableland of the same formation, standing
still higher.
The marly or calcareous beds not only come up nearly horizontally
to the northern and southern foot of the great quartzose
mountains of the Sierra Ventana, but interfold between the
parallel ranges. The superficial beds (for I nowhere obtained
sections more than twenty feet deep) retain, even close to the
mountains, their usual character: the uppermost layer, however,
in one place included pebbles of quartz, and rested on a mass of
detritus of the same rock. At the very foot of the mountains,
there were some few piles of quartz and tosca-rock detritus,
including land-shells; but at the distance of only half a mile
from these lofty, jagged, and battered mountains, I could not, to
my great surprise, find on the boundless surface of the
calcareous plain even a single pebble. Quartz- pebbles, however,
of considerable size have at some period been transported to a
distance of between forty and fifty miles to the shores of Bahia
Blanca. (Schmidtmeyer "Travels in Chile" page 150, states that he
first noticed on the Pampas, very small bits of red granite, when
fifty miles distant from the southern extremity of the mountains
of Cordova, which project on the plain, like a reef into the
sea.)
The highest peak of the St. Ventana is, by Captain Fitzroy's
measurement, 3,340 feet, and the calcareous plain at its foot
(from observations taken by some Spanish officers) 840 feet above
the sea-level. ("La Plata" etc. by Sir W. Parish page 146.) On
the flanks of the mountains, at a height of three hundred or four
hundred feet above the plain, there were a few small patches of
conglomerate and breccia, firmly cemented by ferruginous matter
to the abrupt and battered face of the quartz—traces being thus
exhibited of ancient sea-action. The high plain round this range
sinks quite insensibly to the eye on all sides, except to the
north, where its surface is broken into low cliffs. Round the
Sierras Tapalguen, Guitru-gueyu, and between the latter and the
Ventana we have seen (and shall hereafter see round some hills in
Banda Oriental), that the tosca-rock forms low, flat- topped,
cliff-bounded hills, higher than the surrounding plains of
similar composition. From the horizontal stratification and from
the appearance of the broken cliffs, the greater height of the
Pampean formation round these primary hills ought not to be
altogether or in chief part attributed to these several points
having been uplifted more energetically than the surrounding
country, but to the argillaceo-calcareous mud having collected
round them, when they existed as islets or submarine rocks, at a
greater height, than at the bottom of the adjoining open sea;—the
cliffs having been subsequently worn during the elevation of the
whole country in mass.
Southward of the Ventana, the plain extends farther than the eye
can range; its surface is not very level, having slight
depressions with no drainage exits; it is generally covered by a
few feet in thickness of sandy earth; and in some places,
according to M. Parchappe, by beds of clay two yards thick. (M.
d'Orbigny "Voyage" Part Geolog. pages 47, 48.) On the banks of
the Sauce, four leagues S.E. of the Ventana, there is an
imperfect section about two hundred feet in height, displaying in
the upper part tosca-rock and in the lower part red Pampean mud.
At the settlement of Bahia Blanca, the uppermost plain is
composed of very compact, stratified tosca-rock, containing
rounded grains of quartz distinguishable by the naked eye: the
lower plain, on which the fortress stands, is described by M.
Parchappe as composed of solid tosca-rock (Ibid.); but the
sections which I examined appeared more like a redeposited mass
of this rock, with small pebbles and fragments of quartz. I shall
immediately return to the important sections on the shores of
Bahia Blanca. Twenty miles southward of this place, there is a
remarkable ridge extending W. by N. and E. by S., formed of
small, separate, flat-topped, steep-sided hills, rising between
one hundred and two hundred feet above the Pampean plain at its
southern base, which plain is a little lower than that to the
north. The uppermost stratum in this ridge consists of pale,
highly calcareous, compact tosca-rock, resting (as seen in one
place) on reddish Pampean mud, and this again on a paler kind: at
the foot of the ridge, there is a well in reddish clay or mud. I
have seen no other instance of a chain of hills belonging to the
Pampean formation; and as the strata show no signs of
disturbance, and as the direction of the ridge is the same with
that common to all the metamorphic lines in this whole area, I
suspect that the Pampean sediment has in this instance been
accumulated on and over a ridge of hard rocks, instead of, as in
the case of the above-mentioned Sierras, round their submarine
flanks. South of this little chain of tosca-rock, a plain of
Pampean mud declines towards the banks of the Colorado: in the
middle a well has been dug in red Pampean mud, covered by two
feet of white, softish, highly calcareous tosca-rock, over which
lies sand with small pebbles three feet in thickness—the first
appearance of that vast shingle formation described in the First
Chapter. In the first section after crossing the Colorado, an old
tertiary formation, namely, the Rio Negro sandstone (to be
described in the next chapter), is met with: but from the
accounts given me by the Gauchos, I believe that at the mouth of
the Colorado the Pampean formation extends a little further
southwards.
BAHIA BLANCA.
To return to the shores of this bay. At Monte Hermoso there is a
good section, about one hundred feet in height, of four distinct
strata, appearing to the eye horizontal, but thickening a little
towards the N.W. The uppermost bed, about twenty feet in
thickness, consists of obliquely laminated, soft sandstone,
including many pebbles of quartz, and falling at the surface into
loose sand. The second bed, only six inches thick, is a hard,
dark-coloured sandstone. The third bed is pale-coloured Pampean
mud; and the fourth is of the same nature, but darker coloured,
including in its lower part horizontal layers and lines of
concretions of not very compact pinkish tosca-rock. The bottom of
the sea, I may remark, to a distance of several miles from the
shore, and to a depth of between sixty and one hundred feet, was
found by the anchors to be composed of tosca-rock and reddish
Pampean mud. Professor Ehrenberg has examined for me specimens of
the two lower beds, and finds in them three Polygastrica and six
Phytolitharia.
(The following list is given in the "Monatsberichten der konig.
Akad. zu Berlin" April 1845:— POLYGASTRICA. Fragilaria
rhabdosoma. Gallionella distans. Pinnularia?
PHYTOLITHARIA. Lithodontium Bursa. Lithodontium furcatum.
Lithostylidium exesum. Lithostylidium rude. Lithostylidium Serra.
Spongolithis Fustis?)
Of these, only one (Spongolithis Fustis?) is a marine form; five
of them are identical with microscopical structures of
brackish-water origin, hereafter to be mentioned, which form a
central point in the Pampean formation. In these two beds,
especially in the lower one, bones of extinct mammifers, some
embedded in their proper relative positions and others single,
are very numerous in a small extent of the cliffs. These remains
consist of, first, the head of Ctenomys antiquus, allied to the
living Ctenomys Braziliensis; secondly, a fragment of the remains
of a rodent; thirdly, molar teeth and other bones of a large
rodent, closely allied to, but distinct from, the existing
species of Hydrochoerus, and therefore probably an inhabitant of
fresh water; fourth and fifthly, portions of vertebrae, limbs,
ribs, and other bones of two rodents; sixthly, bones of the
extremities of some great megatheroid quadruped. (See "Fossil
Mammalia" page 109 by Professor Owen, in the "Zoology of the
Voyage of the 'Beagle';" and Catalogue page 36 of Fossil Remains
in Museum of Royal College of Surgeons.) The number of the
remains of rodents gives to this collection a peculiar character,
compared with those found in any other locality. All these bones
are compact and heavy; many of them are stained red, with their
surfaces polished; some of the smaller ones are as black as jet.
Monte Hermoso is between fifty and sixty miles distant in a S.E.
line from the Ventana, with the intermediate country gently
rising towards it, and all consisting of the Pampean formation.
What relation, then, do these beds, at the level of the sea and
under it, bear to those on the flanks of the Ventana, at the
height of 840 feet, and on the flanks of the other neighbouring
sierras, which, from the reasons already assigned, do not appear
to owe their greater height to unequal elevation? When the tosca-
rock was accumulating round the Ventana, and when, with the
exception of a few small rugged primary islands, the whole wide
surrounding plains must have been under water, were the strata at
Monte Hermoso depositing at the bottom of a great open sea,
between eight hundred and one thousand feet in depth? I much
doubt this; for if so, the almost perfect carcasses of the
several small rodents, the remains of which are so very numerous
in so limited a space, must have been drifted to this spot from
the distance of many hundred miles. It appears to me far more
probable, that during the Pampean period this whole area had
commenced slowly rising (and in the cliffs, at several different
heights we have proofs of the land having been exposed to
sea-action at several levels), and that tracts of land had thus
been formed of Pampean sediment round the Ventana and the other
primary ranges, on which the several rodents and other quadrupeds
lived, and that a stream (in which perhaps the extinct aquatic
Hydrochoerus lived) drifted their bodies into the adjoining sea,
into which the Pampean mud continued to be poured from the north.
As the land continued to rise, it appears that this source of
sediment was cut off; and in its place sand and pebbles were
borne down by stronger currents, and conformably deposited over
the Pampean strata.
(FIGURE 15. SECTION OF BEDS WITH RECENT SHELLS AND EXTINCT
MAMMIFERS, AT PUNTA ALTA IN BAHIA BLANCA. (Showing beds from
bottom to top: A, B, C, D.))
Punta Alta is situated about thirty miles higher up on the
northern side of this same bay: it consists of a small plain,
between twenty and thirty feet in height, cut off on the shore by
a line of low cliffs about a mile in length, represented in
Figure 15 with its vertical scale necessarily exaggerated. The
lower bed (A) is more extensive than the upper ones; it consists
of stratified gravel or conglomerate, cemented by calcareo-
arenaceous matter, and is divided by curvilinear layers of
pinkish marl, of which some are precisely like tosca-rock, and
some more sandy. The beds are curvilinear, owing to the action of
currents, and dip in different directions; they include an
extraordinary number of bones of gigantic mammifers and many
shells. The pebbles are of considerable size, and are of hard
sandstone, and of quartz, like that of the Ventana: there are
also a few well-rounded masses of tosca-rock.
The second bed B is about fifteen feet in thickness, but towards
both extremities of the cliff (not included in the diagram) it
either thins out and dies away, or passes insensibly into an
overlying bed of gravel. It consists of red, tough clayey mud,
with minute linear cavities; it is marked with faint horizontal
shades of colour; it includes a few pebbles, and rarely a minute
particle of shell: in one spot, the dermal armour and a few bones
of a Dasypoid quadruped were embedded in it: it fills up furrows
in the underlying gravel. With the exception of the few pebbles
and particles of shells, this bed resembles the true Pampean mud;
but it still more closely resembles the clayey flats (mentioned
in the First Chapter) separating the successively rising parallel
ranges of sand-dunes.
The bed C is of stratified gravel, like the lowest one; it fills
up furrows in the underlying red mud, and is sometimes
interstratified with it, and sometimes insensibly passes into it;
as the red mud thins out, this upper gravel thickens. Shells are
more numerous in it than in the lower gravel; but the bones,
though some are still present, are less numerous. In one part,
however, where this gravel and the red mud passed into each
other, I found several bones and a tolerably perfect head of the
Megatherium. Some of the large Volutas, though embedded in the
gravel-bed C, were filled with the red mud, including great
numbers of the little recent Paludestrina australis. These three
lower beds are covered by an unconformable mantle D of stratified
sandy earth, including many pebbles of quartz, pumice and
phonolite, land and sea-shells.
M. d'Orbigny has been so obliging as to name for me the twenty
species of Mollusca embedded in the two gravel beds: they consist
of:—
1. Volutella angulata, d'Orbigny, "Voyage" Mollusq. and Pal. 2.
Voluta Braziliana, Sol 3. Olicancilleria Braziliensis d'Orbigny.
4. Olicancilleria auricularia, d'Orbigny. 5. Olivina puelchana,
d'Orbigny. 6. Buccinanops cochlidium, d'Orbigny. 7. Buccinanops
globulosum, d'Orbigny. 8. Colombella sertulariarum, d'Orbigny. 9.
Trochus Patagonicus, and var. of ditto, d'Orbigny. 10.
Paludestrina Australis, d'Orbigny. 11. Fissurella Patagonica,
d'Orbigny. 12. Crepidula muricata, Lam. 13. Venus purpurata, Lam.
14. Venus rostrata, Phillippi. 15. Mytilus Darwinianus,
d'Orbigny. 16. Nucula semiornata, d'Orbigny. 17. Cardita
Patagonica, d'Orbigny. 18. Corbula Patagonica (?), d'Orbigny. 19.
Pecten tethuelchus, d'Orbigny. 20. Ostrea puelchana, d'Orbigny.
21. A living species of Balanus. 22 and 23. An Astrae and
encrusting Flustra, apparently identical with species now living
in the bay.
All these shells now live on this coast, and most of them in this
same bay. I was also struck with the fact, that the proportional
numbers of the different kinds appeared to be the same with those
now cast up on the beach: in both cases specimens of Voluta,
Crepidula, Venus, and Trochus are the most abundant. Four or five
of the species are the same with the upraised shells on the
Pampas near Buenos Ayres. All the specimens have a very ancient
and bleached appearance, and do not emit, when heated, an animal
odour: some of them are changed throughout into a white, soft,
fibrous substance; others have the space between the external
walls, either hollow, or filled up with crystalline carbonate of
lime. (A Bulinus, mentioned in the Introduction to the "Fossil
Mammalia" in the "Zoology of the Voyage of the 'Beagle'" has so
much fresher an appearance, than the marine species, that I
suspect it must have fallen amongst the others, and been
collected by mistake.)
The remains of the extinct mammiferous animals, from the two
gravel beds have been described by Professor Owen in the "Zoology
of the Voyage of the 'Beagle':" they consist of, 1st, one nearly
perfect head and three fragments of heads of the Megatherium
Cuvierii; 2nd, a lower jaw of Megalonyx Jeffersonii; 3rd, lower
jaw of Mylodon Darwinii; 4th, fragments of a head of some
gigantic Edental quadruped; 5th, an almost entire skeleton of the
great Scelidotherium leptocephalum, with most of the bones,
including the head, vertebrae, ribs, some of the extremities to
the claw- bone, and even, as remarked by Professor Owen, the
knee-cap, all nearly in their proper relative positions; 6th,
fragments of the jaw and a separate tooth of a Toxodon, belonging
either to T. Platensis, or to a second species lately discovered
near Buenos Ayres; 7th, a tooth of Equus curvidens; 8th, tooth of
a Pachyderm, closely allied to Palaeotherium, of which parts of
the head have been lately sent from Buenos Ayres to the British
Museum; in all probability this pachyderm is identical with the
Macrauchenia Patagonica from Port S. Julian, hereafter to be
referred to. Lastly, and 9thly, in a cliff of the red clayey bed
B, there was a double piece, about three feet long and two wide,
of the bony armour of a large Dasypoid quadruped, with the two
sides pressed nearly close together: as the cliff is now rapidly
washing away, this fossil probably was lately much more perfect;
from between its doubled-up sides, I extracted the middle and
ungual phalanges, united together, of one of the feet, and
likewise a separate phalanx: hence one or more of the limbs must
have been attached to the dermal case, when it was embedded.
Besides these several remains in a distinguishable condition,
there were very many single bones: the greater number were
embedded in a space 200 yards square. The preponderance of the
Edental quadrupeds is remarkable; as is, in contrast with the
beds of Monte Hermoso, the absence of Rodents. Most of the bones
are now in a soft and friable condition, and, like the shells, do
not emit when burnt an animal odour. The decayed state of the
bones may be partly owing to their late exposure to the air and
tidal-waves. Barnacles, Serpulae, and corallines are attached to
many of the bones, but I neglected to observe whether these might
not have grown on them since being exposed to the present tidal
action (After having packed up my specimens at Bahia Blanca, this
point occurred to me, and I noted it; but forgot it on my return,
until the remains had been cleaned and oiled: my attention has
been lately called to the subject by some remarks by M.
d'Orbigny.); but I believe that some of the barnacles must have
grown on the Scelidotherium, soon after being deposited, and
before being WHOLLY covered up by the gravel. Besides the remains
in the condition here described, I found one single fragment of
bone very much rolled, and as black as jet, so as perfectly to
resemble some of the remains from Monte Hermoso.
Very many of the bones had been broken, abraded, and rolled,
before being embedded. Others, even some of those included in the
coarsest parts of the the now hard conglomerate, still retain all
their minutest prominences perfectly preserved; so that I
conclude that they probably were protected by skin, flesh, or
ligaments, whilst being covered up. In the case of the
Scelidotherium, it is quite certain that the whole skeleton was
held together by its ligaments, when deposited in the gravel in
which I found it. Some cervical vertebrae and a humerus of
corresponding size lay so close together, as did some ribs and
the bones of a leg, that I thought that they must originally have
belonged to two skeletons, and not have been washed in single;
but as remains were here very numerous, I will not lay much
stress on these two cases. We have just seen that the armour of
the Dasypoid quadruped was certainly embedded together with some
of the bones of the feet.
Professor Ehrenberg has examined for me specimens of the finer
matter from in contact with these mammiferous remains: he finds
in them two Polygastrica, decidedly marine forms; and six
Phytolitharia, of which one is probably marine, and the others
either of fresh-water or terrestrial origin. ("Monatsberichten
der Akad. zu Berlin" April 1845. The list consists of:—
POLYGASTRICA. Gallionella sulcata. Stauroptera aspera? fragm.
PHYTOLITHARIA. Lithasteriscus tuberculatus. Lithostylidium
Clepsammidium. Lithostylidium quadratum. Lithostylidium rude.
Lithostylidium unidentatum. Spongolithis acicularis.)
Only one of these eight microscopical bodies is common to the
nine from Monte Hermoso: but five of them are in common with
those from the Pampean mud on the banks of the Parana. The
presence of any fresh-water infusoria, considering the aridity of
the surrounding country, is here remarkable: the most probable
explanation appears to be, that these microscopical organisms
were washed out of the adjoining great Pampean formation during
its denudation, and afterwards redeposited.
We will now see what conclusions may be drawn from the facts
above detailed. It is certain that the gravel-beds and
intermediate red mud were deposited within the period, when
existing species of Mollusca held to each other nearly the same
relative proportions as they do on the present coast. These beds,
from the number of littoral species, must have been accumulated
in shallow water; but not, judging from the stratification of the
gravel and the layers of marl, on a beach. From the manner in
which the red clay fills up furrows in the underlying gravel, and
is in some parts itself furrowed by the overlying gravel, whilst
in other parts it either insensibly passes into, or alternates
with, this upper gravel, we may infer several local changes in
the currents, perhaps caused by slight changes, up or down, in
the level of the land. By the elevation of these beds, to which
period the alluvial mantle with pumice-pebbles, land and
sea-shells belongs, the plain of Punta Alta, from twenty to
thirty feet in height, was formed. In this neighbourhood there
are other and higher sea-formed plains and lines of cliffs in the
Pampean formation worn by the denuding action of the waves at
different levels. Hence we can easily understand the presence of
rounded masses of tosca-rock in this lowest plain; and likewise,
as the cliffs at Monte Hermoso with their mammiferous remains
stand at a higher level, the presence of the one much-rolled
fragment of bone which was as black as jet: possibly some few of
the other much-rolled bones may have been similarly derived,
though I saw only the one fragment, in the same condition with
those from Monte Hermoso. M. d'Orbigny has suggested that all
these mammiferous remains may have been washed out of the Pampean
formation, and afterwards redeposited together with the recent
shells. ("Voyage" Part. Geolog. page 49.) Undoubtedly it is a
marvellous fact that these numerous gigantic quadrupeds,
belonging, with the exception of the Equus curvidens, to seven
extinct genera, and one, namely, the Toxodon, not falling into
any existing family, should have co-existed with Mollusca, all of
which are still living species; but analogous facts have been
observed in North America and in Europe. In the first place, it
should not be overlooked, that most of the co-embedded shells
have a more ancient and altered appearance than the bones. In the
second place, is it probable that numerous bones not hardened by
silex or any other mineral, could have retained their delicate
prominences and surfaces perfect if they had been washed out of
one deposit, and re-embedded in another:—this later deposit being
formed of large, hard pebbles, arranged by the action of currents
or breakers in shallow water into variously curved and inclined
layers? The bones which are now in so perfect a state of
preservation, must, I conceive, have been fresh and sound when
embedded, and probably were protected by skin, flesh, or
ligaments. The skeleton of the Scelidotherium indisputably was
deposited entire: shall we say that when held together by its
matrix it was washed out of an old gravel-bed (totally unlike in
character to the Pampean formation), and re-embedded in another
gravel-bed, composed (I speak after careful comparison) of
exactly the same kind of pebbles, in the same kind of cement? I
will lay no stress on the two cases of several ribs and bones of
the extremities having APPARENTLY been embedded in their proper
relative position: but will any one be so bold as to affirm that
it is possible, that a piece of the thin tessellated armour of a
Dasypoid quadruped, at least three feet long and two in width,
and now so tender that I was unable with the utmost care to
extract a fragment more than two or three inches square, could
have been washed out of one bed, and re-embedded in another,
together with some of the small bones of the feet, without having
been dashed into atoms? We must then wholly reject M. d'Orbigny's
supposition, and admit as certain, that the Scelidotherium and
the large Dasypoid quadruped, and as highly probable, that the
Toxodon, Megatherium, etc., some of the bones of which are
perfectly preserved, were embedded for the first time, and in a
fresh condition, in the strata in which they were found entombed.
These gigantic quadrupeds, therefore, though belonging to extinct
genera and families, coexisted with the twenty above-enumerated
Mollusca, the barnacle and two corals, still living on this
coast. From the rolled fragment of black bone, and from the plain
of Punta Alta being lower than that of Monte Hermoso, I conclude
that the coarse sub-littoral deposits of Punta Alta, are of
subsequent origin to the Pampean mud of Monte Hermoso; and the
beds at this latter place, as we have seen, are probably of
subsequent origin to the high tosca-plain round the Sierra
Ventana: we shall, however, return, at the end of this chapter,
to the consideration of these several stages in the great Pampean
formation.
BUENOS AYRES TO ST. FE BAJADA, IN ENTRE RIOS.
For some distance northward of Buenos Ayres, the escarpment of
the Pampean formation does not approach very near to the Plata,
and it is concealed by vegetation: but in sections on the banks
of the Rios Luxan, Areco, and Arrecifes, I observed both pale and
dark reddish Pampean mud, with small, whitish concretions of
tosca; at all these places mammiferous remains have been found.
In the cliffs on the Parana, at San Nicolas, the Pampean mud
contains but little tosca; here M. d'Orbigny found the remains of
two rodents (Ctenomys Bonariensis and Kerodon antiquus) and the
jaw of a Canis: when on the river I could clearly distinguish in
this fine line of cliffs, "horizontal lines of variation both in
tint and compactness." (I quote these words from my note-book, as
written down on the spot, on account of the general absence of
stratification in the Pampean formation having been insisted on
by M. d'Orbigny as a proof of the diluvial origin of this great
deposit.) The plain northward of this point is very level, but
with some depressions and lakes; I estimated its height at from
forty to sixty feet above the Parana. At the A. Medio the bright
red Pampean mud contains scarcely any tosca-rock; whilst at a
short distance the stream of the Pabon, forms a cascade, about
twenty feet in height, over a cavernous mass of two varieties of
tosca-rock; of which one is very compact and semi- crystalline,
with seams of crystallised carbonate of lime: similar compact
varieties are met with on the Salidillo and Seco. The absolute
identity (I speak after a comparison of my specimens) between
some of these varieties, and those from Tapalguen, and from the
ridge south of Bahia Blanca, a distance of 400 miles of latitude,
is very striking.
At Rosario there is but little tosca-rock: near this place I
first noticed at the edge of the river traces of an underlying
formation, which, twenty- five miles higher up in the estancia of
Gorodona, consists of a pale yellowish clay, abounding with
concretionary cylinders of a ferruginous sandstone. This bed,
which is probably the equivalent of the older tertiary marine
strata, immediately to be described in Entre Rios, only just
rises above the level of the Parana when low. The rest of the
cliff at Gorodona, is formed of red Pampean mud, with, in the
lower part, many concretions of tosca, some stalacti-formed, and
with only a few in the upper part: at the height of six feet
above the river, two gigantic skeletons of the Mastodon Andium
were here embedded; their bones were scattered a few feet apart,
but many of them still held their proper relative positions: they
were much decayed and as soft as cheese, so that even one of the
great molar teeth fell into pieces in my hand. We here see that
the Pampean deposit contains mammiferous remains close to its
base. On the banks of the Carcarana, a few miles distant, the
lowest bed visible was pale Pampean mud, with masses of
tosca-rock, in one of which I found a much decayed tooth of the
Mastodon: above this bed, there was a thin layer almost composed
of small concretions of white tosca, out of which I extracted a
well preserved, but slightly broken tooth of Toxodon Platensis:
above this there was an unusual bed of very soft impure
sandstone. In this neighbourhood I noticed many single embedded
bones, and I heard of others having been found in so perfect a
state that they were long used as gate-posts: the Jesuit Falkner
found here the dermal armour of some gigantic Edental quadruped.
In some of the red mud scraped from a tooth of one of the
Mastodons at Gorodona, Professor Ehrenberg finds seven
Polygastrica and thirteen Phytolitharia, all of them, I believe,
with two exceptions, already known species. ("Monatsberichten der
konig. Akad. zu Berlin" April 1845. The list consists of:—
POLYGASTRICA. Campylodiscus clypeus. Coscinodiscus subtilis.
Coscinodiscus al. sp. Eunotia. Gallionella granulata. Himantidium
gracile. Pinnularia borealis.)
Of these twenty, the preponderating number are of fresh-water
origin; only two species of Coscinodiscus and a Spongolithis show
the direct influence of the sea; therefore Professor Ehrenberg
arrives at the important conclusion that the deposit must have
been of brackish-water origin. Of the thirteen Phytolitharia,
nine are met with in the two deposits in Bahia Blanca, where
there is evidence from two other species of Polygastrica that the
beds were accumulated in brackish water. The traces of coral,
sponges, and Polythalamia, found by Dr. Carpenter in the
tosca-rock (of which I must observe the greater number of
specimens were from the upper beds in the southern parts of the
formation), apparently show a more purely marine origin.
At ST. FE BAJADA, in Entre Rios, the cliffs, estimated at between
sixty and seventy feet in height, expose an interesting section:
the lower half consists of tertiary strata with marine shells,
and the upper half of the Pampean formation. The lowest bed is an
obliquely laminated, blackish, indurated mud, with distinct
traces of vegetable remains. (M. d'Orbigny "Voyage" Part. Geolog.
page 37, has given a detailed description of this section, but as
he does not mention this lowest bed, it may have been concealed
when he was there by the river. There is a considerable
discrepancy between his description and mine, which I can only
account for by the beds themselves varying considerably in short
distances.) Above this there is a thick bed of yellowish sandy
clay, with much crystallised gypsum and many shells of Ostreae,
Pectens, and Arcae: above this there generally comes an
arenaceous crystalline limestone, but there is sometimes
interposed a bed, about twelve feet thick, of dark green, soapy
clay, weathering into small angular fragments. The limestone,
where purest, is white, highly crystalline, and full of cavities:
it includes small pebbles of quartz, broken shells, teeth of
sharks, and sometimes, as I was informed, large bones: it often
contains so much sand as to pass into a calcareous sandstone, and
in such parts the great Ostrea Patagonica chiefly abounds.
(Captain Sulivan, R.N., has given me a specimen of this shell,
which he found in the cliffs at Point Cerrito, between twenty and
thirty miles above the Bajada.) In the upper part, the limestone
alternates with layers of fine white sand. The shells included in
these beds have been named for me by M. d'Orbigny: they consist
of:—
1. Ostrea Patagonica, d'Orbigny, "Voyage" Part. Pal. 2. Ostrea
Alvarezii, d'Orbigny, "Voyage" Part. Pal. 3. Pecten Paranensis,
d'Orbigny, "Voyage" Part. Pal. 4. Pecten Darwinianus, d'Orbigny,
"Voyage" Part. Pal. 5. Venus Munsterii, d'Orbigny, "Voyage" Pal.
6. Arca Bonplandiana, d'Orbigny, "Voyage" Pal. 7. Cardium
Platense, d'Orbigny, "Voyage" Pal. 8. Tellina, probably nov.
species, but too imperfect for description.
PHYTOLITHARIA.
Lithasteriscus tuberculatus. Lithodontium bursa. Lithodontium
furcatum. Lithodontium rostratum. Lithostylidium Amphiodon.
Lithostylidium Clepsammidium. Lithostylidium Hamus.
Lithostylidium polyedrum. Lithostylidium quadratum.
Lithostylidium rude. Lithostylidium Serra. Lithostylidium
unidentatum. Spongolithis Fustis.
These species are all extinct: the six first were found by M.
d'Orbigny and myself in the formations of the Rio Negro, S.
Josef, and other parts of Patagonia; and therefore, as first
observed by M. d'Orbigny, these beds certainly belong to the
great Patagonian formation, which will be described in the
ensuing chapter, and which we shall see must be considered as a
very ancient tertiary one. North of the Bajada, M. d'Orbigny
found, in beds which he considers as lying beneath the strata
here described, remains of a Toxodon, which he has named as a
distinct species from the T. Platensis of the Pampean formation.
Much silicified wood is found on the banks of the Parana (and
likewise on the Uruguay), and I was informed that they come out
of these lower beds; four specimens collected by myself are
dicotyledonous.
The upper half of the cliff, to a thickness of about thirty feet,
consists of Pampean mud, of which the lower part is
pale-coloured, and the upper part of a brighter red, with some
irregular layers of an arenaceous variety of tosca, and a few
small concretions of the ordinary kind. Close above the marine
limestone, there is a thin stratum with a concretionary outline
of white hard tosca-rock or marl, which may be considered either
as the uppermost bed of the inferior deposits, or the lowest of
the Pampean formation; at one time I considered this bed as
marking a passage between the two formations: but I have since
become convinced that I was deceived on this point. In the
section on the Parana, I did not find any mammiferous remains;
but at two miles distance on the A. Tapas (a tributary of the
Conchitas), they were extremely numerous in a low cliff of red
Pampean mud with small concretions, precisely like the upper bed
on the Parana. Most of the bones were solitary and much decayed;
but I saw the dermal armour of a gigantic Edental quadruped,
forming a caldron-like hollow, four or five feet in diameter, out
of which, as I was informed, the almost entire skeleton had been
lately removed. I found single teeth of the Mastodon Andium,
Toxodon Platensis, and Equus curvidens, near to each other. As
this latter tooth approaches closely to that of the common horse,
I paid particular attention to its true embedment, for I did not
at that time know that there was a similar tooth hidden in the
matrix with the other mammiferous remains from Punta Alta. It is
an interesting circumstance, that Professor Owen finds that the
teeth of this horse approach more closely in their peculiar
curvature to a fossil specimen brought by Mr. Lyell from North
America, than to those of any other species of Equus. (Lyell
"Travels in North America" volume 1 page 164 and "Proceedings of
Geological Society" volume 4 page 39.)
The underlying marine tertiary strata extend over a wide area: I
was assured that they can be traced in ravines in an east and
west line across Entre Rios to the Uruguay, a distance of about
135 miles. In a S.E. direction I heard of their existence at the
head of the R. Nankay; and at P. Gorda in Banda Oriental, a
distance of 170 miles, I found the same limestone, containing the
same fossil shells, lying at about the same level above the river
as at St. Fe. In a southerly direction, these beds sink in
height, for at another P. Gorda in Entre Rios, the limestone is
seen at a much less height; and there can be little doubt that
the yellowish sandy clay, on a level with the river, between the
Carcarana and S. Nicholas, belongs to this same formation; as
perhaps do the beds of sand at Buenos Ayres, which lie at the
bottom of the Pampean formation, about sixty feet beneath the
surface of the Plata. The southerly declination of these beds may
perhaps be due, not to unequal elevation, but to the original
form of the bottom of the sea, sloping from land situated to the
north; for that land existed at no great distance, we have
evidence in the vegetable remains in the lowest bed at St. Fe;
and in the silicified wood and in the bones of Toxodon
Paranensis, found (according to M. d'Orbigny) in still lower
strata.
BANDA ORIENTAL.
This province lies on the northern side of the Plata, and
eastward of the Uruguay: it has a gentle undulatory surface, with
a basis of primary rocks; and is in most parts covered up with an
unstratified mass, of no great thickness, of reddish Pampean mud.
In the eastern half, near Maldonado, this deposit is more
arenaceous than in the Pampas, it contains many though small
concretions of marl or tosca-rock, and others of highly
ferruginous sandstone; in one section, only a few yards in depth,
it rested on stratified sand. Near Monte Video this deposit in
some spots appears to be of greater thickness; and the remains of
the Glyptodon and other extinct mammifers have been found in it.
In the long line of cliffs, between fifty and sixty feet in
height, called the Barrancas de S. Gregorio, which extend
westward of the Rio S. Lucia, the lower half is formed of coarse
sand of quartz and feldspar without mica, like that now cast up
on the beach near Maldonado; and the upper half of Pampean mud,
varying in colour and containing honeycombed veins of soft
calcareous matter and small concretions of tosca-rock arranged in
lines, and likewise a few pebbles of quartz. This deposit fills
up hollows and furrows in the underlying sand; appearing as if
water charged with mud had invaded a sandy beach. These cliffs
extend far westward, and at a distance of sixty miles, near
Colonia del Sacramiento, I found the Pampean deposit resting in
some places on this sand, and in others on the primary rocks:
between the sand and the reddish mud, there appeared to be
interposed, but the section was not a very good one, a thin bed
of shells of an existing Mytilus, still partially retaining their
colour. The Pampean formation in Banda Oriental might readily be
mistaken for an alluvial deposit: compared with that of the
Pampas, it is often more sandy, and contains small fragments of
quartz; the concretions are much smaller, and there are no
extensive masses of tosca-rock.
In the extreme western parts of this province, between the
Uruguay and a line drawn from Colonia to the R. Perdido (a
tributary of the R. Negro), the formations are far more
complicated. Besides primary rocks, we meet with extensive tracts
and many flat-topped, horizontally stratified, cliff- bounded,
isolated hills of tertiary strata, varying extraordinarily in
mineralogical nature, some identical with the old marine beds of
St. Fe Bajada, and some with those of the much more recent
Pampean formation. There are, also, extensive LOW tracts of
country covered with a deposit containing mammiferous remains,
precisely like that just described in the more eastern parts of
the province. Although from the smooth and unbroken state of the
country, I never obtained a section of this latter deposit close
to the foot of the higher tertiary hills, yet I have not the
least doubt that it is of quite subsequent origin; having been
deposited after the sea had worn the tertiary strata into the
cliff-bounded hills. This later formation, which is certainly the
equivalent of that of the Pampas, is well seen in the valleys in
the estancia of Berquelo, near Mercedes; it here consists of
reddish earth, full of rounded grains of quartz, and with some
small concretions of tosca-rock arranged in horizontal lines, so
as perfectly to resemble, except in containing a little
calcareous matter, the formation in the eastern parts of Banda
Oriental, in Entre Rios, and at other places: in this estancia
the skeleton of a great Edental quadruped was found. In the
valley of the Sarandis, at the distance of only a few miles, this
deposit has a somewhat different character, being whiter, softer,
finer-grained, and full of little cavities, and consequently of
little specific gravity; nor does it contain any concretions or
calcareous matter: I here procured a head, which when first
discovered must have been quite perfect, of the Toxodon
Platensis, another of a Mylodon (This head was at first
considered by Professor Owen (in the "Zoology of the 'Beagle's'
Voyage") as belonging to a distinct genus, namely,
Glossotherium.), perhaps M. Darwinii, and a large piece of dermal
armour, differing from that of the Glyptodon clavipes. These
bones are remarkable from their extraordinarily fresh appearance;
when held over a lamp of spirits of wine, they give out a strong
odour and burn with a small flame; Mr. T. Reeks has been so kind
as to analyse some of the fragments, and he finds that they
contain about 7 per cent of animal matter, and 8 per cent of
water. (Liebig "Chemistry of Agriculture" page 194 states that
fresh dry bones contain from 32 to 33 per cent of dry gelatine.
See also Dr. Daubeny, in "Edinburgh New Philosophical Journal"
volume 37 page 293.)
The older tertiary strata, forming the higher isolated hills and
extensive tracts of country, vary, as I have said,
extraordinarily in composition: within the distance of a few
miles, I sometimes passed over crystalline limestone with agate,
calcareous tuffs, and marly rocks, all passing into each
other,—red and pale mud with concretions of tosca-rock, quite
like the Pampean formation,—calcareous conglomerates and
sandstones,—bright red sandstones passing either into red
conglomerate, or into white sandstone,—hard siliceous sandstones,
jaspery and chalcedonic rocks, and numerous other subordinate
varieties. I was unable to mark out the relations of all these
strata, and will describe only a few distinct sections:—in the
cliffs between P. Gorda on the Uruguay and the A. de Vivoras, the
upper bed is crystalline cellular limestone often passing into
calcareous sandstone, with impressions of some of the same shells
as at St. Fe Bajada; at P. Gorda, this limestone is
interstratified with and rests on, white sand, which covers a bed
about thirty feet thick of pale-coloured clay, with many shells
of the great Ostrea Patagonica (In my "Journal" page 171 1st
edition, I have hastily and inaccurately stated that the Pampean
mud, which is found over the eastern part of B. Oriental, lies
OVER the limestone at P. Gorda; I should have said that there was
reason to infer that it was a subsequent or superior deposit.):
beneath this, in the vertical cliff, nearly on a level with the
river, there is a bed of red mud absolutely like the Pampean
deposit, with numerous often large concretions of perfectly
characterised white, compact tosca-rock. At the mouth of the
Vivoras, the river flows over a pale cavernous tosca-rock, quite
like that in the Pampas, and this APPEARED to underlie the
crystalline limestone; but the section was not unequivocal like
that at P. Gorda. These beds now form only a narrow and much
denuded strip of land; but they must once have extended much
further; for on the next stream, south of the S. Juan, Captain
Sulivan, R.N., found a little cliff, only just above the surface
of the river, with numerous shells of the Venus Munsterii,
D'Orbigny,—one of the species occurring at St. Fe, and of which
there are casts at P. Gorda: the line of cliffs of the
subsequently deposited true Pampean mud, extend from Colonia to
within half a mile of this spot, and no doubt once covered up
this denuded marine stratum. Again at Colonia, a Frenchman found,
in digging the foundations of a house, a great mass of the Ostrea
Patagonica (of which I saw many fragments), packed together just
beneath the surface, and directly superimposed on the gneiss.
These sections are important: M. d'Orbigny is unwilling to
believe that beds of the same nature with the Pampean formation
ever underlie the ancient marine tertiary strata; and I was as
much surprised at it as he could have been; but the vertical
cliff at P. Gorda allowed of no mistake, and I must be permitted
to affirm, that after having examined the country from the
Colorado to St. Fe Bajada, I could not be deceived in the
mineralogical character of the Pampean deposit.
Moreover, in a precipitous part of the ravine of Las Bocas, a red
sandstone is distinctly seen to overlie a thick bed of pale mud,
also quite like the Pampean formation, abounding with concretions
of true tosca-rock. This sandstone extends over many miles of
country: it is as red as the brightest volcanic scoriae; it
sometimes passes into a coarse red conglomerate composed of the
underlying primary rocks; and often passes into a soft white
sandstone with red streaks. At the Calera de los Huerfanos, only
a quarter of a mile south of where I first met with the red
sandstone, the crystalline white limestone is quarried: as this
bed is the uppermost, and as it often passes into calcareous
sandstone, interstratified with pure sand; and as the red
sandstone likewise passes into soft white sandstone, and is also
the uppermost bed, I believe that these two beds, though so
different, are equivalents. A few leagues southward of these two
places, on each side of the low primary range of S. Juan, there
are some flat-topped, cliff-bounded, separate little hills, very
similar to those fringing the primary ranges in the great plain
south of Buenos Ayres: they are composed- -1st, of calcareous
tuff with many particles of quartz, sometimes passing into a
coarse conglomerate; 2nd, of a stone undistinguishable on the
closest inspection from the compacter varieties of tosca-rock;
and 3rd, of semi-crystalline limestone, including nodules of
agate: these three varieties pass insensibly into each other, and
as they form the uppermost stratum in this district, I believe
that they, also, are the equivalents of the pure crystalline
limestone, and of the red and white sandstones and conglomerates.
Between these points and Mercedes on the Rio Negro, there are
scarcely any good sections, the road passing over limestone,
tosca-rock, calcareous and bright red sandstones, and near the
source of the San Salvador over a wide extent of jaspery rocks,
with much milky agate, like that in the limestone near San Juan.
In the estancia of Berquelo, the separate, flat-topped,
cliff-bounded hills are rather higher than in the other parts of
the country; they range in a N.E. and S.W. direction; their
uppermost beds consist of the same bright red sandstone, passing
sometimes into a conglomerate, and in the lower part into soft
white sandstone, and even into loose sand: beneath this
sandstone, I saw in two places layers of calcareous and marly
rocks, and in one place red Pampean-like earth; at the base of
these sections, there was a hard, stratified, white sandstone,
with chalcedonic layers. Near Mercedes, beds of the same nature
and apparently of the same age, are associated with compact,
white, crystalline limestone, including much botryoidal agate,
and singular masses, like porcelain, but really composed of a
calcareo-siliceous paste. In sinking wells in this district the
chalcedonic strata seem to be the lowest. Beds, such as there
described, occur over the whole of this neighbourhood; but twenty
miles further up the R. Negro, in the cliffs of Perika, which are
about fifty feet in height, the upper bed is a prettily
variegated chalcedony, mingled with a pure white tallowy
limestone; beneath this there is a conglomerate of quartz and
granite; beneath this many sandstones, some highly calcareous;
and the whole lower two-thirds of the cliff consists of earthy
calcareous beds of various degrees of purity, with one layer of
reddish Pampean-like mud.
When examining the agates, the chalcedonic and jaspery rocks,
some of the limestones, and even the bright red sandstones, I was
forcibly struck with their resemblance to deposits formed in the
neighbourhood of volcanic action. I now find that M. Isabelle, in
his "Voyage a Buenos Ayres," has described closely similar beds
on Itaquy and Ibicuy (which enter the Uruguay some way north of
the R. Negro) and these beds include fragments of red decomposed
true scoriae hardened by zeolite, and of black retinite: we have
then here good evidence of volcanic action during our tertiary
period. Still further north, near S. Anna, where the Parana makes
a remarkable bend, M. Bonpland found some singular amygdaloidal
rocks, which perhaps may belong to this same epoch. (M. d'Orbigny
"Voyage" Part. Geolog. page 29) I may remark that, judging from
the size and well-rounded condition of the blocks of rock in the
above-described conglomerates, masses of primary formation
probably existed at this tertiary period above water: there is,
also, according to M. Isabelle, much conglomerate further north,
at Salto.
From whatever source and through whatever means the great Pampean
formation originated, we here have, I must repeat, unequivocal
evidence of a similar action at a period before that of the
deposition of the marine tertiary strata with extinct shells, at
Santa Fe and P. Gorda. During also the deposition of these
strata, we have in the intercalated layers of red Pampean-like
mud and tosca-rock, and in the passage near S. Juan of the
semi-crystalline limestones with agate into tosca
undistinguishable from that of the Pampas, evidence of the same
action, though continued only at intervals and in a feeble
manner. We have further seen that in this district, at a period
not only subsequent to the deposition of the tertiary strata, but
to their upheavement and most extensive denudation, true Pampean
mud with its usual characters and including mammiferous remains,
was deposited round and between the hills or islets formed of
these tertiary strata, and over the whole eastern and low primary
districts of Banda Oriental.
EARTHY MASS, WITH EXTINCT MAMMIFEROUS REMAINS, OVER THE
PORPHYRITIC GRAVEL AT S. JULIAN, LATITUDE 49 DEGREES 14' S., IN
PATAGONIA.
(FIGURE 16. SECTION OF THE LOWEST PLAIN AT PORT S. JULIAN.
(Section through beds from top to bottom: A, B, C, D, E, F.)
AA. Superficial bed of reddish earth, with the remains of the
Macrauchenia, and with recent sea-shells on the surface.
B. Gravel of porphyritic rocks.
C. and D. Pumiceous mudstone.—Ancient tertiary formation.
E. and F. Sandstone and argillaceous beds.—Ancient tertiary
formation.)
This case, though not coming strictly under the Pampean
formation, may be conveniently given here. On the south side of
the harbour, there is a nearly level plain (mentioned in the
First Chapter) about seven miles long, and three or four miles
wide, estimated at ninety feet in height, and bordered by
perpendicular cliffs, of which a section is represented in Figure
16.
The lower old tertiary strata (to be described in the next
chapter) are covered by the usual gravel bed; and this by an
irregular earthy, sometimes sandy mass, seldom more than two or
three feet in thickness, except where it fills up furrows or
gullies worn not only through the underlying gravel, but even
through the upper tertiary beds. This earthy mass is of a pale
reddish colour, like the less pure varieties of Pampean mud in
Banda Oriental; it includes small calcareous concretions, like
those of tosca- rock but more arenaceous, and other concretions
of a greenish, indurated argillaceous substance: a few pebbles,
also, from the underlying gravel-bed are also included in it, and
these being occasionally arranged in horizontal lines, show that
the mass is of sub-aqueous origin. On the surface and embedded in
the superficial parts, there are numerous shells, partially
retaining their colours, of three or four of the now commonest
littoral species. Near the bottom of one deep furrow (represented
in Figure 16), filled up with this earthy deposit, I found a
large part of the skeleton of the Macrauchenia Patachonica—a
gigantic and most extraordinary pachyderm, allied, according to
Professor Owen, to the Palaeotherium, but with affinities to the
Ruminants, especially to the American division of the Camelidae.
Several of the vertebrae in a chain, and nearly all the bones of
one of the limbs, even to the smallest bones of the foot, were
embedded in their proper relative positions: hence the skeleton
was certainly united by its flesh or ligaments, when enveloped in
the mud. This earthy mass, with its concretions and mammiferous
remains, filling up furrows in the underlying gravel, certainly
presents a very striking resemblance to some of the sections (for
instance, at P. Alta in B. Blanca, or at the Barrancas de S.
Gregorio) in the Pampean formation; but I must believe that this
resemblance is only accidental. I suspect that the mud which at
the present day is accumulating in deep and narrow gullies at the
head of the harbour, would, after elevation, present a very
similar appearance. The southernmost part of the true Pampean
formation, namely, on the Colorado, lies 560 miles of latitude
north of this point. (In the succeeding chapter I shall have to
refer to a great deposit of extinct mammiferous remains, lately
discovered by Captain Sulivan, R.N., at a point still further
south, namely, at the R. Gallegos; their age must at present
remain doubtful.)
With respect to the age of the Macrauchenia, the shells on the
surface prove that the mass in which the skeleton was enveloped
has been elevated above the sea within the recent period: I did
not see any of the shells embedded at a sufficient depth to
assure me (though it be highly probable) that the whole thickness
of the mass was contemporaneous with these INDIVIDUAL SPECIMENS.
That the Macrauchenia lived subsequently to the spreading out of
the gravel on this plain is certain; and that this gravel, at the
height of ninety feet, was spread out long after the existence of
recent shells, is scarcely less certain. For, it was shown in the
First Chapter, that this line of coast has been upheaved with
remarkable equability, and that over a vast space both north and
south of S. Julian, recent species of shells are strewed on (or
embedded in) the surface of the 250 feet plain, and of the 350
feet plain up to a height of 400 feet. These wide step-formed
plains have been formed by the denuding action of the coast-waves
on the old tertiary strata; and therefore, when the surface of
the 350 feet plain, with the shells on it, first rose above the
level of the sea, the 250 feet plain did not exist, and its
formation, as well as the spreading out of the gravel on its
summit, must have taken place subsequently. So also the
denudation and the gravel-covering of the 90 feet plain must have
taken place subsequently to the elevation of the 250 feet plain,
on which recent shells are also strewed. Hence there cannot be
any doubt that the Macrauchenia, which certainly was entombed in
a fresh state, and which must have been alive after the spreading
out of the gravel on the 90 feet plain, existed, not only
subsequently to the upraised shells on the surface of the 250
feet plain, but also to those on the 350 to 400 feet plain: these
shells, eight in number (namely, three species of Mytilus, two of
Patella, one Fusus, Voluta, and Balanus), are undoubtedly recent
species, and are the commonest kinds now living on this coast. At
Punta Alta in B. Blanca, I remarked how marvellous it was, that
the Toxodon, a mammifer so unlike to all known genera, should
have co-existed with twenty- three still living marine animals;
and now we find that the Macrauchenia, a quadruped only a little
less anomalous than the Toxodon, also co-existed with eight other
still existing Mollusca: it should, moreover, be borne in mind,
that a tooth of a pachydermatous animal was found with the other
remains at Punta Alta, which Professor Owen thinks almost
certainly belonged to the Macrauchenia.
Mr. Lyell has arrived at a highly important conclusion with
respect to the age of the North American extinct mammifers (many
of which are closely allied to, and even identical with, those of
the Pampean formation), namely, that they lived subsequently to
the period when erratic boulders were transported by the agency
of floating ice in temperate latitudes. ("Geological Proceedings"
volume 4 page 36.) Now in the valley of the Santa Cruz, only
fifty miles of latitude south of the spot where the Macrauchenia
was entombed, vast numbers of gigantic, angular boulders, which
must have been transported from the Cordillera on icebergs, lie
strewed on the plain, at the height of 1,400 feet above the level
of the sea. In ascending to this level, several step-formed
plains must be crossed, all of which have necessarily required
long time for their formation; hence the lowest or ninety feet
plain, with its superficial bed containing the remains of the
Macrauchenia, must have been formed very long subsequently to the
period when the 1,400 feet plain was beneath the sea, and
boulders were dropped on it from floating masses of ice. (It must
not be inferred from these remarks, that the ice-action ceased in
South America at this comparatively ancient period; for in Tierra
del Fuego boulders were probably transported contemporaneously
with, if not subsequently to, the formation of the ninety feet
plain at S. Julian, and at other parts of the coast of
Patagonia.) Mr. Lyell's conclusion, therefore, is thus far
confirmed in the southern hemisphere; and it is the more
important, as one is naturally tempted to admit so simple an
explanation, that it was the ice-period that caused the
extinction of the numerous great mammifers which so lately
swarmed over the two Americas.
A SUMMARY AND CONCLUDING REMARKS ON THE PAMPEAN FORMATION.
One of its most striking features is its great extent; I passed
continuously over it from the Colorado to St. Fe Bajada, a
distance of 500 geographical miles; and M. d'Orbigny traced it
for 250 miles further north. In the latitude of the Plata, I
examined this formation at intervals over an east and west line
of 300 miles from Maldonado to the R. Carcarana; and M. d'Orbigny
believes it extends 100 miles further inland: from Mr.
Caldcleugh's travels, however, I should have thought that it had
extended, south of the Cordovese range, to near Mendoza, and I
may add that I heard of great bones having been found high up the
R. Quinto. Hence the area of the Pampean formation, as remarked
by M. d'Orbigny, is probably at least equal to that of France,
and perhaps twice or thrice as great. In a basin, surrounded by
gravel-cliff (at a height of nearly three thousand feet), south
of Mendoza, there is, as described in the Third Chapter, a
deposit very like the Pampean, interstratified with other matter;
and again at S. Julian's, in Patagonia, 560 miles south of the
Colorado, a small irregular bed of a nearly similar nature
contains, as we have just seen, mammiferous remains. In the
provinces of Moxos and Chiquitos (1,000 miles northward of the
Pampas), and in Bolivia, at a height of 4,000 metres, M.
d'Orbigny has described similar deposits, which he believes to
have been formed by the same agency contemporaneously with the
Pampean formation. Considering the immense distances between
these several points, and their different heights, it appears to
me infinitely more probable, that this similarity has resulted
not from contemporaneousness of origin, but from the similarity
of the rocky framework of the continent: it is known that in
Brazil an immense area consists of gneissic rocks, and we shall
hereafter see, over how great a length the plutonic rocks of the
Cordillera, the overlying purple porphyries, and the trachytic
ejections, are almost identical in nature.
Three theories on the origin of the Pampean formation have been
propounded:—First, that of a great debacle by M. d'Orbigny; this
seems founded chiefly on the absence of stratification, and on
the number of embedded remains of terrestrial quadrupeds.
Although the Pampean formation (like so many argillaceous
deposits) is not divided into distinct and separate strata, yet
we have seen that in one good section it was striped with
horizontal zones of colour, and that in several specified places
the upper and lower parts differed, not only considerably in
colour, but greatly in constitution. In the southern part of the
Pampas the upper mass (to a certain extent stratified) generally
consists of hard tosca-rock, and the lower part of red Pampean
mud, often itself divided into two or more masses, varying in
colour and in the quantity of included calcareous matter. In
Western Banda Oriental, beds of a similar nature, but of a
greater age, conformably underlie and are intercalated with the
regularly stratified tertiary formation. As a general rule, the
marly concretions are arranged in horizontal lines, sometimes
united into irregular strata: surely, if the mud had been
tumultuously deposited in mass, the included calcareous matter
would have segregated itself irregularly, and not into nodules
arranged in horizontal lines, one above the other and often far
apart: this arrangement appears to me to prove that mud,
differing slightly in composition, was successively and quietly
deposited. On the theory of a debacle, a prodigious amount of
mud, without a single pebble, is supposed to have been borne over
the wide surface of the Pampas, when under water: on the other
hand, over the whole of Patagonia, the same or another debacle is
supposed to have borne nothing but gravel,—the gravel and the
fine mud in the neighbourhood of the Rios Negro and Colorado
having been borne to an equal distance from the Cordillera, or
imagined line of disturbance: assuredly directly opposite effects
ought not to be attributed to the same agency. Where, again,
could a mass of fine sediment, charged with calcareous matter in
a fit state for chemical segregation, and in quantity sufficient
to cover an area at least 750 miles long, and 400 miles broad, to
a depth of from twenty to thirty feet to a hundred feet, have
been accumulated, ready to be transported by the supposed
debacle? To my mind it is little short of demonstration, that a
great lapse of time was necessary for the production and
deposition of the enormous amount of mudlike matter forming the
Pampas; nor should I have noticed the theory of a debacle, had it
not been adduced by a naturalist so eminent as M. d'Orbigny.
A second theory, first suggested, I believe, by Sir W. Parish, is
that the Pampean formation was thrown down on low and marshy
plains by the rivers of this country before they assumed their
present courses. The appearance and composition of the deposit,
the manner in which it slopes up and round the primary ranges,
the nature of the underlying marine beds, the estuary and
sea-shells on the surface, the overlying sandstone beds at M.
Hermoso, are all quite opposed to this view. Nor do I believe
that there is a single instance of a skeleton of one of the
extinct mammifers having been found in an upright position, as if
it had been mired.
The third theory, of the truth of which I cannot entertain the
smallest doubt, is that the Pampean formation was slowly
accumulated at the mouth of the former estuary of the Plata and
in the sea adjoining it. I have come to this conclusion from the
reasons assigned against the two foregoing theories, and from
simple geographical considerations. From the numerous shells of
the Azara labiata lying loose on the surface of the plains, and
near Buenos Ayres embedded in the tosca-rock, we know that this
formation not only was formerly covered by, but that the
uppermost parts were deposited in, the brackish water of the
ancient La Plata. Southward and seaward of Buenos Ayres, the
plains were upheaved from under water inhabited by true marine
shells. We further know from Professor Ehrenberg's examination of
the twenty microscopical organisms in the mud round the tooth of
the Mastodon high up the course of the Parana, that the bottom-
most part of this formation was of brackish-water origin. A
similar conclusion must be extended to the beds of like
composition, at the level of the sea and under it, at M. Hermoso
in Bahia Blanca. Dr. Carpenter finds that the harder varieties of
tosca-rock, collected chiefly to the south, contain marine
spongoid bodies, minute fragments of shells, corals, and
Polythalamia; these perhaps may have been drifted inwards by the
tides, from the more open parts of the sea. The absence of
shells, throughout this deposit, with the exception of the
uppermost layers near Buenos Ayres, is a remarkable fact: can it
be explained by the brackish condition of the water, or by the
deep mud at the bottom? I have stated that both the reddish mud
and the concretions of tosca-rock are often penetrated by minute,
linear cavities, such as frequently may be observed in
fresh-water calcareous deposits:—were they produced by the
burrowing of small worms? Only on this view of the Pampean
formation having been of estuary origin, can the extraordinary
numbers (presently to be alluded to) of the embedded mammiferous
remains be explained. (It is almost superfluous to give the
numerous cases (for instance, in Sumatra; Lyell "Principles"
volume 3 page 325 sixth edition, of the carcasses of animals
having been washed out to sea by swollen rivers; but I may refer
to a recent account by Mr. Bettington "Asiatic Society" 1845 June
21st, of oxen, deer, and bears being carried into the Gulf of
Cambray; see also the account in my "Journal" 2nd edition page
133, of the numbers of animals drowned in the Plata during the
great, often recurrent, droughts.)
With respect to the first origin of the reddish mud, I will only
remark, that the enormous area of Brazil consists in chief part
of gneissic and other granitic rocks, which have suffered
decomposition, and been converted into a red, gritty,
argillaceous mass, to a greater depth than in any other country
which I have seen. The mixture of rounded grains, and even of
small fragments and pebbles of quartz, in the Pampean mud of
Banda Oriental, is evidently due to the neighbouring and
underlying primary rocks. The estuary mud was drifted during the
Pampean period in a much more southerly course, owing probably to
the east and west primary ridges south of the Plata not having
been then elevated, than the mud of the Plata at present is; for
it was formerly deposited as far south as the Colorado. The
quantity of calcareous matter in this formation, especially in
those large districts where the whole mass passes into
tosca-rock, is very great: I have already remarked on the close
resemblance in external and microscopical appearance, between
this tosca-rock and the strata at Coquimbo, which have certainly
resulted from the decay and attrition of recent shells: I dare
not, however, extend this conclusion to the calcareous rocks of
the Pampas, more especially as the underlying tertiary strata in
western Banda Oriental show that at that period there was a
copious emission of carbonate of lime, in connection with
volcanic action. (I may add, that there are nearly similar
superficial calcareous beds at King George's Sound in Australia;
and these undoubtedly have been formed by the disintegration of
marine remains see "Volcanic Islands" etc. page 144. There is,
however, something very remarkable in the frequency of
superficial, thin beds of earthy calcareous matter, in districts
where the surrounding rocks are not calcareous. Major Charters,
in a Paper read before the Geographical Society April 13, 1840
and abstracted in the "Athenaeum" page 317, states that this is
the case in parts of Mexico, and that he has observed similar
appearances in many parts of South Africa. The circumstance of
the uppermost stratum round the ragged Sierra Ventana, consisting
of calcareous or marly matter, without any covering of alluvial
matter, strikes me as very singular, in whatever manner we view
the deposition and elevation of the Pampean formation.)
The Pampean formation, judging from its similar composition, and
from the apparent absolute specific identity of some of its
mammiferous remains, and from the generic resemblance of others,
belongs over its vast area— throughout Banda Oriental, Entre
Rios, and the wide extent of the Pampas as far south as the
Colorado,—to the same geological epoch. The mammiferous remains
occur at all depths from the top to the bottom of the deposit;
and I may add that nowhere in the Pampas is there any appearance
of much superficial denudation: some bones which I found near the
Guardia del Monte were embedded close to the surface; and this
appears to have been the case with many of those discovered in
Banda Oriental: on the Matanzas, twenty miles south of Buenos
Ayres, a Glyptodon was embedded five feet beneath the surface;
numerous remains were found by S. Muniz, near Luxan, at an
average depth of eighteen feet; in Buenos Ayres a skeleton was
disinterred at sixty feet depth, and on the Parana I have
described two skeletons of the Mastodon only five or six feet
above the very base of the deposit. With respect to the age of
this formation, as judged of by the ordinary standard of the
existence of Mollusca, the only evidence within the limits of the
true Pampas which is at all trustworthy, is afforded by the still
living Azara labiata being embedded in tosca-rock near Buenos
Ayres. At Punta Alta, however, we have seen that several of the
extinct mammifers, most characteristic of the Pampean formation,
co-existed with twenty species of Mollusca, a barnacle and two
corals, all still living on this same coast;— for when we
remember that the shells have a more ancient appearance than the
bones; that many of the bones, though embedded in a coarse
conglomerate, are perfectly preserved; that almost all the parts
of the skeleton of the Scelidotherium, even to the knee-cap, were
lying in their proper relative positions; and that a large piece
of the fragile dermal armour of a Dasypoid quadruped, connected
with some of the bones of the foot, had been entombed in a
condition allowing the two sides to be doubled together, it must
assuredly be admitted that these mammiferous remains were
embedded in a fresh state, and therefore that the living animals
co-existed with the co-embedded shells. Moreover, the
Macrauchenia Patachonica (of which, according to Professor Owen,
remains also occur in the Pampas of Buenos Ayres, and at Punta
Alta) has been shown by satisfactory evidence of another kind, to
have lived on the plains of Patagonia long after the period when
the adjoining sea was first tenanted by its present commonest
molluscous animals. We must, therefore, conclude that the Pampean
formation belongs, in the ordinary geological sense of the word,
to the Recent Period. (M. d'Orbigny believes "Voyage" Part.
Geolog. page 81, that this formation, though "tres voisine de la
notre, est neanmoins de beaucoup anterieure a notre creation.")
At St. Fe Bajada, the Pampean estuary formation, with its
mammiferous remains, conformably overlies the marine tertiary
strata, which (as first shown by M. d'Orbigny) are
contemporaneous with those of Patagonia, and which, as we shall
hereafter see, belong to a very ancient tertiary stage. When
examining the junction between these two formations, I thought
that the concretionary layer of marl marked a passage between the
marine and estuary stages. M. d'Orbigny disputes this view (as
given in my "Journal"), and I admit that it is erroneous, though
in some degree excusable, from their conformability and from both
abounding with calcareous matter. It would, indeed, have been a
great anomaly if there had been a true passage between a deposit
contemporaneous with existing species of mollusca, and one in
which all the mollusca appear to be extinct. Northward of Santa
Fe, M. d'Orbigny met with ferruginous sandstones, marly rocks,
and other beds, which he considers as a distinct and lower
formation; but the evidence that they are not parts of the same
with an altered mineralogical character, does not appear to me
quite satisfactory.
In Western Banda Oriental, while the marine tertiary strata were
accumulating, there were volcanic eruptions, much silex and lime
were precipitated from solution, coarse conglomerates were
formed, being derived probably from adjoining land, and layers of
red mud and marly rocks, like those of the Pampean formation,
were occasionally deposited. The true Pampean deposit, with
mammiferous remains, instead of as at Santa Fe overlying
conformably the tertiary strata, is here seen at a lower level
folding round and between the flat-topped, cliff-bounded hills,
formed by a upheaval and denudation of these same tertiary
strata. The upheaval, having occurred here earlier than at Santa
Fe, may be naturally accounted for by the contemporaneous
volcanic action. At the Barrancas de S. Gregorio, the Pampean
deposit, as we have seen, overlies and fills up furrows in coarse
sand, precisely like that now accumulating on the shores near the
mouth of the Plata. I can hardly believe that this loose and
coarse sand is contemporaneous with the old tertiary and often
crystalline strata of the more western parts of the province; and
am induced to suspect that it is of subsequent origin. If that
section near Colonia could be implicitly trusted, in which, at a
height of only fifteen feet above the Plata, a bed of
fresh-looking mussels, of an existing littoral species, appeared
to lie between the sand and the Pampean mud, I should conclude
that Banda Oriental must have stood, when the coarse sand was
accumulating, at only a little below its present level, and had
then subsided, allowing the estuary Pampean mud to cover far and
wide its surface up to a height of some hundred feet; and that
after this subsidence the province had been uplifted to its
present level.
In Western Banda Oriental, we know, from two unequivocal sections
that there is a mass, absolutely undistinguishable from the true
Pampean deposit, beneath the old tertiary strata. This inferior
mass must be very much more ancient than the upper deposit with
its mammiferous remains, for it lies beneath the tertiary strata
in which all the shells are extinct. Nevertheless, the lower and
upper masses, as well as some intermediate layers, are so similar
in mineralogical character, that I cannot doubt that they are all
of estuary origin, and have been derived from the same great
source. At first it appeared to me extremely improbable, that mud
of the same nature should have been deposited on nearly the same
spot, during an immense lapse of time, namely, from a period
equivalent perhaps to the Eocene of Europe to that of the Pampean
formation. But as, at the very commencement of the Pampean
period, if not at a still earlier period, the Sierra Ventana
formed a boundary to the south,—the Cordillera or the plains in
front of them to the west,—the whole province of Corrientes
probably to the north, for, according to M. d'Orbigny, it is not
covered by the Pampean deposit,—and Brazil, as known by the
remains in the caves, to the north-east; and as again, during the
older tertiary period, land already existed in Western Banda
Oriental and near St. Fe Bajada, as may be inferred from the
vegetable debris, from the quantities of silicified wood, and
from the remains of a Toxodon found, according to M. d'Orbigny,
in still lower strata, we may conclude, that at this ancient
period a great expanse of water was surrounded by the same rocky
framework which now bounds the plains of Pampean formation. This
having been the case, the circumstance of sediment of the same
nature having been deposited in the same area during an immense
lapse of time, though highly remarkable, does not appear
incredible.
The elevation of the Pampas, at least of the southern parts, has
been slow and interrupted by several periods of rest, as may be
inferred from the plains, cliffs, and lines of sand-dunes (with
shells and pumice-pebbles) standing at different heights. I
believe, also, that the Pampean mud continued to be deposited,
after parts of this formation had already been elevated, in the
same manner as mud would continue to be deposited in the estuary
of the Plata, if the mud-banks on its shores were now uplifted
and changed into plains: I believe in this from the improbability
of so many skeletons and bones having been accumulated at one
spot, where M. Hermoso now stands, at a depth of between eight
hundred and one thousand feet, and at a vast distance from any
land except small rocky islets,—as must have been the case, if
the high tosca-plain round the Ventana and adjoining Sierras, had
not been already uplifted and converted into land, supporting
mammiferous animals. At Punta Alta we have good evidence that the
gravel- strata, which certainly belong to the true Pampean
period, were accumulated after the elevation in that
neighbourhood of the main part of the Pampean deposit, whence the
rounded masses of tosca-rock were derived, and that rolled
fragment of black bone in the same peculiar condition with the
remains at Monte Hermoso.
The number of the mammiferous remains embedded in the Pampas is,
as I have remarked, wonderful: it should be borne in mind that
they have almost exclusively been found in the cliffs and steep
banks of rivers, and that, until lately, they excited no
attention amongst the inhabitants: I am firmly convinced that a
deep trench could not be cut in any line across the Pampas,
without intersecting the remains of some quadruped. It is
difficult to form an opinion in what part of the Pampas they are
most numerous; in a limited spot they could not well have been
more numerous than they were at P. Alta; the number, however,
lately found by Senor F. Muniz, near Luxan, in a central spot in
the Pampas, is extraordinarily great: at the end of this chapter
I will give a list of all the localities at which I have heard of
remains having been discovered. Very frequently the remains
consist of almost perfect skeletons; but there are, also,
numerous single bones, as for instance at St. Fe. Their state of
preservation varies much, even when embedded near each other: I
saw none others so perfectly preserved as the heads of the
Toxodon and Mylodon from the white soft earthy bed on the
Sarandis in Banda Oriental. It is remarkable that in two limited
sections I found no less than five teeth separately embedded, and
I heard of teeth having been similarly found in other parts: may
we suppose that the skeletons or heads were for a long time
gently drifted by currents over the soft muddy bottom, and that
the teeth occasionally, here and there, dropped out?
It may be naturally asked, where did these numerous animals live?
From the remarkable discoveries of MM. Lund and Clausen, it
appears that some of the species found in the Pampas inhabited
the highlands of Brazil: the Mastodon Andium is embedded at great
heights in the Cordillera from north of the equator to at least
as far south as Tarija (Humboldt states that the Mastodon has
been discovered in New Granada: it has been found in Quito. When
at Lima, I saw a tooth of a Mastodon in the possession of Don M.
Rivero, found at Playa Chica on the Maranon, near the Guallaga.
Every one has heard of the numerous remains of Mastodon in
Bolivia.); and as there is no higher land, there can be little
doubt that this Mastodon must have lived on the plains and
valleys of that great range. These countries, however, appear too
far distant for the habitation of the individuals entombed in the
Pampas: we must probably look to nearer points, for instance to
the province of Corrientes, which, as already remarked, is said
not to be covered by the Pampean formation, and may therefore, at
the period of its deposition, have existed as dry land. I have
already given my reasons for believing that the animals embedded
at M. Hermoso and at P. Alta in Bahia Blanca, lived on adjoining
land, formed of parts of the already elevated Pampean deposit.
With respect to the food of these many great extinct quadrupeds,
I will not repeat the facts given in my "Journal" (second edition
page 85), showing that there is no correlation between the
luxuriance of the vegetation of a country and the size of its
mammiferous inhabitants. I do not doubt that large animals could
now exist, as far as the amount, not kind, of vegetation is
concerned, on the sterile plains of Bahia Blanca and of the R.
Negro, as well as on the equally, if not more sterile plains of
Southern Africa. The climate, however, may perhaps have somewhat
deteriorated since the mammifers embedded at Bahia Blanca lived
there; for we must not infer, from the continued existence of the
same shells on the present coasts, that there has been no change
in climate; for several of these shells now range northward along
the shores of Brazil, where the most luxuriant vegetation
flourishes under a tropical temperature. With respect to the
extinction, which at first fills the mind with astonishment, of
the many great and small mammifers of this period, I may also
refer to the work above cited (second edition page 173), in which
I have endeavoured to show, that however unable we may be to
explain the precise cause, we ought not properly to feel more
surprised at a species becoming extinct than at one being rare;
and yet we are accustomed to view the rarity of any particular
species as an ordinary event, not requiring any extraordinary
agency.
The several mammifers embedded in the Pampean formation, which
mostly belong to extinct genera, and some even to extinct
families or orders, and which differ nearly, if not quite, as
much as do the Eocene mammifers of Europe from living quadrupeds
having existed contemporaneously with mollusca, all still
inhabiting the adjoining sea, is certainly a most striking fact.
It is, however, far from being an isolated one; for, during the
late tertiary deposits of Britain, an elephant, rhinoceros, and
hippopotamus co-existed with many recent land and fresh-water
shells; and in North America, we have the best evidence that a
mastodon, elephant, megatherium, megalonyx, mylodon, an extinct
horse and ox, likewise co- existed with numerous land,
fresh-water, and marine recent shells. (Many original
observations, and a summary on this subject, are given in Mr.
Lyell's paper in the "Geological Proceedings" volume 4 page 3 and
in his "Travels in North America" volume 1 page 164 and volume 2
page 60. For the European analogous cases see Mr. Lyell's
"Principles of Geology" 6th edition volume 1 page 37.) The
enumeration of these extinct North American animals naturally
leads me to refer to the former closer relation of the
mammiferous inhabitants of the two Americas, which I have
discussed in my "Journal," and likewise to the vast extent of
country over which some of them ranged: thus the same species of
the Megatherium, Megalonyx, Equus (as far as the state of their
remains permits of identification), extended from the Southern
United States of North America to Bahia Blanca, in latitude 39
degrees S., on the coast of Patagonia. The fact of these animals
having inhabited tropical and temperate regions, does not appear
to me any great difficulty, seeing that at the Cape of Good Hope
several quadrupeds, such as the elephant and hippopotamus, range
from the equator to latitude 35 degrees south. The case of the
Mastodon Andium is one of more difficulty, for it is found from
latitude 36 degrees S., over, as I have reason to believe, nearly
the whole of Brazil, and up the Cordillera to regions which,
according to M. d'Orbigny, border on perpetual snow, and which
are almost destitute of vegetation: undoubtedly the climate of
the Cordillera must have been different when the mastodon
inhabited it; but we should not forget the case of the Siberian
mammoth and rhinoceros, as showing how severe a climate the
larger pachydermata can endure; nor overlook the fact of the
guanaco ranging at the present day over the hot low deserts of
Peru, the lofty pinnacles of the Cordillera, and the damp
forest-clad land of Southern Tierra del Fuego; the puma, also, is
found from the equator to the Strait of Magellan, and I have seen
its footsteps only a little below the limits of perpetual snow in
the Cordillera of Chile.
At the period, so recent in a geological sense, when these
extinct mammifers existed, the two Americas must have swarmed
with quadrupeds, many of them of gigantic size; for, besides
those more particularly referred to in this chapter, we must
include in this same period those wonderfully numerous remains,
some few of them specifically, and others generically related to
those of the Pampas, discovered by MM. Lund and Clausen in the
caves of Brazil. Finally, the facts here given show how cautious
we ought to be in judging of the antiquity of a formation from
even a great amount of difference between the extinct and living
species in any one class of animals;—we ought even to be cautious
in accepting the general proposition, that change in organic
forms and lapse of time are at all, necessarily, correlatives.
...
LOCALITIES WITHIN THE REGION OF THE PAMPAS WHERE GREAT BONES HAVE
BEEN FOUND.
The following list, which includes every account which I have
hitherto met with of the discovery of fossil mammiferous remains
in the Pampas, may be hereafter useful to a geologist
investigating this region, and it tends to show their
extraordinary abundance. I heard of and saw many fossils, the
original position of which I could not ascertain; and I received
many statements too vague to be here inserted. Beginning to the
south:—we have the two stations in Bahia Blanca, described in
this chapter, where at P. Alta, the Megatherium, Megalonyx,
Scelidotherium, Mylodon, Holophractus (or an allied genus),
Toxodon, Macrauchenia, and an Equus were collected; and at M.
Hermoso a Ctenomys, Hydrochaerus, some other rodents and the
bones of a great megatheroid quadruped. Close north-east of the
S. Tapalguen, we have the Rios 'Huesos' (i.e. "bones"), which
probably takes its name from large fossil bones. Near Villa
Nuevo, and at Las Averias, not far from the Salado, three nearly
perfect skeletons, one of the Megatherium, one of the Glyptodon
clavipes, and one of some great Dasypoid quadruped, were found by
the agent of Sir W. Parish (see his work "Buenos Ayres" etc. page
171). I have seen the tooth of a Mastodon from the Salado; a
little northward of this river, on the borders of a lake near the
G. del Monte, I saw many bones, and one large piece of dermal
armour; higher up the Salado, there is a place called Monte
"Huesos." On the Matanzas, about twenty miles south of Buenos
Ayres, the skeleton (vide page 178 of "Buenos Ayres" etc. by Sir
W. Parish) of a Glyptodon was found about five feet beneath the
surface; here also (see Catalogue of Royal College of Surgeons)
remains of Glyptodon clavipes, G. ornatus, and G. reticulatus
were found. Signor Angelis, in a letter which I have seen, refers
to some great remains found in Buenos Ayres, at a depth of twenty
varas from the surface. Seven leagues north of this city the same
author found the skeletons of Mylodon robustus and Glyptodon
ornatus. From this neighbourhood he has lately sent to the
British Museum the following fossils:—Remains of three or four
individuals of Megatherium; of three species of Glyptodon; of
three individuals of the Mastodon Andium; of Macrauchenia; of a
second species of Toxodon, different from T. Platensis; and
lastly, of the Machairodus, a wonderful large carnivorous animal.
M. d'Orbigny has lately received from the Recolate "Voyage" Pal.
page 144), near Buenos Ayres, a tooth of Toxodon Platensis.
Proceeding northward, along the west bank of the Parana, we come
to the Rio Luxan, where two skeletons of the Megatherium have
been found; and lately, within eight leagues of the town of
Luxan, Dr. F. X. Muniz has collected ("British Packet" Buenos
Ayres September 25, 1841), from an average depth of eighteen
feet, very numerous remains, of no less than, as he believes,
nine distinct species of mammifers. At Areco, large bones have
been found, which are believed, by the inhabitants, to have been
changed from small bones, by the water of the river! At
Arrecifes, the Glyptodon, sent to the College of Surgeons, was
found; and I have seen two teeth of a Mastodon from this quarter.
At S. Nicolas, M. d'Orbigny found remains of a Canis, Ctenomys,
and Kerodon; and M. Isabelle ("Voyage" page 332) refers to a
gigantic Armadillo found there. At S. Carlos, I heard of great
bones. A little below the mouth of the Carcarana, the two
skeletons of Mastodon were found; on the banks of this river,
near S. Miguel, I found teeth of the Mastodon and Toxodon; and
"Falkner" (page 55) describes the osseous armour of some great
animal; I heard of many other bones in this neighbourhood. I have
seen, I may add, in the possession of Mr. Caldcleugh, the tooth
of a Mastodon Andium, said to have been found in Paraguay; I may
here also refer to a statement in this gentleman's travels
(volume 1 page 48), of a great skeleton having been found in the
province of Bolivia in Brazil, on the R. de las Contas. The
furthest point westward in the Pampas, at which I have HEARD of
fossil bones, was high up on the banks of R. Quinto.
In Entre Rios, besides the remains of the Mastodon, Toxodon,
Equus, and a great Dasypoid quadruped near St. Fe Bajada, I
received an account of bones having been found a little S.E. of
P. Gorda (on the Parana), and of an entire skeleton at Matanzas,
on the Arroyo del Animal.
In Banda Oriental, besides the remains of the Toxodon, Mylodon,
and two skeletons of great animals with osseous armour (distinct
from that of the Glyptodon), found on the Arroyos Sarandis and
Berquelo, M. Isabelle ("Voyage" page 322) says, many bones have
been found near the R. Negro, and on the R. Arapey, an affluent
of the Paraguay, in latitude 30 degrees 40 minutes south. I heard
of bones near the source of the A. Vivoras. I saw the remains of
a Dasypoid quadruped from the Arroyo Seco, close to M. Video; and
M. d'Orbigny refers ("Voyage" Geolog. page 24), to another found
on the Pedernal, an affluent of the St. Lucia; and Signor
Angelis, in a letter, states that a third skeleton of this family
has been found, near Canelones. I saw a tooth of the Mastodon
from Talas, another affluent of the St. Lucia. The most eastern
point at which I heard of great bones having been found, was at
Solis Grande, between M. Video and Maldonado.
CHAPTER V. ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND
CHILE.
Rio Negro. S. Josef. Port Desire, white pumiceous mudstone with
Infusoria. Port S. Julian. Santa Cruz, basaltic lava of. P. Gallegos.
Eastern Tierra del Fuego; leaves of extinct beech-trees. Summary on
the Patagonian tertiary formations. Tertiary formations of the
Western Coast. Chonos and Chiloe groups, volcanic rocks of.
Concepcion. Navidad. Coquimbo. Summary. Age of the tertiary
formations. Lines of elevation. Silicified wood. Comparative ranges
of the extinct and living mollusca on the West Coast of S. America.
Climate of the tertiary period. On the causes of the absence of
recent conchiferous deposits on the coast of S. America. On the
contemporaneous deposition and preservation of sedimentary
formations.
RIO NEGRO.
I can add little to the details given by M. d'Orbigny on the
sandstone formation of this district. ("Voyage" Part Geolog.
pages 57-65.) The cliffs to the south of the river are about two
hundred feet in height, and are composed of sandstone of various
tints and degrees of hardness. One layer, which thinned out at
both ends, consisted of earthy matter, of a pale reddish colour,
with some gypsum, and very like (I speak after comparison of the
specimens brought home) Pampean mud: above this was a layer of
compact marly rock with dendritic manganese. Many blocks of a
conglomerate of pumice-pebbles embedded in hard sandstone were
strewed at the foot of the cliff, and had evidently fallen from
above. A few miles N.E. of the town, I found, low down in the
sandstone, a bed, a few inches in thickness, of a white, friable,
harsh-feeling sediment, which adheres to the tongue, is of easy
fusibility, and of little specific gravity; examined under the
microscope, it is seen to be pumiceous tuff, formed of broken
transparent crystals. In the cliffs south of the river, there is,
also, a thin layer of nearly similar nature, but finer grained,
and not so white; it might easily have been mistaken for a
calcareous tuff, but it contains no lime: this substance
precisely resembles a most widely extended and thick formation in
Southern Patagonia, hereafter to be described, and which is
remarkable for being partially formed of infusoria. These beds,
conjointly with the conglomerate of pumice, are interesting, as
showing the nature of the volcanic action in the Cordillera
during this old tertiary period.
In a bed at the base of the southern cliffs, M. d'Orbigny found
two extinct fresh-water shells, namely, a Unio and Chilina. This
bed rested on one with bones of an extinct rodent, namely, the
Megamys Patagoniensis; and this again on another with extinct
marine shells. The species found by M. d'Orbigny in different
parts of this formation consist of:—
1. Ostrea Patagonica, d'Orbigny, "Voyage, Pal." (also at St. Fe,
and whole coast of Patagonia). 2. Ostrea Ferrarisi, d'Orbigny,
"Voyage, Pal." 3. Ostrea Alvarezii, d'Orbigny, "Voyage, Pal."
(also at St. Fe, and S. Josef). 4. Pecten Patagoniensis,
d'Orbigny, "Voyage, Pal." 5. Venus Munsterii, d'Orbigny, "Voyage,
Pal." (also at St. Fe). 6. Arca Bonplandiana, d'Orbigny, "Voyage,
Pal." (also at St. Fe).
According to M. d'Orbigny, the sandstone extends westward along
the coast as far as Port S. Antonio, and up the R. Negro far into
the interior: northward I traced it to the southern side of the
Rio Colorado, where it forms a low denuded plain. This formation,
though contemporaneous with that of the rest of Patagonia, is
quite different in mineralogical composition, being connected
with it only by the one thin white layer: this difference may be
reasonably attributed to the sediment brought down in ancient
times by the Rio Negro; by which agency, also, we can understand
the presence of the fresh-water shells, and of the bones of land
animals. Judging from the identity of four of the above shells,
this formation is contemporaneous (as remarked by M. d'Orbigny)
with that under the Pampean deposit in Entre Rios and in Banda
Oriental. The gravel capping the sandstone plain, with its
calcareous cement and nodules of gypsum, is probably, from the
reasons given in the First Chapter, contemporaneous with the
uppermost beds of the Pampean formation on the upper plain north
of the Colorado.
SAN JOSEF.
My examination here was very short: the cliffs are about a
hundred feet high; the lower third consists of yellowish-brown,
soft, slightly calcareous, muddy sandstone, parts of which when
struck emit a fetid smell. In this bed the great Ostraea
Patagonica, often marked with dendritic manganese and small
coral-lines, were extraordinarily numerous. I found here the
following shells:—
1. Ostrea Patagonica, d'Orbigny, "Voyage, Pal." (also at St. Fe
and whole coast of Patagonia). 2. Ostrea Alvarezii, d'Orbigny,
"Voyage, Pal." (also at St. Fe and R. Negro). 3. Pecten
Paranensis, d'Orbigny, "Voyage, Pal." (also at St. Fe, S. Julian,
and Port Desire). 4. Pecten Darwinianus, d'Orbigny, "Voyage,
Pal." (also at St. Fe). 5. Pecten actinodes, G.B. Sowerby. 6.
Terebratula Patagonica, G.B. Sowerby (also S. Julian). 7. Casts
of a Turritella.
The four first of these species occur at St. Fe in Entre Rios,
and the two first in the sandstone of the Rio Negro. Above this
fossiliferous mass, there is a stratum of very fine-grained, pale
brown mudstone, including numerous laminae of selenite. All the
strata appear horizontal, but when followed by the eye for a long
distance, they are seen to have a small easterly dip. On the
surface we have the porphyritic gravel, and on it sand with
recent shells.
NUEVO GULF.
From specimens and notes given me by Lieutenant Stokes, it
appears that the lower bed consists of soft muddy sandstone, like
that of S. Josef, with many imperfect shells, including the
Pecten Paranensis, d'Orbigny, casts of a Turritella and Scutella.
On this there are two strata of the pale brown mudstone, also
like that of S. Josef, separated by a darker-coloured, more
argillaceous variety, including the Ostrea Patagonica. Professor
Ehrenberg has examined this mudstone for me: he finds in it three
already known microscopic organisms, enveloped in a fine-grained
pumiceous tuff, which I shall have immediately to describe in
detail. Specimens brought to me from the uppermost bed, north of
the Rio Chupat, consist of this same substance, but of a whiter
colour.
Tertiary strata, such as here described, appear to extend along
the whole coast between Rio Chupat and Port Desire, except where
interrupted by the underlying claystone porphyry, and by some
metamorphic rocks; these hard rocks, I may add, are found at
intervals over a space of about five degrees of latitude, from
Point Union to a point between Port S. Julian and S. Cruz, and
will be described in the ensuing chapter. Many gigantic specimens
of the Ostraea Patagonica were collected in the Gulf of St.
George.
PORT DESIRE.
A good section of the lowest fossiliferous mass, about forty feet
in thickness, resting on claystone porphyry, is exhibited a few
miles south of the harbour. The shells sufficiently perfect to be
recognised consist of:—
1. Ostrea Patagonica, d'Orbigny, (also at St. Fe, and whole coast
of Patagonia). 2. Pecten Paranensis, d'Orbigny, "Voyage, Pal."
(also at St. Fe, S. Josef, S. Julian). 3. Pecten centralis, G.B.
Sowerby (also at S. Julian and S. Cruz). 4. Cucullaea alta, G.B.
Sowerby (also at S. Cruz). 5. Nucula ornata, G.B. Sowerby. 6.
Turritella Patagonica, G.B. Sowerby.
The fossiliferous strata, when not denuded, are conformably
covered by a considerable thickness of the fine-grained pumiceous
mudstone, divided into two masses: the lower half is very
fine-grained, slightly unctuous, and so compact as to break with
a semi-conchoidal fracture, though yielding to the nail; it
includes laminae of selenite: the upper half precisely resembles
the one layer at the Rio Negro, and with the exception of being
whiter, the upper beds at San Josef and Nuevo Gulf. In neither
mass is there any trace to the naked eye of organic forms. Taking
the entire deposit, it is generally quite white, or yellowish, or
feebly tinted with green; it is either almost friable under the
finger, or as hard as chalk; it is of easy fusibility, of little
specific gravity, is not harsh to the touch, adheres to the
tongue, and when breathed on exhales a strong aluminous odour; it
sometimes contains a very little calcareous matter, and traces
(besides the included laminae) of gypsum. Under the microscope,
according to Professor Ehrenberg, it consists of minute,
triturated, cellular, glassy fragments of pumice, with some
broken crystals. ("Monatsberichten de konig. Akad. zu Berlin" vom
April 1845.) In the minute glassy fragments, Professor Ehrenberg
recognises organic structures, which have been affected by
volcanic heat: in the specimens from this place, and from Port S.
Julian, he finds sixteen Polygastrica and twelve Phytolitharia.
Of these organisms, seven are new forms, the others being
previously known: all are of marine, and chiefly of oceanic,
origin. This deposit to the naked eye resembles the crust which
often appears on weathered surfaces of feldspathic rocks; it
likewise resembles those beds of earthy feldspathic matter,
sometimes interstratified with porphyritic rocks, as is the case
in this very district with the underlying purple claystone
porphyry. From examining specimens under a common microscope, and
comparing them with other specimens undoubtedly of volcanic
origin, I had come to the same conclusion with Professor
Ehrenberg, namely, that this great deposit, in its first origin,
is of volcanic nature.
PORT S. JULIAN.
(FIGURE 17. SECTION OF THE STRATA EXHIBITED IN THE CLIFFS OF THE
NINETY FEET PLAIN AT PORT S. JULIAN.
(Section through beds from top to bottom: A, B, C, D, E, F.))
On the south side of the harbour, Figure 17 gives the nature of
the beds seen in the cliffs of the ninety feet plain. Beginning
at the top:—
1st, the earthy mass (AA), including the remains of the
Macrauchenia, with recent shells on the surface.
Second, the porphyritic shingle (B), which in its lower part is
interstratified (owing, I believe, to redisposition during
denudation) with the white pumiceous mudstone.
Third, this white mudstone, about twenty feet in thickness, and
divided into two varieties (C and D), both closely resembling the
lower, fine- grained, more unctuous and compact kind at Port
Desire; and, as at that place, including much selenite.
Fourth, a fossiliferous mass, divided into three main beds, of
which the uppermost is thin, and consists of ferruginous
sandstone, with many shells of the great oyster and Pecten
Paranensis; the middle bed (E) is a yellowish earthy sandstone
abounding with Scutellae; and the lowest bed (F) is an indurated,
greenish, sandy clay, including large concretions of calcareous
sandstone, many shells of the great oyster, and in parts almost
made up of fragments of Balanidae. Out of these three beds, I
procured the following twelve species, of which the two first
were exceedingly numerous in individuals, as were the
Terebratulae and Turritellae in certain layers:—
1. Ostrea Patagonica, d'Orbigny, "Voyage, Pal." (also at St. Fe,
and whole coast of Patagonia). 2. Pecten Paranensis, d'Orbigny,
"Voyage, Pal." (St. Fe, S. Josef, Port Desire). 3. Pecten
centralis, G.B. Sowerby (also at Port Desire and S. Cruz). 4.
Pecten geminatus, G.B. Sowerby. 5. Terebratula Patagonica, G.B.
Sowerby (also S. Josef). 6. Struthiolaria ornata, G.B. Sowerby
(also S. Cruz). 7. Fusus Patagonicus, G.B. Sowerby. 8. Fusus
Noachinus, G.B. Sowerby. 9. Scalaria rugulosa, G.B. Sowerby. 10.
Turritella ambulacrum, G.B. Sowerby (also S. Cruz). 11. Pyrula,
cast of, like P. ventricosa of Sowerby, Tank Cat. 12. Balanus
varians, G.B. Sowerby. 13. Scutella, differing from the species
from Nuevo Gulf.
At the head of the inner harbour of Port S. Julian, the
fossiliferous mass is not displayed, and the sea-cliffs from the
water's edge to a height of between one and two hundred feet are
formed of the white pumiceous mudstone, which here includes
innumerable, far-extended, sometimes horizontal, sometimes
inclined or vertical laminae of transparent gypsum, often about
an inch in thickness. Further inland, with the exception of the
superficial gravel, the whole thickness of the truncated hills,
which represent a formerly continuous plain 950 feet in height,
appears to be formed of this white mudstone: here and there,
however, at various heights, thin earthy layers, containing the
great oyster, Pecten Paranensis and Turritella ambulacrum, are
interstratified; thus showing that the whole mass belongs to the
same epoch. I nowhere found even a fragment of a shell actually
in the white deposit, and only a single cast of a Turritella. Out
of the eighteen microscopic organisms discovered by Ehrenberg in
the specimens from this place, ten are common to the same deposit
at Port Desire. I may add that specimens of this white mudstone,
with the same identical characters were brought me from two
points,—one twenty miles north of S. Julian, where a wide
gravel-capped plain, 350 feet in height, is thus composed; and
the other forty miles south of S. Julian, where, on the old
charts, the cliffs are marked as "Chalk Hills."
SANTA CRUZ.
The gravel-capped cliffs at the mouth of the river are 355 feet
in height: the lower part, to a thickness of fifty or sixty feet,
consists of a more or less hardened, darkish, muddy, or
argillaceous sandstone (like the lowest bed of Port Desire),
containing very many shells, some silicified and some converted
into yellow calcareous spar. The great oyster is here numerous in
layers; the Trigonocelia and Turritella are also very numerous:
it is remarkable that the Pecten Paranensis, so common in all
other parts of the coast, is here absent: the shells consist of:—
1. Ostrea Patagonica, d'Orbigny; "Voyage Pal." (also at St. Fe and
whole coast of Patagonia). 2. Pecten centralis, G.B. Sowerby (also P.
Desire and S. Julian). 3. Venus meridionalis of G.B. Sowerby. 4.
Crassatella Lyellii, G.B. Sowerby. 5. Cardium puelchum, G.B. Sowerby.
6. Cardita Patagonica, G.B. Sowerby. 7. Mactra rugata, G.B. Sowerby. 8.
Mactra Darwinii, G.B. Sowerby. 9. Cucullaea alta, G.B. Sowerby (also P.
Desire). 10. Trigonocelia insolita, G.B. Sowerby. 11. Nucula (?)
glabra, G.B. Sowerby. 12. Crepidula gregaria, G.B. Sowerby. 13. Voluta
alta, G.B. Sowerby. 14. Trochus collaris, G.B. Sowerby. 15. Natica
solida (?), G.B. Sowerby 16. Struthiolaria ornata, G.B. Sowerby (also
P. Desire). 17. Turritella ambulacrum, G.B. Sowerby (also P. S.
Julian). Imperfect fragments of the genera Byssoarca, Artemis, and
Fusus.
The upper part of the cliff is generally divided into three great
strata, differing slightly in composition, but essentially
resembling the pumiceous mudstone of the places farther north;
the deposit, however, here is more arenaceous, of greater
specific gravity, and not so white: it is interlaced with
numerous thin veins, partially or quite filled with transverse
fibres of gypsum; these fibres were too short to reach across the
vein, have their extremities curved or bent: in the same veins
with the gypsum, and likewise in separate veins as well as in
little nests, there is much powdery sulphate of magnesia (as
ascertained by Mr. Reeks) in an uncompressed form: I believe that
this salt has not heretofore been found in veins. Of the three
beds, the central one is the most compact, and more like ordinary
sandstone: it includes numerous flattened spherical concretions,
often united like a necklace, composed of hard calcareous
sandstone, containing a few shells: some of these concretions
were four feet in diameter, and in a horizontal line nine feet
apart, showing that the calcareous matter must have been drawn to
the centres of attraction, from a distance of four feet and a
half on both sides. In the upper and lower finer-grained strata,
there were other concretions of a grey colour, containing
calcareous matter, and so fine-grained and compact, as almost to
resemble porcelain- rock: I have seen exactly similar concretions
in a volcanic tufaceous bed in Chiloe. Although in this upper
fine-grained strata, organic remains were very rare, yet I
noticed a few of the great oyster; and in one included soft
ferruginous layer, there were some specimens of the Cucullaea
alta (found at Port Desire in the lower fossiliferous mass) and
of the Mactra rugata, which latter shell has been partially
converted into gypsum.
(FIGURE 18. SECTION OF THE PLAINS OF PATAGONIA, ON THE BANKS OF
THE S. CRUZ.
(Section through strata (from top to bottom)): Surface of plain
with erratic boulders; 1,146 feet above the sea. a. Gravel and
boulders, 212 feet thick. b. Basaltic lava, 322 feet thick. c, d
and e. Sedimentary layers, bed of small pebbles and talus
respectively, total 592 feet thick. River of S. Cruz; here 280
feet above sea.)
In ascending the valley of the S. Cruz, the upper strata of the
coast- cliffs are prolonged, with nearly the same characters, for
fifty miles: at about this point, they begin in the most gradual
and scarcely perceptible manner, to be banded with white lines;
and after ascending ten miles farther, we meet with distinct thin
layers of whitish, greenish, and yellowish fine-grained, fusible
sediments. At eighty miles from the coast, in a cliff thus
composed, there were a few layers of ferruginous sandstone, and
of an argillaceous sandstone with concretions of marl like those
in the Pampas. (At this spot, for a space of three-quarters of a
mile along the north side of the river, and for a width of half a
mile, there has been a great slip, which has formed hills between
sixty and seventy feet in height, and has tilted the strata into
highly inclined and even vertical positions. The strata generally
dipped at an angle of 45 degrees towards the cliff from which
they had slided. I have observed in slips, both on a small and
large scale, that this inward dip is very general. Is it due to
the hydrostatic pressure of water percolating with difficulty
through the strata acting with greater force at the base of the
mass than against the upper part?) At one hundred miles from the
coast, that is at a central point between the Atlantic and the
Cordillera, we have the section in Figure 18.
The upper half of the sedimentary mass, under the basaltic lava,
consists of innumerable zones of perfectly white bright green,
yellowish and brownish, fine-grained, sometimes incoherent,
sedimentary matter. The white, pumiceous, trachytic tuff-like
varieties are of rather greater specific gravity than the
pumiceous mudstone on the coast to the north; some of the layers,
especially the browner ones, are coarser, so that the broken
crystals are distinguishable with a weak lens. The layers vary in
character in short distances. With the exception of a few of the
Ostrea Patagonica, which appeared to have rolled down from the
cliff above, no organic remains were found. The chief difference
between these layers taken as a whole, and the upper beds both at
the mouth of the river and on the coast northward, seems to lie
in the occasional presence of more colouring matter, and in the
supply having been intermittent; these characters, as we have
seen, very gradually disappear in descending the valley, and this
fact may perhaps be accounted for by the currents of a more open
sea having blended together the sediment from a distant and
intermittent source.
The coloured layers in the foregoing section rest on a mass,
apparently of great thickness (but much hidden by the talus), of
soft sandstone, almost composed of minute pebbles, from one-tenth
to two-tenths of an inch in diameter, of the rocks (with the
entire exception of the basaltic lava) composing the great
boulders on the surface of the plain, and probably composing the
neighbouring Cordillera. Five miles higher up the valley, and
again thirty miles higher up (that is twenty miles from the
nearest range of the Cordillera), the lower plain included within
the upper escarpments, is formed, as seen on the banks of the
river, of a nearly similar but finer-grained, more earthy,
laminated sandstone, alternating with argillaceous beds, and
containing numerous moderately sized pebbles of the same rocks,
and some shells of the great Ostrea Patagonica. (I found at both
places, but not in situ, quantities of coniferous and ordinary
dicotyledonous silicified wood, which was examined for me by Mr.
R. Brown.) As most of these shells had been rolled before being
here embedded, their presence does not prove that the sandstone
belongs to the great Patagonian tertiary formation, for they
might have been redeposited in it, when the valley existed as a
sea-strait; but as amongst the pebbles there were none of basalt,
although the cliffs on both sides of the valley are composed of
this rock, I believe that the sandstone does belong to this
formation. At the highest point to which we ascended, twenty
miles distant from the nearest slope of the Cordillera, I could
see the horizontally zoned white beds, stretching under the black
basaltic lava, close up to the mountains; so that the valley of
the S. Cruz gives a fair idea of the constitution of the whole
width of Patagonia.
BASALTIC LAVA OF THE S. CRUZ.
This formation is first met with sixty-seven miles from the mouth
of the river; thence it extends uninterruptedly, generally but
not exclusively on the northern side of the valley, close up to
the Cordillera. The basalt is generally black and fine-grained,
but sometimes grey and laminated; it contains some olivine, and
high up the valley much glassy feldspar, where, also, it is often
amygdaloidal; it is never highly vesicular, except on the sides
of rents and on the upper and lower, spherically laminated
surfaces. It is often columnar; and in one place I saw
magnificent columns, each face twelve feet in width, with their
interstices filled up with calcareous tuff. The streams rest
conformably on the white sedimentary beds, but I nowhere saw the
actual junction; nor did I anywhere see the white beds actually
superimposed on the lava; but some way up the valley at the foot
of the uppermost escarpments, they must be thus superimposed.
Moreover, at the lowest point down the valley, where the streams
thin out and terminate in irregular projections, the spaces or
intervals between these projections are filled up to the level of
the now denuded and gravel-capped surfaces of the plains, with
the white-zoned sedimentary beds; proving that this matter
continued to be deposited after the streams had flowed. Hence we
may conclude that the basalt is contemporaneous with the upper
parts of the great tertiary formation.
The lava where first met with is 130 feet in thickness: it there
consists of two, three, or perhaps more streams, divided from
each other by vesicular spheroids like those on the surface. From
the streams having, as it appears, extended to different
distances, the terminal points are of unequal heights. Generally
the surface of the basalt is smooth them in one part high up the
valley, it was so uneven and hummocky, that until I afterwards
saw the streams extending continuously on both sides of the
valley up to a height of about three thousand feet close to the
Cordillera, I thought that the craters of eruption were probably
close at hand. This hummocky surface I believe to have been
caused by the crossing and heaping up of different streams. In
one place, there were several rounded ridges about twenty feet in
height, some of them as broad as high, and some broader, which
certainly had been formed whilst the lava was fluid, for in
transverse sections each ridge was seen to be concentrically
laminated, and to be composed of imperfect columns radiating from
common centres, like the spokes of wheels.
The basaltic mass where first met with is, as I have said, 130
feet in thickness, and, thirty-five miles higher up the valley,
it increases to 322 feet. In the first fourteen and a half miles
of this distance, the upper surface of the lava, judging from
three measurements taken above the level of the river (of which
the apparently very uniform inclination has been calculated from
its total height at a point 135 miles from the mouth), slopes
towards the Atlantic at an angle of only 0 degrees 7 minutes
twenty seconds: this must be considered only as an approximate
measurement, but it cannot be far wrong. Taking the whole
thirty-five miles, the upper surface slopes at an angle of 0
degrees 10 minutes 53 seconds; but this result is of no value in
showing the inclination of any one stream, for halfway between
the two points of measurement, the surface suddenly rises between
one hundred and two hundred feet, apparently caused by some of
the uppermost streams having extended thus far and no farther.
From the measurement made at these two points, thirty-five miles
apart, the mean inclination of the sedimentary beds, over which
the lava has flowed, is NOW (after elevation from under the sea)
only 0 degrees 7 minutes 52 seconds: for the sake of comparison,
it may be mentioned that the bottom of the present sea in a line
from the mouth of the S. Cruz to the Falkland Islands, from a
depth of seventeen fathoms to a depth of eighty-five fathoms,
declines at an angle of 0 degrees 1 minute 22 seconds; between
the beach and the depth of seventeen fathoms, the slope is
greater. From a point about half-way up the valley, the basaltic
mass rises more abruptly towards the foot of the Cordillera,
namely, from a height of 1,204 feet, to about 3,000 feet above
the sea.
This great deluge of lava is worthy, in its dimensions, of the
great continent to which it belongs. The aggregate streams have
flowed from the Cordillera to a distance (unparalleled, I
believe, in any case yet known) of about one hundred geographical
miles. Near their furthest extremity their total thickness is 130
feet, which increase thirty-five miles farther inland, as we have
just seen, to 322 feet. The least inclination given by M. E. de
Beaumont of the upper surface of a lava-stream, namely 0 degrees
30 minutes, is that of the great subaerial eruption in 1783 from
Skaptar Jukul in Iceland; and M. E. de Beaumont shows that it
must have flowed down a mean inclination of less than 0 degrees
20 minutes. ("Memoires pour servir" etc. pages 178 and 217.) But
we now see that under the pressure of the sea, successive streams
have flowed over a smooth bottom with a mean inclination of not
more than 0 degrees 7 minutes 52 seconds; and that the upper
surface of the terminal portion (over a space of fourteen and a
half miles) has an inclination of not more than 0 degrees 7
minutes 20 seconds. If the elevation of Patagonia has been
greater nearer the Cordillera than near the Atlantic (as is
probable), then these angles are now all too large. I must
repeat, that although the foregoing measurements, which were all
carefully taken with the barometer, may not be absolutely
correct, they cannot be widely erroneous.
Southward of the S. Cruz, the cliffs of the 840 feet plain extend
to Coy Inlet, and owing to the naked patches of the white
sediment, they are said on the charts to be "like the coast of
Kent." At Coy Inlet the high plain trends inland, leaving
flat-topped outliers. At Port Gallegos (latitude 51 degrees 35
minutes, and ninety miles south of S. Cruz), I am informed by
Captain Sulivan, R.N., that there is a gravel-capped plain from
two to three hundred feet in height, formed of numerous strata,
some fine-grained and pale-coloured, like the upper beds at the
mouth of the S. Cruz, others rather dark and coarser, so as to
resemble gritstones or tuffs; these latter include rather large
fragments of apparently decomposed volcanic rocks; there are,
also, included layers of gravel. This formation is highly
remarkable, from abounding with mammiferous remains, which have
not as yet been examined by Professor Owen, but which include
some large, but mostly small, species of Pachydermata, Edentata,
and Rodentia. From the appearance of the pale-coloured,
fine-grained beds, I was inclined to believe that they
corresponded with the upper beds of the S. Cruz; but Professor
Ehrenberg, who has examined some of the specimens, informs me
that the included microscopical organisms are wholly different,
being fresh and brackish-water forms. Hence the two to three
hundred feet plain at Port Gallegos is of unknown age, but
probably of subsequent origin to the great Patagonian tertiary
formation.
EASTERN TIERRA DEL FUEGO.
Judging from the height, the general appearance, and the white
colour of the patches visible on the hill sides, the uppermost
plain, both on the north and western side of the Strait of
Magellan, and along the eastern coast of Tierra del Fuego as far
south as near Port St. Polycarp, probably belongs to the great
Patagonian tertiary formation, These higher table- ranges are
fringed by low, irregular, extensive plains, belonging to the
boulder formation (Described in the "Geological Transactions"
volume 6 page 415.), and composed of coarse unstratified masses,
sometimes associated (as north of C. Virgin's) with fine,
laminated, muddy sandstones. The cliffs in Sebastian Bay are 200
feet in height, and are composed of fine sandstones, often in
curvilinear layers, including hard concretions of calcareous
sandstone, and layers of gravel. In these beds there are
fragments of wood, legs of crabs, barnacles encrusted with
corallines still partially retaining their colour, imperfect
fragments of a Pholas distinct from any known species, and of a
Venus, approaching very closely to, but slightly different in
form from, the V. lenticularis, a species living on the coast of
Chile. Leaves of trees are numerous between the laminae of the
muddy sandstone; they belong, as I am informed by Dr. J.D.
Hooker, to three species of deciduous beech, different from the
two species which compose the great proportion of trees in this
forest-clad land. ("Botany of the Antarctic Voyage" page 212.)
From these facts it is difficult to conjecture, whether we here
see the basal part of the great Patagonian formation, or some
later deposit.
A SUMMARY ON THE PATAGONIAN TERTIARY FORMATION.
Four out of the seven fossil shells, from St. Fe in Entre Rios,
were found by M. d'Orbigny in the sandstone of the Rio Negro, and
by me at San Josef. Three out of the six from San Josef are
identical with those from Port Desire and S. Julian, which two
places have together fifteen species, out of which three are
common to both. Santa Cruz has seventeen species, out of which
five are common to Port Desire and S. Julian. Considering the
difference in latitude between these several places, and the
small number of species altogether collected, namely thirty-six,
I conceive the above proportional number of species in common, is
sufficient to show that the lower fossiliferous mass belongs
nearly, I do not say absolutely, to the same epoch. What this
epoch may be, compared with the European tertiary stages, M.
d'Orbigny will not pretend to determine. The thirty-six species
(including those collected by myself and by M. d'Orbigny) are all
extinct, or at least unknown; but it should be borne in mind,
that the present coast consists of shingle, and that no one, I
believe, has dredged here for shells; hence it is not improbable
that some of the species may hereafter be found living. Some few
of the species are closely related with existing ones; this is
especially the case, according to M. d'Orbigny and Mr. Sowerby,
with the Fusus Patagonicus; and, according to Mr. Sowerby, with
the Pyrula, the Venus meridionalis, the Crepidula gregaria, and
the Turritella ambulacrum, and T. Patagonica. At least three of
the genera, namely, Cucullaea, Crassatella, and (as determined by
Mr. Sowerby) Struthiolaria, are not found in this quarter of the
world; and Trigonocelia is extinct. The evidence taken altogether
indicates that this great tertiary formation is of considerable
antiquity; but when treating of the Chilean beds, I shall have to
refer again to this subject.
The white pumiceous mudstone, with its abundant gypsum, belongs
to the same general epoch with the underlying fossiliferous mass,
as may be inferred from the shells included in the intercalated
layers at Nuevo Gulf, S. Julian, and S. Cruz. Out of the
twenty-seven marine microscopic structures found by Professor
Ehrenberg in the specimens from S. Julian and Port Desire, ten
are common to these two places: the three found at Nuevo Gulf are
distinct. I have minutely described this deposit, from its
remarkable characters and its wide extension. From Coy Inlet to
Port Desire, a distance of 230 miles, it is certainly continuous;
and I have reason to believe that it likewise extends to the Rio
Chupat, Nuevo Gulf, and San Josef, a distance of 570 miles: we
have, also, seen that a single layer occurs at the Rio Negro. At
Port S. Julian it is from eight to nine hundred feet in
thickness; and at S. Cruz it extends, with a slightly altered
character, up to the Cordillera. From its microscopic structure,
and from its analogy with other formations in volcanic districts,
it must be considered as originally of volcanic origin: it may
have been formed by the long-continued attrition of vast
quantities of pumice, or judging from the manner in which the
mass becomes, in ascending the valley of S. Cruz, divided into
variously coloured layers, from the long-continued eruption of
clouds of fine ashes. In either case, we must conclude, that the
southern volcanic orifices of the Cordillera, now in a dormant
state, were at about this period over a wide space, and for a
great length of time, in action. We have evidence of this fact,
in the latitude of the Rio Negro, in the sandstone-conglomerate
with pumice, and demonstrative proof of it, at S. Cruz, in the
vast deluges of basaltic lava: at this same tertiary period,
also, there is distinct evidence of volcanic action in Western
Banda Oriental.
The Patagonian tertiary formation extends continuously, judging
from fossils alone, from S. Cruz to near the Rio Colorado, a
distance of above six hundred miles, and reappears over a wide
area in Entre Rios and Banda Oriental, making a total distance of
1,100 miles; but this formation undoubtedly extends (though no
fossils were collected) far south of the S. Cruz, and, according
to M. d'Orbigny, 120 miles north of St. Fe. At S. Cruz we have
seen that it extends across the continent; being on the coast
about eight hundred feet in thickness (and rather more at S.
Julian), and rising with the contemporaneous lava-streams to a
height of about three thousand feet at the base of the
Cordillera. It rests, wherever any underlying formation can be
seen, on plutonic and metamorphic rocks. Including the newer
Pampean deposit, and those strata in Eastern Tierra del Fuego of
doubtful age, as well as the boulder formation, we have a line of
more than twenty-seven degrees of latitude, equal to that from
the Straits of Gibraltar to the south of Iceland, continuously
composed of tertiary formations. Throughout this great space the
land has been upraised, without the strata having been in a
single instance, as far as my means of observation went,
unequally tilted or dislocated by a fault.
TERTIARY FORMATIONS ON THE WEST COAST.
CHONOS ARCHIPELAGO.
The numerous islands of this group, with the exception of Lemus,
Ypun, consist of metamorphic schists; these two islands are
formed of softish grey and brown, fusible, often laminated
sandstones, containing a few pebbles, fragments of black lignite,
and numerous mammillated concretions of hard calcareous
sandstone. Out of these concretions at Ypun (latitude 40 degrees
30 minutes S.), I extracted the four following extinct species of
shells:—
1. Turritella suturalis, G.B. Sowerby (also Navidad). 2.
Sigaretus subglobosus, G.B. Sowerby (also Navidad). 3. Cytheraea
(?) sulculosa (?), G.B. Sowerby (also Chiloe and Huafo?). 4.
Voluta, fragments of.
In the northern parts of this group there are some cliffs of
gravel and of the boulder formation. In the southern part (at P.
Andres in Tres Montes), there is a volcanic formation, probably
of tertiary origin. The lavas attain a thickness of from two to
three hundred feet; they are extremely variable in colour and
nature, being compact, or brecciated, or cellular, or
amygdaloidal with zeolite, agate and bole, or porphyritic with
glassy albitic feldspar. There is also much imperfect rubbly
pitchstone, with the interstices charged with powdery carbonate
of lime apparently of contemporaneous origin. These lavas are
conformably associated with strata of breccia and of brown tuff
containing lignite. The whole mass has been broken up and tilted
at an angle of 45 degrees, by a series of great volcanic dikes,
one of which was thirty yards in breadth. This volcanic formation
resembles one, presently to be described, in Chiloe.
HUAFO.
This island lies between the Chonos and Chiloe groups: it is
about eight hundred feet high, and perhaps has a nucleus of
metamorphic rocks. The strata which I examined consisted of
fine-grained muddy sandstones, with fragments of lignite and
concretions of calcareous sandstone. I collected the following
extinct shells, of which the Turritella was in great numbers:—
1. Bulla cosmophila, G.B. Sowerby. 2. Pleurotoma subaequalis,
G.B. Sowerby. 3. Fusus cleryanus, d'Orbigny, "Voyage Pal." (also
at Coquimbo). 4. Triton leucostomoides, G.B. Sowerby. 5.
Turritella Chilensis, G.B. Sowerby (also Mocha). 6. Venus,
probably a distinct species, but very imperfect. 7. Cytheraea (?)
sulculosa (?), probably a distinct species, but very imperfect.
8. Dentalium majus, G.B. Sowerby.
CHILOE.
This fine island is about one hundred miles in length. The entire
southern part, and the whole western coast, consists of
mica-schist, which likewise is seen in the ravines of the
interior. The central mountains rise to a height of 3,000 feet,
and are said to be partly formed of granite and greenstone: there
are two small volcanic districts. The eastern coast, and large
parts of the northern extremity of the island are composed of
gravel, the boulder formation, and underlying horizontal strata.
The latter are well displayed for twenty miles north and south of
Castro; they vary in character from common sandstone to
fine-grained, laminated mudstones: all the specimens which I
examined are easily fusible, and some of the beds might be called
volcanic grit-stones. These latter strata are perhaps related to
a mass of columnar trachyte which occurs behind Castro. The
sandstone occasionally includes pebbles, and many fragments and
layers of lignite; of the latter, some are apparently formed of
wood and others of leaves: one layer on the N.W. side of Lemuy is
nearly two feet in thickness. There is also much silicified wood,
both common dicotyledonous and coniferous: a section of one
specimen in the direction of the medullary rays has, as I am
informed by Mr. R. Brown, the discs in a double row placed
alternately, and not opposite as in the true Araucaria. I found
marine remains only in one spot, in some concretions of hard
calcareous sandstone: in several other districts I have observed
that organic remains were exclusively confined to such
concretions; are we to account for this fact, by the supposition
that the shells lived only at these points, or is it not more
probable that their remains were preserved only where concretions
were formed? The shells here are in a bad state, they consist
of:—
1. Tellinides (?) oblonga, G.B. Sowerby (a solenella in M.
d'Orbigny's opinion). 2. Natica striolata, G.B. Sowerby. 3.
Natica (?) pumila, G.B. Sowerby. 4. Cytheraea (?) sulculosa, G.B.
Sowerby (also Ypun and Huafo?).
At the northern extremity of the island, near S. Carlos, there is
a large volcanic formation, between five and seven hundred feet
in thickness. The commonest lava is blackish-grey or brown,
either vesicular, or amygdaloidal with calcareous spar and bole:
most even of the darkest varieties fuse into a pale-coloured
glass. The next commonest variety is a rubbly, rarely well
characterised pitchstone (fusing into a white glass) which passes
in the most irregular manner into stony grey lavas. This
pitchstone, as well as some purple claystone porphyry, certainly
flowed in the form of streams. These various lavas often pass, at
a considerable depth from the surface, in the most abrupt and
singular manner into wacke. Great masses of the solid rock are
brecciated, and it was generally impossible to discover whether
the recementing process had been an igneous or aqueous action.
(In a cliff of the hardest fragmentary mass, I found several
tortuous, vertical veins, varying in thickness from a few tenths
of an inch to one inch and a half, of a substance which I have
not seen described. It is glossy, and of a brown colour; it is
thinly laminated, with the laminae transparent and elastic; it is
a little harder than calcareous spar; it is infusible under the
blowpipe, sometimes decrepitates, gives out water, curls up,
blackens, and becomes magnetic. Borax easily dissolves a
considerable quantity of it, and gives a glass tinged with green.
I have no idea what its true nature is. On first seeing it, I
mistook it for lignite!) The beds are obscurely separated from
each other; they are sometimes parted by seams of tuff and layers
of pebbles. In one place they rested on, and in another place
were capped by, tuffs and girt-stones, apparently of submarine
origin.
The neighbouring peninsula of Lacuy is almost wholly formed of
tufaceous deposits, connected probably in their origin with the
volcanic hills just described. The tuffs are pale-coloured,
alternating with laminated mudstones and sandstones (all easily
fusible), and passing sometimes into fine-grained white beds
strikingly resembling the great upper infusorial deposit of
Patagonia, and sometimes into brecciolas with pieces of pumice in
the last stage of decay; these again pass into ordinary coarse
breccias and conglomerates of hard rocks. Within very short
distances, some of the finer tuffs often passed into each other
in a peculiar manner, namely, by irregular polygonal concretions
of one variety increasing so much and so suddenly in size, that
the second variety, instead of any longer forming the entire
mass, was left merely in thin veins between the concretions. In a
straight line of cliffs, at Point Tenuy, I examined the following
remarkable section (Figure 19):—
(FIGURE 19.)
On the left hand, the lower part (AA) consists of regular,
alternating strata of brown tuffs and greenish laminated
mudstone, gently inclined to the right, and conformably covered
by a mass (B left) of a white, tufaceous and brecciolated
deposit. On the right hand, the whole cliff (BB right) consists
of the same white tufaceous matter, which on this side presents
scarcely a trace of stratification, but to the left becomes very
gradually and rather indistinctly divided into strata quite
conformable with the underlying beds (AA): moreover, a few
hundred yards further to the left, where the surface has been
less denuded, the tufaceous strata (B left) are conformably
covered by another set of strata, like the underlying ones (AA)
of this section. In the middle of the diagram, the beds (AA) are
seen to be abruptly cut off, and to abut against the tufaceous
non-stratified mass; but the line of junction has been
accidentally not represented steep enough, for I particularly
noticed that before the beds had been tilted to the right, this
line must have been nearly vertical. It appears that a current of
water cut for itself a deep and steep submarine channel, and at
the same time or afterwards filled it up with the tufaceous and
brecciolated matter, and spread the same over the surrounding
submarine beds; the matter becoming stratified in these more
distant and less troubled parts, and being moreover subsequently
covered up by other strata (like AA) not shown in the diagram. It
is singular that three of the beds (of AA) are prolonged in their
proper direction, as represented, beyond the line of junction
into the white tufaceous matter: the prolonged portions of two of
the beds are rounded; in the third, the terminal fragment has
been pushed upwards: how these beds could have been left thus
prolonged, I will not pretend to explain. In another section on
the opposite side of a promontory, there was at the foot of this
same line of junction, that is at the bottom of the old submarine
channel, a pile of fragments of the strata (AA), with their
interstices filled up with white tufaceous matter: this is
exactly what might have been anticipated under such
circumstances.
(FIGURE 20. GROUND PLAN SHOWING THE RELATION BETWEEN VEINS AND
CONCRETIONARY ZONES IN A MASS OF TUFF.)
The various tufaceous and other beds at this northern end of
Chiloe probably belong to about the same age with those near
Castro, and they contain, as there, many fragments of black
lignite and of silicified and pyritous wood, often embedded close
together. They also contain many and singular concretions: some
are of hard calcareous sandstone, in which it would appear that
broken volcanic crystals and scales of mica have been better
preserved (as in the case of the organic remains near Castro)
than in the surrounding mass. Other concretions in the white
brecciola are of a hard, ferruginous, yet fusible, nature; they
are as round as cannon-balls, and vary from two or three inches
to two feet in diameter; their insides generally consist either
of fine, scarcely coherent volcanic sand (The frequent tendency
in iron to form hollow concretions or shell containing incoherent
matter is singular; D'Aubuisson ("Traite de Geogn." tome 1 page
318) remarks on this circumstance.), or of an argillaceous tuff;
in this latter case, the external crust was quite thin and hard.
Some of these spherical balls were encircled in the line of their
equators, by a necklace-like row of smaller concretions. Again
there were other concretions, irregularly formed, and composed of
a hard, compact, ash- coloured stone, with an almost porcelainous
fracture, adhesive to the tongue, and without any calcareous
matter. These beds are, also, interlaced by many veins,
containing gypsum, ferruginous matter, calcareous spar, and
agate. It was here seen with remarkable distinctness, how
intimately concretionary action and the production of fissures
and veins are related together. Figure 20 is an accurate
representation of a horizontal space of tuff, about four feet
long by two and a half in width: the double lines represent the
fissures partially filled with oxide of iron and agate: the
curvilinear lines show the course of the innumerable, concentric,
concretionary zones of different shades of colour and of
coarseness in the particles of tuff. The symmetry and complexity
of the arrangement gave the surface an elegant appearance. It may
be seen how obviously the fissures determine (or have been
determined by) the shape, sometimes of the whole concretion, and
sometimes only of its central parts. The fissures also determine
the curvatures of the long undulating zones of concretionary
action. From the varying composition of the veins and
concretions, the amount of chemical action which the mass has
undergone is surprisingly great; and it would likewise appear
from the difference in size in the particles of the concretionary
zones, that the mass, also, has been subjected to internal
mechanical movements.
In the peninsula of Lacuy, the strata over a width of four miles
have been upheaved by three distinct, and some other indistinct,
lines of elevation, ranging within a point of north and south.
One line, about two hundred feet in height, is regularly
anticlinal, with the strata dipping away on both sides, at an
angle of 15 degrees, from a central "valley of elevation," about
three hundred yards in width. A second narrow steep ridge, only
sixty feet high, is uniclinal, the strata throughout dipping
westward; those on both flanks being inclined at an angle of from
ten to fifteen degrees; whilst those on the ridge dip in the same
direction at an angle of between thirty and forty degrees. This
ridge, traced northwards, dies away; and the beds at its terminal
point, instead of dipping westward, are inclined 12 degrees to
the north. This case interested me, as being the first in which I
found in South America, formations perhaps of tertiary origin,
broken by lines of elevation.
VALDIVIA: ISLAND OF MOCHA.
The formations of Chiloe seem to extend with nearly the same
character to Valdivia, and for some leagues northward of it: the
underlying rocks are micaceous schists, and are covered up with
sandstone and other sedimentary beds, including, as I was
assured, in many places layers of lignite. I did not land on
Mocha (latitude 38 degrees 20 minutes), but Mr. Stokes brought me
specimens of the grey, fine-grained, slightly calcareous
sandstone, precisely like that of Huafo, containing lignite and
numerous Turritellae. The island is flat topped, 1,240 feet in
height, and appears like an outlier of the sedimentary beds on
the mainland. The few shells collected consist of:—
1. Turritella Chilensis, G.B. Sowerby (also at Huafo). 2. Fusus,
very imperfect, somewhat resembling F. subreflexus of Navidad,
but probably different. 3. Venus, fragments of.
CONCEPCION.
Sailing northward from Valdivia, the coast-cliffs are seen, first
to assume near the R. Tolten, and thence for 150 miles northward,
to be continued with the same mineralogical characters,
immediately to be described at Concepcion. I heard in many places
of beds of lignite, some of it fine and glossy, and likewise of
silicified wood; near the Tolten the cliffs are low, but they
soon rise in height; and the horizontal strata are prolonged,
with a nearly level surface, until coming to a more lofty tract
between points Rumena and Lavapie. Here the beds have been broken
up by at least eight or nine parallel lines of elevation, ranging
E. or E.N.E. and W. or W.S.W. These lines can be followed with
the eye many miles into the interior; they are all uniclinal, the
strata in each dipping to a point between S. and S.S.E. with an
inclination in the central lines of about forty degrees, and in
the outer ones of under twenty degrees. This band of
symmetrically troubled country is about eight miles in width.
The island of Quiriquina, in the Bay of Concepcion, is formed of
various soft and often ferruginous sandstones, with bands of
pebbles, and with the lower strata sometimes passing into a
conglomerate resting on the underlying metamorphic schists. These
beds include subordinate layers of greenish impure clay, soft
micaceous and calcareous sandstones, and reddish friable earthy
matter with white specks like decomposed crystals of feldspar;
they include, also, hard concretions, fragments of shells,
lignite, and silicified wood. In the upper part they pass into
white, soft sediments and brecciolas, very like those described
at Chiloe; as indeed is the whole formation. At Lirguen and other
places on the eastern side of the bay, there are good sections of
the lower sandstones, which are generally ferruginous, but which
vary in character, and even pass into an argillaceous nature;
they contain hard concretions, fragments of lignite, silicified
wood, and pebbles (of the same rocks with the pebbles in the
sandstones of Quiriquina), and they alternate with numerous,
often very thin layers of imperfect coal, generally of little
specific gravity. The main bed here is three feet thick; and only
the coal of this one bed has a glossy fracture. Another
irregular, curvilinear bed of brown, compact lignite, is
remarkable for being included in a mass of coarse gravel. These
imperfect coals, when placed in a heap, ignite spontaneously. The
cliffs on this side of the bay, as well as on the island of
Quiriquina, are capped with red friable earth, which, as stated
in the Second Chapter, is of recent formation. The stratification
in this neighbourhood is generally horizontal; but near Lirguen
the beds dip N.W. at an angle of 23 degrees; near Concepcion they
are also inclined: at the northern end of Quiriquina they have
been tilted at an angle of 30 degrees, and at the southern end at
angles varying from 15 degrees to 40 degrees: these dislocations
must have taken place under the sea.
A collection of shells, from the island of Quiriquina, has been
described by M. d'Orbigny: they are all extinct, and from their
generic character, M. d'Orbigny inferred that they were of
tertiary origin: they consist of:—
1. Scalaria Chilensis, d'Orbigny, "Voyage, Part Pal." 2. Natica
Araucana, d'Orbigny, "Voyage, Part Pal." 3. Natica australis,
d'Orbigny, "Voyage, Part Pal." 4. Fusus difficilis, d'Orbigny,
"Voyage, Part Pal." 5. Pyrula longirostra, d'Orbigny, "Voyage,
Part Pal." 6. Pleurotoma Araucana, d'Orbigny, "Voyage, Part Pal."
7. Cardium auca, d'Orbigny, "Voyage, Part Pal." 8. Cardium
acuticostatum, d'Orbigny, "Voyage, Part Pal." 9. Venus auca,
d'Orbigny, "Voyage, Part Pal." 10. Mactra cecileana, d'Orbigny,
"Voyage, Part Pal." 11. Mactra Araucana, d'Orbigny, "Voyage, Part
Pal." 12. Arca Araucana, d'Orbigny, "Voyage, Part Pal." 13.
Nucula Largillierti, d'Orbigny, "Voyage, Part Pal." 14. Trigonia
Hanetiana, d'Orbigny, "Voyage, Part Pal."
During a second visit of the "Beagle" to Concepcion, Mr. Kent
collected for me some silicified wood and shells out of the
concretions in the sandstone from Tome, situated a short distance
north of Lirguen. They consist of:—
1. Natica australis, d'Orbigny, "Voyage, Part Pal." 2. Mactra
Araucana, d'Orbigny, "Voyage, Part Pal." 3. Trigonia Hanetiana,
d'Orbigny, "Voyage, Part Pal." 4. Pecten, fragments of, probably
two species, but too imperfect for description. 5. Baculites
vagina, E. Forbes. 6. Nautilus d'Orbignyanus, E. Forbes.
Besides these shells, Captain Belcher found here an Ammonite,
nearly three feet in diameter, and so heavy that he could not
bring it away; fragments are deposited at Haslar Hospital: he
also found the silicified vertebrae of some very large animal.
("Zoology of Captain Beechey's Voyage" page 163.) From the
identity in mineralogical nature of the rocks, and from Captain
Belcher's minute description of the coast between Lirguen and
Tome, the fossiliferous concretions at this latter place
certainly belong to the same formation with the beds examined by
myself at Lirguen; and these again are undoubtedly the same with
the strata of Quiriquina; moreover; the three first of the shells
from Tome, though associated in the same concretions with the
Baculite, are identical with the species from Quiriquina. Hence
all the sandstone and lignitiferous beds in this neighbourhood
certainly belong to the same formation. Although the generic
character of the Quiriquina fossils naturally led M. d'Orbigny to
conceive that they were of tertiary origin, yet as we now find
them associated with the Baculites vagina and with an Ammonite,
we must, in the opinion of M. d'Orbigny, and if we are guided by
the analogy of the northern hemisphere, rank them in the
Cretaceous system. Moreover, the Baculites vagina, which is in a
tolerable state of preservation, appears to Professor E. Forbes
certainly to be identical with a species, so named by him, from
Pondicherry in India; where it is associated with numerous
decidedly cretaceous species, which approach most nearly to Lower
Greensand or Neocomian forms: this fact, considering the vast
distance between Chile and India, is truly surprising. Again, the
Nautilus d'Orbignyanus, as far as its imperfect state allows of
comparison, resembles, as I am informed by Professor Forbes, both
in its general form and in that of its chambers, two species from
the Upper Greensand. It may be added that every one of the
above-named genera from Quiriquina, which have an apparently
tertiary character, are found in the Pondicherry strata. There
are, however, some difficulties on this view of the formations at
Concepcion being cretaceous, which I shall afterwards allude to;
and I will here only state that the Cardium auca is found also at
Coquimbo, the beds at which place, there can be no doubt, are
tertiary.
NAVIDAD. (I was guided to this locality by the Report on M. Gay's
"Geological Researches" in the "Annales des Scienc. Nat." 1st
series tome 28.)
The Concepcion formation extends some distance northward, but how
far I know not; for the next point at which I landed was at
Navidad, 160 miles north of Concepcion, and 60 miles south of
Valparaiso. The cliffs here are about eight hundred feet in
height: they consist, wherever I could examine them, of
fine-grained, yellowish, earthy sandstones, with ferruginous
veins, and with concretions of hard calcareous sandstone. In one
part, there were many pebbles of the common metamorphic
porphyries of the Cordillera: and near the base of the cliff, I
observed a single rounded boulder of greenstone, nearly a yard in
diameter. I traced this sandstone formation beneath the
superficial covering of gravel, for some distance inland: the
strata are slightly inclined from the sea towards the Cordillera,
which apparently has been caused by their having been accumulated
against or round outlying masses of granite, of which some points
project near the coast. The sandstone contains fragments of wood,
either in the state of lignite or partially silicified, sharks'
teeth, and shells in great abundance, both high up and low down
the sea-cliffs. Pectunculus and Oliva were most numerous in
individuals, and next to them Turritella and Fusus. I collected
in a short time, though suffering from illness, the following
thirty-one species, all of which are extinct, and several of the
genera do not now range (as we shall hereafter show) nearly so
far south:—
1. Gastridium cepa, G.B. Sowerby. 2. Monoceros, fragments of,
considered by M. d'Orbigny as a new species. 3. Voluta alta, G.B.
Sowerby (considered by M. d'Orbigny as distinct from the V. alta
of Santa Cruz). 4. Voluta triplicata, G.B. Sowerby. 5. Oliva
dimidiata, G.B. Sowerby. 6. Pleurotoma discors, G.B. Sowerby. 7.
Pleurotoma turbinelloides, G.B. Sowerby. 8. Fusus subreflexus,
G.B. Sowerby. 9. Fusus pyruliformis, G.B. Sowerby. 10. Fusus,
allied to F. regularis (considered by M. d'Orbigny as a distinct
species). 11. Turritella suturalis, G.B. Sowerby. 12. Turritella
Patagonica, G.B. Sowerby (fragments of). 13. Trochus laevis, G.B.
Sowerby. 14. Trochus collaris, G.B. Sowerby (considered by M.
d'Orbigny as the young of the T. laevis). 15. Cassis monilifer,
G.B. Sowerby. 16. Pyrula distans, G.B. Sowerby. 17. Triton
verruculosus, G.B. Sowerby. 18. Sigaretus subglobosus, G.B.
Sowerby. 19. Natica solida, G.B. Sowerby. (It is doubtful whether
the Natica solida of S. Cruz is the same species with this.) 20.
Terebra undulifera, G.B. Sowerby. 21. Terebra costellata, G.B.
Sowerby. 22. Bulla (fragments of). 23. Dentalium giganteum, do.
24. Dentalium sulcosum, do. 25. Corbis (?) laevigata, do. 26.
Cardium multiradiatum, do. 27. Venus meridionalis, do. 28.
Pectunculus dispar, (?) Desh. (considered by M. d'Orbigny as a
distinct species). 29, 30. Cytheraea and Mactra, fragments of
(considered by M. d'Orbigny as new species). 31. Pecten,
fragments of.
COQUIMBO.
(FIGURE 21. SECTION OF THE TERTIARY FORMATION AT COQUIMBO.
From Level of Sea to Surface of plain, 252 feet above sea,
through levels F, E, D and C:
F.—Lower sandstone, with concretions and silicified bones, with
fossil shells, all, or nearly all, extinct.
E.—Upper ferruginous sandstone, with numerous Balani, with fossil
shells, all, or nearly all, extinct.
C and D.—Calcareous beds with recent shells.
A.—Stratified sand in a ravine, also with recent shells.)
For more than two hundred miles northward of Navidad, the coast
consists of plutonic and metamorphic rocks, with the exception of
some quite insignificant superficial beds of recent origin. At
Tonguay, twenty-five miles south of Coquimbo, tertiary beds
recommence. I have already minutely described in the Second
Chapter, the step-formed plains of Coquimbo, and the upper
calcareous beds (from twenty to thirty feet in thickness)
containing shells of recent species, but in different proportions
from those on the beach. There remains to be described only the
underlying ancient tertiary beds, represented in Figure 21 by the
letters F and E:—
I obtained good sections of bed F only in Herradura Bay: it
consists of soft whitish sandstone, with ferruginous veins, some
pebbles of granite, and concretionary layers of hard calcareous
sandstone. These concretions are remarkable from the great number
of large silicified bones, apparently of cetaceous animals, which
they contain; and likewise of a shark's teeth, closely resembling
those of the Carcharias megalodon. Shells of the following
species, of which the gigantic Oyster and Perna are the most
conspicuous, are numerously embedded in the concretions:—
1. Bulla ambigua, d'Orbigny "Voyage" Pal. 2. Monoceros
Blainvillii, d'Orbigny "Voyage" Pal. 3. Cardium auca, d'Orbigny
"Voyage" Pal. 4. Panopaea Coquimbensis, d'Orbigny "Voyage" Pal.
5. Perna Gaudichaudi, d'Orbigny "Voyage" Pal. 6. Artemis
ponderosa; Mr. Sowerby can find no distinguishing character
between this fossil and the recent A. ponderosa; it is certainly
an Artemis, as shown by the pallial impression. 7. Ostrea
Patagonica (?); Mr. Sowerby can point out no distinguishing
character between this species and that so eminently
characteristic of the great Patagonian formation; but he will not
pretend to affirm that they are identical. 8. Fragments of a
Venus and Natica.
The cliffs on one side of Herradura Bay are capped by a mass of
stratified shingle, containing a little calcareous matter, and I
did not doubt that it belonged to the same recent formation with
the gravel on the surrounding plains, also cemented by calcareous
matter, until to my surprise, I found in the midst of it, a
single thin layer almost entirely composed of the above gigantic
oyster.
At a little distance inland, I obtained several sections of the
bed E, which, though different in appearance from the lower bed
F, belongs to the same formation: it consists of a highly
ferruginous sandy mass, almost composed, like the lowest bed at
Port S. Julian, of fragments of Balanidae; it includes some
pebbles, and layers of yellowish-brown mudstone. The embedded
shells consist of:—
1. Monoceros Blainvillii, d'Orbigny "Voyage" Pal. 2. Monoceros
ambiguus, G.B. Sowerby. 3. Anomia alternans, G.B. Sowerby. 4.
Pecten rudis, G.B. Sowerby. 5. Perna Gaudichaudi, d'Orbigny
"Voyage" Pal. 6. Ostrea Patagonica (?), d'Orbigny "Voyage" Pal.
7. Ostrea, small species, in imperfect state; it appeared to me
like a small kind now living in, but very rare in the bay. 8.
Mytilus Chiloensis; Mr. Sowerby can find no distinguishing
character between this fossil, as far as its not very perfect
condition allows of comparison, and the recent species. 9.
Balanus Coquimbensis, G.B. Sowerby. 10. Balanus psittacus? King.
This appears to Mr. Sowerby and myself identical with a very
large and common species now living on the coast.
The uppermost layers of this ferrugino-sandy mass are conformably
covered by, and impregnated to the depth of several inches with,
the calcareous matter of the bed D called losa: hence I at one
time imagined that there was a gradual passage between them; but
as all the species are recent in the bed D, whilst the most
characteristic shells of the uppermost layers of E are the
extinct Perna, Pecten, and Monoceros, I agree with M. d'Orbigny,
that this view is erroneous, and that there is only a
mineralogical passage between them, and no gradual transition in
the nature of their organic remains. Besides the fourteen species
enumerated from these two lower beds, M. d'Orbigny has described
ten other species given to him from this locality; namely:—
1. Fusus Cleryanus, d'Orbigny "Voyage" Pal. 2. Fusus petitianus,
d'Orbigny "Voyage" Pal. 3. Venus hanetiana, d'Orbigny "Voyage"
Pal. 4. Venus incerta (?) d'Orbigny "Voyage" Pal. 5. Venus
Cleryana, d'Orbigny "Voyage" Pal. 6. Venus petitiana, d'Orbigny
"Voyage" Pal. 7. Venus Chilensis, d'Orbigny "Voyage" Pal. 8.
Solecurtus hanetianus, d'Orbigny "Voyage" Pal. 9. Mactra auca,
d'Orbigny "Voyage" Pal. 10. Oliva serena, d'Orbigny "Voyage" Pal.
Of these twenty-four shells, all are extinct, except, according
to Mr. Sowerby, the Artemis ponderosa, Mytilus Chiloensis, and
probably the great Balanus.
COQUIMBO TO COPIAPO.
A few miles north of Coquimbo, I met with the ferruginous,
balaniferous mass E with many silicified bones; I was informed
that these silicified bones occur also at Tonguay, south of
Coquimbo: their number is certainly remarkable, and they seem to
take the place of the silicified wood, so common on the
coast-formations of Southern Chile. In the valley of Chaneral, I
again saw this same formation, capped with the recent calcareous
beds. I here left the coast, and did not see any more of the
tertiary formations, until descending to the sea at Copiapo: here
in one place I found variously coloured layers of sand and soft
sandstone, with seams of gypsum, and in another place, a
comminuted shelly mass, with layers of rotten-stone and seams of
gypsum, including many of the extinct gigantic oyster: beds with
these oysters are said to occur at English Harbour, a few miles
north of Copiapo.
COAST OF PERU.
With the exception of deposits containing recent shells and of
quite insignificant dimensions, no tertiary formations have been
observed on this coast, for a space of twenty-two degrees of
latitude north of Copiapo, until coming to Payta, where there is
said to be a considerable calcareous deposit: a few fossils have
been described by M. d'Orbigny from this place, namely:—
1. Rostellaria Gaudichaudi, d'Orbigny "Voyage" Pal. 2.
Pectunculus Paytensis, d'Orbigny "Voyage" Pal. 3. Venus
petitiana, d'Orbigny "Voyage" Pal. 4. Ostrea Patagonica? This
great oyster (of which specimens have been given me) cannot be
distinguished by Mr. Sowerby from some of the varieties from
Patagonia; though it would be hazardous to assert it is the same
with that species, or with that from Coquimbo.
CONCLUDING REMARKS.
The formations described in this chapter, have, in the case of
Chiloe and probably in that of Concepcion and Navidad, apparently
been accumulated in troughs formed by submarine ridges extending
parallel to the ancient shores of the continent; in the case of
the islands of Mocha and Huafo it is highly probable, and in that
of Ypun and Lemus almost certain, that they were accumulated
round isolated rocky centres or nuclei, in the same manner as mud
and sand are now collecting round the outlying islets and reefs
in the West Indian Archipelago. Hence, I may remark, it does not
follow that the outlying tertiary masses of Mocha and Huafo were
ever continuously united at the same level with the formations on
the mainland, though they may have been of contemporaneous
origin, and been subsequently upraised to the same height. In the
more northern parts of Chile, the tertiary strata seem to have
been separately accumulated in bays, now forming the mouths of
valleys.
The relation between these several deposits on the shores of the
Pacific, is not nearly so clear as in the case of the tertiary
formations on the Atlantic. Judging from the form and height of
the land (evidence which I feel sure is here much more
trustworthy than it can ever be in such broken continents as that
of Europe), from the identity of mineralogical composition, from
the presence of fragments of lignite and of silicified wood, and
from the intercalated layers of imperfect coal, I must believe
that the coast-formations from Central Chiloe to Concepcion, a
distance of 400 miles, are of the same age: from nearly similar
reasons, I suspect that the beds of Mocha, Huafo, and Ypun,
belong also to the same period. The commonest shell in Mocha and
Huafo is the same species of Turritella; and I believe the same
Cytheraea is found on the islands of Huafo, Chiloe, and Ypun; but
with these trifling exceptions, the few organic remains found at
these places are distinct. The numerous shells from Navidad, with
the exception of two, namely, the Sigaretus and Turritella found
at Ypun, are likewise distinct from those found in any other part
of this coast. Coquimbo has Cardium auca in common with
Concepcion, and Fusus Cleryanus with Huafo; I may add, that
Coquimbo has Venus petitiana, and a gigantic oyster (said by M.
d'Orbigny also to be found a little south of Concepcion) in
common with Payta, though this latter place is situated
twenty-two degrees northward of latitude 27 degrees, to which
point the Coquimbo formation extends.
From these facts, and from the generic resemblance of the fossils
from the different localities, I cannot avoid the suspicion that
they all belong to nearly the same epoch, which epoch, as we
shall immediately see, must be a very ancient tertiary one. But
as the Baculite, especially considering its apparent identity
with the Cretaceous Pondicherry species, and the presence of an
Ammonite, and the resemblance of the Nautilus to two upper
greensand species, together afford very strong evidence that the
formation of Concepcion is a Secondary one; I will, in my remarks
on the fossils from the other localities, put on one side those
from Concepcion and from Eastern Chiloe, which, whatever their
age may be, appear to me to belong to one group. I must, however,
again call attention to the fact that the Cardium auca is found
both at Concepcion and in the undoubtedly tertiary strata of
Coquimbo: nor should the possibility be overlooked, that as
Trigonia, though known in the northern hemisphere only as a
Secondary genus, has living representatives in the Australian
seas, so a Baculite, Ammonite, and Trigonia may have survived in
this remote part of the southern ocean to a somewhat later period
than to the north of the equator.
Before passing in review the fossils from the other localities,
there are two points, with respect to the formations between
Concepcion and Chiloe, which deserve some notice. First, that
though the strata are generally horizontal, they have been
upheaved in Chiloe in a set of parallel anticlinal and uniclinal
lines ranging north and south,—in the district near P. Rumena by
eight or nine far-extended, most symmetrical, uniclinal lines
ranging nearly east and west,—and in the neighbourhood of
Concepcion by less regular single lines, directed both N.E. and
S.W., and N.W. and S.E. This fact is of some interest, as showing
that within a period which cannot be considered as very ancient
in relation to the history of the continent, the strata between
the Cordillera and the Pacific have been broken up in the same
variously directed manner as have the old plutonic and
metamorphic rocks in this same district. The second point is,
that the sandstone between Concepcion and Southern Chiloe is
everywhere lignitiferous, and includes much silicified wood;
whereas the formations in Northern Chile do not include beds of
lignite or coal, and in place of the fragments of silicified wood
there are silicified bones. Now, at the present day, from Cape
Horn to near Concepcion, the land is entirely concealed by
forests, which thin out at Concepcion, and in Central and
Northern Chile entirely disappear. This coincidence in the
distribution of the fossil wood and the living forests may be
quite accidental; but I incline to take a different view of it;
for, as the difference in climate, on which the presence of
forests depends, is here obviously in chief part due to the form
of the land, and as the Cordillera undoubtedly existed when the
lignitiferous beds were accumulating, I conceive it is not
improbable that the climate, during the lignitiferous period,
varied on different parts of the coast in a somewhat similar
manner as it now does. Looking to an earlier epoch, when the
strata of the Cordillera were depositing, there were islands
which even in the latitude of Northern Chile, where now all is
irreclaimably desert, supported large coniferous forests.
TABLE 4.
Column 1. Genera, with living and tertiary species on the west
coast of South America. (M. d'Orbigny states that the genus
Natica is not found on the coast of Chile; but Mr. Cuming found
it at Valparaiso. Scalaria was found at Valparaiso; Arca, at
Iquique, in latitude 20, by Mr. Cuming; Arca, also, was found by
Captain King, at Juan Fernandez, in latitude 33 degrees 30'S.)
Column 2. Latitudes, in which found fossil on the coasts of Chile
and Peru. (In degrees and minutes.)
Column 3. Southernmost latitude, in which found living on the
west coast of South America. (In degrees and minutes.)
Bulla : 30 to 43 30 : 12 near Lima.
Cassis : 34 : 1 37.
Pyrula : 34 (and 36 30 at Concepcion) : 5 Payta.
Fusus : 30 and 43 30 : 23 Mexillones; reappears at the St. of
Magellan.
Pleurotoma : 34 to 43 30 : 2 18 St. Elena.
Terebra : 34 : 5 Payta.
Sigaretus : 34 to 44 30 : 12 Lima.
Anomia : 30 : 7 48.
Perna : 30 : 1 23 Xixappa.
Cardium : 30 to 34 (and 36 30 at Concepcion) : 5 Payta.
Artemis : 30 : 5 Payta.
Voluta : 34 to 44 30 : Mr. Cuming does not know of any species
living on the west coast, between the equator and latitude 43
south; from this latitude a species is found as far south as
Tierra del Fuego.
Seventy-nine species of fossil shells, in a tolerably
recognisable condition, from the coast of Chile and Peru, are
described in this volume, and in the Palaeontological part of M.
d'Orbigny's "Voyage": if we put on one side the twenty species
exclusively found at Concepcion and Chiloe, fifty-nine species
from Navidad and the other specified localities remain. Of these
fifty-nine species only an Artemis, a Mytilus and Balanus, all
from Coquimbo, are (in the opinion of Mr. Sowerby, but not in
that of M. d'Orbigny) identical with living shells; and it would
certainly require a better series of specimens to render this
conclusion certain. Only the Turritella Chilensis from Huafo and
Mocha, the T. Patagonica and Venus meridionalis from Navidad,
come very near to recent South American shells, namely, the two
Turritellas to T. cingulata, and the Venus to V. exalbida: some
few other species come rather less near; and some few resemble
forms in the older European tertiary deposits: none of the
species resemble secondary forms. Hence I conceive there can be
no doubt that these formations are tertiary,—a point necessary to
consider, after the case of Concepcion. The fifty-nine species
belong to thirty-two genera; of these, Gastridium is extinct, and
three or four of the genera (viz. Panopaea, Rostellaria, Corbis
(?), and I believe Solecurtus) are not now found on the west
coast of South America. Fifteen of the genera have on this coast
living representatives in about the same latitudes with the
fossil species; but twelve genera now range very differently to
what they formerly did. The idea of Table 4, in which the
difference between the extension in latitude of the fossil and
existing species is shown, is taken from M. d'Orbigny's work; but
the range of the living shells is given on the authority of Mr.
Cuming, whose long-continued researches on the conchology of
South America are well-known.
When we consider that very few, if any, of the fifty-nine fossil
shells are identical with, or make any close approach to, living
species; when we consider that some of the genera do not now
exist on the west coast of South America, and that no less than
twelve genera out of the thirty-two formerly ranged very
differently from the existing species of the same genera, we must
admit that these deposits are of considerable antiquity, and that
they probably verge on the commencement of the tertiary era. May
we not venture to believe, that they are of nearly
contemporaneous origin with the Eocene formations of the northern
hemisphere?
Comparing the fossil remains from the coast of Chile (leaving
out, as before, Concepcion and Chiloe) with those from Patagonia,
we may conclude, from their generic resemblance, and from the
small number of the species which from either coast approach
closely to living forms, that the formations of both belong to
nearly the same epoch; and this is the opinion of M. D'Orbigny.
Had not a single fossil shell been common to the two coasts, it
could not have been argued that the formations belonged to
different ages; for Messrs. Cuming and Hinds have found, on the
comparison of nearly two thousand living species from the
opposite sides of South America, only one in common, namely, the
Purpura lapillus from both sides of the Isthmus of Panama: even
the shells collected by myself amongst the Chonos Islands and on
the coast of Patagonia, are dissimilar, and we must descend to
the apex of the continent, to Tierra del Fuego, to find these two
great conchological provinces united into one. Hence it is
remarkable that four or five of the fossil shells from Navidad,
namely, Voluta alta, Turritella Patagonica, Trochus collaris,
Venus meridionalis, perhaps Natica solida, and perhaps the large
oyster from Coquimbo, are considered by Mr. Sowerby as identical
with species from Santa Cruz and P. Desire. M. d'Orbigny,
however, admits the perfect identity only of the Trochus.
ON THE TEMPERATURE OF THE TERTIARY PERIOD.
As the number of the fossil species and genera from the western
and eastern coasts is considerable, it will be interesting to
consider the probable nature of the climate under which they
lived. We will first take the case of Navidad, in latitude 34
degrees, where thirty-one species were collected, and which, as
we shall presently see, must have inhabited shallow water, and
therefore will necessarily well exhibit the effects of
temperature. Referring to Table 4 we find that the existing
species of the genera Cassis, Pyrula, Pleurotoma, Terebra, and
Sigaretus, which are generally (though by no means invariably)
characteristic of warmer latitudes, do not at the present day
range nearly so far south on this line of coast as the fossil
species formerly did. Including Coquimbo, we have Perna in the
same predicament. The first impression from this fact is, that
the climate must formerly have been warmer than it now is; but we
must be very cautious in admitting this, for Cardium, Bulla, and
Fusus (and, if we include Coquimbo, Anomia and Artemis) likewise
formerly ranged farther south than they now do; and as these
genera are far from being characteristic of hot climates, their
former greater southern range may well have been owing to causes
quite distinct from climate: Voluta, again, though generally so
tropical a genus, is at present confined on the west coast to
colder or more southern latitudes than it was during the tertiary
period. The Trochus collaris, moreover, and, as we have just seen
according to Mr. Sowerby, two or three other species, formerly
ranged from Navidad as far south as Santa Cruz in latitude 50
degrees. If, instead of comparing the fossils of Navidad, as we
have hitherto done, with the shells now living on the west coast
of South America, we compare them with those found in other parts
of the world, under nearly similar latitudes; for instance, in
the southern parts of the Mediterranean or of Australia, there is
no evidence that the sea off Navidad was formerly hotter than
what might have been expected from its latitude, even if it was
somewhat warmer than it now is when cooled by the great southern
polar current. Several of the most tropical genera have no
representative fossils at Navidad; and there are only single
species of Cassis, Pyrula, and Sigaretus, two of Pleurotoma and
two of Terebra, but none of these species are of conspicuous
size. In Patagonia, there is even still less evidence in the
character of the fossils, of the climate having been formerly
warmer. (It may be worth while to mention that the shells living
at the present day on this eastern side of South America, in
latitude 40 degrees, have perhaps a more tropical character than
those in corresponding latitudes on the shores of Europe: for at
Bahia Blanca and S. Blas, there are two fine species of Voluta
and four of Oliva.) As from the various reasons already assigned,
there can be little doubt that the formations of Patagonia and at
least of Navidad and Coquimbo in Chile, are the equivalents of an
ancient stage in the tertiary formations of the northern
hemisphere, the conclusion that the climate of the southern seas
at this period was not hotter than what might have been expected
from the latitude of each place, appears to me highly important;
for we must believe, in accordance with the views of Mr. Lyell,
that the causes which gave to the older tertiary productions of
the quite temperate zones of Europe a tropical character, WERE OF
A LOCAL CHARACTER AND DID NOT AFFECT THE ENTIRE GLOBE. On the
other hand, I have endeavoured to show, in the "Geological
Transactions," that, at a much later period, Europe and North and
South America were nearly contemporaneously subjected to ice-
action, and consequently to a colder, or at least more equable,
climate than that now characteristic of the same latitudes.
ON THE ABSENCE OF EXTENSIVE MODERN CONCHIFEROUS DEPOSITS IN SOUTH
AMERICA; AND ON THE CONTEMPORANEOUSNESS OF THE OLDER TERTIARY
DEPOSITS AT DISTANT POINTS BEING DUE TO CONTEMPORANEOUS MOVEMENTS
OF SUBSIDENCE.
Knowing from the researches of Professor E. Forbes, that
molluscous animals chiefly abound within a depth of 100 fathoms
and under, and bearing in mind how many thousand miles of both
coasts of South America have been upraised within the recent
period by a slow, long-continued, intermittent movement,- -seeing
the diversity in nature of the shores and the number of shells
now living on them,—seeing also that the sea off Patagonia and
off many parts of Chile, was during the tertiary period highly
favourable to the accumulation of sediment,—the absence of
extensive deposits including recent shells over these vast spaces
of coast is highly remarkable. The conchiferous calcareous beds
at Coquimbo, and at a few isolated points northward, offer the
most marked exception to this statement; for these beds are from
twenty to thirty feet in thickness, and they stretch for some
miles along shore, attaining, however, only a very trifling
breadth. At Valdivia there is some sandstone with imperfect casts
of shells, which POSSIBLY may belong to the recent period: parts
of the boulder formation and the shingle-beds on the lower plains
of Patagonia probably belong to this same period, but neither are
fossiliferous: it also so happens that the great Pampean
formation does not include, with the exception of the Azara, any
mollusca. There cannot be the smallest doubt that the upraised
shells along the shores of the Atlantic and Pacific, whether
lying on the bare surface, or embedded in mould or in
sand-hillocks, will in the course of ages be destroyed by
alluvial action: this probably will be the case even with the
calcareous beds of Coquimbo, so liable to dissolution by
rain-water. If we take into consideration the probability of
oscillations of level and the consequent action of the
tidal-waves at different heights, their destruction will appear
almost certain. Looking to an epoch as far distant in futurity as
we now are from the past Miocene period, there seems to me
scarcely a chance, under existing conditions, of the numerous
shells now living in those zones of depths most fertile in life,
and found exclusively on the western and south-eastern coasts of
South America, being preserved to this imaginary distant epoch. A
whole conchological series will in time be swept away, with no
memorials of their existence preserved in the earth's crust.
Can any light be thrown on this remarkable absence of recent
conchiferous deposits on these coasts, on which, at an ancient
tertiary epoch, strata abounding with organic remains were
extensively accumulated? I think there can, namely, by
considering the conditions necessary for the preservation of a
formation to a distant age. Looking to the enormous amount of
denudation which on all sides of us has been effected,—as
evidenced by the lofty cliffs cutting off on so many coasts
horizontal and once far-extended strata of no great antiquity (as
in the case of Patagonia),—as evidenced by the level surface of
the ground on both sides of great faults and dislocations,—by
inland lines of escarpments, by outliers, and numberless other
facts, and by that argument of high generality advanced by Mr.
Lyell, namely, that every SEDIMENTARY formation, whatever its
thickness may be, and over however many hundred square miles it
may extend, is the result and the measure of an equal amount of
wear and tear of pre-existing formations; considering these
facts, we must conclude that, as an ordinary rule, a formation to
resist such vast destroying powers, and to last to a distant
epoch, must be of wide extent, and either in itself, or together
with superincumbent strata, be of great thickness. In this
discussion, we are considering only formations containing the
remains of marine animals, which, as before mentioned, live, with
some exceptions within (most of them much within) depths of 100
fathoms. How, then, can a thick and widely extended formation be
accumulated, which shall include such organic remains? First, let
us take the case of the bed of the sea long remaining at a
stationary level: under these circumstances it is evident that
CONCHIFEROUS strata can accumulate only to the same thickness
with the depth at which the shells can live; on gently inclined
coasts alone can they accumulate to any considerable width; and
from the want of superincumbent pressure, it is probable that the
sedimentary matter will seldom be much consolidated: such
formations have no very good chance, when in the course of time
they are upraised, of long resisting the powers of denudation.
The chance will be less if the submarine surface, instead of
having remained stationary, shall have gone on slowly rising
during the deposition of the strata, for in this case their total
thickness must be less, and each part, before being consolidated
or thickly covered up by superincumbent matter, will have had
successively to pass through the ordeal of the beach; and on most
coasts, the waves on the beach tend to wear down and disperse
every object exposed to their action. Now, both on the
south-eastern and western shores of South America, we have had
clear proofs that the land has been slowly rising, and in the
long lines of lofty cliffs, we have seen that the tendency of the
sea is almost everywhere to eat into the land. Considering these
facts, it ceases, I think, to be surprising, that extensive
recent conchiferous deposits are entirely absent on the southern
and western shores of America.
Let us take the one remaining case, of the bed of the sea slowly
subsiding during a length of time, whilst sediment has gone on
being deposited. It is evident that strata might thus accumulate
to any thickness, each stratum being deposited in shallow water,
and consequently abounding with those shells which cannot live at
great depths: the pressure, also, I may observe, of each fresh
bed would aid in consolidating all the lower ones. Even on a
rather steep coast, though such must ever be unfavourable to
widely extended deposits, the formations would always tend to
increase in breadth from the water encroaching on the land. Hence
we may admit that periods of slow subsidence will commonly be
most favourable to the accumulation of CONCHIFEROUS deposits, of
sufficient thickness, extension, and hardness, to resist the
average powers of denudation.
We have seen that at an ancient tertiary epoch, fossiliferous
deposits were extensively deposited on the coasts of South
America; and it is a very interesting fact, that there is
evidence that these ancient tertiary beds were deposited during a
period of subsidence. Thus, at Navidad, the strata are about
eight hundred feet in thickness, and the fossil shells are
abundant both at the level of the sea and some way up the cliffs;
having sent a list of these fossils to Professor E. Forbes, he
thinks they must have lived in water between one and ten fathoms
in depth: hence the bottom of the sea on which these shells once
lived must have subsided at least 700 feet to allow of the
superincumbent matter being deposited. I must here remark, that,
as all these and the following fossil shells are extinct species,
Professor Forbes necessarily judges of the depths at which they
lived only from their generic character, and from the analogical
distribution of shells in the northern hemisphere; but there is
no just cause from this to doubt the general results. At Huafo
the strata are about the same thickness, namely, 800 feet, and
Professor Forbes thinks the fossils found there cannot have lived
at a greater depth than fifty fathoms, or 300 feet. These two
points, namely, Navidad and Huafo, are 570 miles apart, but
nearly halfway between them lies Mocha, an island 1,200 feet in
height, apparently formed of tertiary strata up to its level
summit, and with many shells, including the same Turritella with
that found at Huafo, embedded close to the level of the sea. In
Patagonia, shells are numerous at Santa Cruz, at the foot of the
350 feet plain, which has certainly been formed by the denudation
of the 840 feet plain, and therefore was originally covered by
strata that number of feet in thickness, and these shells,
according to Professor Forbes, probably lived at a depth of
between seven and fifteen fathoms: at Port S. Julian, sixty miles
to the north, shells are numerous at the foot of the ninety feet
plain (formed by the denudation of the 950 feet plain), and
likewise occasionally at the height of several hundred feet in
the upper strata; these shells must have lived in water somewhere
between five and fifty fathoms in depth. Although in other parts
of Patagonia I have no direct evidence of shoal-water shells
having been buried under a great thickness of superincumbent
submarine strata, yet it should be borne in mind that the lower
fossiliferous strata with several of the same species of
Mollusca, the upper tufaceous beds, and the high summit-plain,
stretch for a considerable distance southward, and for hundreds
of miles northward; seeing this uniformity of structure, I
conceive it may be fairly concluded that the subsidence by which
the shells at Santa Cruz and S. Julian were carried down and
covered up, was not confined to these two points, but was
co-extensive with a considerable portion of the Patagonian
tertiary formation. In a succeeding chapter it will be seen, that
we are led to a similar conclusion with respect to the secondary
fossiliferous strata of the Cordillera, namely, that they also
were deposited during a long- continued and great period of
subsidence. From the foregoing reasoning, and from the facts just
given, I think we must admit the probability of the following
proposition: namely, that when the bed of the sea is either
stationary or rising, circumstances are far less favourable, than
when the level is sinking, to the accumulation of CONCHIFEROUS
deposits of sufficient thickness and extension to resist, when
upheaved, the average vast amount of denudation. This result
appears to me, in several respects, very interesting: every one
is at first inclined to believe that at innumerable points,
wherever there is a supply of sediment, fossiliferous strata are
now forming, which at some future distant epoch will be upheaved
and preserved; but on the views above given, we must conclude
that this is far from being the case; on the contrary, we require
(1st), a long-continued supply of sediment; (2nd), an extensive
shallow area; and (3rd), that this area shall slowly subside to a
great depth, so as to admit the accumulation of a widely extended
thick mass of superincumbent strata. In how few parts of the
world, probably, do these conditions at the present day concur!
We can thus, also, understand the general want of that close
sequence in fossiliferous formations which we might theoretically
have anticipated; for, without we suppose a subsiding movement to
go on at the same spot during an enormous period, from one
geological era to another, and during the whole of this period
sediment to accumulate at the proper rate, so that the depth
should not become too great for the continued existence of
molluscous animals, it is scarcely possible that there should be
a perfect sequence at the same spot in the fossil shells of the
two geological formations. (Professor H.D. Rogers, in his
excellent address to the Association of American Geologists
("Silliman's Journal" volume 47 page 277) makes the following
remark: "I question if we are at all aware how COMPLETELY the
whole history of all departed time lies indelibly recorded with
the amplest minuteness of detail in the successive sediments of
the globe, how effectually, in other words, every period of time
HAS WRITTEN ITS OWN HISTORY, carefully preserving every created
form and every trace of action." I think the correctness of such
remarks is more than doubtful, even if we except (as I suppose he
would) all those numerous organic forms which contain no hard
parts.) So far from a very long-continued subsidence being
probable, many facts lead to the belief that the earth's surface
oscillates up and down; and we have seen that during the
elevatory movements there is but a small chance of DURABLE
fossiliferous deposits accumulating.
Lastly, these same considerations appear to throw some light on
the fact that certain periods appear to have been favourable to
the deposition, or at least to the preservation, of
contemporaneous formations at very distant points. We have seen
that in South America an enormous area has been rising within the
recent period; and in other quarters of the globe immense spaces
appear to have risen contemporaneously. From my examination of
the coral- reefs of the great oceans, I have been led to conclude
that the bed of the sea has gone on slowly sinking within the
present era, over truly vast areas: this, indeed, is in itself
probable, from the simple fact of the rising areas having been so
large. In South America we have distinct evidence that at nearly
the same tertiary period, the bed of the sea off parts of the
coast of Chile and off Patagonia was sinking, though these
regions are very remote from each other. If, then, it holds good,
as a general rule, that in the same quarter of the globe the
earth's crust tends to sink and rise contemporaneously over vast
spaces, we can at once see, that we have at distant points, at
the same period, those very conditions which appear to be
requisite for the accumulation of fossiliferous masses of
sufficient extension, thickness, and hardness, to resist
denudation, and consequently to last unto an epoch distant in
futurity. (Professor Forbes has some admirable remarks on this
subject, in his "Report on the Shells of the Aegean Sea." In a
letter to Mr. Maclaren ("Edinburgh New Philosophical Journal"
January 1843), I partially entered into this discussion, and
endeavoured to show that it was highly improbable, that upraised
atolls or barrier-reefs, though of great thickness, should, owing
to their small extension or breadth, be preserved to a distant
future period.)
CHAPTER VI. PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND
FOLIATION.
Brazil, Bahia, gneiss with disjointed metamorphosed dikes. Strike of
foliation. Rio de Janeiro, gneiss-granite, embedded fragment in,
decomposition of. La Plata, metamorphic and old volcanic rocks of. S.
Ventana. Claystone porphyry formation of Patagonia; singular
metamorphic rocks; pseudo-dikes. Falkland Islands, Palaeozoic fossils
of. Tierra del Fuego, clay-slate formation, cretaceous fossils of;
cleavage and foliation; form of land. Chonos Archipelago,
mica-schists, foliation disturbed by granitic axis; dikes. Chiloe.
Concepcion, dikes, successive formation of. Central and Northern
Chile. Concluding remarks on cleavage and foliation. Their close
analogy and similar origin. Stratification of metamorphic schists.
Foliation of intrusive rocks. Relation of cleavage and foliation to
the lines of tension during metamorphosis.
The metamorphic and plutonic formations of the several districts
visited by the "Beagle" will be here chiefly treated of, but only
such cases as appear to me new, or of some special interest, will
be described in detail; at the end of the chapter I will sum up
all the facts on cleavage and foliation,— to which I particularly
attended.
BAHIA, BRAZIL: latitude 13 degrees south.
The prevailing rock is gneiss, often passing, by the
disappearance of the quartz and mica, and by the feldspar losing
its red colour, into a brilliantly grey primitive greenstone. Not
unfrequently quartz and hornblende are arranged in layers in
almost amorphous feldspar. There is some fine-grained syenitic
granite, orbicularly marked by ferruginous lines, and weathering
into vertical, cylindrical holes, almost touching each other. In
the gneiss, concretions of granular feldspar and others of
garnets with mica occur. The gneiss is traversed by numerous
dikes composed of black, finely crystallised, hornblendic rock,
containing a little glassy feldspar and sometimes mica, and
varying in thickness from mere threads to ten feet: these
threads, which are often curvilinear, could sometimes be traced
running into the larger dikes. One of these dikes was remarkable
from having been in two or three places laterally disjointed,
with unbroken gneiss interposed between the broken ends, and in
one part with a portion of the gneiss driven, apparently whilst
in a softened state, into its side or wall. In several
neighbouring places, the gneiss included angular, well- defined,
sometimes bent, masses of hornblende rock, quite like, except in
being more perfectly crystallised, that forming the dikes, and,
at least in one instance, containing (as determined by Professor
Miller) augite as well as hornblende. In one or two cases these
angular masses, though now quite separate from each other by the
solid gneiss, had, from their exact correspondence in size and
shape, evidently once been united; hence I cannot doubt that most
or all of the fragments have been derived from the breaking up of
the dikes, of which we see the first stage in the above-
mentioned laterally disjointed one. The gneiss close to the
fragments generally contained many large crystals of hornblende,
which are entirely absent or rare in other parts: its folia or
laminae were gently bent round the fragments, in the same manner
as they sometimes are round concretions. Hence the gneiss has
certainly been softened, its composition modified, and its folia
arranged, subsequently to the breaking up of the dikes, these
latter also having been at the same time bent and softened.
(Professor Hitchcock "Geology of Massachusetts" volume 2 page
673, gives a closely similar case of a greenstone dike in
syenite.)
I must here take the opportunity of premising, that by the term
CLEAVAGE I imply those planes of division which render a rock,
appearing to the eye quite or nearly homogeneous, fissile. By the
term FOLIATION, I refer to the layers or plates of different
mineralogical nature of which most metamorphic schists are
composed; there are, also, often included in such masses,
alternating, homogeneous, fissile layers or folia, and in this
case the rock is both foliated and has a cleavage. By
STRATIFICATION, as applied to these formations, I mean those
alternate, parallel, large masses of different composition, which
are themselves frequently either foliated or fissile,—such as the
alternating so-called strata of mica-slate, gneiss, glossy
clay-slate, and marble.
The folia of the gneiss within a few miles round Bahia generally
strike irregularly, and are often curvilinear, dipping in all
directions at various angles: but where best defined, they
extended most frequently in a N.E. by N. (or East 50 degrees N.)
and S.W. by S. line, corresponding nearly with the coast-line
northwards of the bay. I may add that Mr. Gardner found in
several parts of the province of Ceara, which lies between four
and five hundred miles north of Bahia, gneiss with the folia
extending E. 45 degrees N.; and in Guyana according to Sir R.
Schomburgk, the same rock strikes E. 57 degrees N. Again,
Humboldt describes the gneiss-granite over an immense area in
Venezuela and even in Colombia, as striking E. 50 degrees N., and
dipping to the N.W. at an angle of fifty degrees. (Gardner
"Geological Section of the British Association" 1840. For Sir R.
Schomburgk's observations see "Geographical Journal" 1842 page
190. See also Humboldt's discussion on Loxodrism in the "Personal
Narrative.") Hence all the observations hitherto made tend to
show that the gneissic rocks over the whole of this part of the
continent have their folia extending generally within almost a
point of the compass of the same direction. (I landed at only one
place north of Bahia, namely, at Pernambuco. I found there only
soft, horizontally stratified matter, formed from disintegrated
granitic rocks, and some yellowish impure limestone, probably of
a tertiary epoch. I have described a most singular natural bar of
hard sandstone, which protects the harbour, in the 19th volume
1841 page 258 of the "London and Edinburgh Philosophical
Magazine."
ABROLHOS ISLETS, Latitude 18 degrees S. off the coast of Brazil.
Although not strictly in place, I do not know where I can more
conveniently describe this little group of small islands. The
lowest bed is a sandstone with ferruginous veins; it weathers
into an extraordinary honeycombed mass; above it there is a
dark-coloured argillaceous shale; above this a coarser
sandstone—making a total thickness of about sixty feet; and
lastly, above these sedimentary beds, there is a fine conformable
mass of greenstone, in some parts having a columnar structure.
All the strata, as well as the surface of the land, dip at an
angle of about 12 degrees to N. by W. Some of the islets are
composed entirely of the sedimentary, others of the trappean
rocks, generally, however, with the sandstone, cropping out on
the southern shores.)
RIO DE JANEIRO.
This whole district is almost exclusively formed of gneiss,
abounding with garnets, and porphyritic with large crystals, even
three and four inches in length, of orthoclase feldspar: in these
crystals mica and garnets are often enclosed. At the western base
of the Corcovado, there is some ferruginous carious quartz-rock;
and in the Tijeuka range, much fine- grained granite. I observed
boulders of greenstone in several places; and on the islet of
Villegagnon, and likewise on the coast some miles northward, two
large trappean dikes. The porphyritic gneiss, or gneiss- granite
as it has been called by Humboldt, is only so far foliated that
the constituent minerals are arranged with a certain degree of
regularity, and may be said to have a "GRAIN," but they are not
separated into distinct folia or laminae. There are, however,
several other varieties of gneiss regularly foliated, and
alternating with each other in so-called strata. The
stratification and foliation of the ordinary gneisses, and the
foliation or "grain" of the gneiss-granite, are parallel to each
other, and generally strike within a point of N.E. and S.W.
dipping at a high angle (between 50 and 60 degrees) generally to
S.E.: so that here again we meet with the strike so prevalent
over the more northern parts of this continent. The mountains of
gneiss-granite are to a remarkable degree abruptly conical, which
seems caused by the rock tending to exfoliate in thick, conically
concentric layers: these peaks resemble in shape those of
phonolite and other injected rocks on volcanic islands; nor is
the grain or foliation (as we shall afterwards see) any
difficulty on the idea of the gneiss-granite having been an
intrusive rather than a metamorphic formation. The lines of
mountains, but not always each separate hill, range nearly in the
same direction with the foliation and so-called stratification,
but rather more easterly.
(FIGURE 22. FRAGMENT OF GNEISS EMBEDDED IN ANOTHER VARIETY OF THE
SAME ROCK.)
On a bare gently inclined surface of the porphyritic gneiss in
Botofogo Bay, I observed the appearance represented in Figure 22.
A fragment seven yards long and two in width, with angular and
distinctly defined edges, composed of a peculiar variety of
gneiss with dark layers of mica and garnets, is surrounded on all
sides by the ordinary gneiss- granite; both having been
dislocated by a granitic vein. The folia in the fragment and in
the surrounding rock strike in the same N.N.E. and S.S.W. line;
but in the fragment they are vertical, whereas in the
gneiss-granite they dip at a small angle, as shown by the arrows,
to S.S.E. This fragment, considering its great size, its solitary
position, and its foliated structure parallel to that of the
surrounding rock, is, as far as I know, a unique case: and I will
not attempt any explanation of its origin.
The numerous travellers in this country, have all been greatly
surprised at the depth to which the gneiss and other granitic
rocks, as well as the talcose slates of the interior, have been
decomposed. (Spix and Martius have collected in an Appendix to
their "Travels," the largest body of facts on this subject. See
also some remarks by M. Lund in his communications to the Academy
at Copenhagen; and others by M. Gaudichaud in Freycinet
"Voyage.") Near Rio, every mineral except the quartz has been
completely softened, in some places to a depth little less than
one hundred feet. (Dr. Benza describes granitic rock, "Madras
Journal of Literature" etc. October 183? page 246), in the
Neelgherries, decomposed to a depth of forty feet.) The minerals
retain their positions in folia ranging in the usual direction;
and fractured quartz veins may be traced from the solid rock,
running for some distance into the softened, mottled, highly
coloured, argillaceous mass. It is said that these decomposed
rocks abound with gems of various kinds, often in a fractured
state, owing, as some have supposed, to the collapse of geodes,
and that they contain gold and diamonds. At Rio, it appeared to
me that the gneiss had been softened before the excavation (no
doubt by the sea) of the existing, broad, flat-bottomed valleys;
for the depth of decomposition did not appear at all conformable
with the present undulations of the surface. The porphyritic
gneiss, where now exposed to the air, seems to withstand
decomposition remarkably well; and I could see no signs of any
tendency to the production of argillaceous masses like those here
described. I was also struck with the fact, that where a bare
surface of this rock sloped into one of the quiet bays, there
were no marks of erosion at the level of the water, and the parts
both beneath and above it preserved a uniform curve. At Bahia,
the gneiss rocks are similarly decomposed, with the upper parts
insensibly losing their foliation, and passing, without any
distinct line of separation, into a bright red argillaceous
earth, including partially rounded fragments of quartz and
granite. From this circumstance, and from the rocks appearing to
have suffered decomposition before the excavation of the valleys,
I suspect that here, as at Rio, the decomposition took place
under the sea. The subject appeared to me a curious one, and
would probably well repay careful examination by an able
mineralogist.
THE NORTHERN PROVINCES OF LA PLATA.
According to some observations communicated to me by Mr. Fox, the
coast from Rio de Janeiro to the mouth of the Plata seems
everywhere to be granitic, with a few trappean dikes. At Port
Alegre, near the boundary of Brazil, there are porphyries and
diorites. (M. Isabelle "Voyage a Buenos Ayres" page 479.) At the
mouth of the Plata, I examined the country for twenty-five miles
west, and for about seventy miles north of Maldonado: near this
town, there is some common gneiss, and much, in all parts of the
country, of a coarse-grained mixture of quartz and reddish
feldspar, often, however, assuming a little dark-green imperfect
hornblende, and then immediately becoming foliated. The abrupt
hillocks thus composed, as well as the highly inclined folia of
the common varieties of gneiss, strike N.N.E. or a little more
easterly, and S.S.W. Clay-slate is occasionally met with, and
near the L. del Potrero, there is white marble, rendered fissile
from the presence of hornblende, mica, and asbestus; the cleavage
of these rocks and their stratification, that is the alternating
masses thus composed, strike N.N.E. and S.S.W. like the foliated
gneisses, and have an almost vertical dip. The Sierra Larga, a
low range five miles west of Maldonado, consists of quartzite,
often ferruginous, having an arenaceous feel, and divided into
excessively thin, almost vertical laminae or folia by
microscopically minute scales, apparently of mica, and striking
in the usual N.N.E. and S.S.W. direction. The range itself is
formed of one principal line with some subordinate ones; and it
extends with remarkable uniformity far northward (it is said even
to the confines of Brazil), in the same line with the vertically
ribboned quartz rock of which it is composed. The S. de Las
Animas is the highest range in the country; I estimated it at
1,000 feet; it runs north and south, and is formed of feldspathic
porphyry; near its base there is a N.N.W. and S.S.E. ridge of a
conglomerate in a highly porphyritic basis.
Northward of Maldonado, and south of Las Minas, there is an E.
and W. hilly band of country, some miles in width, formed of
siliceous clay-slate, with some quartz, rock, and limestone,
having a tortuous irregular cleavage, generally ranging east and
west. E. and S.E. of Las Minas there is a confused district of
imperfect gneiss and laminated quartz, with the hills ranging in
various directions, but with each separate hill generally running
in the same line with the folia of the rocks of which it is
composed: this confusion appears to have been caused by the
intersection of the [E. and W.] and [N.N.E. and S.S.W.] strikes.
Northward of Las Minas, the more regular northerly ranges
predominate: from this place to near Polanco, we meet with the
coarse-grained mixture of quartz and feldspar, often with the
imperfect hornblende, and then becoming foliated in a N. and S.
line—with imperfect clay-slate, including laminae of red
crystallised feldspar—with white or black marble, sometimes
containing asbestus and crystals of gypsum—with quartz-rock—with
syenite—and lastly, with much granite. The marble and granite
alternate repeatedly in apparently vertical masses: some miles
northward of the Polanco, a wide district is said to be entirely
composed of marble. It is remarkable, how rare mica is in the
whole range of country north and westward of Maldonado.
Throughout this district, the cleavage of the clay-slate and
marble—the foliation of the gneiss and the quartz—the
stratification or alternating masses of these several rocks—and
the range of the hills, all coincide in direction; and although
the country is only hilly, the planes of division are almost
everywhere very highly inclined or vertical.
Some ancient submarine volcanic rocks are worth mentioning, from
their rarity on this eastern side of the continent. In the valley
of the Tapas (fifty or sixty miles N. of Maldonado) there is a
tract three or four miles in length, composed of various trappean
rocks with glassy feldspar—of apparently metamorphosed
grit-stones—of purplish amygdaloids with large kernels of
carbonate of lime (Near the Pan de Azucar there is some greenish
porphyry, in one place amygdaloidal with agate.)—and much of a
harshish rock with glassy feldspar intermediate in character
between claystone porphyry and trachyte. This latter rock was in
one spot remarkable from being full of drusy cavities, lined with
quartz crystals, and arranged in planes, dipping at an angle of
50 degrees to the east, and striking parallel to the foliation of
an adjoining hill composed of the common mixture of quartz,
feldspar, and imperfect hornblende: this fact perhaps indicates
that these volcanic rocks have been metamorphosed, and their
constituent parts rearranged, at the same time and according to
the same laws, with the granitic and metamorphic formations of
this whole region. In the valley of the Marmaraya, a few miles
south of the Tapas, a band of trappean and amygdaloidal rock is
interposed between a hill of granite and an extensive surrounding
formation of red conglomerate, which (like that at the foot of
the S. Animas) has its basis porphyritic with crystals of
feldspar, and which hence has certainly suffered metamorphosis.
MONTE VIDEO.
The rocks here consist of several varieties of gneiss, with the
feldspar often yellowish, granular and imperfectly crystallised,
alternating with, and passing insensibly into, beds, from a few
yards to nearly a mile in thickness, of fine or coarse grained,
dark-green hornblendic slate; this again often passing into
chloritic schist. These passages seem chiefly due to changes in
the mica, and its replacement by other minerals. At Rat Island I
examined a mass of chloritic schist, only a few yards square,
irregularly surrounded on all sides by the gneiss, and
intricately penetrated by many curvilinear veins of quartz, which
gradually BLEND into the gneiss: the cleavage of the chloritic
schist and the foliation of the gneiss were exactly parallel.
Eastward of the city there is much fine- grained, dark-coloured
gneiss, almost assuming the character of hornblende- slate, which
alternates in thin laminae with laminae of quartz, the whole mass
being transversely intersected by numerous large veins of quartz:
I particularly observed that these veins were absolutely
continuous with the alternating laminae of quartz. In this case
and at Rat Island, the passage of the gneiss into imperfect
hornblendic or into chloritic slate, seemed to be connected with
the segregation of the veins of quartz. (Mr. Greenough page 78
"Critical Examination" etc., observes that quartz in mica-slate
sometimes appears in beds and sometimes in veins. Von Buch also
in his "Travels in Norway" page 236, remarks on alternating
laminae of quartz and hornblende-slate replacing mica-schist.)
The Mount, a hill believed to be 450 feet in height, from which
the place takes its name, is much the highest land in this
neighbourhood: it consists of hornblendic slate, which (except on
the eastern and disturbed base) has an east and west nearly
vertical cleavage; the longer axis of the hill also ranges in
this same line. Near the summit the hornblende-slate gradually
becomes more and more coarsely crystallised, and less plainly
laminated, until it passes into a heavy, sonorous greenstone,
with a slaty conchoidal fracture; the laminae on the north and
south sides near the summit dip inwards, as if this upper part
had expanded or bulged outwards. This greenstone must, I
conceive, be considered as metamorphosed hornblende- slate. The
Cerrito, the next highest, but much less elevated point, is
almost similarly composed. In the more western parts of the
province, besides gneiss, there is quartz-rock, syenite, and
granite; and at Colla, I heard of marble.
Near M. Video, the space which I more accurately examined was
about fifteen miles in an east and west line, and here I found
the foliation of the gneiss and the cleavage of the slates
generally well developed, and extending parallel to the
alternating strata composed of the gneiss, hornblendic and
chloritic schists. These planes of division all range within one
point of east and west, frequently east by south and west by
north; their dip is generally almost vertical, and scarcely
anywhere under 45 degrees: this fact, considering how slightly
undulatory the surface of the country is, deserves attention.
Westward of M. Video, towards the Uruguay, wherever the gneiss is
exposed, the highly inclined folia are seen striking in the same
direction; I must except one spot where the strike was N.W. by W.
The little Sierra de S. Juan, formed of gneiss and laminated
quartz, must also be excepted, for it ranges between [N. to N.E.]
and [S. to S.W.] and seems to belong to the same system with the
hills in the Maldonado district. Finally, we have seen that, for
many miles northward of Maldonado and for twenty-five miles
westward of it, as far as the S. de las Animas, the foliation,
cleavage, so-called stratification and lines of hills, all range
N.N.E. and S.S.W., which is nearly coincident with the adjoining
coast of the Atlantic. Westward of the S. de las Animas, as far
as even the Uruguay, the foliation, cleavage, and stratification
(but not lines of hills, for there are no defined ones) all range
about E. by S. and W. by N., which is nearly coincident with the
direction of the northern shore of the Plata; in the confused
country near Las Minas, where these two great systems appear to
intersect each other, the cleavage, foliation, and stratification
run in various directions, but generally coincide with the line
of each separate hill.
SOUTHERN LA PLATA.
The first ridge, south of the Plata, which projects through the
Pampean formation, is the Sierra Tapalguen and Vulcan, situated
200 miles southward of the district just described. This ridge is
only a few hundred feet in height, and runs from C. Corrientes in
a W.N.W. line for at least 150 miles into the interior: at
Tapalguen, it is composed of unstratified granular quartz,
remarkable from forming tabular masses and small plains,
surrounded by precipitous cliffs: other parts of the range are
said to consist of granite: and marble is found at the S. Tinta.
It appears from M. Parchappe's observations, that at Tandil there
is a range of quartzose gneiss, very like the rocks of the S.
Larga near Maldonado, running in the same N.N.E. and S.S.W.
direction; so that the framework of the country here is very
similar to that on the northern shore of the Plata. (M.
d'Orbigny's "Voyage" Part. Geolog. page 46. I have given a short
account of the peculiar forms of the quartz hills of Tapalguen,
so unusual in a metamorphic formation, in my "Journal of
Researches" 2nd edition page 116.)
The Sierra Guitru-gueyu is situated sixty miles south of the S.
Tapalguen: it consists of numerous parallel, sometimes blended
together ridges, about twenty-three miles in width, and five
hundred feet in height above the plain, and extending in a N.W.
and S.E. direction. Skirting round the extreme S.E. termination,
I ascended only a few points, which were composed of a
fine-grained gneiss, almost composed of feldspar with a little
mica, and passing in the upper parts of the hills into a rather
compact purplish clay-slate. The cleavage was nearly vertical,
striking in a N.W. by W. and S.E. by E. line, nearly, though not
quite, coincident with the direction of the parallel ridges.
The Sierra Ventana lies close south of that of Guitru-gueyu; it
is remarkable from attaining a height, very unusual on this side
of the continent, of 3,340 feet. It consists up to its summit, of
quartz, generally pure and white, but sometimes reddish, and
divided into thick laminae or strata: in one part there is a
little glossy clay-slate with a tortuous cleavage. The thick
layers of quartz strike in a W. 30 degrees N. line, dipping
southerly at an angle of 45 degrees and upwards. The principal
line of mountains, with some quite subordinate parallel ridges,
range about W. 45 degrees N.: but at their S.E. termination, only
W. 25 degrees N. This Sierra is said to extend between twenty and
thirty leagues into the interior.
PATAGONIA.
With the exception perhaps of the hill of S. Antonio (600 feet
high) in the Gulf of S. Matias, which has never been visited by a
geologist, crystalline rocks are not met with on the coast of
Patagonia for a space of 380 miles south of the S. Ventana. At
this point (latitude 43 degrees 50 minutes), at Points Union and
Tombo, plutonic rocks are said to appear, and are found, at
rather wide intervals, beneath the Patagonian tertiary formation
for a space of about three hundred miles southward, to near Bird
Island, in latitude 48 degrees 56 minutes. Judging from specimens
kindly collected for me by Mr. Stokes, the prevailing rock at
Ports St. Elena, Camerones, Malaspina, and as far south as the
Paps of Pineda, is a purplish-pink or brownish claystone
porphyry, sometimes laminated, sometimes slightly vesicular, with
crystals of opaque feldspar and with a few grains of quartz;
hence these porphyries resemble those immediately to be described
at Port Desire, and likewise a series which I have seen from P.
Alegre on the southern confines of Brazil. This porphyritic
formation further resembles in a singularly close manner the
lowest stratified formation of the Cordillera of Chile, which, as
we shall hereafter see, has a vast range, and attains a great
thickness. At the bottom of the Gulf of St. George, only tertiary
deposits appear to be present. At Cape Blanco, there is quartz
rock, very like that of the Falkland Islands, and some hard, blue
siliceous clay-slate.
At Port Desire there is an extensive formation of the claystone
porphyry, stretching at least twenty-five miles into the
interior: it has been denuded and deeply worn into gullies before
being covered up by the tertiary deposits, through which it here
and there projects in hills; those north of the bay being 440
feet in height. The strata have in several places been tilted at
small angles, generally either to N.N.W. or S.S.E. By gradual
passages and alternations, the porphyries change incessantly in
nature. I will describe only some of the principal mineralogical
changes, which are highly instructive, and which I carefully
examined. The prevailing rock has a compact purplish base, with
crystals of earthy or opaque feldspar, and often with grains of
quartz. There are other varieties, with an almost truly trachytic
base, full of little angular vesicles and crystals of glassy
feldspar; and there are beds of black perfect pitchstone, as well
as of a concretionary imperfect variety. On a casual inspection,
the whole series would be thought to be of the same plutonic or
volcanic nature with the trachytic varieties and pitchstone; but
this is far from being the case, as much of the porphyry is
certainly of metamorphic origin. Besides the true porphyries,
there are many beds of earthy, quite white or yellowish, friable,
easily fusible matter, resembling chalk, which under the
microscope is seen to consist of minute broken crystals, and
which, as remarked in a former chapter, singularly resembles the
upper tufaceous beds of the Patagonian tertiary formation. This
earthy substance often becomes coarser, and contains minute
rounded fragments of porphyries and rounded grains of quartz, and
in one case so many of the latter as to resemble a common
sandstone. These beds are sometimes marked with true lines of
aqueous deposition, separating particles of different degrees of
coarseness; in other cases there are parallel ferruginous lines
not of true deposition, as shown by the arrangement of the
particles, though singularly resembling them. The more indurated
varieties often include many small and some larger angular
cavities, which appear due to the removal of earthy matter: some
varieties contain mica. All these earthy and generally white
stones insensibly pass into more indurated sonorous varieties,
breaking with a conchoidal fracture, yet of small specific
gravity; many of these latter varieties assume a pale purple
tint, being singularly banded and veined with different shades,
and often become plainly porphyritic with crystals of feldspar.
The formation of these crystals could be most clearly traced by
minute angular and often partially hollow patches of earthy
matter, first assuming a FIBROUS STRUCTURE, then passing into
opaque imperfectly shaped crystals, and lastly, into perfect
glassy crystals. When these crystals have appeared, and when the
basis has become compact, the rock in many places could not be
distinguished from a true claystone porphyry without a trace of
mechanical structure.
In some parts, these earthy or tufaceous beds pass into jaspery
and into beautifully mottled and banded porcelain rocks, which
break into splinters, translucent at their edges, hard enough to
scratch glass, and fusible into white transparent beads: grains
of quartz included in the porcelainous varieties can be seen
melting into the surrounding paste. In other parts, the earthy or
tufaceous beds either insensibly pass into, or alternate with,
breccias composed of large and small fragments of various
purplish porphyries, with the matrix generally porphyritic: these
breccias, though their subaqueous origin is in many places shown
both by the arrangement of their smaller particles and by an
oblique or current lamination, also pass into porphyries, in
which every trace of mechanical origin and stratification has
been obliterated.
Some highly porphyritic though coarse-grained masses, evidently
of sedimentary origin, and divided into thin layers, differing
from each other chiefly in the number of embedded grains of
quartz, interested me much from the peculiar manner in which here
and there some of the layers terminated in abrupt points, quite
unlike those produced by a layer of sediment naturally thinning
out, and apparently the result of a subsequent process of
metamorphic aggregation. In another common variety of a finer
texture, the aggregating process had gone further, for the whole
mass consisted of quite short, parallel, often slightly curved
layers or patches, of whitish or reddish finely
granulo-crystalline feldspathic matter, generally terminating at
both ends in blunt points; these layers or patches further tended
to pass into wedge or almond-shaped little masses, and these
finally into true crystals of feldspar, with their centres often
slightly drusy. The series was so perfect that I could not doubt
that these large crystals, which had their longer axes placed
parallel to each other, had primarily originated in the
metamorphosis and aggregation of alternating layers of tuff; and
hence their parallel position must be attributed (unexpected
though the conclusion may be), not to laws of chemical action,
but to the original planes of deposition. I am tempted briefly to
describe three other singular allied varieties of rock; the first
without examination would have passed for a stratified
porphyritic breccia, but all the included angular fragments
consisted of a border of pinkish crystalline feldspathic matter,
surrounding a dark translucent siliceous centre, in which grains
of quartz not quite blended into the paste could be
distinguished: this uniformity in the nature of the fragments
shows that they are not of mechanical, but of concretionary
origin, having resulted perhaps from the self-breaking up and
aggregation of layers of indurated tuff containing numerous
grains of quartz,—into which, indeed, the whole mass in one part
passed. The second variety is a reddish non-porphyritic
claystone, quite full of spherical cavities, about half an inch
in diameter, each lined with a collapsed crust formed of crystals
of quartz. The third variety also consists of a pale purple
non-porphyritic claystone, almost wholly formed of concretionary
balls, obscurely arranged in layers, of a less compact and paler
coloured claystone; each ball being on one side partly hollow and
lined with crystals of quartz.
PSEUDO-DIKES.
Some miles up the harbour, in a line of cliffs formed of slightly
metamorphosed tufaceous and porphyritic claystone beds, I
observed three vertical dikes, so closely resembling in general
appearance ordinary volcanic dikes, that I did not doubt, until
closely examining their composition, that they had been injected
from below. The first is straight, with parallel sides, and about
four feet wide; it consists of whitish, indurated tufaceous
matter, precisely like some of the beds intersected by it. The
second dike is more remarkable; it is slightly tortuous, about
eighteen inches thick, and can be traced for a considerable
distance along the beach; it is of a purplish-red or brown
colour, and is formed chiefly of ROUNDED grains of quartz, with
broken crystals of earthy feldspar, scales of black mica, and
minute fragments of claystone porphyry, all firmly united
together in a hard sparing base. The structure of this dike shows
obviously that it is of mechanical and sedimentary origin; yet it
thinned out upwards, and did not cut through the uppermost strata
in the cliffs. This fact at first appears to indicate that the
matter could not have been washed in from above (Upfilled
fissures are known to occur both in volcanic and in ordinary
sedimentary formations. At the Galapagos Archipelago "Volcanic
Islands" etc., there are some striking examples of pseudo-dikes
composed of hard tuff.); but if we reflect on the suction which
would result from a deep-seated fissure being formed, we may
admit that if the fissure were in any part open to the surface,
mud and water might well be drawn into it along its whole course.
The third dike consisted of a hard, rough, white rock, almost
composed of broken crystals of glassy feldspar, with numerous
scales of black mica, cemented in a scanty base; there was little
in the appearance of this rock, to preclude the idea of its
having been a true injected feldspathic dike. The matter
composing these three pseudo-dikes, especially the second one,
appears to have suffered, like the surrounding strata, a certain
degree of metamorphic action; and this has much aided the
deceptive appearance. At Bahia, in Brazil, we have seen that a
true injected hornblendic dike, not only has suffered
metamorphosis, but has been dislocated and even diffused in the
surrounding gneiss, under the form of separate crystals and of
fragments.
FALKLAND ISLANDS.
I have described these islands in a paper published in the third
volume of the "Geological Journal." The mountain-ridges consist
of quartz, and the lower country of clay-slate and sandstone, the
latter containing Palaeozoic fossils. These fossils have been
separately described by Messrs. Morris and Sharpe: some of them
resemble Silurian, and others Devonian forms. In the eastern part
of the group the several parallel ridges of quartz extend in a
west and east line; but further westward the line becomes W.N.W.
and E.S.E., and even still more northerly. The cleavage-planes of
the clay- slate are highly inclined, generally at an angle of
above 50 degrees, and often vertical; they strike almost
invariably in the same direction with the quartz ranges. The
outline of the indented shores of the two main islands, and the
relative positions of the smaller islets, accord with the strike
both of the main axes of elevation and of the cleavage of the
clay- slate.
TIERRA DEL FUEGO.
My notes on the geology of this country are copious, but as they
are unimportant, and as fossils were found only in one district,
a brief sketch will be here sufficient. The east coast from the
S. of Magellan (where the boulder formation is largely developed)
to St. Polycarp's Bay is formed of horizontal tertiary strata,
bounded some way towards the interior by a broad mountainous band
of clay-slate. This great clay-slate formation extends from St.
Le Maire westward for 140 miles, along both sides of the Beagle
Channel to near its bifurcation. South of this channel, it forms
all Navarin Island, and the eastern half of Hoste Island and of
Hardy Peninsula; north of the Beagle Channel it extends in a
north-west line on both sides of Admiralty Sound to Brunswick
Peninsula in the St. of Magellan, and I have reason to believe,
stretches far up the eastern side of the Cordillera. The western
and broken side of Tierra del Fuego towards the Pacific is formed
of metamorphic schists, granite and various trappean rocks: the
line of separation between the crystalline and clay-slate
formations can generally be distinguished, as remarked by Captain
King, by the parallelism in the clay-slate districts of the
shores and channels, ranging in a line between [W. 20 degrees to
40 degrees N.] and [E. 20 degrees to 40 degrees S.].
("Geographical Journal" volume 1 page 155.)
The clay-slate is generally fissile, sometimes siliceous or
ferruginous, with veins of quartz and calcareous spar; it often
assumes, especially on the loftier mountains, an altered
feldspathic character, passing into feldspathic porphyry:
occasionally it is associated with breccia and grauwacke. At Good
Success Bay, there is a little intercalated black crystalline
limestone. At Port Famine much of the clay-slate is calcareous,
and passes either into a mudstone or into grauwacke, including
odd-shaped concretions of dark argillaceous limestone. Here
alone, on the shore a few miles north of Port Famine, and on the
summit of Mount Tarn (2,600 feet high), I found organic remains;
they consist of:—
1. Ancyloceras simplex, d'Orbigny "Pal Franc" Mount Tarn. 2.
Fusus (in imperfect state), d'Orbigny "Pal Franc" Mount Tarn. 3.
Natica, d'Orbigny "Pal Franc" Mount Tarn. 4. Pentacrimus,
d'Orbigny "Pal Franc" Mount Tarn. 5. Lucina excentrica, G.B.
Sowerby, Port Famine. 6. Venus (in imperfect state), G.B.
Sowerby, Port Famine. 7. Turbinolia (?), G.B. Sowerby, Port
Famine. 8. Hamites elatior, G.B. Sowerby, Port Famine.
M. d'Orbigny states that MM. Hombron and Grange found in this
neighbourhood an Ancyloceras, perhaps A. simplex, an Ammonite, a
Plicatula and Modiola. ("Voyage" Part Geolog. page 242.) M.
d'Orbigny believes from the general character of these fossils,
and from the Ancyloceras being identical (as far as its imperfect
condition allows of comparison) with the A. simplex of Europe,
that the formation belongs to an early stage of the Cretaceous
system. Professor E. Forbes, judging only from my specimens,
concurs in the probability of this conclusion. The Hamites
elatior of the above list, of which a description has been given
by Mr. Sowerby, and which is remarkable from its large size, has
not been seen either by M. d'Orbigny or Professor E. Forbes, as,
since my return to England, the specimens have been lost. The
great clay-slate formation of Tierra del Fuego being cretaceous,
is certainly a very interesting fact,—whether we consider the
appearance of the country, which, without the evidence afforded
by the fossils, would form the analogy of most known districts,
probably have been considered as belonging to the Palaeozoic
series,—or whether we view it as showing that the age of this
terminal portion of the great axis of South America, is the same
(as will hereafter be seen) with the Cordillera of Chile and
Peru.
The clay-slate in many parts of Tierra del Fuego, is broken by
dikes and by great masses of greenstone, often highly hornblendic
(In a greenstone-dike in the Magdalen Channel, the feldspar
cleaved with the angle of albite. This dike was crossed, as well
as the surrounding slate, by a large vein of quartz, a
circumstance of unusual occurrence.): almost all the small islets
within the clay-slate districts are thus composed. The slate near
the dikes generally becomes paler-coloured, harder, less fissile,
of a feldspathic nature, and passes into a porphyry or
greenstone: in one case, however, it became more fissile, of a
red colour, and contained minute scales of mica, which were
absent in the unaltered rock. On the east side of Ponsonby Sound
some dikes composed of a pale sonorous feldspathic rock,
porphyritic with a little feldspar, were remarkable from their
number,—there being within the space of a mile at least one
hundred,—from their nearly equalling in bulk the intermediate
slate,—and more especially from the excessive fineness (like the
finest inlaid carpentry) and perfect parallelism of their
junctions with the almost vertical laminae of clay-slate. I was
unable to persuade myself that these great parallel masses had
been injected, until I found one dike which abruptly thinned out
to half its thickness, and had one of its walls jagged, with
fragments of the slate embedded in it.
In Southern Tierra del Fuego, the clay-slate towards its S.W.
boundary, becomes much altered and feldspathic. Thus on Wollaston
Island slate and grauwacke can be distinctly traced passing into
feldspathic rocks and greenstones, including iron pyrites and
epidote, but still retaining traces of cleavage with the usual
strike and dip. One such metamorphosed mass was traversed by
large vein-like masses of a beautiful mixture (as ascertained by
Professor Miller) of green epidote, garnets, and white calcareous
spar. On the northern point of this same island, there were
various ancient submarine volcanic rocks, consisting of
amygdaloids with dark bole and agate,—of basalt with decomposed
olivine—of compact lava with glassy feldspar,—and of a coarse
conglomerate of red scoriae, parts being amygdaloidal with
carbonate of lime. The southern part of Wollaston Island and the
whole of Hermite and Horn Islands, seem formed of cones of
greenstone; the outlying islets of Il Defenso and D. Raminez are
said to consist of porphyritic lava. (Determined by Professor
Jameson. Weddell's "Voyage" page 169.) In crossing Hardy
Peninsula, the slate still retaining traces of its usual
cleavage, passes into columnar feldspathic rocks, which are
succeeded by an irregular tract of trappean and basaltic rocks,
containing glassy feldspar and much iron pyrites: there is, also,
some harsh red claystone porphyry, and an almost true trachyte,
with needles of hornblende, and in one spot a curious slaty rock
divided into quadrangular columns, having a base almost like
trachyte, with drusy cavities lined by crystals, too imperfect,
according to Professor Miller, to be measured, but resembling
Zeagonite. (See Mr. Brooke's Paper in the "London Philosophical
Magazine" volume 10. This mineral occurs in an ancient volcanic
rock near Rome.) In the midst of these singular rocks, no doubt
of ancient submarine volcanic origin, a high hill of feldspathic
clay-slate projected, retaining its usual cleavage. Near this
point, there was a small hillock, having the aspect of granite,
but formed of white albite, brilliant crystals of hornblende
(both ascertained by the reflecting goniometer) and mica; but
with no quartz. No recent volcanic district has been observed in
any part of Tierra del Fuego.
Five miles west of the bifurcation of the Beagle Channel, the
slate- formation, instead of becoming, as in the more southern
parts of Tierra del Fuego, feldspathic, and associated with
trappean or old volcanic rocks, passes by alternations into a
great underlying mass of fine gneiss and glossy clay-slate, which
at no great distance is succeeded by a grand formation of
mica-slate containing garnets. The folia of these metamorphic
schists strike parallel to the cleavage-planes of the clay-slate,
which have a very uniform direction over the whole of this part
of the country: the folia, however, are undulatory and tortuous,
whilst the cleavage- laminae of the slate are straight. These
schists compose the chief mountain-chain of Southern Tierra del
Fuego, ranging along the north side of the northern arm of the
Beagle Channel, in a short W.N.W. and E.S.E. line, with two
points (Mounts Sarmiento and Darwin) rising to heights of 6,800
and 6,900 feet. On the south-western side of this northern arm of
the Beagle Channel, the clay-slate is seen with its STRATA
dipping from the great chain, so that the metamorphic schists
here form a ridge bordered on each side by clay-slate. Further
north, however, to the west of this great range, there is no
clay-slate, but only gneiss, mica, and hornblendic slates,
resting on great barren hills of true granite, and forming a
tract about sixty miles in width. Again, westward of these rocks,
the outermost islands are of trappean formation, which, from
information obtained during the voyages of the "Adventure" and
"Beagle," seem, together with granite, chiefly to prevail along
the western coast as far north as the entrance of the St. of
Magellan (See the Paper by Captain King in the "Geographical
Journal"; also a Letter to Dr. Fitton in "Geological Proceedings"
volume 1 page 29; also some observations by Captain Fitzroy
"Voyages" volume 1 page 375. I am indebted also to Mr. Lyell for
a series of specimens collected by Lieutenant Graves.): a little
more inland, on the eastern side of Clarence Island and S.
Desolation, granite, greenstone, mica-slate, and gneiss appear to
predominate. I am tempted to believe, that where the clay-slate
has been metamorphosed at great depths beneath the surface,
gneiss, mica- slate, and other allied rocks have been formed, but
where the action has taken place nearer the surface, feldspathic
porphyries, greenstones, etc., have resulted, often accompanied
by submarine volcanic eruptions.
Only one other rock, met with in both arms of the Beagle Channel,
deserves any notice, namely a granulo-crystalline mixture of
white albite, black hornblende (ascertained by measurement of the
crystals, and confirmed by Professor Miller), and more or less of
brown mica, but without any quartz. This rock occurs in large
masses, closely resembling in external form granite or syenite:
in the southern arm of the Channel, one such mass underlies the
mica-slate, on which clay-slate was superimposed: this peculiar
plutonic rock which, as we have seen, occurs also in Hardy
Peninsula, is interesting, from its perfect similarity with that
(hereafter often to be referred to under the name of andesite)
forming the great injected axes of the Cordillera of Chile.
The stratification of the clay-slate is generally very obscure,
whereas the cleavage is remarkably well defined: to begin with
the extreme eastern parts of Tierra del Fuego; the
cleavage-planes near the St. of Le Maire strike either W. and E.
or W.S.W. and E.N.E., and are highly inclined; the form of the
land, including Staten Island, indicates that the axes of
elevation have run in this same line, though I was unable to
distinguish the planes of stratification. Proceeding westward, I
accurately examined the cleavage of the clay-slate on the
northern, eastern, and western sides (thirty-five miles apart) of
Navarin Island, and everywhere found the laminae ranging with
extreme regularity, W.N.W. and E.S.E., seldom varying more than
one point of the compass from this direction. (The clay-slate in
this island was in many places crossed by parallel smooth joints.
Out of five cases, the angle of intersection between the strike
of these joints and that of the cleavage-laminae was in two cases
45 degrees and in two others 79 degrees.) Both on the east and
west coasts, I crossed at right angles the cleavage-planes for a
space of about eight miles, and found them dipping at an angle of
between 45 degrees and 90 degrees, generally to S.S.W., sometimes
to N.N.E., and often quite vertically. The S.S.W. dip was
occasionally succeeded abruptly by a N.N.E. dip, and this by a
vertical cleavage, or again by the S.S.W. dip; as in a lofty
cliff on the eastern end of the island the laminae of slate were
seen to be folded into very large steep curves, ranging in the
usual W.N.W. line, I suspect that the varying and opposite dips
may possibly be accounted for by the cleavage- laminae, though to
the eye appearing straight, being parts of large abrupt curves,
with their summits cut off and worn down.
In several places I was particularly struck with the fact, that
the fine laminae of the clay-slate, where cutting straight
through the bands of stratification, and therefore indisputably
true cleavage-planes, differed slightly in their greyish and
greenish tints of colour, in compactness, and in some of the
laminae having a rather more jaspery appearance than others. I
have not seen this fact recorded, and it appears to me important,
for it shows that the same cause which has produced the highly
fissile structure, has altered in a slight degree the
mineralogical character of the rock in the same planes. The bands
of stratification, just alluded to, can be distinguished in many
places, especially in Navarin Island, but only on the weathered
surfaces of the slate; they consist of slightly undulatory zones
of different shades of colour and of thicknesses, and resemble
the marks (more closely than anything else to which I can compare
them) left on the inside of a vessel by the draining away of some
dirty slightly agitated liquid: no difference in composition,
corresponding with these zones, could be seen in freshly
fractured surfaces. In the more level parts of Navarin Island,
these bands of stratification were nearly horizontal; but on the
flanks of the mountains they were inclined from them, but in no
instance that I saw at a very high angle. There can, I think, be
no doubt that these zones, which appear only on the weathered
surfaces, are the last vestiges of the original planes of
stratification, now almost obliterated by the highly fissile and
altered structure which the mass has assumed.
The clay-slate cleaves in the same W.N.W. and E.S.E. direction,
as on Navarin Island, on both sides of the Beagle Channel, on the
eastern side of Hoste Island, on the N.E. side of Hardy
Peninsula, and on the northern point of Wollaston Island;
although in these two latter localities the cleavage has been
much obscured by the metamorphosed and feldspathic condition of
the slate. Within the area of these several islands, including
Navarin Island, the direction of the stratification and of the
mountain- chains is very obscure; though the mountains in several
places appeared to range in the same W.N.W. line with the
cleavage: the outline of the coast, however, does not correspond
with this line. Near the bifurcation of the Beagle Channel, where
the underlying metamorphic schists are first seen, they are
foliated (with some irregularities), in this same W.N.W. line,
and parallel, as before stated, to the main mountain-axis of this
part of the country. Westward of this main range, the metamorphic
schists are foliated, though less plainly, in the same direction,
which is likewise common to the zone of old erupted trappean
rocks, forming the outermost islets. Hence the area, over which
the cleavage of the slate and the foliation of the metamorphic
schists extends with an average W.N.W. and E.S.E. strike, is
about forty miles in a north and south line, and ninety miles in
an east and west line.
Further northward, near Port Famine, the stratification of the
clay-slate and of the associated rocks, is well defined, and
there alone the cleavage and strata-planes are parallel. A little
north of this port there is an anticlinal axis ranging N.W. (or a
little more westerly) and S.E.: south of the port, as far as
Admiralty Sound and Gabriel Channel, the outline of the land
clearly indicates the existence of several lines of elevation in
this same N.W. direction, which, I may add, is so uniform in the
western half of the St. of Magellan, that, as Captain King has
remarked, "a parallel ruler placed on the map upon the projecting
points of the south shore, and extended across the strait, will
also touch the headlands on the opposite coast." ("Geographical
Journal" volume 1 page 170.) It would appear, from Captain King's
observations, that over all this area the cleavage extends in the
same line. Deep-water channels, however, in all parts of Tierra
del Fuego have burst through the trammels both of stratification
and cleavage; most of them may have been formed during the
elevation of the land by long- continued erosion, but others, for
instance the Beagle Channel, which stretches like a narrow canal
for 120 miles obliquely through the mountains, can hardly have
thus originated.
Finally, we have seen that in the extreme eastern point of Tierra
del Fuego, the cleavage and coast-lines extend W. and E. and even
W.S.W. and E.N.E.: over a large area westward, the cleavage, the
main range of mountains, and some subordinate ranges, but not the
outlines of the coast, strike W.N.W., and E.S.E.: in the central
and western parts of the St. of Magellan, the stratification, the
mountain-ranges, the outlines of the coast, and the cleavage all
strike nearly N.W. and S.E. North of the strait, the outline of
the coast, and the mountains on the mainland, run nearly north
and south. Hence we see, at this southern point of the continent,
how gradually the Cordillera bend, from their north and south
course of so many thousand miles in length, into an E. and even
E.N.E. direction.
WEST COAST, FROM THE SOUTHERN CHONOS ISLANDS TO NORTHERN CHILE.
The first place at which we landed north of the St. of Magellan
was near Cape Tres Montes, in latitude 47 degrees S. Between this
point and the Northern Chonos Islands, a distance of 200 miles,
the "Beagle" visited several points, and specimens were collected
for me from the intermediate spaces by Lieutenant Stokes. The
predominant rock is mica-slate, with thick folia of quartz, very
frequently alternating with and passing into a chloritic, or into
a black, glossy, often striated, slightly anthracitic schist,
which soils paper, and becomes white under a great heat, and then
fuses. Thin layers of feldspar, swelling at intervals into well
crystallised kernels, are sometimes included in these black
schists; and I observed one mass of the ordinary black variety
insensibly lose its fissile structure, and pass into a singular
mixture of chlorite, epidote, feldspar, and mica. Great veins of
quartz are numerous in the mica-schists; wherever these occur the
folia are much convoluted. In the southern part of the Peninsula
of Tres Montes, a compact altered feldspathic rock with crystals
of feldspar and grains of quartz is the commonest variety; this
rock exhibits occasionally traces of an original brecciated
structure, and often presents (like the altered state of Tierra
del Fuego) traces of cleavage- planes, which strike in the same
direction with the folia of mica-schist further northward. (The
peculiar, abruptly conical form of the hills in this
neighbourhood, would have led any one at first to have supposed
that they had been formed of injected or intrusive rocks. At
Inchemo Island, a similar rock gradually becomes
granulo-crystalline and acquires scales of mica; and this variety
at S. Estevan becomes highly laminated, and though still
exhibiting some rounded grains of quartz, passes into the black,
glossy, slightly anthracitic schist, which, as we have seen,
repeatedly alternates with and passes into the micaceous and
chloritic schists. Hence all the rocks on this line of coast
belong to one series, and insensibly vary from an altered
feldspathic clay-slate into largely foliated, true mica-schist.
The cleavage of the homogeneous schists, the foliation of those
composed of more or less distinct minerals in layers, and the
planes of alternation of the different varieties or so-called
stratification, are all parallel, and preserve over this 200
miles of coast a remarkable degree of uniformity in direction. At
the northern end of the group, at Low's Harbour, the well-
defined folia of mica-schist everywhere ranged within eight
degrees (or less than one point of the compass) of N. 19 degrees
W. and S. 19 degrees E.; and even the point of dip varied very
little, being always directed to the west and generally at an
angle of forty degrees; I should mention that I had here good
opportunities of observation, for I followed the naked rock on
the beach, transversely to the strike, for a distance of four
miles and a half, and all the way attended to the dip. Along the
outer islands for 100 miles south of Low's Harbour, Lieutenant
Stokes, during his boat- survey, kindly observed for me the
strike of the foliation, and he assures me that it was invariably
northerly, and the dip with one single exception to the west.
Further south at Vallenar Bay, the strike was almost universally
N. 25 degrees W. and the dip, generally at an angle of about 40
degrees to W. 25 degrees S., but in some places almost vertical.
Still farther south, in the neighbourhood of the harbours of Anna
Pink, S. Estevan and S. Andres, and (judging from a distance)
along the southern part of Tres Montes, the foliation and
cleavage extended in a line between [N. 11 degrees to 22 degrees
W.] and [S. 11 degrees to 22 degrees E.]; and the planes dipped
generally westerly, but often easterly, at angles varying from a
gentle inclination to vertical. At A. Pink's Harbour, where the
schists generally dipped easterly, wherever the angle became very
high, the strike changed from N. 11 degrees W. to even as much as
N. 45 degrees W.: in an analogous manner at Vallenar Bay, where
the dip was westerly (viz. on an average directed to W. 25
degrees S.), as soon as the angle became very high, the planes
struck in a line more than 25 degrees west of north. The average
result from all the observations on this 200 miles of coast, is a
strike of N. 19 degrees W. and S. 19 degrees E.: considering that
in each specified place my examination extended over an area of
several miles, and that Lieutenant Stokes' observations apply to
a length of 100 miles, I think this remarkable uniformity is
pretty well established. The prevalence, throughout the northern
half of this line of coast, of a dip in one direction, that is to
the west, instead of being sometimes west and sometimes east, is,
judging from what I have elsewhere seen, an unusual circumstance.
In Brazil, La Plata, the Falkland Islands, and Tierra del Fuego,
there is generally an obvious relation between the axis of
elevation, the outline of the coast, and the strike of the
cleavage or foliation: in the Chonos Archipelago, however,
neither the minor details of the coast-line, nor the chain of the
Cordillera, nor the subordinate transverse mountain-axes, accord
with the strike of the foliation and cleavage: the seaward face
of the numerous islands composing this Archipelago, and
apparently the line of the Cordillera, range N. 11 degrees E.,
whereas, as we have just seen, the average strike of the
foliation is N. 19 degrees W.
There is one interesting exception to the uniformity in the
strike of the foliation. At the northern point of Tres Montes
(latitude 45 degrees 52 minutes) a bold chain of granite, between
two and three thousand feet in height, runs from the coast far
into the interior, in an E.S.E. line, or more strictly E. 28
degrees S. and W. 28 degrees N. (In the distance, other mountains
could be seen apparently ranging N.N.E. and S.S.W. at right
angles to this one. I may add, that not far from Vallenar Bay
there is a fine range, apparently of granite, which has burst
through the mica-slate in a N.E. by E. and S.W. by S. line.) In a
bay, at the northern foot of this range, there are a few islets
of mica-slate, with the folia in some parts horizontal, but
mostly inclined at an average angle of 20 degrees to the north.
On the northern steep flank of the range, there are a few patches
(some quite isolated, and not larger than half a-crown!) of the
mica-schist, foliated with the same northerly dip. On the broad
summit, as far as the southern crest, there is much mica-slate,
in some places even 400 feet in thickness, with the folia all
dipping north, at angles varying from 5 degrees to 20 degrees,
but sometimes mounting up to 30 degrees. The southern flank
consists of bare granite. The mica-slate is penetrated by small
veins of granite, branching from the main body. (The granite
within these veins, as well as generally at the junction with the
mica-slate, is more quartzose than elsewhere. The granite, I may
add, is traversed by dikes running for a very great length in the
line of the mountains; they are composed of a somewhat laminated
eurite, containing crystals of feldspar, hornblende, and octagons
of quartz.) Leaving out of view the prevalent strike of the folia
in other parts of this Archipelago, it might have been expected
that they would have dipped N. 28 degrees E., that is directly
from the ridge, and, considering its abruptness, at a high
inclination; but the real dip, as we have just seen, both at the
foot and on the northern flank, and over the entire summit, is at
a small angle, and directed nearly due north. From these
considerations it occurred to me, that perhaps we here had the
novel and curious case of already inclined laminae obliquely
tilted at a subsequent period by the granitic axis. Mr. Hopkins,
so well known from his mathematical investigations, has most
kindly calculated the problem: the proposition sent was,—Take a
district composed of laminae, dipping at an angle of 40 degrees
to W. 19 degrees S., and let an axis of elevation traverse it in
an E. 28 degrees S. line, what will the position of the laminae
be on the northern flank after a tilt, we will first suppose, of
45 degrees? Mr. Hopkins informs me, that the angle of the dip
will be 28 degrees 31 minutes, and its direction to north 30
degrees 33 minutes west. (On the south side of the axis (where,
however, I did not see any mica-slate) the dip of the folia would
be at an angle of 77 degrees 55 minutes, directed to west 35
degrees 33 minutes south. Hence the two points of dip on the
opposite sides of the range, instead of being as in ordinary
cases directly opposed to each other at an angle of 180 degrees,
would here be only 86 degrees 50 minutes apart.) By varying the
supposed angle of the tilt, our previously inclined folia can be
thrown into any angle between 26 degrees, which is the least
possible angle, and 90 degrees; but if a small inclination be
thus given to them, their point of dip will depart far from the
north, and therefore not accord with the actual position of the
folia of mica-schist on our granitic range. Hence it appears very
difficult, without varying considerably the elements of the
problem, thus to explain the anomalous strike and dip of the
foliated mica- schist, especially in those parts, namely, at the
base of the range, where the folia are almost horizontal. Mr.
Hopkins, however, adds, that great irregularities and lateral
thrusts might be expected in every great line of elevation, and
that these would account for considerable deviations from the
calculated results: considering that the granitic axis, as shown
by the veins, has indisputably been injected after the perfect
formation of the mica-slate, and considering the uniformity of
the strike of the folia throughout the rest of the Archipelago, I
cannot but still think that their anomalous position at this one
point is someway directly and mechanically related to the
intrusion of this W.N.W. and E.S.E. mountain-chain of granite.
Dikes are frequent in the metamorphic schists of the Chonos
Islands, and seem feebly to represent that great band of trappean
and ancient volcanic rocks on the south-western coast of Tierra
del Fuego. At S. Andres I observed in the space of half-a-mile,
seven broad, parallel dikes, composed of three varieties of trap,
running in a N.W. and S.E. line, parallel to the neighbouring
mountain-ranges of altered clay-slate; but they must be of long
subsequent origin to these mountains; for they intersected the
volcanic formation described in the last chapter. North of Tres
Montes, I noticed three dikes differing from each other in
composition, one of them having a euritic base including large
octagons of quartz; these dikes, as well as several of
porphyritic greenstone at Vallenar Bay, extended N.E. and S.W.,
nearly at right angles to the foliation of the schists, but in
the line of their joints. At Low's Harbour, however, a set of
great parallel dikes, one ninety yards and another sixty yards in
width, have been guided by the foliation of the mica-schist, and
hence are inclined westward at an angle of 45 degrees: these
dikes are formed of various porphyritic traps, some of which are
remarkable from containing numerous rounded grains of quartz. A
porphyritic trap of this latter kind, passed in one of the dikes
into a most curious hornstone, perfectly white, with a waxy
fracture and pellucid edges, fusible, and containing many grains
of quartz and specks of iron pyrites. In the ninety-yard dike
several large, apparently now quite isolated, fragments of
mica-slate were embedded: but as their foliation was exactly
parallel to that of the surrounding solid rock, no doubt these
new separate fragments originally formed wedge-shaped depending
portions of a continuous vault or crust, once extending over the
dike, but since worn down and denuded.
CHILOE, VALDIVIA, CONCEPCION.
In Chiloe, a great formation of mica-schist strikingly resembles
that of the Chonos Islands. For a space of eleven miles on the
S.E. coast, the folia were very distinct, though slightly
convoluted, and ranged within a point of N.N.W. and S.S.E.,
dipping either E.N.E. or more commonly W.S.W., at an average
angle of 22 degrees (in one spot, however, at 60 degrees), and
therefore decidedly at a lesser inclination than amongst the
Chonos Islands. On the west and north-western shores, the
foliation was often obscure, though, where best defined, it
ranged within a point of N. by W. and S. by E., dipping either
easterly or westerly, at varying and generally very small angles.
Hence, from the southern part of Tres Montes to the northern end
of Chiloe, a distance of 300 miles, we have closely allied rocks
with their folia striking on an average in the same direction,
namely between N. 11 degrees and 22 degrees W. Again, at
Valdivia, we meet with the same mica-schist, exhibiting nearly
the same mineralogical passages as in the Chonos Archipelago,
often, however, becoming more ferruginous, and containing so much
feldspar as to pass into gneiss. The folia were generally well
defined; but nowhere else in South America did I see them varying
so much in direction: this seemed chiefly caused by their forming
parts, as I could sometimes distinctly trace, of large flat
curves: nevertheless, both near the settlement and towards the
interior, a N.W. and S.E. strike seemed more frequent than any
other direction; the angle of the dip was generally small. At
Concepcion, a highly glossy clay-slate had its cleavage often
slightly curvilinear, and inclined, seldom at a high angle,
towards various points of the compass: but here, as at Valdivia,
a N.W. and S.E. strike seemed to be the most frequent one.
((FIGURE 23.) I observed in some parts that the tops of the
laminae of the clay-slate (b in Figure 23) under the superficial
detritus and soil (a) were bent, sometimes without being broken,
as represented in Figure 23, which is copied from one given by
Sir H. De la Beche (page 42 "Geological Manual") of an exactly
similar phenomenon in Devonshire. Mr. R.A.C. Austen, also, in his
excellent paper on S.E. Devon ("Geological Transactions" volume 6
page 437), has described this phenomenon; he attributes it to the
action of frosts, but at the same time doubts whether the frosts
of the present day penetrate to a sufficient depth. As it is
known that earthquakes particularly affect the surface of the
ground, it occurred to me that this appearance might perhaps be
due, at least at Concepcion, to their frequent occurrence; the
superficial layers of detritus being either jerked in one
direction, or, where the surface was inclined, pushed a little
downwards during each strong vibration. In North Wales I have
seen a somewhat analogous but less regular appearance, though on
a greater scale ("London Philosophical Magazine" volume 21 page
184), and produced by a quite different cause, namely, by the
stranding of great icebergs; this latter appearance has also been
observed in N. America.)
In certain spots large quartz veins were numerous, and near them,
the cleavage, as was the case with the foliation of the schists
in the Chonos Archipelago, became extremely tortuous.
At the northern end of Quiriquina Island, in the Bay of
Concepcion, at least eight rudely parallel dikes, which have been
guided to a certain extent by the cleavage of the slate, occur
within the space of a quarter of a mile. They vary much in
composition, resembling in many respects the dikes at Low's
Harbour: the greater number consist of feldspathic porphyries,
sometimes containing grains of quartz: one, however, was black
and brilliant, like an augitic rock, but really formed of
feldspar; others of a feldspathic nature were perfectly white,
with either an earthy or crystalline fracture, and including
grains and regular octagons of quartz; these white varieties
passed into ordinary greenstones. Although, both here and at
Low's Harbour, the nature of the rock varied considerably in the
same dike, yet I cannot but think that at these two places and in
other parts of the Chonos group, where the dikes, though close to
each other and running parallel, are of different composition,
that they must have been formed at different periods. In the case
of Quiriquina this is a rather interesting conclusion, for these
eight parallel dikes cut through the metamorphic schists in a
N.W. and S.E. line, and since their injection the overlying
cretaceous or tertiary strata have been tilted (whilst still
under the sea) from a N.W. by N. and S.E. by S. line; and again,
during the great earthquake of February 1835, the ground in this
neighbourhood was fissured in N.W. and S.E. lines; and from the
manner in which buildings were thrown down, it was evident that
the surface undulated in this same direction. ("Geological
Transactions" volume 6 pages 602 and 617. "Journal of Researches"
2nd edition page 307.)
CENTRAL AND NORTHERN CHILE.
Northward of Concepcion, as far as Copiapo, the shores of the
Pacific consist, with the exception of some small tertiary
basins, of gneiss, mica- schist, altered clay-slate, granite,
greenstone and syenite: hence the coast from Tres Montes to
Copiapo, a distance of 1,200 miles, and I have reason to believe
for a much greater space, is almost similarly constituted.
Near Valparaiso the prevailing rock is gneiss, generally
including much hornblende: concretionary balls formed of
feldspar, hornblende and mica, from two or three feet in
diameter, are in very many places conformably enfolded by the
foliated gneiss: veins of quartz and feldspar, including black
schorl and well-crystallised epidote, are numerous. Epidote
likewise occurs in the gneiss in thin layers, parallel to the
foliation of the mass. One large vein of a coarse granitic
character was remarkable from in one part quite changing its
character, and insensibly passing into a blackish porphyry,
including acicular crystals of glassy feldspar and of hornblende:
I have never seen any other such case. (Humboldt "Personal
Narrative" volume 4 page 60, has described with much surprise,
concretionary balls, with concentric divisions, composed of
partially vitreous feldspar, hornblende, and garnets, included
within great veins of gneiss, which cut across the mica-slate
near Venezuela.)
I shall in the few following remarks on the rocks of Chile allude
exclusively to their foliation and cleavage. In the gneiss round
Valparaiso the strike of the foliation is very variable, but I
think about N. by W. and S. by E. is the commonest direction;
this likewise holds good with the cleavage of the altered
feldspathic clay-slates, occasionally met with on the coast for
ninety miles north of Valparaiso. Some feldspathic slate,
alternating with strata of claystone porphyry in the Bell of
Quillota and at Jajuel, and therefore, perhaps, belonging to a
later period than the metamorphic schists on the coast, cleaved
in this same direction. In the Eastern Cordillera, in the
Portillo Pass, there is a grand mass of mica- slate, foliated in
a north and south line, and with a high westerly dip: in the
Uspallata range, clay-slate and grauwacke have a highly inclined,
nearly north and south cleavage, though in some parts the strike
is irregular: in the main or Cumbre range, the direction of the
cleavage in the feldspathic clay-slate is N.W. and S.E.
Between Coquimbo and Guasco there are two considerable formations
of mica- slate, in one of which the rock passed sometimes into
common clay-slate and sometimes into a glossy black variety, very
like that in the Chonos Archipelago. The folia and cleavage of
these rocks ranged between [N. and N.W. by N.] and [S. and S.W.
by S.]. Near the Port of Guasco several varieties of altered
clay-slate have a quite irregular cleavage. Between Guasco and
Copiapo, there are some siliceous and talcaceous slates cleaving
in a north and south line, with an easterly dip of between 60 and
70 degrees: high up, also, the main valley of Copiapo, there is
mica-slate with a high easterly dip. In the whole space between
Valparaiso and Copiapo an easterly dip is much more common than
an opposite or westerly one.
CONCLUDING REMARKS ON CLEAVAGE AND FOLIATION.
In this southern part of the southern hemisphere, we have seen
that the cleavage-laminae range over wide areas with remarkable
uniformity, cutting straight through the planes of
stratification, but yet being parallel in strike to the main axes
of elevation, and generally to the outlines of the coast. (In my
paper on the Falkland Islands "Geological Journal" volume 3 page
267, I have given a curious case on the authority of Captain
Sulivan, R.N., of much folded beds of clay-slate, in some of
which the cleavage is perpendicular to the horizon, and in others
it is perpendicular to each curvature or fold of the bed: this
appears a new case.) The dip, however, is as variable, both in
angle and in direction (that is, sometimes being inclined to the
one side and sometimes to the directly opposite side), as the
strike is uniform. In all these respects there is a close
agreement with the facts given by Professor Sedgwick in his
celebrated memoir in the "Geological Transactions," and by Sir
R.I. Murchison in his various excellent discussions on this
subject. The Falkland Islands, and more especially Tierra del
Fuego, offer striking instances of the lines of cleavage, the
principle axes of elevation, and the outlines of the coast,
gradually changing together their courses. The direction which
prevails throughout Tierra del Fuego and the Falkland Islands,
namely, from west with some northing to east with some southing,
is also common to the several ridges in Northern Patagonia and in
the western parts of Banda Oriental: in this latter province, in
the Sierra Tapalguen, and in the Western Falkland Island, the W.
by N., or W.N.W. and E.S.E., ridges, are crossed at right angles
by others ranging N.N.E. and S.S.W.
The fact of the cleavage-laminae in the clay-slate of Tierra del
Fuego, where seen cutting straight through the planes of
stratification, and where consequently there could be no doubt
about their nature, differing slightly in colour, texture, and
hardness, appears to me very interesting. In a thick mass of
laminated, feldspathic and altered clay-slate, interposed between
two great strata of porphyritic conglomerate in Central Chile,
and where there could be but little doubt about the bedding, I
observed similar slight differences in composition, and likewise
some distinct thin layers of epidote, parallel to the highly
inclined cleavage of the mass. Again, I incidentally noticed in
North Wales, where glaciers had passed over the truncated edges
of the highly inclined laminae of clay-slate, that the surface,
though smooth, was worn into small parallel undulations, caused
by the competent laminae being of slightly different degrees of
hardness. ("London Philosophical Magazine" volume 21 page 182.)
With reference to the slates of North Wales, Professor Sedgwick
describes the planes of cleavage, as "coated over with chlorite
and semi-crystalline matter, which not only merely define the
planes in question, but strike in parallel flakes through the
whole mass of the rock." ("Geological Transactions" volume 3 page
471.) In some of those glossy and hard varieties of clay-slate,
which may often be seen passing into mica-schist, it has appeared
to me that the cleavage- planes were formed of excessively thin,
generally slighted convoluted, folia, composed of microscopically
minute scales of mica. From these several facts, and more
especially from the case of the clay-slate in Tierra del Fuego,
it must, I think, be concluded, that the same power which has
impressed on the slate its fissile structure or cleavage has
tended to modify its mineralogical character in parallel planes.
Let us now turn to the foliation of the metamorphic schists, a
subject which has been much less attended to. As in the case of
cleavage-laminae, the folia preserve over very large areas a
uniform strike: thus Humboldt found for a distance of 300 miles
in Venezuela, and indeed over a much larger space, gneiss,
granite, mica, and clay-slate, striking very uniformly N.E. and
S.W., and dipping at an angle of between 60 and 70 degrees to
N.W. ("Personal Narrative" volume 6 page 59 et seq.); it would
even appear from the facts given in this chapter, that the
metamorphic rocks throughout the north-eastern part of South
America are generally foliated within two points of N.E. and S.W.
Over the eastern parts of Banda Oriental, the foliation strikes
with a high inclination, very uniformly N.N.E. to S.S.W., and
over the western parts, in a W. by N. and E. by S. line. For a
space of 300 miles on the shores of the Chonos and Chiloe
Islands, we have seen that the foliation seldom deviates more
than a point of the compass from a N. 19 degrees W. and S. 19
degrees E. strike. As in the case of cleavage, the angle of the
dip in foliated rocks is generally high but variable, and
alternates from one side of the line of strike to the other side,
sometimes being vertical: in the Northern Chonos Islands,
however, the folia are inclined almost always to the west; in
nearly the same manner, the cleavage-laminae in Southern Tierra
del Fuego certainly dip much more frequently to S.S.W. than to
the opposite point. In Eastern Banda Oriental, in parts of
Brazil, and in some other districts, the foliation runs in the
same direction with the mountain-ranges and adjoining
coast-lines: amongst the Chonos Islands, however, this
coincidence fails, and I have given my reasons for suspecting
that one granitic axis has burst through and tilted the already
inclined folia of mica-schist: in the case of cleavage, the
coincidence between its strike and that of the main
stratification seems sometimes to fail. (Cases are given by Mr.
Jukes in his "Geology of Newfoundland" page 130.) Foliation and
cleavage resemble each other in the planes winding round
concretions, and in becoming tortuous where veins of quartz
abound. (I have seen in Brazil and Chile concretions thus
enfolded by foliated gneiss; and Macculloch "Highlands" volume 1
page 64, has described a similar case. For analogous cases in
clay-slate, see Professor Henslow's Memoir in "Cambridge
Philosophical Transactions" volume 1 page 379, and Macculloch's
"Classification of Rocks" page 351. With respect to both
foliation and cleavage becoming tortuous where quartz-veins
abound, I have seen instances near Monte Video, at Concepcion,
and in the Chonos Islands. See also Mr. Greenough's "Critical
Examination" page 78.) On the flanks of the mountains both in
Tierra del Fuego and in other countries, I have observed that the
cleavage-planes frequently dip at a high angle inwards; and this
was long ago observed by Von Buch to be the case in Norway: this
fact is perhaps analogous to the folded, fan-like or radiating
structure in the metamorphic schists of the Alps, in which the
folia in the central crests are vertical and on the two flanks
inclined inwards. (Studer in "Edinburgh New Philosophical
Journal" volume 23 page 144.) Where masses of fissile and
foliated rocks alternate together, the cleavage and foliation, in
all cases which I have seen, are parallel. Where in one district
the rocks are fissile, and in another adjoining district they are
foliated, the planes of cleavage and foliation are likewise
generally parallel: this is the case with the feldspathic
homogeneous slates in the southern part of the Chonos group,
compared with the fine foliated mica-schists of the northern
part; so again the clay- slate of the whole eastern side of
Tierra del Fuego cleaves in exactly the same line with the
foliated gneiss and mica-slate of the western coast; other
analogous instances might have been adduced. (I have given a case
in Australia. See my "Volcanic Islands.")
With respect to the origin of the folia of quartz, mica,
feldspar, and other minerals composing the metamorphic schists,
Professor Sedgwick, Mr. Lyell, and most authors believe, that the
constituent parts of each layer were separately deposited as
sediment, and then metamorphosed. This view, in the majority of
cases, I believe to be quite untenable. In those not uncommon
instances, where a mass of clay-slate, in approaching granite,
gradually passes into gneiss, we clearly see that folia of
distinct minerals can originate through the metamorphosis of a
homogeneous fissile rock. (I have described in "Volcanic Islands"
a good instance of such a passage at the Cape of Good Hope.) The
deposition, it may be remarked, of numberless alternations of
pure quartz, and of the elements of mica or feldspar does not
appear a probable event. (See some excellent remarks on this
subject, in D'Aubuisson's "Traite de Geog." tome 1 page 297. Also
some remarks by Mr. Dana in "Silliman's American Journal" volume
45 page 108.) In those districts in which the metamorphic schists
are foliated in planes parallel to the cleavage of the rocks in
an adjoining district, are we to believe that the folia are due
to sedimentary layers, whilst the cleavage- laminae, though
parallel, have no relation whatever to such planes of deposition?
On this view, how can we reconcile the vastness of the areas over
which the strike of the foliation is uniform, with what we see in
disturbed districts composed of true strata: and especially, how
can we understand the high and even vertical dip throughout many
wide districts, which are not mountainous, and throughout some,
as in Western Banda Oriental, which are not even hilly? Are we to
admit that in the northern part of the Chonos Archipelago,
mica-slate was first accumulated in parallel horizontal folia to
a thickness of about four geographical miles, and then upturned
at an angle of forty degrees; whilst, in the southern part of
this same Archipelago, the cleavage-laminae of closely allied
rocks, which none would imagine had ever been horizontal, dip at
nearly the same angle, to nearly the same point?
Seeing, then, that foliated schists indisputably are sometimes
produced by the metamorphosis of homogeneous fissile rocks;
seeing that foliation and cleavage are so closely analogous in
the several above-enumerated respects; seeing that some fissile
and almost homogeneous rocks show incipient mineralogical changes
along the planes of their cleavage, and that other rocks with a
fissile structure alternate with, and pass into varieties with a
foliated structure, I cannot doubt that in most cases foliation
and cleavage are parts of the same process: in cleavage there
being only an incipient separation of the constituent minerals;
in foliation a much more complete separation and crystallisation.
The fact often referred to in this chapter, of the foliation and
the so- called strata in the metamorphic series,—that is, the
alternating masses of different varieties of gneiss, mica-schist,
and hornblende-slate, etc.,- -being parallel to each other, at
first appears quite opposed to the view, that the folia have no
relation to the planes of original deposition. Where the
so-called beds are not very thick and of widely different
mineralogical composition from each other, I do not think that
there is any difficulty in supposing that they have originated in
an analogous manner with the separate folia. We should bear in
mind what thick strata, in ordinary sedimentary masses, have
obviously been formed by a concretionary process. In a pile of
volcanic rocks on the Island of Ascension, there are strata,
differing quite as much in appearance as the ordinary varieties
of the metamorphic schists, which undoubtedly have been produced,
not by successive flowings of lava, but by internal molecular
changes. Near Monte Video, where the stratification, as it would
be called, of the metamorphic series is, in most parts,
particularly well developed, being as usual, parallel to the
foliation, we have seen that a mass of chloritic schist, netted
with quartz-veins, is entangled in gneiss, in such a manner as to
show that it had certainly originated in some process of
segregation: again, in another spot, the gneiss tended to pass
into hornblendic schist by alternating with layers of quartz; but
these layers of quartz almost certainly had never been separately
deposited, for they were absolutely continuous with the numerous
intersecting veins of quartz. I have never had an opportunity of
tracing for any distance, along the line both of strike and of
dip, the so-called beds in the metamorphic schists, but I
strongly suspect that they would not be found to extend with the
same character, very far in the line either of their dip or
strike. Hence I am led to believe, that most of the so-called
beds are of the nature of complex folia, and have not been
separately deposited. Of course, this view cannot be extended to
THICK masses included in the metamorphic series, which are of
totally different composition from the adjoining schists, and
which are far extended, as is sometimes the case with quartz and
marble; these must generally be of the nature of true strata.
(Macculloch "Classification of Rocks" page 364, states that
primary limestones are often found in irregular masses or great
nodules, "which can scarcely be said to possess a stratified
shape!") Such strata, however, will almost always strike in the
same direction with the folia, owing to the axes of elevation
being in most countries parallel to the strike of the foliation;
but they will generally dip at a different angle from that of the
foliation; and the angle of the foliation in itself almost always
varies much: hence, in crossing a metamorphosed schistose
district, it would require especial attention to discriminate
between true strata of deposition and complex foliated masses.
The mere presence of true strata in the midst of a set of
metamorphic schists, is no argument that the foliation is of
sedimentary origin, without it be further shown in each case,
that the folia not only strike, but dip throughout in parallel
planes with those of the true stratification.
As in some cases it appears that where a fissile rock has been
exposed to partial metamorphic action, for instance from the
irruption of granite, the foliation has supervened on the already
existing cleavage-planes; so perhaps in some instances, the
foliation of a rock may have been determined by the original
planes of deposition or of oblique current-laminae: I have,
however, myself, never seen such a case, and I must maintain that
in most extensive metamorphic areas, the foliation is the extreme
result of that process, of which cleavage is the first effect.
That foliation may arise without any previous structural
arrangement in the mass, we may infer from injected, and
therefore once liquified, rocks, both of volcanic and plutonic
origin, sometimes having a "grain" (as expressed by Professor
Sedgwick), and sometimes being composed of distinct folia or
laminae of different compositions. In my work on "Volcanic
Islands," I have given several instances of this structure in
volcanic rocks, and it is not uncommonly seen in plutonic
masses—thus, in the Cordillera of Chile, there are gigantic
mountain-like masses of red granite, which have been injected
whilst liquified, and which, nevertheless, display in parts a
decidedly laminar structure. (As remarked in a former part of
this chapter, I suspect that the boldly conical mountains of
gneiss-granite, near Rio de Janeiro, in which the constituent
minerals are arranged in parallel planes, are of intrusive
origin. We must not, however, forget the lesson of caution taught
by the curious claystone porphyries of Port Desire, in which we
have seen that the breaking up and aggregation of a thinly
stratified tufaceous mass, has yielded a rock semi-porphyritic
with crystals of feldspar, arranged in the planes of original
deposition.)
Finally, we have seen that the planes of cleavage and of
foliation, that is, of the incipient process and of the final
result, generally strike parallel to the principal axes of
elevation, and to the outline of the land: the strike of the axes
of elevation (that is, of the lines of fissures with the strata
on their edges upturned), according to the reasoning of Mr.
Hopkins, is determined by the form of the area undergoing changes
of level, and the consequent direction of the lines of tension
and fissure. Now, in that remarkable pile of volcanic rocks at
Ascension, which has several times been alluded to (and in some
other cases), I have endeavoured to show, that the lamination of
the several varieties, and their alternations, have been caused
by the moving mass, just before its final consolidation, having
been subjected (as in a glacier) to planes of different tension;
this difference in the tension affecting the crystalline and
concretionary processes. (In "Volcanic Islands.") One of the
varieties of rock thus produced at Ascension, at first sight,
singularly resembles a fine-grained gneiss; it consists of quite
straight and parallel zones of excessive tenuity, of more or less
coloured crystallised feldspar, of distinct crystals of quartz,
diopside, and oxide of iron. These considerations,
notwithstanding the experiments made by Mr. Fox, showing the
influence of electrical currents in producing a structure like
that of cleavage, and notwithstanding the apparently inexplicable
variation, both in the inclination of the cleavage-laminae and in
their dipping first to one side and then to the other side of the
line of strike, lead me to suspect that the planes of cleavage
and foliation are intimately connected with the planes of
different tension, to which the area was long subjected, AFTER
the main fissures or axes of upheavement had been formed, but
BEFORE the final consolidation of the mass and the total
cessation of all molecular movement.
CHAPTER VII. CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.
Central Chile. Basal formations of the Cordillera. Origin of the
porphyritic clay-stone conglomerate. Andesite. Volcanic rocks.
Section of the Cordillera by the Peuquenes are Portillo Pass. Great
gypseous formation. Peuquenes line; thickness of strata, fossils of.
Portillo line. Conglomerate, orthitic granite, mica-schist, volcanic
rocks of. Concluding remarks on the denudation and elevation of the
Portillo line. Section by the Cumbre, or Uspallata Pass. Porphyries.
Gypseous strata. Section near the Puente del Inca; fossils of. Great
subsidence. Intrusive porphyries. Plain of Uspallata. Section of the
Uspallata chain. Structure and nature of the strata. Silicified
vertical trees. Great subsidence. Granitic rocks of axis. Concluding
remarks on the Uspallata range; origin subsequent to that of the main
Cordillera; two periods of subsidence; comparison with the Portillo
chain.
The district between the Cordillera and the Pacific, on a rude
average, is from about eighty to one hundred miles in width. It
is crossed by many chains of mountains, of which the principal
ones, in the latitude of Valparaiso and southward of it, range
nearly north and south; but in the more northern parts of the
province, they run in almost every possible direction. Near the
Pacific, the mountain-ranges are generally formed of syenite or
granite, and or of an allied euritic porphyry; in the low
country, besides these granitic rocks and greenstone, and much
gneiss, there are, especially northward of Valparaiso, some
considerable districts of true clay-slate with quartz veins,
passing into a feldspathic and porphyritic slate; there is also
some grauwacke and quartzose and jaspery rocks, the latter
occasionally assuming the character of the basis of claystone
porphyry: trap-dikes are numerous. Nearer the Cordillera the
ranges (such as those of S. Fernando, the Prado (Meyen "Reise um
Erde" th. 1 s. 235.), and Aconcagua) are formed partly of
granitic rocks, and partly of purple porphyritic conglomerates,
claystone porphyry, greenstone porphyry, and other rocks, such as
we shall immediately see, form the basal strata of the main
Cordillera. In the more northern parts of Chile, this porphyritic
series extends over large tracts of country far from the
Cordillera; and even in Central Chile such occasionally occur in
outlying positions.
I will describe the Campana of Quillota, which stands only
fifteen miles from the Pacific, as an instance of one of these
outlying masses. This hill is conspicuous from rising to the
height of 6,400 feet: its summit shows a nucleus, uncovered for a
height of 800 feet, of fine greenstone, including epidote and
octahedral magnetic iron ore; its flanks are formed of great
strata of porphyritic claystone conglomerate associated with
various true porphyries and amygdaloids, alternating with thick
masses of a highly feldspathic, sometimes porphyritic,
pale-coloured slaty rock, with its cleavage-laminae dipping
inwards at a high angle. At the base of the hill there are
syenites, a granular mixture of quartz and feldspar, and harsh
quartzose rocks, all belonging to the basal metamorphic series. I
may observe that at the foot of several hills of this class,
where the porphyries are first seen (as near S. Fernando, the
Prado, Las Vacas, etc.), similar harsh quartzose rocks and
granular mixtures of quartz and feldspar occur, as if the more
fusible constituent parts of the granitic series had been drawn
off to form the overlying porphyries.
In Central Chile, the flanks of the main Cordillera, into which I
penetrated by four different valleys, generally consist of
distinctly stratified rocks. The strata are inclined at angles
varying from sometimes even under ten, to twenty degrees, very
rarely exceeding forty degrees: in some, however, of the quite
small, exterior, spur-like ridges, the inclination was not
unfrequently greater. The dip of the strata in the main outer
lines was usually outwards or from the Cordillera, but in
Northern Chile frequently inwards,—that is, their basset-edges
fronted the Pacific. Dikes occur in extraordinary numbers. In the
great, central, loftiest ridges, the strata, as we shall
presently see, are almost always highly inclined and often
vertical. Before giving a detailed account of my two sections
across the Cordillera, it will, I think, be convenient to
describe the basal strata as seen, often to a thickness of four
or five thousand feet, on the flanks of the outer lines.
BASAL STRATA OF THE CORDILLERA.
The prevailing rock is a purplish or greenish, porphyritic
claystone conglomerate. The embedded fragments vary in size from
mere particles to blocks as much as six or eight inches (rarely
more) in diameter; in many places, where the fragments were
minute, the signs of aqueous deposition were unequivocally
distinct; where they were large, such evidence could rarely be
detected. The basis is generally porphyritic with perfect
crystals of feldspar, and resembles that of a true injected
claystone porphyry: often, however, it has a mechanical or
sedimentary aspect, and sometimes (as at Jajuel) is jaspery. The
included fragments are either angular, or partially or quite
rounded (Some of the rounded fragments in the porphyritic
conglomerate near the Baths of Cauquenes, were marked with radii
and concentric zones of different shades of colour: any one who
did not know that pebbles, for instance flint pebbles from the
chalk, are sometimes zoned concentrically with their worn and
rounded surfaces, might have been led to infer, that these balls
of porphyry were not true pebbles, but had originated in
concretionary action.); in some parts the rounded, in others the
angular fragments prevail, and usually both kinds are mixed
together: hence the word BRECCIA ought strictly to be appended to
the term PORPHYRITIC CONGLOMERATE. The fragments consist of many
varieties of claystone porphyry, usually of nearly the same
colour with the surrounding basis, namely, purplish-reddish,
brownish, mottled or bright green; occasionally fragments of a
laminated, pale-coloured, feldspathic rock, like altered
clay-slate are included; as are sometimes grains of quartz, but
only in one instance in Central Chile (namely, at the mines of
Jajuel) a few pebbles of quartz. I nowhere observed mica in this
formation, and rarely hornblende; where the latter mineral did
occur, I was generally in doubt whether the mass really belonged
to this formation, or was of intrusive origin. Calcareous spar
occasionally occurs in small cavities; and nests and layers of
epidote are common. In some few places in the finer-grained
varieties (for instance, at Quillota), there were short,
interrupted layers of earthy feldspar, which could be traced,
exactly as at Port Desire, passing into large crystals of
feldspar: I doubt, however, whether in this instance the layers
had ever been separately deposited as tufaceous sediment.
All the varieties of porphyritic conglomerates and breccias pass
into each other, and by innumerable gradations into porphyries no
longer retaining the least trace of mechanical origin: the
transition appears to have been effected much more easily in the
finer-grained, than in the coarser-grained varieties. In one
instance, near Cauquenes, I noticed that a porphyritic
conglomerate assumed a spheroidal structure, and tended to become
columnar. Besides the porphyritic conglomerates and the perfectly
characterised porphyries, of metamorphic origin, there are other
porphyries, which, though differing not at all or only slightly
in composition, certainly have had a different origin: these
consist of pink or purple claystone porphyries, sometimes
including grains of quartz,—of greenstone porphyry, and of other
dusky rocks, all generally porphyritic with fine, large, tabular,
opaque crystals, often placed crosswise, of feldspar cleaving
like albite (judging from several measurements), and often
amygdaloidal with silex, agate, carbonate of lime, green and
brown bole. (This bole is a very common mineral in the
amygdaloidal rocks; it is generally of a greenish- brown colour,
with a radiating structure; externally it is black with an almost
metallic lustre, but often coated by a bright green film. It is
soft and can be scratched by a quill; under the blowpipe swells
greatly and becomes scaly, then fuses easily into a black
magnetic bead. This substance is evidently similar to that which
often occurs in submarine volcanic rocks. An examination of some
very curious specimens of a fine porphyry (from Jajuel) leads me
to suspect that some of these amygdaloidal balls, instead of
having been deposited in pre-existing air-vesicles, are of
concretionary origin; for in these specimens, some of the
pea-shaped little masses (often externally marked with minute
pits) are formed of a mixture of green earth with stony matter,
like the basis of the porphyry, including minute imperfect
crystals of feldspar; and these pea-shaped little masses are
themselves amygdaloidal with minute spheres of the green earth,
each enveloped by a film of white, apparently feldspathic, earthy
matter: so that the porphyry is doubly amygdaloidal. It should
not, however, be overlooked, that all the strata here have
undergone metamorphic action, which may have caused crystals of
feldspar to appear, and other changes to be effected, in the
originally simple amygdaloidal balls. Mr. J.D. Dana, in an
excellent paper on Trap-rocks "Edinburgh New Philosophical
Journal" volume 41 page 198, has argued with great force, that
all amygdaloidal minerals have been deposited by aqueous
infiltration. I may take this opportunity of alluding to a
curious case, described in my work on "Volcanic Islands," of an
amygdaloid with many of its cells only half filled up with a
mesotypic mineral. M. Rose has described an amygdaloid, brought
by Dr. Meyen "Reise um Erde" Th. 1. s. 316, from Chile, as
consisting of crystallised quartz, with crystals of stilbite
within, and lined externally by green earth.) These several
porphyritic and amygdaloidal varieties never show any signs of
passing into masses of sedimentary origin: they occur both in
great and small intrusive masses, and likewise in strata
alternating with those of the porphyritic conglomerate, and with
the planes of junction often quite distinct, yet not seldom
blended together. In some of these intrusive masses, the
porphyries exhibit, more or less plainly, a brecciated structure,
like that often seen in volcanic masses. These brecciated
porphyries could generally be distinguished at once from the
metamorphosed, porphyritic breccia- conglomerates, by all the
fragments being angular and being formed of the same variety, and
by the absence of every trace of aqueous deposition. One of the
porphyries above specified, namely, the greenstone porphyry with
large tabular crystals of albite, is particularly abundant, and
in some parts of the Cordillera (as near St. Jago) seemed more
common even than the purplish porphyritic conglomerate. Numerous
dikes likewise consist of this greenstone porphyry; others are
formed of various fine-grained trappean rocks; but very few of
claystone porphyry: I saw no true basaltic dikes.
In several places in the lower part of the series, but not
everywhere, thick masses of a highly feldspathic, often
porphyritic, slaty rock occur interstratified with the
porphyritic conglomerate; I believe in one or two cases blackish
limestone has been found in a similar position. The feldspathic
rock is of a pale grey or greenish colour; it is easily fusible;
where porphyritic, the crystals of feldspar are generally small
and vitreous: it is distinctly laminated, and sometimes includes
parallel layers of epidote (This mineral is extremely common in
all the formations of Chile; in the gneiss near Valparaiso and in
the granitic veins crossing it, in the injected greenstone
crowning the C. of Quillota, in some granitic porphyries, in the
porphyritic conglomerate, and in the feldspathic clay-slates.);
the lamination appears to be distinct from stratification.
Occasionally this rock is somewhat curious; and at one spot,
namely, at the C. of Quillota, it had a brecciated structure.
Near the mines of Jajuel, in a thick stratum of this feldspathic,
porphyritic slate, there was a layer of hard, blackish,
siliceous, infusible, compact clay-slate, such as I saw nowhere
else; at the same place I was able to follow for a considerable
distance the junction between the slate and the conformably
underlying porphyritic conglomerate, and they certainly passed
gradually into each other. Wherever these slaty feldspathic rocks
abound, greenstone seems common; at the C. of Quillota a bed of
well-crystallised greenstone lay conformably in the midst of the
feldspathic slate, with the upper and lower junctions passing
insensibly into it. From this point, and from the frequently
porphyritic condition of the slate, I should perhaps have
considered this rock as an erupted one (like certain laminated
feldspathic lavas in the trachytic series), had I not seen in
Tierra del Fuego how readily true clay-slate becomes feldspathic
and porphyritic, and had I not seen at Jajuel the included layer
of black, siliceous clay-slate, which no one could have thought
of igneous origin. The gentle passage of the feldspathic slate,
at Jajuel, into the porphyritic conglomerate, which is certainly
of aqueous origin, should also be taken in account.
The alternating strata of porphyries and porphyritic
conglomerate, and with the occasionally included beds of
feldspathic slate, together make a grand formation; in several
places within the Cordillera, I estimated its thickness at from
six to seven thousand feet. It extends for many hundred miles,
forming the western flank of the Chilean Cordillera; and even at
Iquique in Peru, 850 miles north of the southernmost point
examined by me in Chile, the coast-escarpment which rises to a
height of between two and three thousand feet is thus composed.
In several parts of Northern Chile this formation extends much
further towards the Pacific, over the granitic and metamorphic
lower rocks, than it does in Central Chile; but the main
Cordillera may be considered as its central line, and its breadth
in an east and west direction is never great. At first the origin
of this thick, massive, long but narrow formation, appeared to me
very anomalous: whence were derived, and how were dispersed the
innumerable fragments, often of large size, sometimes angular and
sometimes rounded, and almost invariably composed of porphyritic
rocks? Seeing that the interstratified porphyries are never
vesicular and often not even amygdaloidal, we must conclude that
the pile was formed in deep water; how then came so many
fragments to be well rounded and so many to remain angular,
sometimes the two kinds being equally mingled, sometimes one and
sometimes the other preponderating? That the claystone,
greenstone, and other porphyries and amygdaloids, which lie
CONFORMABLY between the beds of conglomerate, are ancient
submarine lavas, I think there can be no doubt; and I believe we
must look to the craters whence these streams were erupted, as
the source of the breccia- conglomerate; after the great
explosion, we may fairly imagine that the water in the heated and
scarcely quiescent crater would remain for a considerable time
sufficiently agitated to triturate and round the loose fragments,
few or many in number, would be shot forth at the next eruption,
associated with few or many angular fragments, according to the
strength of the explosion. (This certainly seems to have taken
place in some recent volcanic archipelagos, as at the Galapagos,
where numerous craters are exclusively formed of tuff and
fragments of lava.) The porphyritic conglomerate being purple or
reddish, even when alternating with dusty- coloured or bright
green porphyries and amygdaloids, is probably an analogous
circumstance to the scoriae of the blackish basalts being often
bright red. The ancient submarine orifices whence the porphyries
and their fragments were ejected having been arranged in a band,
like most still active volcanoes, accounts for the thickness, the
narrowness, and linear extension of this formation.
This whole great pile of rock has suffered much metamorphic
action, as is very obvious in the gradual formation and
appearance of the crystals of albitic feldspar and of epidote—in
the bending together of the fragments— in the appearance of a
laminated structure in the feldspathic slate—and, lastly, in the
disappearance of the planes of stratification, which could
sometimes be seen on the same mountain quite distinct in the
upper part, less and less plain on the flanks, and quite
obliterated at the base. Partly owing to this metamorphic action,
and partly to the close relationship in origin, I have seen
fragments of porphyries—taken from a metamorphosed
conglomerate—from a neighbouring stream of lava—from the nucleus
or centre (as it appeared to me) of the whole submarine volcano—
and lastly from an intrusive mass of quite subsequent origin, all
of which were absolutely undistinguishable in external
characters.
One other rock, of plutonic origin, and highly important in the
history of the Cordillera, from having been injected in most of
the great axes of elevation, and from having apparently been
instrumental in metamorphosing the superincumbent strata, may be
conveniently described in this preliminary discussion. It has
been called by some authors ANDESITE: it mainly consists of
well-crystallised white albite (as determined with the goniometer
in numerous specimens both by Professor Miller and myself), of
less perfectly crystallised green hornblende, often associated
with much mica, with chlorite and epidote, and occasionally with
a few grains of quartz: in one instance in Northern Chile, I
found crystals of orthitic or potash feldspar, mingled with those
of albite. (I here, and elsewhere, call by this name, those
feldspathic minerals which cleave like albite: but it now appears
("Edinburgh New Philosophical Journal" volume 24 page 181) that
Abich has analysed a mineral from the Cordillera, associated with
hornblende and quartz (probably the same rock with that here
under discussion), which cleaves like albite, but which is a new
and distinct kind, called by him ANDESINE. It is allied to
leucite, with the greater proportion of its potash replaced by
lime and soda. This mineral seems scarcely distinguishable from
albite, except by analysis.) Where the mica and quartz are
abundant, the rock cannot be distinguished from granite; and it
may be called andesitic granite. Where these two minerals are
quite absent, and when, as often then happens, the crystals of
albite are imperfect and blend together, the rock may be called
andesitic porphyry, which bears nearly the same relation to
andesitic granite that euritic porphyry does to common granite.
These andesitic rocks form mountain masses of a white colour,
which, in their general outline and appearance—in their joints—in
their occasionally including dark-coloured, angular fragments,
apparently of some pre-existing rock—and in the great dikes
branching from them into the superincumbent strata, manifest a
close and striking resemblance to masses of common granite and
syenite: I never, however, saw in these andesitic rocks, those
granitic veins of segregation which are so common in true
granites. We have seen that andesite occurs in three places in
Tierra del Fuego; in Chile, from S. Fernando to Copiapo, a
distance of 450 miles, I found it under most of the axes of
elevation; in a collection of specimens from the Cordillera of
Lima in Peru, I immediately recognised it; and Erman states that
it occurs in Eastern Kamtschatka. ("Geographical Journal" volume
9 page 510.) From its wide range, and from the important part it
has played in the history of the Cordillera, I think this rock
has well deserved its distinct name of Andesite.
The few still active volcanoes in Chile are confined to the
central and loftiest ranges of the Cordillera; and volcanic
matter, such as appears to have been of subaerial eruption, is
everywhere rare. According to Meyen, there is a hill of pumice
high up the valley of the Maypu, and likewise a trachytic
formation at Colina, a village situated north of St. Jago.
("Reise um Erde" Th. 1 ss. 338 and 362.) Close to this latter
city, there are two hills formed of a pale feldspathic porphyry,
remarkable from being doubly columnar, great cylindrical columns
being subdivided into smaller four- or five-sided ones; and a
third hillock (Cerro Blanco) is formed of a fragmentary mass of
rock, which I believed to be of volcanic origin, intermediate in
character between the above feldspathic porphyry and common
trachyte, and containing needles of hornblende and granular oxide
of iron. Near the Baths of Cauquenes, between two short parallel
lines of elevation, where they are intersected by the valley,
there is a small, though distinct volcanic district; the rock is
a dark grey (andesitic) trachyte, which fuses into a
greenish-grey bead, and is formed of long crystals of fractured
glassy albite (judging from one measurement) mingled with well-
formed crystals, often twin, of augite. The whole mass is
vesicular, but the surface is darker coloured and much more
vesicular than any other part. This trachyte forms a
cliff-bounded, horizontal, narrow strip on the steep southern
side of the valley, at the height of four or five hundred feet
above the river-bed; judging from an apparently corresponding
line of cliff on the northern side, the valley must once have
been filled up to this height by a field of lava. On the summit
of a lofty mountain some leagues higher up this same valley of
the Cachapual, I found columnar pitchstone porphyritic with
feldspar; I do not suppose this rock to be of volcanic origin,
and only mention it here, from its being intersected by masses
and dikes of a VESICULAR rock, approaching in character to
trachyte; in no other part of Chile did I observe vesicular or
amygdaloidal dikes, though these are so common in ordinary
volcanic districts.
PASSAGE OF THE ANDES BY THE PORTILLO OR PEQUENES PASS.
Although I crossed the Cordillera only once by this pass, and
only once by that of the Cumbre or Uspallata (presently to be
described), riding slowly and halting occasionally to ascend the
mountains, there are many circumstances favourable to obtaining a
more faithful sketch of their structure than would at first be
thought possible from so short an examination. The mountains are
steep and absolutely bare of vegetation; the atmosphere is
resplendently clear; the stratification distinct; and the rocks
brightly and variously coloured: some of the natural sections
might be truly compared for distinctness to those coloured ones
in geological works. Considering how little is known of the
structure of this gigantic range, to which I particularly
attended, most travellers having collected only specimens of the
rocks, I think my sketch-sections, though necessarily imperfect,
possess some interest. Section 1/1 in Plate 1 which I will now
describe in detail, is on a horizontal scale of a third of an
inch to a nautical mile, and on a vertical scale of one inch to a
mile (or 6,000 feet). The width of the range (excluding a few
outlying hillocks), from the plain on which St. Jago the capital
of Chile stands, to the Pampas, is sixty miles, as far as I can
judge from the maps, which differ from each other and are all
EXCEEDINGLY imperfect. The St. Jago plain at the mouth of the
Maypu, I estimate from adjoining known points at 2,300 feet, and
the Pampas at 3,500 feet, both above the level of the sea. The
height of the Pequenes line, according to Dr. Gillies, is 13,210
feet ("Journal of Natural and Geographical Science" August
1830.); and that of the Portillo line (both in the gaps where the
road crosses them) is 14,345 feet; the lowest part of the
intermediate valley of Tenuyan is 7,530 feet—all above the level
of the sea.
The Cordillera here, and indeed I believe throughout Chile,
consist of several parallel, anticlinal and uniclinal
mountain-lines, ranging north, or north with a little westing,
and south. Some exterior and much lower ridges often vary
considerably from this course, projecting like oblique spurs from
the main ranges: in the district towards the Pacific, the
mountains, as before remarked, extend in various directions, even
east and west. In the main exterior lines, the strata, as also
before remarked, are seldom inclined at a high angle; but in the
central lofty ridges they are almost always highly inclined,
broken by many great faults, and often vertical. As far as I
could judge, few of the ranges are of great length: and in the
central parts of the Cordillera, I was frequently able to follow
with my eye a ridge gradually becoming higher and higher, as the
stratification increased in inclination, from one end where its
height was trifling and its strata gently inclined to the other
end where vertical strata formed snow-clad pinnacles. Even
outside the main Cordillera, near the baths of Cauquenes, I
observed one such case, where a north and south ridge had its
strata in the valley inclined at 37 degrees, and less than a mile
south of it at 67 degrees: another parallel and similarly
inclined ridge rose at the distance of about five miles, into a
lofty mountain with absolutely vertical strata. Within the
Cordillera, the height of the ridges and the inclination of the
strata often became doubled and trebled in much shorter distances
than five miles; this peculiar form of upheaval probably
indicates that the stratified crust was thin, and hence yielded
to the underlying intrusive masses unequally, at certain points
on the lines of fissure.
The valleys, by which the Cordillera are drained, follow the
anticlinal or rarely synclinal troughs, which deviate most from
the usual north and south course; or still more commonly those
lines of faults or of unequal curvature (that is, lines with the
strata on both hands dipping in the same direction, but at a
somewhat different angle) which deviate most from a northerly
course. Occasionally the torrents run for some distance in the
north and south valleys, and then recover their eastern or
western course by bursting through the ranges at those points
where the strata have been least inclined and the height
consequently is less. Hence the valleys, along which the roads
run, are generally zigzag; and, in drawing an east and west
section, it is necessary to contract greatly that which is
actually seen on the road.
Commencing at the western end of Section 1/1 where the R. Maypu
debouches on the plain of St. Jago, we immediately enter on the
porphyritic conglomerate formation, and in the midst of it find
some hummocks [A] of granite and syenite, which probably (for I
neglected to collect specimens) belong to the andesitic class.
These are succeeded by some rugged hills [B] of dark-green,
crystalline, feldspathic and in some parts slaty rocks, which I
believe belong to the altered clay-slate formation. From this
point, great mountains of purplish and greenish, generally thinly
stratified, highly porphyritic conglomerates, including many
strata of amygdaloidal and greenstone porphyries, extend up the
valley to the junction of the rivers Yeso and Volcan. As the
valley here runs in a very southerly course, the width of the
porphyritic conglomerate formation is quite conjectural; and from
the same cause, I was unable to make out much about the
stratification. In most of the exterior mountains the dip was
gentle and directed inwards; and at only one spot I observed an
inclination as high as 50 degrees. Near the junction of the R.
Colorado with the main stream, there is a hill of whitish,
brecciated, partially decomposed feldspathic porphyry, having a
volcanic aspect but not being really of that nature: at Tolla,
however, in this valley, Dr. Meyen met with a hill of pumice
containing mica. ("Reise um Erde" Th.1 ss. 338, 341.) At the
junction of the Yeso and Volcan [D] there is an extensive mass,
in white conical hillocks, of andesite, containing some mica, and
passing either into andesitic granite, or into a spotted,
semi-granular mixture of albitic (?) feldspar and hornblende: in
the midst of this formation Dr. Meyen found true trachyte. The
andesite is covered by strata of dark-coloured, crystalline,
obscurely porphyritic rocks, and above them by the ordinary
porphyritic conglomerates,—the strata all dipping away at a small
angle from the underlying mass. The surrounding lofty mountains
appear to be entirely composed of the porphyritic conglomerate,
and I estimated its thickness here at between six and seven
thousand feet. Beyond the junction of the Yeso and Volcan, the
porphyritic strata appear to dip towards the hillocks of andesite
at an angle of 40 degrees; but at some distant points on the same
ridge they are bent up and vertical. Following the valley of the
Yeso, trending N.E. (and therefore still unfavourable for our
transverse section), the same porphyritic conglomerate formation
is prolonged to near the Cuestadel Indio, situated at the western
end of the basin (like a drained lake) of Yeso. Some way before
arriving at this point, distant lofty pinnacles capped by
coloured strata belonging to the great gypseous formation could
first be seen. From the summit of the Cuesta, looking southward,
there is a magnificent sectional view of a mountain-mass, at
least 2,000 feet in thickness [E], of fine andesite granite
(containing much black mica, a little chlorite and quartz), which
sends great white dikes far into the superincumbent,
dark-coloured, porphyritic conglomerates. At the line of junction
the two formations are wonderfully interlaced together: in the
lower part of the porphyritic conglomerate, the stratification
has been quite obliterated, whilst in the upper part it is very
distinct, the beds composing the crests of the surrounding
mountains being inclined at angles of between 70 and 80 degrees,
and some being even vertical. On the northern side of the valley,
there is a great corresponding mass of andesitic granite, which
is encased by porphyritic conglomerate, dipping both on the
western and eastern sides, at about 80 degrees to west, but on
the eastern side with the tips of the strata bent in such a
manner, as to render it probable that the whole mass has been on
that side thrown over and inverted.
In the valley basin of the Yeso, which I estimated at 7,000 feet
above the level of the sea, we first reach at [F] the gypseous
formation. Its thickness is very great. It consists in most parts
of snow-white, hard, compact gypsum, which breaks with a
saccharine fracture, having translucent edges; under the blowpipe
gives out much vapour; it frequently includes nests and
exceedingly thin layers of crystallised, blackish carbonate of
lime. Large, irregularly shaped concretions (externally still
exhibiting lines of aqueous deposition) of blackish-grey, but
sometimes white, coarsely and brilliantly crystallised, hard
anhydrite, abound within the common gypsum. Hillocks, formed of
the hardest and purest varieties of the white gypsum, stand up
above the surrounding parts, and have their surfaces cracked and
marked, just like newly baked bread. There is much pale brown,
soft argillaceous gypsum; and there were some intercalated green
beds which I had not time to reach. I saw only one fragment of
selenite or transparent gypsum, and that perhaps may have come
from some subsequently formed vein. From the mineralogical
characters here given, it is probable that these gypseous beds
have undergone some metamorphic action. The strata are much
hidden by detritus, but they appeared in most parts to be highly
inclined; and in an adjoining lofty pinnacle they could be
distinctly seen bending up, and becoming vertical, conformably
with the underlying porphyritic conglomerate. In very many parts
of the great mountain-face [F], composed of thin gypseous beds,
there were innumerable masses, irregularly shaped and not like
dikes, yet with well-defined edges, of an imperfectly granular,
pale greenish, or yellowish-white rock, essentially composed of
feldspar, with a little chlorite or hornblende, epidote,
iron-pyrites, and ferruginous powder: I believe that these
curious trappean masses have been injected from the not far
distant mountain-mass [E] of andesite whilst still fluid, and
that owing to the softness of the gypseous strata they have not
acquired the ordinary forms of dikes. Subsequently to the
injection of these feldspathic rocks, a great dislocation has
taken place; and the much shattered gypseous strata here overlie
a hillock [G], composed of vertical strata of impure limestone
and of black highly calcareous shale including threads of gypsum:
these rocks, as we shall presently see, belong to the upper parts
of the gypseous series, and hence must here have been thrown down
by a vast fault.
Proceeding up the valley-basin of the Yeso, and taking our
section sometimes on one hand and sometimes on the other, we come
to a great hill of stratified porphyritic conglomerate [H]
dipping at 45 degrees to the west; and a few hundred yards
farther on, we have a bed between three or four hundred feet
thick of gypsum [I] dipping eastward at a very high angle: here
then we have a fault and anticlinal axis. On the opposite side of
the valley, a vertical mass of red conglomerate, conformably
underlying the gypsum, appears gradually to lose its
stratification and passes into a mountain of porphyry. The gypsum
[I] is covered by a bed [K], at least 1,000 feet in thickness, of
a purplish-red, compact, heavy, fine-grained sandstone or
mudstone, which fuses easily into a white enamel, and is seen
under a lens to contain triturated crystals. This is succeeded by
a bed [L], 1,000 feet thick (I believe I understate the
thickness) of gypsum, exactly like the beds before described; and
this again is capped by another great bed [M] of purplish-red
sandstone. All these strata dip eastward; but the inclination
becomes less and less, as we leave the first and almost vertical
bed [I] of gypsum.
Leaving the basin-plain of Yeso, the road rapidly ascends,
passing by mountains composed of the gypseous and associated
beds, with their stratification greatly disturbed and therefore
not easily intelligible: hence this part of the section has been
left uncoloured. Shortly before reaching the great Pequenes
ridge, the lowest stratum visible [N] is a red sandstone or
mudstone, capped by a vast thickness of black, compact,
calcareous, shaly rock [O], which has been thrown into four
lofty, though small ridges: looking northward, the strata in
these ridges are seen gradually to rise in inclination, becoming
in some distant pinnacles absolutely vertical.
The ridge of Pequenes, which divides the waters flowing into the
Pacific and Atlantic Oceans, extends in a nearly N.N.W. and
S.S.E. line; its strata dip eastward at an angle of between 30
and 45 degrees, but in the higher peaks bending up and becoming
almost vertical. Where the road crosses this range, the height is
13,210 feet above the sea-level, and I estimated the neighbouring
pinnacles at from fourteen to fifteen thousand feet. The lowest
stratum visible in this ridge is a red stratified sandstone [P];
on it are superimposed two great masses [Q and S] of black, hard,
compact, even having a conchoidal fracture, calcareous, more or
less laminated shale, passing into limestone: this rock contains
organic remains, presently to be enumerated. The compacter
varieties fuse easily in a white glass; and this I may add is a
very general character with all the sedimentary beds in the
Cordillera: although this rock when broken is generally quite
black, it everywhere weathers into an ash-grey tint. Between
these two great masses [Q and S], a bed [R] of gypsum is
interposed, about three hundred feet in thickness, and having the
same characters as heretofore described. I estimated the total
thickness of these three beds [Q, R, S] at nearly three thousand
feet; and to this must be added, as will be immediately seen, a
great overlying mass of red sandstone.
In descending the eastern slope of this great central range, the
strata, which in the upper part dip eastward at about an angle of
40 degrees, become more and more curved, till they are nearly
vertical; and a little further onwards there is seen on the
further side of a ravine, a thick mass of strata of bright red
sandstone [T], with their upper extremities slightly curved,
showing that they were once conformably prolonged over the beds
[S]: on the southern and opposite side of the road, this red
sandstone and the underlying black shaly rocks stand vertical,
and in actual juxtaposition. Continuing to descend, we come to a
synclinal valley filled with rubbish, beyond which we have the
red sandstone [T2] corresponding with [T], and now dipping, as is
seen both north and south of the road, at 45 degrees to the west;
and under it, the beds [S2, R2, Q2, and I believe P2] in
corresponding order and of similar composition, with those on the
western flank of the Pequenes range, but dipping westward. Close
to the synclinal valley the dip of these strata is 45 degrees,
but at the eastern or farther end of the series it increases to
60 degrees. Here the great gypseous formation abruptly
terminates, and is succeeded eastward by a pile of more modern
strata. Considering how violently these central ranges have been
dislocated, and how very numerous dikes are in the exterior and
lower parts of the Cordillera, it is remarkable that I did not
here notice a single dike. The prevailing rock in this
neighbourhood is the black, calcareous, compact shale, whilst in
the valley-basin of the Yeso the purplish red sandstone or
mudstone predominates,—both being associated with gypseous strata
of exactly the same nature. It would be very difficult to
ascertain the relative superposition of these several masses, for
we shall afterwards see in the Cumbre Pass that the gypseous and
intercalated beds are lens-shaped, and that they thin out, even
where very thick, and disappear in short horizontal distances: it
is quite possible that the black shales and red sandstones may be
contemporaneous, but it is more probable that the former compose
the uppermost parts of the series.
The fossils above alluded to in the black calcareous shales are
few in number, and are in an imperfect condition; they consist,
as named for me by M. d'Orbigny, of:—
1. Ammonite, indeterminable, near to A. recticostatus, d'Orbigny,
"Pal. Franc." (Neocomian formation). 2. Gryphaea, near to G.
Couloni (Neocomian formations of France and Neufchatel). 3.
Natica, indeterminable. 4. Cyprina rostrata, d'Orbigny, "Pal.
Franc." (Neocomian formation). 5. Rostellaria angulosa (?),
d'Orbigny, "Pal. de l'Amer. Mer." 6. Terebratula (?).
Some of the fragments of Ammonites were as thick as a man's arm:
the Gryphaea is much the most abundant shell. These fossils M.
d'Orbigny considers as belonging to the Neocomian stage of the
Cretaceous system. Dr. Meyen, who ascended the valley of the Rio
Volcan, a branch of the Yeso, found a nearly similar, but
apparently more calcareous formation, with much gypsum, and no
doubt the equivalent of that here described ("Reise um Erde" etc.
Th. 1 s. 355.): the beds were vertical, and were prolonged up to
the limits of perpetual snow; at the height of 9,000 feet above
the sea, they abounded with fossils, consisting, according to Von
Buch ("Descript. Phys. des Iles Canaries" page 471.), of:—
1. Exogyra (Gryphaea) Couloni, absolutely identical with
specimens from the Jura and South of France. 2. Trigonia costata,
identical with those found in the upper Jurassic beds at
Hildesheim. 3. Pecten striatus, identical with those found in the
upper Jurassic beds at Hildesheim. 4. Cucullaea, corresponding in
form to C. longirostris, so frequent in the upper Jurassic beds
of Westphalia. 5. Ammonites resembling A. biplex.
Von Buch concludes that this formation is intermediate between
the limestone of the Jura and the chalk, and that it is analogous
with the uppermost Jurassic beds forming the plains of
Switzerland. Hence M. D'Orbigny and Von Buch, under different
terms, compare these fossils to those from the same late stage in
the secondary formations of Europe.
Some of the fossils which I collected were found a good way down
the western slope of the main ridge, and hence must originally
have been covered up by a great thickness of the black shaly
rock, independently of the now denuded, thick, overlying masses
of red sandstone. I neglected at the time to estimate how many
hundred or rather thousand feet thick the superincumbent strata
must have been: and I will not now attempt to do so. This,
however, would have been a highly interesting point, as
indicative of a great amount of subsidence, of which we shall
hereafter find in other parts of the Cordillera analogous
evidence during this same period. The altitude of the Peuquenes
Range, considering its not great antiquity, is very remarkable;
many of the fossils were embedded at the height of 13,210 feet,
and the same beds are prolonged up to at least from fourteen to
fifteen thousand feet above the level of the sea.
THE PORTILLO OR EASTERN CHAIN.
The valley of Tenuyan, separating the Peuquenes and Portillo
lines, is, as estimated by Dr. Gillies and myself, about twenty
miles in width; the lowest part, where the road crosses the
river, being 7,500 feet above the sea-level. The pass on the
Portillo line is 14,365 feet high (1,100 feet higher than that on
the Peuquenes), and the neighbouring pinnacles must, I conceive,
rise to nearly 16,000 feet above the sea. The river draining the
intermediate valley of Tenuyan, passes through the Portillo line.
To return to our section:—shortly after leaving the lower beds
[P2] of the gypseous formation, we come to grand masses of a
coarse, red conglomerate [V], totally unlike any strata hitherto
seen in the Cordillera. This conglomerate is distinctly
stratified, some of the beds being well defined by the greater
size of the pebbles: the cement is calcareous and sometimes
crystalline, though the mass shows no signs of having been
metamorphosed. The included pebbles are either perfectly or only
partially rounded: they consist of purplish sandstones, of
various porphyries, of brownish limestone, of black calcareous,
compact shale precisely like that in situ in the Peuquenes range,
and CONTAINING SOME OF THE SAME FOSSIL SHELLS; also very many
pebbles of quartz, some of micaceous schist, and numerous,
broken, rounded crystals of a reddish orthitic or potash feldspar
(as determined by Professor Miller), and these from their size
must have been derived from a coarse-grained rock, probably
granite. From this feldspar being orthitic, and even from its
external appearance, I venture positively to affirm that it has
not been derived from the rocks of the western ranges; but, on
the other hand, it may well have come, together with the quartz
and metamorphic schists, from the eastern or Portillo line, for
this line mainly consists of coarse orthitic granite. The pebbles
of the fossiliferous slate and of the purple sandstone, certainly
have been derived from the Peuquenes or western ranges.
The road crosses the valley of Tenuyan in a nearly east and west
line, and for several miles we have on both hands the
conglomerate, everywhere dipping west and forming separate great
mountains. The strata, where first met with, after leaving the
gypseous formation, are inclined westward at an angle of only 20
degrees, which further on increases to about 45 degrees. The
gypseous strata, as we have seen, are also inclined westward:
hence, when looking from the eastern side of the valley towards
the Peuquenes range, a most deceptive appearance is presented, as
if the newer beds of conglomerate dipped directly under the much
older beds of the gypseous formation. In the middle of the
valley, a bold mountain of unstratified lilac-coloured porphyry
(with crystals of hornblende) projects; and further on, a little
south of the road, there is another mountain, with its strata
inclined at a small angle eastwards, which in its general aspect
and colour, resembles the porphyritic conglomerate formation, so
rare on this side of the Peuquenes line and so grandly developed
throughout the western ranges.
The conglomerate is of great thickness: I do not suppose that the
strata forming the separate mountain-masses [V,V,V] have ever
been prolonged over each other, but that one mass has been broken
up by several, distinct, parallel, uniclinal lines of elevation.
Judging therefore of the thickness of the conglomerate, as seen
in the separate mountain-masses, I estimated it at least from one
thousand five hundred to two thousand feet. The lower beds rest
conformably on some singularly coloured, soft strata [W], which I
could not reach to examine; and these again rest conformably on a
thick mass of micaceous, thinly laminated, siliceous sandstone
[X], associated with a little black clay-slate. These lower beds
are traversed by several dikes of decomposing porphyry. The
laminated sandstone is directly superimposed on the vast masses
of granite [Y,Y] which mainly compose the Portillo range. The
line of junction between this latter rock, which is of a bright
red colour, and the whitish sandstone was beautifully distinct;
the sandstone being penetrated by numerous, great, tortuous dikes
branching from the granite, and having been converted into a
granular quartz rock (singularly like that of the Falkland
Islands), containing specks of an ochrey powder, and black
crystalline atoms, apparently of imperfect mica. The quartzose
strata in one spot were folded into a regular dome.
The granite which composes the magnificent bare pinnacles and the
steep western flank of the Portillo chain, is of a brick-red
colour, coarsely crystallised, and composed of orthitic or potash
feldspar, quartz, and imperfect mica in small quantity, sometimes
passing into chlorite. These minerals occasionally assume a
laminar or foliated arrangement. The fact of the feldspar being
orthitic in this range, is very remarkable, considering how rare,
or rather, as I believe, entirely absent, this mineral is
throughout the western ranges, in which soda-feldspar, or at
least a variety cleaving like albite, is so extremely abundant.
In one spot on the western flank, and on the eastern flank near
Los Manantiales and near the crest, I noticed some great masses
of a whitish granite, parts of it fine- grained, and parts
containing large crystals of feldspar; I neglected to collect
specimens, so I do not know whether this feldspar is also
orthitic, though I am inclined to think so from its general
appearance. I saw also some syenite and one mass which resembled
andesite, but of which I likewise neglected to collect specimens.
From the manner in which the whitish granites formed separate
mountain-masses in the midst of the brick-red variety, and from
one such mass near the crest being traversed by numerous veins of
flesh-coloured and greenish eurite (into which I occasionally
observed the brick-red granite insensibly passing), I conclude
that the white granites probably belong to an older formation,
almost overwhelmed and penetrated by the red granite.
On the crest I saw also, at a short distance, some coloured
stratified beds, apparently like those [W] at the western base,
but was prevented examining them by a snowstorm: Mr. Caldcleugh,
however, collected here specimens of ribboned jasper, magnesian
limestone, and other minerals. ("Travels" etc. volume 1 page
308.) A little way down the eastern slope a few fragments of
quartz and mica-slate are met with; but the great formation of
this latter rock [Z], which covers up much of the eastern flank
and base of the Portillo range, cannot be conveniently examined
until much lower down at a place called Mal Paso. The mica-schist
here consists of thick layers of quartz, with intervening folia
of finely-scaly mica, often passing into a substance like black
glossy clay-slate: in one spot, the layers of the quartz having
disappeared, the whole mass became converted into glossy
clay-slate. Where the folia were best defined, they were inclined
at a high angle westward, that is, towards the range. The line of
junction between the dark mica-slate and the coarse red granite
was most clearly distinguishable from a vast distance: the
granite sent many small veins into the mica-slate, and included
some angular fragments of it. As the sandstone on the western
base has been converted by the red granite into a granular
quartz-rock, so this great formation of mica-schist may possibly
have been metamorphosed at the same time and by the same means;
but I think it more probable, considering its more perfect
metamorphic character and its well-pronounced foliation, that it
belongs to an anterior epoch, connected with the white granites:
I am the more inclined to this view, from having found at the
foot of the range the mica-schist surrounding a hummock [Y2],
exclusively composed of white granite. Near Los Arenales, the
mountains on all sides are composed of the mica-slate; and
looking backwards from this point up to the bare gigantic peaks
above, the view was eminently interesting. The colours of the red
granite and the black mica-slate are so distinct, that with a
bright light these rocks could be readily distinguished even from
the Pampas, at a level of at least 9,000 feet below. The red
granite, from being divided by parallel joints, has weathered
into sharp pinnacles, on some of which, even on some of the
loftiest, little caps of mica-schist could be clearly seen: here
and there isolated patches of this rock adhered to the
mountain-flanks, and these often corresponded in height and
position on the opposite sides of the immense valleys. Lower down
the schist prevailed more and more, with only a few quite small
points of granite projecting through. Looking at the entire
eastern face of the Portillo range, the red colour far exceeds in
area the black; yet it was scarcely possible to doubt that the
granite had once been almost wholly encased by the mica-schist.
At Los Arenales, low down on the eastern flank, the mica-slate is
traversed by several closely adjoining, broad dikes, parallel to
each other and to the foliation of the schist. The dikes are
formed of three different varieties of rock, of which a pale
brown feldspathic porphyry with grains of quartz was much the
most abundant. These dikes with their granules of quartz, as well
as the mica-schist itself, strikingly resemble the rocks of the
Chonos Archipelago. At a height of about twelve hundred feet
above the dikes, and perhaps connected with them, there is a
range of cliffs formed of successive lava-streams [AA], between
three and four hundred feet in thickness, and in places finely
columnar. The lava consists of dark- greyish, harsh rocks,
intermediate in character between trachyte and basalt, containing
glassy feldspar, olivine, and a little mica, and sometimes
amygdaloidal with zeolite: the basis is either quite compact, or
crenulated with air-vesicles arranged in laminae. The streams are
separated from each other by beds of fragmentary brown scoriae,
firmly cemented together, and including a few well-rounded
pebbles of lava. From their general appearance, I suspect that
these lava-streams flowed at an ancient period under the pressure
of the sea, when the Atlantic covered the Pampas and washed the
eastern foot of the Cordillera. (This conclusion might, perhaps,
even have been anticipated, from the general rarity of volcanic
action, except near the sea or large bodies of water. Conformably
with this rule, at the present day, there are no active volcanoes
on this eastern side of the Cordillera; nor are severe
earthquakes experienced here.) On the opposite and northern side
of the valley there is another line of lava- cliffs at a
corresponding height; the valley between being of considerable
breadth, and as nearly as I could estimate 1,500 feet in depth.
This field of lava is confined on both sides by the mountains of
mica-schist, and slopes down rapidly but irregularly to the edge
of the Pampas, where, having a thickness of about two hundred
feet, it terminates against a little range of claystone porphyry.
The valley in this lower part expands into a bay-like, gentle
slope, bordered by the cliffs of lava, which must certainly once
have extended across this wide expanse. The inclination of the
streams from Los Arenales to the mouth of the valley is so great,
that at the time (though ignorant of M. Elie de Beaumont's
researches on the extremely small slope over which lava can flow,
and yet retain a compact structure and considerable thickness) I
concluded that they must subsequently to their flowing have been
upheaved and tilted from the mountains; of this conclusion I can
now entertain not the smallest doubt.
At the mouth of the valley, within the cliffs of the above
lava-field, there are remnants, in the form of separate small
hillocks and of lines of low cliffs, of a considerable deposit of
compact white tuff (quarried for filtering-stones), composed of
broken pumice, volcanic crystals, scales of mica, and fragments
of lava. This mass has suffered much denudation; and the hard
mica-schist has been deeply worn, since the period of its
deposition; and this period must have been subsequent to the
denudation of the basaltic lava-streams, as attested by their
encircling cliffs standing at a higher level. At the present day,
under the existing arid climate, ages might roll past without a
square yard of rock of any kind being denuded, except perhaps in
the rarely moistened drainage-channel of the valley. Must we then
look back to that ancient period, when the waves of the sea beat
against the eastern foot of the Cordillera, for a power
sufficient to denude extensively, though superficially, this
tufaceous deposit, soft although it be?
There remains only to mention some little water-worn hillocks
[BB], a few hundred feet in height, and mere mole-hills compared
with the gigantic mountains behind them, which rise out of the
sloping, shingle-covered margin of the Pampas. The first little
range is composed of a brecciated purple porphyritic claystone,
with obscurely marked strata dipping at 70 degrees to the S.W.;
the other ranges consist of—a pale-coloured feldspathic
porphyry,—a purple claystone porphyry with grains of quartz,— and
a rock almost exclusively composed of brick-red crystals of
feldspar. These outermost small lines of elevation extend in a
N.W. by W. and S.E. by S. direction.
CONCLUDING REMARKS ON THE PORTILLO RANGE.
When on the Pampas and looking southward, and whilst travelling
northward, I could see for very many leagues the red granite and
dark mica-schist forming the crest and eastern flank of the
Portillo line. This great range, according to Dr. Gillies, can be
traced with little interruption for 140 miles southward to the R.
Diamante, where it unites with the western ranges: northward,
according to this same author, it terminates where the R. Mendoza
debouches from the mountains; but a little further north in the
eastern part of the Cumbre section, there are, as we shall
hereafter see, some mountain-masses of a brick-red porphyry, the
last injected amidst many other porphyries, and having so close
an analogy with the coarse red granite of the Portillo line, that
I am tempted to believe that they belong to the same axis of
injection; if so, the Portillo line is at least 200 miles in
length. Its height, even in the lowest gap in the road, is 14,365
feet, and some of the pinnacles apparently attain an elevation of
about 16,000 feet above the sea. The geological history of this
grand chain appears to me eminently interesting. We may safely
conclude, that at a former period the valley of Tenuyan existed
as an arm of the sea, about twenty-miles in width, bordered on
one hand by a ridge or chain of islets of the black calcareous
shales and purple sandstones of the gypseous formation; and on
the other hand, by a ridge or chain of islets composed of
mica-slate, white granite, and perhaps to a partial extent of red
granite. These two chains, whilst thus bordering the old
sea-channel, must have been exposed for a vast lapse of time to
alluvial and littoral action, during which the rocks were
shattered, the fragments rounded, and the strata of conglomerate
accumulated to a thickness of at least fifteen hundred or two
thousand feet. The red orthitic granite now forms, as we have
seen, the main part of the Portillo chain: it is injected in
dikes not only into the mica-schist and white granites, but into
the laminated sandstone, which it has metamorphosed, and which it
has thrown off, together with the conformably overlying coloured
beds and stratified conglomerate, at an angle of forty-five
degrees. To have thrown off so vast a pile of strata at this
angle, is a proof that the main part of the red granite (whether
or not portions, as perhaps is probable, previously existed) was
injected in a liquified state after the accumulation both of the
laminated sandstone and of the conglomerate; this conglomerate,
we know, was accumulated, not only after the deposition of the
fossiliferous strata of the Peuquenes line, but after their
elevation and long-continued denudation: and these fossiliferous
strata belong to the early part of the Cretaceous system. Late,
therefore, in a geological sense, as must be the age of the main
part of the red granite, I can conceive nothing more impressive
than the eastern view of this great range, as forcing the mind to
grapple with the idea of the thousands of thousands of years
requisite for the denudation of the strata which originally
encased it,—for that the fluidified granite was once encased, its
mineralogical composition and structure, and the bold conical
shape of the mountain-masses, yield sufficient evidence. Of the
encasing strata we see the last vestiges in the coloured beds on
the crest, in the little caps of mica-schist on some of the
loftiest pinnacles, and in the isolated patches of this same rock
at corresponding heights on the now bare and steep flanks.
The lava-streams at the eastern foot of the Portillo are
interesting, not so much from the great denudation which they
have suffered at a comparatively late period as from the evidence
they afford by their inclination taken conjointly with their
thickness and compactness, that after the great range had assumed
its present general outline, it continued to rise as an axis of
elevation. The plains extending from the base of the Cordillera
to the Atlantic show that the continent has been upraised in mass
to a height of 3,500 feet, and probably to a much greater height,
for the smooth shingle-covered margin of the Pampas is prolonged
in a gentle unbroken slope far up many of the great valleys. Nor
let it be assumed that the Peuquenes and Portillo ranges have
undergone only movements of elevation; for we shall hereafter
see, that the bottom of the sea subsided several thousand feet
during the deposition of strata, occupying the same relative
place in the Cordillera, with those of the Peuquenes ridge;
moreover, we shall see from the unequivocal evidence of buried
upright trees, that at a somewhat later period, during the
formation of the Uspallata chain, which corresponds
geographically with that of the Portillo, there was another
subsidence of many thousand feet: here, indeed, in the valley of
Tenuyan, the accumulation of the coarse stratified conglomerate
to a thickness of fifteen hundred or two thousand feet, offers
strong presumptive evidence of subsidence; for all existing
analogies lead to the belief that large pebbles can be
transported only in shallow water, liable to be affected by
currents and movements of undulation—and if so, the shallow bed
of the sea on which the pebbles were first deposited must
necessarily have sunk to allow of the accumulation of the
superincumbent strata. What a history of changes of level, and of
wear and tear, all since the age of the latter secondary
formations of Europe, does the structure of this one great
mountain-chain reveal!
PASSAGE OF THE ANDES BY THE CUMBRE OR USPALLATA PASS.
This Pass crosses the Andes about sixty miles north of that just
described: the section given in Plate 1, Section 1/2, is on the
same scale as before, namely, at one-third of an inch to a mile
in distance, and one inch to a mile (or 6,000 feet) in height.
Like the last section, it is a mere sketch, and cannot pretend to
accuracy, though made under favourable circumstances. We will
commence as before, with the western half, of which the main
range bears the name of the Cumbre (that is the Ridge), and
corresponds to the Peuquenes line in the former section; as does
the Uspallata range, though on a much smaller scale, to that of
the Portillo. Near the point where the river Aconcagua debouches
on the basin plain of the same name, at a height of about two
thousand three hundred feet above the sea, we meet with the usual
purple and greenish porphyritic claystone conglomerate. Beds of
this nature, alternating with numerous compact and amygdaloidal
porphyries, which have flowed as submarine lavas, and associated
with great mountain- masses of various, injected, non-stratified
porphyries, are prolonged the whole distance up to the Cumbre or
central ridge. One of the commonest stratified porphyries is of a
green colour, highly amygdaloidal with the various minerals
described in the preliminary discussion, and including fine
tabular crystals of albite. The mountain-range north (often with
a little westing) and south. The stratification, wherever I could
clearly distinguish it, was inclined westward or towards the
Pacific, and, except near the Cumbre, seldom at angles above 25
degrees. Only at one spot on this western side, on a lofty
pinnacle not far from the Cumbre, I saw strata apparently
belonging to the gypseous formation, and conformably capping a
pile of stratified porphyries. Hence, both in composition and in
stratification, the structure of the mountains on this western
side of the divortium aquarum, is far more simple than in the
corresponding part of the Peuquenes section. In the porphyritic
claystone conglomerate, the mechanical structure and the planes
of stratification have generally been much obscured and even
quite obliterated towards the base of the series, whilst in the
upper parts, near the summits of the mountains, both are
distinctly displayed. In these upper portions the porphyries are
generally lighter coloured. In three places [X, Y, Z] masses of
andesite are exposed: at [Y], this rock contained some quartz,
but the greater part consisted of andesitic porphyry, with only a
few well-developed crystals of albite, and forming a great white
mass, having the external aspect of granite, capped by much dark
unstratified porphyry. In many parts of the mountains, there are
dikes of a green colour, and other white ones, which latter
probably spring from underlying masses of andesite.
The Cumbre, where the road crosses it, is, according to Mr.
Pentland, 12,454 feet above the sea; and the neighbouring peaks,
composed of dark purple and whitish porphyries, some obscurely
stratified with a westerly dip, and others without a trace of
stratification, must exceed 13,000 feet in height. Descending the
eastern slope of the Cumbre, the structure becomes very
complicated, and generally differs on the two sides of the east
and west line of road and section. First we come to a great mass
[A] of nearly vertical, singularly contorted strata, composed of
highly compact red sandstones, and of often calcareous
conglomerates, and penetrated by green, yellow, and reddish
dikes; but I shall presently have an opportunity of describing in
some detail an analogous pile of strata. These vertical beds are
abruptly succeeded by others [B], of apparently nearly the same
nature but more metamorphosed, alternating with porphyries and
limestones; these dip for a short space westward, but there has
been here an extraordinary dislocation, which, on the north side
of the road, appears to have determined the excavation of the
north and south valley of the R. de las Cuevas. On this northern
side of the road, the strata [B] are prolonged till they come in
close contact with a jagged lofty mountain [D] of dark- coloured,
unstratified, intrusive porphyry, where the beds have been more
highly inclined and still more metamorphosed. This mountain of
porphyry seems to form a short axis of elevation, for south of
the road in its line there is a hill [C] of porphyritic
conglomerate with absolutely vertical strata.
We now come to the gypseous formation: I will first describe the
structure of the several mountains, and then give in one section
a detailed account of the nature of the rocks. On the north side
of the road, which here runs in an east and west valley, the
mountain of porphyry [D] is succeeded by a hill [E] formed of the
upper gypseous strata tilted, at an angle of between 70 and 80
degrees to the west, by a uniclinal axis of elevation which does
not run parallel to the other neighbouring ranges, and which is
of short length; for on the south side of the valley its
prolongation is marked only by a small flexure in a pile of
strata inclined by a quite separate axis. A little further on the
north and south valley of Horcones enters at right angles our
line of section; its western side is bounded by a hill of
gypseous strata [F] dipping westward at about 45 degrees, and its
eastern side by a mountain of similar strata [G] inclined
westward at 70 degrees, and superimposed by an oblique fault on
another mass of the same strata [H], also inclined westward, but
at an angle of about 30 degrees: the complicated relation of
these three masses [F, G, H] is explained by the structure of a
great mountain-range lying some way to the north, in which a
regular anticlinal axis (represented in the section by dotted
lines) is seen, with the strata on its eastern side again bending
up and forming a distinct uniclinal axis, of which the beds
marked [H] form the lower part. This great uniclinal line is
intersected, near the Puente del Inca, by the valley along which
the road runs, and the strata composing it will be immediately
described. On the south side of the road, in the space
corresponding with the mountains [E, F, and G], the strata
everywhere dip westward generally at an angle of 30 degrees,
occasionally mounting up to 45 degrees, but not in an unbroken
line, for there are several vertical faults, forming separate
uniclinal masses, all dipping in the same direction,—a form of
elevation common in the Cordillera. We thus see that within a
narrow space, the gypseous strata have been upheaved and crushed
together by a great uniclinal, anticlinal, and one lesser
uniclinal line [E] of elevation; and that between these three
lines and the Cumbre, in the sandstones, conglomerates and
porphyritic formation, there have been at least two or three
other great elevatory axes.
The uniclinal axis [I] intersected near the Puente del Inca (of
which the strata at [H] form a part) ranges N. by W. and S. by
E., forming a chain of mountains, apparently little inferior in
height to the Cumbre: the strata, as we have seen, dip at an
average angle of 30 degrees to the west. (At this place, there
are some hot and cold springs, the warmest having a temperature,
according to Lieutenant Brand "Travels," page 240, of 91 degrees;
they emit much gas. According to Mr. Brande, of the Royal
Institution, ten cubical inches contain forty-five grains of
solid matter, consisting chiefly of salt, gypsum, carbonate of
lime, and oxide of iron. The water is charged with carbonic acid
and sulphuretted hydrogen. These springs deposit much tufa in the
form of spherical balls. They burst forth, as do those of
Cauquenes, and probably those of Villa Vicencio, on a line of
elevation.) The flanks of the mountains are here quite bare and
steep, affording an excellent section; so that I was able to
inspect the strata to a thickness of about 4,000 feet, and could
clearly distinguish their general nature for 1,000 feet higher,
making a total thickness of 5,000 feet, to which must be added
about 1,000 feet of the inferior strata seen a little lower down
the valley, I will describe this one section in detail, beginning
at the bottom.
1st. The lowest mass is the altered clay-slate described in the
preliminary discussion, and which in this line of section was
here first met with. Lower down the valley, at the R. de las
Vacas, I had a better opportunity of examining it; it is there in
some parts well characterised, having a distinct, nearly
vertical, tortuous cleavage, ranging N.W. and S.E., and
intersected by quartz veins: in most parts, however, it is
crystalline and feldspathic, and passes into a true greenstone
often including grains of quartz. The clay-slate, in its upper
half, is frequently brecciated, the embedded angular fragments
being of nearly the same nature with the paste.
2nd. Several strata of purplish porphyritic conglomerate, of no
very great thickness, rest conformably upon the feldspathic
slate. A thick bed of fine, purple, claystone porphyry, obscurely
brecciated (but not of metamorphosed sedimentary origin), and
capped by porphyritic conglomerate, was the lowest bed actually
examined in this section at the Puente del Inca.
3rd. A stratum, eighty feet thick, of hard and very compact
impure whitish limestone, weathering bright red, with included
layers brecciated and re- cemented. Obscure marks of shell are
distinguishable in it.
4th. A red, quartzose, fine-grained conglomerate, with grains of
quartz, and with patches of white earthy feldspar, apparently due
to some process of concretionary crystalline action; this bed is
more compact and metamorphosed than any of the overlying
conglomerates.
5th. A whitish cherty limestone, with nodules of bluish
argillaceous limestone.
6th. A white conglomerate, with many particles of quartz, almost
blending into the paste.
7th. Highly siliceous, fine-grained white sandstone.
8th and 9th. Red and white beds not examined.
10th. Yellow, fine-grained, thinly stratified, magnesian (judging
from its slow dissolution in acids) limestone: it includes some
white quartz pebbles, and little cavities, lined with calcareous
spar, some retaining the form of shells.
11th. A bed between twenty and thirty feet thick, quite
conformable with the underlying ones, composed of a hard basis,
tinged lilac-grey porphyritic with NUMEROUS crystals of whitish
feldspar, with black mica and little spots of soft ferruginous
matter: evidently a submarine lava.
12th. Yellow magnesian limestone, as before, part-stained purple.
13th. A most singular rock; basis purplish grey, obscurely
crystalline, easily fusible into a dark green glass, not hard,
thickly speckled with crystals more or less perfect of white
carbonate of lime, of red hydrous oxide of iron, of a white and
transparent mineral like analcime, and of a green opaque mineral
like soap-stone; the basis is moreover amygdaloidal with many
spherical balls of white crystallised carbonate of lime, of which
some are coated with the red oxide of iron. I have no doubt, from
the examination of a superincumbent stratum (19), that this is a
submarine lava; though in Northern Chile, some of the
metamorphosed sedimentary beds are almost as crystalline, and of
as varied composition.
14th. Red sandstone, passing in the upper part into a coarse,
hard, red conglomerate, 300 feet thick, having a calcareous
cement, and including grains of quartz and broken crystals of
feldspar; basis infusible; the pebbles consist of dull purplish
porphyries, with some of quartz, from the size of a nut to a
man's head. This is the coarsest conglomerate in this part of the
Cordillera: in the middle there was a white layer not examined.
15th. Grand thick bed, of a very hard, yellowish-white rock, with
a crystalline feldspathic base, including large crystals of white
feldspar, many little cavities mostly full of soft ferruginous
matter, and numerous hexagonal plates of black mica. The upper
part of this great bed is slightly cellular; the lower part
compact: the thickness varied a little in different parts.
Manifestly a submarine lava; and is allied to bed 11.
16th and 17th. Dull purplish, calcareous, fine-grained, compact
sandstones, which pass into coarse white conglomerates with
numerous particles of quartz.
18th. Several alternations of red conglomerate, purplish
sandstone, and submarine lava, like that singular rock forming
bed 13.
19th. A very heavy, compact, greenish-black stone, with a
fine-grained obviously crystalline basis, containing a few specks
of white calcareous spar, many specks of the crystallised hydrous
red oxide of iron, and some specks of a green mineral; there are
veins and nests filled with epidote: certainly a submarine lava.
20th. Many thin strata of compact, fine-grained, pale purple
sandstone.
21st. Gypsum in a nearly pure state, about three hundred feet in
thickness: this bed, in its concretions of anhydrite and layers
of small blackish crystals of carbonate of lime, exactly
resembles the great gypseous beds in the Peuquenes range.
22nd. Pale purple and reddish sandstone, as in bed 20: about
three hundred feet in thickness.
23rd. A thick mass composed of layers, often as thin as paper and
convoluted, of pure gypsum with others very impure, of a purplish
colour.
24th. Pure gypsum, thick mass.
25th. Red sandstones, of great thickness.
26th. Pure gypsum, of great thickness.
27th. Alternating layers of pure and impure gypsum, of great
thickness.
I was not able to ascend to these few last great strata, which
compose the neighbouring loftiest pinnacles. The thickness, from
the lowest to the uppermost bed of gypsum, cannot be less than
2,000 feet: the beds beneath I estimated at 3,000 feet, and this
does not include either the lower parts of the porphyritic
conglomerate, or the altered clay-slate; I conceive the total
thickness must be about six thousand feet. I distinctly observed
that not only the gypsum, but the alternating sandstones and
conglomerates were lens-shaped, and repeatedly thinned out and
replaced each other: thus in the distance of about a mile, a bed
300 feet thick of sandstone between two beds of gypsum, thinned
out to nothing and disappeared. The lower part of this section
differs remarkably,—in the much greater diversity of its
mineralogical composition,—in the abundance of calcareous
matter,—in the greater coarseness of some of the
conglomerates,—and in the numerous particles and well-rounded
pebbles, sometimes of large size, of quartz,— from any other
section hitherto described in Chile. From these peculiarities and
from the lens-form of the strata, it is probable that this great
pile of strata was accumulated on a shallow and very uneven
bottom, near some pre-existing land formed of various porphyries
and quartz-rock. The formation of porphyritic claystone
conglomerate does not in this section attain nearly its ordinary
thickness; this may be PARTLY attributed to the metamorphic
action having been here much less energetic than usual, though
the lower beds have been affected to a certain degree. If it had
been as energetic as in most other parts of Chile, many of the
beds of sandstone and conglomerate, containing rounded masses of
porphyry, would doubtless have been converted into porphyritic
conglomerate; and these would have alternated with, and even
blended into, crystalline and porphyritic strata without a trace
of mechanical structure,—namely, into those which, in the present
state of the section, we see are unquestionably submarine lavas.
The beds of gypsum, together with the red alternating sandstones
and conglomerates, present so perfect and curious a resemblance
with those seen in our former section in the basin-valley of
Yeso, that I cannot doubt the identity of the two formations: I
may add, that a little westward of the P. del Inca, a mass of
gypsum passed into a fine-grained, hard, brown sandstone, which
contained some layers of black, calcareous, compact, shaly rock,
precisely like that seen in such vast masses on the Peuquenes
range.
Near the Puente del Inca, numerous fragments of limestone,
containing some fossil remains, were scattered on the ground:
these fragments so perfectly resemble the limestone of bed No. 3,
in which I saw impressions of shells, that I have no doubt they
have fallen from it. The yellow magnesian limestone of bed No.
10, which also includes traces of shells, has a different
appearance. These fossils (as named by M. d'Orbigny) consist of:-
-
Gryphaea, near to G. Couloni (Neocomian formation). Arca, perhaps
A. Gabrielis, d'Orbigny, "Pal. Franc." (Neocomian formation).
Mr. Pentland made a collection of shells from this same spot, and
Von Buch considers them as consisting of:—
Trigonia, resembling in form T. costata. Pholadomya, like one
found by M. Dufresnoy near Alencon. Isocardi excentrica, Voltz.,
identical with that from the Jura. ("Description Phys. des Iles
Can." page 472.)
Two of these shells, namely, the Gryphaea and Trigonia, appear to
be identical with species collected by Meyen and myself on the
Peuquenes range; and in the opinion of Von Buch and M. d'Orbigny,
the two formations belong to the same age. I must here add, that
Professor E. Forbes, who has examined my specimens from this
place and from the Peuquenes range, has likewise a strong
impression that they indicate the Cretaceous period, and probably
an early epoch in it: so that all the palaeontologists who have
seen these fossils nearly coincide in opinion regarding their
age. The limestone, however, with these fossils here lies at the
very base of the formation, just above the porphyritic
conglomerate, and certainly several thousand feet lower in the
series, than the equivalent, fossiliferous, black, shaly rocks
high up on the Peuquenes range.
It is well worthy of remark that these shells, or at least those
of which I saw impressions in the limestone (bed No. 3), must
have been covered up, on the LEAST computation, by 4,000 feet of
strata: now we know from Professor E. Forbes's researches, that
the sea at greater depths than 600 feet becomes exceedingly
barren of organic beings,—a result quite in accordance with what
little I have seen of deep-sea soundings. Hence, after this
limestone with its shells was deposited, the bottom of the sea
where the main line of the Cordillera now stands, must have
subsided some thousand feet to allow of the deposition of the
superincumbent submarine strata. Without supposing a movement of
this kind, it would, moreover, be impossible to understand the
accumulation of the several lower strata of COARSE, well-rounded
conglomerates, which it is scarcely possible to believe were
spread out in profoundly deep water, and which, especially those
containing pebbles of quartz, could hardly have been rounded in
submarine craters and afterwards ejected from them, as I believe
to have been the case with much of the porphyritic conglomerate
formation. I may add that, in Professor Forbes's opinion, the
above-enumerated species of mollusca probably did not live at a
much greater depth than twenty fathoms, that is only 120 feet.
To return to our section down the valley; standing on the great
N. by W. and S. by E. uniclinal axis of the Puente del Inca, of
which a section has just been given, and looking north-east,
greater tabular masses of gypseous formation (KK) could be seen
in the distance, very slightly inclined towards the east. Lower
down the valley, the mountains are almost exclusively composed of
porphyries, many of them of intrusive origin and non-stratified,
others stratified, but with the stratification seldom
distinguishable except in the upper parts. Disregarding local
disturbances, the beds are either horizontal or inclined at a
small angle eastwards: hence, when standing on the plain of
Uspallata and looking to the west or backwards, the Cordillera
appear composed of huge, square, nearly horizontal, tabular
masses: so wide a space, with such lofty mountains so equably
elevated, is rarely met with within the Cordillera. In this line
of section, the interval between the Puente del Inca and the
neighbourhood of the Cumbre, includes all the chief axes of
dislocation.
The altered clay-slate formation, already described, is seen in
several parts of the valley as far down as Las Vacas, underlying
the porphyritic conglomerate. At the Casa de Pujios [L], there is
a hummock of (andesitic?) granite; and the stratification of the
surrounding mountains here changes from W. by S. to S.W. Again,
near the R. Vacas there is a larger formation of (andesitic?)
granite [M], which sends a meshwork of veins into the
superincumbent clay-slate, and which locally throws off the
strata, on one side to N.W. and on the other to S.E. but not at a
high angle: at the junction, the clay-slate is altered into
fine-grained greenstone. This granitic axis is intersected by a
green dike, which I mention, because I do not remember having
elsewhere seen dikes in this lowest and latest intrusive rock.
From the R. Vacas to the plain of Uspallata, the valley runs
N.E., so that I have had to contract my section; it runs
exclusively through porphyritic rocks. As far as the Pass of
Jaula, the claystone conglomerate formation, in most parts highly
porphyritic, and crossed by numerous dikes of greenstone
porphyry, attains a great thickness: there is also much intrusive
porphyry. From the Jaula to the plain, the stratification has
been in most places obliterated, except near the tops of some of
the mountains; and the metamorphic action has been extremely
great. In this space, the number and bulk of the intrusive masses
of differently coloured porphyries, injected one into another and
intersected by dikes, is truly extraordinary. I saw one mountain
of whitish porphyry, from which two huge dikes, thinning out,
branched DOWNWARDS into an adjoining blackish porphyry. Another
hill of white porphyry, which had burst through dark- coloured
strata, was itself injected by a purple, brecciated, and
recemented porphyry, both being crossed by a green dike, and both
having been upheaved and injected by a granitic dome. One
brick-red porphyry, which above the Jaula forms an isolated mass
in the midst of the porphyritic conglomerate formation, and lower
down the valley a magnificent group of peaked mountains, differs
remarkably from all the other porphyries. It consists of a red
feldspathic base, including some rather large crystals of red
feldspar, numerous large angular grains of quartz, and little
bits of a soft green mineral answering in most of its characters
to soapstone. The crystals of red feldspar resemble in external
appearance those of orthite, though, from being partially
decomposed, I was unable to measure them; and they certainly are
quite unlike the variety, so abundantly met with in almost all
the other rocks of this line of section, and which, wherever I
tried it, cleaved like albite. This brick-red porphyry appears to
have burst through all the other porphyries, and numerous red
dikes traversing the neighbouring mountains have proceeded from
it: in some few places, however, it was intersected by white
dikes. From this posteriority of intrusive origin,—from the close
general resemblance between this red porphyry and the red granite
of the Portillo line, the only difference being that the feldspar
here is less perfectly granular, and that soapstone replaces the
mica, which is there imperfect and passes into chlorite,—and from
the Portillo line a little southward of this point appearing to
blend (according to Dr. Gillies) into the western ranges,—I am
strongly urged to believe (as formerly remarked) that the grand
mountain-masses composed of this brick-red porphyry belong to the
same axis of injection with the granite of the Portillo line; if
so, the injection of this porphyry probably took place, as long
subsequently to the several axes of elevation in the gypseous
formation near the Cumbre, as the injection of the Portillo
granite has been shown to have been subsequent to the elevation
of the gypseous strata composing the Peuquenes range; and this
interval, we have seen, must have been a very long one.
The Plain of Uspallata has been briefly described in Chapter 3;
it resembles the basin-plains of Chile; it is ten or fifteen
miles wide, and is said to extend for 180 miles northward; its
surface is nearly six thousand feet above the sea; it is
composed, to a thickness of some hundred feet of loosely
aggregated, stratified shingle, which is prolonged with a gently
sloping surface up the valleys in the mountains on both sides.
One section in this plain [Z] is interesting, from the unusual
circumstance of alternating layers of almost loose red and white
sand with lines of pebbles (from the size of a nut to that of an
apple), and beds of gravel, being inclined at an angle of 45
degrees, and in some spots even at a higher angle. (I find that
Mr. Smith of Jordan Hill has described ("Edinburgh New
Philosophical Journal" volume 25 page 392) beds of sand and
gravel, near Edinburgh, tilted at an angle of 60 degrees, and
dislocated by miniature faults.) These beds are dislocated by
small faults: and are capped by a thick mass of horizontally
stratified gravel, evidently of subaqueous origin. Having been
accustomed to observe the irregularities of beds accumulated
under currents, I feel sure that the inclination here has not
been thus produced. The pebbles consist chiefly of the brick-red
porphyry just described and of white granite, both probably
derived from the ranges to the west, and of altered clay-slate
and of certain porphyries, apparently belonging to the rocks of
the Uspallata chain. This plain corresponds geographically with
the valley of Tenuyan between the Portillo and Peuquenes ranges;
but in that valley the shingle, which likewise has been derived
both from the eastern and western ranges, has been cemented into
a hard conglomerate, and has been throughout tilted at a
considerable inclination; the gravel there apparently attains a
much greater thickness, and is probably of higher antiquity.
THE USPALLATA RANGE.
The road by the Villa Vicencio Pass does not strike directly
across the range, but runs for some leagues northward along its
western base: and I must briefly describe the rocks here seen,
before continuing with the coloured east and west section. At the
mouth of the valley of Canota, and at several points northwards,
there is an extensive formation of a glossy and harsh, and of a
feldspathic clay-slate, including strata of grauwacke, and having
a tortuous, nearly vertical cleavage, traversed by numerous
metalliferous veins and others of quartz. The clay-slate is in
many parts capped by a thick mass of fragments of the same rock,
firmly recemented; and both together have been injected and
broken up by very numerous hillocks, ranging north and south, of
lilac, white, dark and salmon- coloured porphyries: one steep,
now denuded, hillock of porphyry had its face as distinctly
impressed with the angles of a fragmentary mass of the slate,
with some of the points still remaining embedded, as sealing-wax
could be by a seal. At the mouth of this same valley of Canota,
in a fine escarpment having the strata dipping from 50 to 60
degrees to the N.E. (Nearly opposite to this escarpment, there is
another corresponding one, with the strata dipping not to the
exactly opposite point, or S.W., but to S.S.W.: consequently the
two escarpments trend towards each other, and some miles
southward they become actually united: this is a form of
elevation which I have not elsewhere seen.), the clay-slate
formation is seen to be covered by—(1st) a purple, claystone
porphyry resting unconformably in some parts on the solid slate,
and in others on a thick fragmentary mass; (2nd), a conformable
stratum of compact blackish rock, having a spheroidal structure,
full of minute acicular crystals of glassy feldspar, with red
spots of oxide of iron; (3rd), a great stratum of purplish-red
claystone porphyry, abounding with crystals of opaque feldspar,
and laminated with thin, parallel, often short, layers, and
likewise with great irregular patches of white, earthy,
semi-crystalline feldspar; this rock (which I noticed in other
neighbouring places) perfectly resembles a curious variety
described at Port Desire, and occasionally occurs in the great
porphyritic conglomerate formation of Chile; (4th), a thin
stratum of greenish white, indurated tuff, fusible and containing
broken crystals and particles of porphyries; (5th), a grand mass,
imperfectly columnar and divided into three parallel and closely
joined strata, of cream-coloured claystone porphyry; (6th), a
thick stratum of lilac-coloured porphyry, which I could see was
capped by another bed of the cream-coloured variety; I was unable
to examine the still higher parts of the escarpment. These
conformably stratified porphyries, though none are either
vesicular are amygdaloidal, have evidently flowed as submarine
lavas: some of them are separated from each other by seams of
indurated tuff, which, however, are quite insignificant in
thickness compared with the porphyries. This whole pile
resembles, but not very closely, some of the less brecciated
parts of the great porphyritic conglomerate formation of Chile;
but it does not probably belong to the same age, as the
porphyries here rest unconformably on the altered feldspathic
clay-slate, whereas the porphyritic conglomerate formation
alternates with and rests conformably on it. These porphyries,
moreover, with the exception of the one blackish stratum, and of
the one indurated, white tufaceous bed, differ from the beds
composing the Uspallata range in the line of the Villa Vicencio
Pass.
I will now give, first, a sketch of the structure of the range,
as represented in the section, and will then describe its
composition and interesting history. At its western foot, a
hillock [N] is seen to rise out of the plain, with its strata
dipping at 70 degrees to the west, fronted by strata [O] inclined
at 45 degrees to the east, thus forming a little north and south
anticlinal axis. Some other little hillocks of similar
composition, with their strata highly inclined, range N.E. and
S.W., obliquely to the main Uspallata line. The cause of these
dislocations, which, though on a small scale, have been violent
and complicated, is seen to lie in hummocks of lilac, purple and
red porphyries, which have been injected in a liquified state
through and into the underlying clay-slate formation. Several
dykes were exposed here, but in no other part, that I saw of this
range. As the strata consist of black, white, greenish and
brown-coloured rocks, and as the intrusive porphyries are so
brightly tinted, a most extraordinary view was presented, like a
coloured geological drawing. On the gently inclined main western
slope [PP], above the little anticlinal ridges just mentioned,
the strata dip at an average angle of 25 degrees to the west; the
inclination in some places being only 19 degrees, in some few
others as much as 45 degrees. The masses having these different
inclinations, are separated from each other by parallel vertical
faults [as represented at Pa], often giving rise to separate,
parallel, uniclinal ridges. The summit of the main range is broad
and undulatory, with the stratification undulatory and irregular:
in a few places granitic and porphyritic masses [Q] protrude,
which, from the small effect they have locally produced in
deranging the strata, probably form the upper points of a
regular, great underlying dome. These denuded granitic points, I
estimated at about nine thousand feet in height above the sea. On
the eastern slope, the strata in the upper part are regularly
inclined at about 25 degrees to the east, so that the summit of
this chain, neglecting small irregularities, forms a broad
anticlinal axis. Lower down, however, near Los Hornillos [R],
there is a well-marked synclinal axis, beyond which the strata
are inclined at nearly the same angle, namely from 20 to 30
degrees, inwards or westward. Owing to the amount of denudation
which this chain has suffered, the outline of the gently inclined
eastern flank scarcely offers the slightest indication of this
synclinal axis. The stratified beds, which we have hitherto
followed across the range, a little further down are seen to lie,
I believe unconformably, on a broad mountainous band of
clay-slate and grauwacke. The strata and laminae of this latter
formation, on the extreme eastern flank, are generally nearly
vertical; further inwards they become inclined from 45 to 80
degrees to the west: near Villa Vicencio [S] there is apparently
an anticlinal axis, but the structure of this outer part of the
clay-slate formation is so obscure, that I have not marked the
planes of stratification in the section. On the margin of the
Pampas, some low, much dislocated spurs of this same formation,
project in a north- easterly line, in the same oblique manner as
do the ridges on the western foot, and as is so frequently the
case with those at the base of the main Cordillera.
I will now describe the nature of the beds, beginning at the base
on the eastern side. First, for the clay-slate formation: the
slate is generally hard and bluish, with the laminae coated by
minute micaceous scales; it alternates many times with a
coarse-grained, greenish grauwacke, containing rounded fragments
of quartz and bits of slate in a slightly calcareous basis. The
slate in the upper part generally becomes purplish, and the
cleavage so irregular that the whole consists of mere splinters.
Transverse veins of quartz are numerous. At the Calera, some
leagues distant, there is a dark crystalline limestone,
apparently included in this formation. With the exception of the
grauwacke being here more abundant, and the clay-slate less
altered, this formation closely resembles that unconformably
underlying the porphyries at the western foot of this same range;
and likewise that alternating with the porphyritic conglomerate
in the main Cordillera. This formation is a considerable one, and
extends several leagues southward to near Mendoza: the mountains
composed of it rise to a height of about two thousand feet above
the edge of the Pampas, or about seven thousand feet above the
sea. (I infer this from the height of V. Vicencio, which was
ascertained by Mr. Miers to be 5,328 feet above the sea.)
Secondly: the most usual bed on the clay-slate is a coarse,
white, slightly calcareous conglomerate, of no great thickness,
including broken crystals of feldspar, grains of quartz, and
numerous pebbles of brecciated claystone porphyry, but without
any pebbles of the underlying clay-slate. I nowhere saw the
actual junction between this bed and the clay-slate, though I
spent a whole day in endeavouring to discover their relations. In
some places I distinctly saw the white conglomerate and overlying
beds inclined at from 25 to 30 degrees to the west, and at the
bottom of the same mountain, the clay-slate and grauwacke
inclined to the same point, but at an angle from 70 to 80
degrees: in one instance, the clay-slate dipped not only at a
different angle, but to a different point from the overlying
formation. In these cases the two formations certainly appeared
quite unconformable: moreover, I found in the clay-slate one
great, vertical, dike-like fissure, filled up with an indurated
whitish tuff, quite similar to some of the upper beds presently
to be described; and this shows that the clay-slate must have
been consolidated and dislocated before their deposition. On the
other hand, the stratification of the slate and grauwacke, in
some cases gradually and entirely disappeared in approaching the
overlying white conglomerate; in other cases the stratification
of the two formations became strictly conformable; and again in
other cases, there was some tolerably well characterised
clay-slate lying above the conglomerate. (The coarse, mechanical
structure of many grauwackes has always appeared to me a
difficulty; for the texture of the associated clay-slate and the
nature of the embedded organic remains where present, indicate
that the whole has been a deep-water deposit. Whence have the
sometimes included angular fragments of clay-slate, and the
rounded masses of quartz and other rocks, been derived? Many
deep-water limestones, it is well known, have been brecciated,
and then firmly recemented.) The most probable conclusion appears
to be, that after the clay-slate formation had been dislocated
and tilted, but whilst under the sea, a fresh and more recent
deposition of clay-slate took place, on which the white
conglomerate was conformably deposited, with here and there a
thin intercalated bed of clay-slate. On this view the white
conglomerates and the presently to be described tuffs and lavas
are really unconformable to the main part of the clay-slate; and
this, as we have seen, certainly is the case with the clay-stone
lavas in the valley of Canota, at the western and opposite base
of the range.
Thirdly: on the white conglomerate, strata several hundred feet
in thickness are superimposed, varying much in nature in short
distances: the commonest variety is a white, much indurated tuff,
sometimes slightly calcareous, with ferruginous spots and
water-lines, often passing into whitish or purplish compact,
fine-grained grit or sandstones; other varieties become
semi-porcellanic, and tinted faint green or blue; others pass
into an indurated shale: most of these varieties are easily
fusible.
Fourthly: a bed, about one hundred feet thick of a compact,
partially columnar, pale-grey, feldspathic lava, stained with
iron, including very numerous crystals of opaque feldspar, and
with some crystallised and disseminated calcareous matter. The
tufaceous stratum on which this feldspathic lava rests is much
hardened, stained purple, and has a spherico-concretionary
structure; it here contains a good many pebbles of claystone
porphyry.
Fifthly: thin beds, 400 feet in thickness, varying much in
nature, consisting of white and ferruginous tuffs, in some parts
having a concretionary structure, in others containing rounded
grains and a few pebbles of quartz; also passing into hard
gritstones and into greenish mudstones: there is, also, much of a
bluish-grey and green semi-porcellanic stone.
Sixthly: a volcanic stratum, 250 feet in thickness, of so varying
a nature that I do not believe a score of specimens would show
all the varieties; much is highly amygdaloidal, much compact;
there are greenish, blackish, purplish, and grey varieties,
rarely including crystals of green augite and minute acicular
ones of feldspar, but often crystals and amygdaloidal masses of
white, red, and black carbonate of lime. Some of the blackish
varieties of this rock have a conchoidal fracture and resemble
basalt; others have an irregular fracture. Some of the grey and
purplish varieties are thickly speckled with green earth and with
white crystalline carbonate of lime; others are largely
amygdaloidal with green earth and calcareous spar. Again, other
earthy varieties, of greenish, purplish and grey tints, contain
much iron, and are almost half composed of amygdaloidal balls of
dark brown bole, of a whitish indurated feldspathic matter, of
bright green earth, of agate, and of black and white crystallised
carbonate of lime. All these varieties are easily fusible. Viewed
from a distance, the line of junction with the underlying
semi-porcellanic strata was distinct; but when examined closely,
it was impossible to point out within a foot where the lava ended
and where the sedimentary mass began: the rock at the time of
junction was in most places hard, of a bright green colour, and
abounded with irregular amygdaloidal masses of ferruginous and
pure calcareous spar, and of agate.
Seventhly: strata, eighty feet in thickness, of various indurated
tuffs, as before; many of the varieties have a fine basis
including rather coarse extraneous particles; some of them are
compact and semi-porcellanic, and include vegetable impressions.
Eighthly: a bed, about fifty feet thick, of greenish-grey,
compact, feldspathic lava, with numerous small crystals of opaque
feldspar, black augite, and oxide of iron. The junction with the
bed on which it rested, was ill defined; balls and masses of the
feldspathic rock being enclosed in much altered tuff.
Ninthly: indurated tuffs, as before.
Tenthly: a conformable layer, less than two feet in thickness, of
pitchstone, generally brecciated, and traversed by veins of agate
and of carbonate of lime: parts are composed of apparently
concretionary fragments of a more perfect variety, arranged in
horizontal lines in a less perfectly characterised variety. I
have much difficulty in believing that this thin layer of
pitchstone flowed as lava.
Eleventhly: sedimentary and tufaceous beds as before, passing
into sandstone, including some conglomerate: the pebbles in the
latter are of claystone porphyry, well rounded, and some as large
as cricket-balls.
Twelfthly: a bed of compact, sonorous, feldspathic lava, like
that of bed No. 8, divided by numerous joints into large angular
blocks.
Thirteenthly: sedimentary beds as before.
Fourteenthly: a thick bed of greenish or greyish black, compact
basalt (fusing into a black enamel), with small crystals,
occasionally distinguishable, of feldspar and augite: the
junction with the underlying sedimentary bed, differently from
that in most of the foregoing streams, here was quite
distinct:—the lava and tufaceous matter preserving their perfect
characters within two inches of each other. This rock closely
resembles certain parts of that varied and singular lava-stream
No. 6; it likewise resembles, as we shall immediately see, many
of the great upper beds on the western flank and on the summit of
this range.
The pile of strata here described attains a great thickness; and
above the last-mentioned volcanic stratum, there were several
other great tufaceous beds alternating with submarine lavas,
which I had not time to examine; but a corresponding series,
several thousand feet in thickness, is well exhibited on the
crest and western flank of the range. Most of the lava- streams
on the western side are of a jet-black colour and basaltic
nature; they are either compact and fine-grained, including
minute crystals of augite and feldspar, or they are
coarse-grained and abound with rather large coppery-brown
crystals of an augitic mineral. (Very easily fusible into a
jet-black bead, attracted by the magnet: the crystals are too
much tarnished to be measured by the goniometer.) Another variety
was of a dull- red colour, having a claystone brecciated basis,
including specks of oxide of iron and of calcareous spar, and
amygdaloidal with green earth: there were apparently several
other varieties. These submarine lavas often exhibit a
spheroidal, and sometimes an imperfect columnar structure: their
upper junctions are much more clearly defined than their lower
junctions; but the latter are not so much blended into the
underlying sedimentary beds as is the case in the eastern flank.
On the crest and western flank of the range, the streams, viewed
as a whole, are mostly basaltic; whilst those on the eastern
side, which stand lower in the series, are, as we have seen,
mostly feldspathic.
The sedimentary strata alternating with the lavas on the crest
and western side, are of an almost infinitely varying nature; but
a large proportion of them closely resemble those already
described on the eastern flank: there are white and brown,
indurated, easily fusible tuffs,—some passing into pale blue and
green semi-porcellanic rocks,—others into brownish and purplish
sandstones and gritstones, often including grains of quartz,—
others into mudstone containing broken crystals and particles of
rock, and occasionally single large pebbles. There was one
stratum of a bright red, coarse, volcanic gritstone; another of
conglomerate; another of a black, indurated, carbonaceous shale
marked with imperfect vegetable impressions; this latter bed,
which was thin, rested on a submarine lava, and followed all the
considerable inequalities of its upper surface. Mr. Miers states
that coal has been found in this range. Lastly, there was a bed
(like No. 10 on the eastern flank) evidently of sedimentary
origin, and remarkable from closely approaching in character to
an imperfect pitchstone, and from including extremely thin layers
of perfect pitchstone, as well as nodules and irregular fragments
(but not resembling extraneous fragments) of this same rock
arranged in horizontal lines: I conceive that this bed, which is
only a few feet in thickness, must have assumed its present state
through metamorphic and concretionary action. Most of these
sedimentary strata are much indurated, and no doubt have been
partially metamorphosed: many of them are extraordinarily heavy
and compact; others have agate and crystalline carbonate of lime
disseminated throughout them. Some of the beds exhibit a singular
concretionary arrangement, with the curves determined by the
lines of fissure. There are many veins of agate and calcareous
spar, and innumerable ones of iron and other metals, which have
blackened and curiously affected the strata to considerable
distances on both sides.
Many of these tufaceous beds resemble, with the exception of
being more indurated, the upper beds of the Great Patagonian
tertiary formation, especially those variously coloured layers
high up the River Santa Cruz, and in a remarkable degree the
tufaceous formation at the northern end of Chiloe. I was so much
struck with this resemblance, that I particularly looked out for
silicified wood, and found it under the following extraordinary
circumstances. High up on this western flank, at a height
estimated at 7,000 feet above the sea, in a broken escarpment of
thin strata, composed of compact green gritstone passing into a
fine mudstone, and alternating with layers of coarser, brownish,
very heavy mudstone, including broken crystals and particles of
rock almost blended together, I counted the stumps of fifty-two
trees. (For the information of any future traveller, I will
describe the spot in detail. Proceeding eastward from the Agua
del Zorro, and afterwards leaving on the north side of the road a
rancho attached to some old goldmines, you pass through a gully
with low but steep rocks on each hand: the road then bends, and
the ascent becomes steeper. A few hundred yards farther on, a
stone's throw on the south side of the road, the white calcareous
stumps may be seen. The spot is about half a mile east of the
Agua del Zorro.) They projected between two and five feet above
the ground, and stood at exactly right angles to the strata,
which were here inclined at an angle of about 25 degrees to the
west. Eleven of these trees were silicified and well preserved;
Mr. R. Brown has been so kind as to examine the wood when sliced
and polished; he says it is coniferous, partaking of the
characters of the Araucarian tribe, with some curious points of
affinity with the Yew. The bark round the trunks must have been
circularly furrowed with irregular lines, for the mudstone round
them is thus plainly marked. One cast consisted of dark
argillaceous limestone; and forty of them of coarsely
crystallised carbonate of lime, with cavities lined by quartz
crystals: these latter white calcareous columns do not retain any
internal structure, but their external form plainly shows their
origin. All the stumps have nearly the same diameter, varying
from one foot to eighteen inches; some of them stand within a
yard of each other; they are grouped in a clump within a space of
about sixty yards across, with a few scattered round at the
distance of 150 yards. They all stand at about the same level.
The longest stump stood seven feet out of the ground: the roots,
if they are still preserved, are buried and concealed. No one
layer of the mudstone appeared much darker than the others, as if
it had formerly existed as soil, nor could this be expected, for
the same agents which replaced with silex and lime the wood of
the trees, would naturally have removed all vegetable matter from
the soil. Besides the fifty-two upright trees, there were a few
fragments, like broken branches, horizontally embedded. The
surrounding strata are crossed by veins of carbonate of lime,
agate, and oxide of iron; and a poor gold vein has been worked
not far from the trees.
The green and brown mudstone beds including the trees, are
conformably covered by much indurated, compact, white or
ferruginous tuffs, which pass upwards into a fine-grained,
purplish sedimentary rock: these strata, which, together, are
from four to five hundred feet in thickness, rest on a thick bed
of submarine lava, and are conformably covered by another great
mass of fine-grained basalt, which I estimated at 1,000 feet in
thickness, and which probably has been formed by more than one
stream. (This rock is quite black, and fuses into a black bead,
attracted strongly by the magnet; it breaks with a conchoidal
fracture; the included crystals of augite are distinguishable by
the naked eye, but are not perfect enough to be measured: there
are many minute acicular crystals of glassy feldspar.) Above this
mass I could clearly distinguish five conformable alternations,
each several hundred feet in thickness, of stratified sedimentary
rocks and lavas, such as have been previously described.
Certainly the upright trees have been buried under several
thousand feet in thickness of matter, accumulated under the sea.
As the trees obviously must once have grown on dry land, what an
enormous amount of subsidence is thus indicated! Nevertheless,
had it not been for the trees there was no appearance which would
have led any one even to have conjectured that these strata had
subsided. As the land, moreover, on which the trees grew, is
formed of subaqueous deposits, of nearly if not quite equal
thickness with the superincumbent strata, and as these deposits
are regularly stratified and fine-grained, not like the matter
thrown up on a sea-beach, a previous upward movement, aided no
doubt by the great accumulation of lavas and sediment, is also
indicated. (At first I imagined, that the strata with the trees
might have been accumulated in a lake: but this seems highly
improbable; for, first, a very deep lake was necessary to receive
the matter below the trees, then it must have been drained for
their growth, and afterwards re-formed and made profoundly deep,
so as to receive a subsequent accumulation of matter SEVERAL
THOUSAND feet in thickness. And all this must have taken place
necessarily before the formation of the Uspallata range, and
therefore on the margin of the wide level expanse of the Pampas!
Hence I conclude, that it is infinitely more probable that the
strata were accumulated under the sea: the vast amount of
denudation, moreover, which this range has suffered, as shown by
the wide valleys, by the exposure of the very trees and by other
appearances, could have been effected, I conceive, only by the
long-continued action of the sea; and this shows that the range
was either upheaved from under the sea, or subsequently let down
into it. From the natural manner in which the stumps (fifty-two
in number) are GROUPED IN A CLUMP, and from their all standing
vertically to the strata, it is superfluous to speculate on the
chance of the trees having been drifted from adjoining land, and
deposited upright: I may, however, mention that the late Dr.
Malcolmson assured me, that he once met in the Indian Ocean,
fifty miles from land, several cocoa-nut trees floating upright,
owing to their roots being loaded with earth.)
In nearly the middle of the range, there are some hills [Q],
before alluded to, formed of a kind of granite externally
resembling andesite, and consisting of a white, imperfectly
granular, feldspathic basis, including some perfect crystals
apparently of albite (but I was unable to measure them), much
black mica, epidote in veins, and very little or no quartz.
Numerous small veins branch from this rock into the surrounding
strata; and it is a singular fact that these veins, though
composed of the same kind of feldspar and small scales of mica as
in the solid rock, abound with innumerable minute ROUNDED grains
of quartz: in the veins or dikes also, branching from the great
granitic axis in the peninsula of Tres Montes, I observed that
quartz was more abundant in them than in the main rock: I have
heard of other analogous cases: can we account for this fact, by
the long-continued vicinity of quartz when cooling, and by its
having been thus more easily sucked into fissures than the other
constituent minerals of granite? (See a paper by M. Elie de
Beaumont, "Soc. Philomath." May 1839 "L'Institut." 1839 page
161.) The strata encasing the flanks of these granitic or
andesite masses, and forming a thick cap on one of their summits,
appear originally to have been of the same tufaceous nature with
the beds already described, but they are now changed into
porcellanic, jaspery, and crystalline rocks, and into others of a
white colour with a harsh texture, and having a siliceous aspect,
though really of a feldspathic nature and fusible. Both the
granitic intrusive masses and the encasing strata are penetrated
by innumerable metallic veins, mostly ferruginous and auriferous,
but some containing copper-pyrites and a few silver: near the
veins, the rocks are blackened as if blasted by gunpowder. The
strata are only slightly dislocated close round these hills, and
hence, perhaps, it may be inferred that the granitic masses form
only the projecting points of a broad continuous axis-dome, which
has given to the upper parts of this range its anticlinal
structure.
CONCLUDING REMARKS ON THE USPALLATA RANGE.
I will not attempt to estimate the total thickness of the pile of
strata forming this range, but it must amount to many thousand
feet. The sedimentary and tufaceous beds have throughout a
general similarity, though with infinite variations. The
submarine lavas in the lower part of the series are mostly
feldspathic, whilst in the upper part, on the summit and western
flank, they are mostly basaltic. We are thus reminded of the
relative position in most recent volcanic districts of the
trachytic and basaltic lavas,—the latter from their greater
weight having sunk to a lower level in the earth's crust, and
having consequently been erupted at a later period over the
lighter and upper lavas of the trachytic series. (See on this
subject, "Volcanic Islands" etc. by the Author.) Both the
basaltic and feldspathic submarine streams are very compact; none
being vesicular, and only a few amygdaloidal: the effects which
some of them, especially those low in the series, have produced
on the tufaceous beds over which they have flowed is highly
curious. Independently of this local metamorphic action, all the
strata undoubtedly display an indurated and altered character;
and all the rocks of this range—the lavas, the alternating
sediments, the intrusive granite and porphyries, and the
underlying clay- slate—are intersected by metalliferous veins.
The lava-strata can often be seen extending for great distances,
conformably with the under and overlying beds; and it was obvious
that they thickened towards the west. Hence the points of
eruption must have been situated westward of the present range,
in the direction of the main Cordillera: as, however, the flanks
of the Cordillera are entirely composed of various porphyries,
chiefly claystone and greenstone, some intrusive, and others
belonging to the porphyritic conglomerate formation, but all
quite unlike these submarine lava-streams, we must in all
probability look to the plain of Uspallata for the now deeply
buried points of eruption.
Comparing our section of the Uspallata range with that of the
Cumbre, we see, with the exception of the underlying clay-slate,
and perhaps of the intrusive rocks of the axes, a striking
dissimilarity in the strata composing them. The great porphyritic
conglomerate formation has not extended as far as this range; nor
have we here any of the gypseous strata, the magnesian and other
limestones, the red sandstones, the siliceous beds with pebbles
of quartz, and comparatively little of the conglomerates, all of
which form such vast masses over the basal series in the main
Cordillera. On the other hand, in the Cordillera, we do not find
those endless varieties of indurated tuffs, with their numerous
veins and concretionary arrangement, and those grit and mud
stones, and singular semi-porcellanic rocks, so abundant in the
Uspallata range. The submarine lavas, also, differ considerably;
the feldspathic streams of the Cordillera contain much mica,
which is absent in those of the Uspallata range: in this latter
range we have seen on how grand a scale, basaltic lava has been
poured forth, of which there is not a trace in the Cordillera.
This dissimilarity is the more striking, considering that these
two parallel chains are separated by a plain only between ten and
fifteen miles in width; and that the Uspallata lavas, as well as
no doubt the alternating tufaceous beds, have proceeded from the
west, from points apparently between the two ranges. To imagine
that these two piles of strata were contemporaneously deposited
in two closely adjoining, very deep, submarine areas, separated
from each other by a lofty ridge, where a plain now extends,
would be a gratuitous hypothesis. And had they been
contemporaneously deposited, without any such dividing ridge,
surely some of the gypseous and other sedimentary matter forming
such immensely thick masses in the Cordillera, would have
extended this short distance eastwards; and surely some of the
Uspallata tuffs and basalts also accumulated to so great a
thickness, would have extended a little westward. Hence I
conclude, that it is far from probable that these two series are
not contemporaneous; but that the strata of one of the chains
were deposited, and even the chain itself uplifted, before the
formation of the other:—which chain, then, is the oldest?
Considering that in the Uspallata range the lowest strata on the
western flank lie unconformably on the clay- slate, as probably
is the case with those on the eastern flank, whereas in the
Cordillera all the overlying strata lie conformably on this
formation:- -considering that in the Uspallata range some of the
beds, both low down and high up in the series, are marked with
vegetable impressions, showing the continued existence of
neighbouring land;—considering the close general resemblance
between the deposits of this range and those of tertiary origin
in several parts of the continent;—and lastly, even considering
the lesser height and outlying position of the Uspallata range,—I
conclude that the strata composing it are in all probability of
subsequent origin, and that they were accumulated at a period
when a deep sea studded with submarine volcanoes washed the
eastern base of the already partially elevated Cordillera.
This conclusion is of much importance, for we have seen that in
the Cordillera, during the deposition of the Neocomian strata,
the bed of the sea must have subsided many thousand feet: we now
learn that at a later period an adjoining area first received a
great accumulation of strata, and was upheaved into land on which
coniferous trees grew, and that this area then subsided several
thousand feet to receive the superincumbent submarine strata,
afterwards being broken up, denuded, and elevated in mass to its
present height. I am strengthened in this conclusion of there
having been two distinct, great periods of subsidence, by
reflecting on the thick mass of coarse stratified conglomerate in
the valley of Tenuyan, between the Peuquenes and Portillo lines;
for the accumulation of this mass seems to me, as previously
remarked, almost necessarily to have required a prolonged
subsidence; and this subsidence, from the pebbles in the
conglomerate having been to a great extent derived from the
gypseous or Neocomian strata of the Peuquenes line, we know must
have been quite distinct from, and subsequent to, that sinking
movement which probably accompanied the deposition of the
Peuquenes strata, and which certainly accompanied the deposition
of the equivalent beds near the Puente del Inca, in this line of
section.
The Uspallata chain corresponds in geographical position, though
on a small scale, with the Portillo line; and its clay-slate
formation is probably the equivalent of the mica-schist of the
Portillo, there metamorphosed by the old white granites and
syenites. The coloured beds under the conglomerate in the valley
of Tenuyan, of which traces are seen on the crest of the
Portillo, and even the conglomerate itself, may perhaps be
synchronous with the tufaceous beds and submarine lavas of the
Uspallata range; an open sea and volcanic action in the latter
case, and a confined channel between two bordering chains of
islets in the former case, having been sufficient to account for
the mineralogical dissimilarity of the two series. From this
correspondence between the Uspallata and Portillo ranges, perhaps
in age and certainly in geographical position, one is tempted to
consider the one range as the prolongation of the other; but
their axes are formed of totally different intrusive rocks; and
we have traced the apparent continuation of the red granite of
the Portillo in the red porphyries diverging into the main
Cordillera. Whether the axis of the Uspallata range was injected
before, or as perhaps is more probable, after that of the
Portillo line, I will not pretend to decide; but it is well to
remember that the highly inclined lava-streams on the eastern
flank of the Portillo line, prove that its angular upheavement
was not a single and sudden event; and therefore that the
anticlinal elevation of the Uspallata range may have been
contemporaneous with some of the later angular movements by which
the gigantic Portillo range gained its present height above the
adjoining plain.
CHAPTER VIII. NORTHERN CHILE. CONCLUSION.
Section from Illapel to Combarbala; gypseous formation with
silicified wood. Panuncillo. Coquimbo; mines of Arqueros; section up
valley; fossils. Guasco, fossils of. Copiapo, section up valley; Las
Amolanas, silicified wood. Conglomerates, nature of former land,
fossils, thickness of strata, great subsidence. Valley of Despoblado,
fossils, tufaceous deposit, complicated dislocations of. Relations
between ancient orifices of eruption and subsequent axes of
injection. Iquique, Peru, fossils of, salt-deposits. Metalliferous
veins. Summary on the porphyritic conglomerate and gypseous
formations. Great subsidence with partial elevations during the
cretaceo-oolitic period. On the elevation and structure of the
Cordillera. Recapitulation on the tertiary series. Relation between
movements of subsidence and volcanic action. Pampean formation.
Recent elevatory movements. Long-continued volcanic action in the
Cordillera. Conclusion.
VALPARAISO TO COQUIMBO.
I have already described the general nature of the rocks in the
low country north of Valparaiso, consisting of granites,
syenites, greenstones, and altered feldspathic clay-slate. Near
Coquimbo there is much hornblendic rock and various
dusky-coloured porphyries. I will describe only one section in
this district, namely, from near Illapel in a N.E. line to the
mines of Los Hornos, and thence in a north by east direction to
Combarbala, at the foot of the main Cordillera.
Near Illapel, after passing for some distance over granite,
andesite, and andesitic porphyry, we come to a greenish
stratified feldspathic rock, which I believe is altered
clay-slate, conformably capped by porphyries and porphyritic
conglomerate of great thickness, dipping at an average angle of
20 degrees to N.E. by N. The uppermost beds consist of
conglomerates and sandstone only a little metamorphosed, and
conformably covered by a gypseous formation of very great
thickness, but much denuded. This gypseous formation, where first
met with, lies in a broad valley or basin, a little southward of
the mines of Los Hornos: the lower half alone contains gypsum,
not in great masses as in the Cordillera, but in innumerable thin
layers, seldom more than an inch or two in thickness. The gypsum
is either opaque or transparent, and is associated with carbonate
of lime. The layers alternate with numerous varying ones of a
calcareous clay-shale (with strong aluminous odour, adhering to
the tongue, easily fusible into a pale green glass), more or less
indurated, either earthy and cream-coloured, or greenish and
hard. The more indurated varieties have a compact, homogeneous,
almost crystalline fracture, and contain granules of crystallised
oxide of iron. Some of the varieties almost resemble honestones.
There is also a little black, hardly fusible, siliceo- calcareous
clay-slate, like some of the varieties alternating with gypsum on
the Peuquenes range.
The upper half of this gypseous formation is mainly formed of the
same calcareous clay-shale rock, but without any gypsum, and
varying extremely in nature: it passes from a soft, coarse,
earthy, ferruginous state, including particles of quartz, into
compact claystones with crystallised oxide of iron,—into
porcellanic layers, alternating with seams of calcareous
matter,—and into green porcelain-jasper, excessively hard, but
easily fusible. Strata of this nature alternate with much black
and brown siliceo-calcareous slate, remarkable from the wonderful
number of huge embedded logs of silicified wood. This wood,
according to Mr. R. Brown, is (judging from several specimens)
all coniferous. Some of the layers of the black siliceous slate
contained irregular angular fragments of imperfect pitchstone,
which I believe, as in the Uspallata range, has originated in a
metamorphic process. There was one bed of a marly tufaceous
nature, and of little specific gravity. Veins of agate and
calcareous spar are numerous. The whole of this gypseous
formation, especially the upper half, has been injected,
metamorphosed, and locally contorted by numerous hillocks of
intrusive porphyries crowded together in an extraordinary manner.
These hillocks consist of purple claystone and of various other
porphyries, and of much white feldspathic greenstone passing into
andesite; this latter variety included in one case crystals of
orthitic and albitic feldspar touching each other, and others of
hornblende, chlorite, and epidote. The strata surrounding these
intrusive hillocks at the mines of Los Hornos, are intersected by
many veins of copper-pyrites, associated with much micaceous
iron-ore, and by some of gold: in the neighbourhood of these
veins the rocks are blackened and much altered. The gypsum near
the intrusive masses is always opaque. One of these hillocks of
porphyry was capped by some stratified porphyritic conglomerate,
which must have been brought up from below, through the whole
immense thickness of the overlying gypseous formation. The lower
beds of the gypseous formation resemble the corresponding and
probably contemporaneous strata of the main Cordillera; whilst
the upper beds in several respects resemble those of the
Uspallata chain, and possibly may be contemporaneous with them;
for I have endeavoured to show that the Uspallata beds were
accumulated subsequently to the gypseous or Neocomian formations
of the Cordillera.
This pile of strata dips at an angle of about 20 degrees to N.E.
by N., close up to the foot of the Cuesta de Los Hornos, a
crooked range of mountains formed of intrusive rocks of the same
nature with the above described hillocks. Only in one or two
places, on this south-eastern side of the range, I noticed a
narrow fringe of the upper gypseous strata brushed up and
inclined south-eastward from it. On its north-eastern flank, and
likewise on a few of the summits, the stratified porphyritic
conglomerate is inclined N.E.: so that, if we disregard the very
narrow anticlinal fringe of gypseous strata at its S.E. foot,
this range forms a second uniclinal axis of elevation. Proceeding
in a north-by-east direction to the village of Combarbala, we
come to a third escarpment of the porphyritic conglomerate,
dipping eastwards, and forming the outer range of the main
Cordillera. The lower beds were here more jaspery than usual, and
they included some white cherty strata and red sandstones,
alternating with purple claystone porphyry. Higher up in the
Cordillera there appeared to be a line of andesitic rocks; and
beyond them, a fourth escarpment of the porphyritic conglomerate,
again dipping eastwards or inwards. The overlying gypseous
strata, if they ever existed here, have been entirely removed.
COPPER MINES OF PANUNCILLO.
From Combarbala to Coquimbo, I traversed the country in a zigzag
direction, crossing and recrossing the porphyritic conglomerate
and finding in the granitic districts an unusual number of
mountain-masses composed of various intrusive, porphyritic rocks,
many of them andesitic. One common variety was greenish-black,
with large crystals of blackish albite. At Panuncillo a short
N.N.W. and S.S.E. ridge, with a nucleus formed of greenstone and
of a slate-coloured porphyry including crystals of glassy
feldspar, deserves notice, from the very singular nature of the
almost vertical strata composing it. These consist chiefly of a
finer and coarser granular mixture, not very compact, of white
carbonate of lime, of protoxide of iron and of yellowish garnets
(ascertained by Professor Miller), each grain being an almost
perfect crystal. Some of the varieties consist exclusively of
granules of the calcareous spar; and some contain grains of
copper ore, and, I believe, of quartz. These strata alternate
with a bluish, compact, fusible, feldspathic rock. Much of the
above granular mixture has, also, a pseudo-brecciated structure,
in which fragments are obscurely arranged in planes parallel to
those of the stratification, and are conspicuous on the weathered
surfaces. The fragments are angular or rounded, small or large,
and consist of bluish or reddish compact feldspathic matter, in
which a few acicular crystals of feldspar can sometimes be seen.
The fragments often blend at their edges into the surrounding
granular mass, and seem due to a kind of concretionary action.
These singular rocks are traversed by many copper veins, and
appear to rest conformably on the granular mixture (in parts as
fine-grained as a sandstone) of quartz, mica, hornblende, and
feldspar; and this on fine- grained, common gneiss; and this on a
laminated mass, composed of pinkish ORTHITIC feldspar, including
a few specks of hornblende; and lastly, this on granite, which
together with andesitic rocks, form the surrounding district.
COQUIMBO: MINING DISTRICT OF ARQUEROS.
At Coquimbo the porphyritic conglomerate formation approaches
nearer to the Pacific than in any other part of Chile visited by
me, being separated from the coast by a tract only a few miles
broad of the usual plutonic rocks, with the addition of a
porphyry having a red euritic base. In proceeding to the mines of
Arqueros, the strata of porphyritic conglomerate are at first
nearly horizontal, an unusual circumstance, and afterwards they
dip gently to S.S.E. After having ascended to a considerable
height, we come to an undulatory district in which the famous
silver mines are situated; my examination was chiefly confined to
those of S. Rosa. Most of the rocks in this district are
stratified, dipping in various directions, and many of them are
of so singular a nature, that at the risk of being tedious I must
briefly describe them. The commonest variety is a dull-red,
compact, finely brecciated stone, containing much iron and
innumerable white crystallised particles of carbonate of lime,
and minute extraneous fragments. Another variety is almost
equally common near S. Rosa; it has a bright green, scanty basis,
including distinct crystals and patches of white carbonate of
lime, and grains of red, semi-micaceous oxide of iron; in parts
the basis becomes dark green, and assumes an obscure crystalline
arrangement, and occasionally in parts it becomes soft and
slightly translucent like soapstone. These red and green rocks
are often quite distinct, and often pass into each other; the
passage being sometimes affected by a fine brecciated structure,
particles of the red and green matter being mingled together.
Some of the varieties appear gradually to become porphyritic with
feldspar; and all of them are easily fusible into pale or
dark-coloured beads, strongly attracted by the magnet. I should
perhaps have mistaken several of these stratified rocks for
submarine lavas, like some of those described at the Puente del
Inca, had I not examined, a few leagues eastward of this point, a
fine series of analogous but less metamorphosed, sedimentary beds
belonging to the gypseous formation, and probably derived from a
volcanic source.
This formation is intersected by numerous metalliferous veins,
running, though irregularly, N.W. and S.E., and generally at
right angles to the many dikes. The veins consist of native
silver, of muriate of silver, an amalgam of silver, cobalt,
antimony, and arsenic, generally embedded in sulphate of barytes.
(See the Report on M. Domeyko's account of those mines, in the
"Comptes Rendus" tome 14 page 560.) I was assured by Mr. Lambert,
that native copper without a trace of silver has been found in
the same vein with native silver without a trace of copper. At
the mines of Aristeas, the silver veins are said to be
unproductive as soon as they pass into the green strata, whereas
at S. Rosa, only two or three miles distant, the reverse happens;
and at the time of my visit, the miners were working through a
red stratum, in the hope of the vein becoming productive in the
underlying green sedimentary mass. I have a specimen of one of
these green rocks, with the usual granules of white calcareous
spar and red oxide of iron, abounding with disseminated particles
of glittering native and muriate of silver, yet taken at the
distance of one yard from any vein,—a circumstance, as I was
assured, of very rare occurrence.
A SECTION EASTWARD, UP THE VALLEY OF COQUIMBO.
After passing for a few miles over the coast granitic series, we
come to the porphyritic conglomerate, with its usual characters,
and with some of the beds distinctly displaying their mechanical
origin. The strata, where first met with, are, as before stated,
only slightly inclined; but near the Hacienda of Pluclaro, we
come to an anticlinal axis, with the beds much dislocated and
shifted by a great fault, of which not a trace is externally seen
in the outline of the hill. I believe that this anticlinal axis
can be traced northwards, into the district of Arqueros, where a
conspicuous hill called Cerro Blanco, formed of a harsh,
cream-coloured euritic rock, including a few crystals of reddish
feldspar, and associated with some purplish claystone porphyry,
seems to fall on a line of elevation. In descending from the
Arqueros district, I crossed on the northern border of the
valley, strata inclined eastward from the Pluclaro axis: on the
porphyritic conglomerate there rested a mass, some hundred feet
thick, of brown argillaceous limestone, in parts crystalline, and
in parts almost composed of Hippurites Chilensis, d'Orbigny;
above this came a black calcareous shale, and on it a red
conglomerate. In the brown limestone, with the Hippurites, there
was an impression of a Pecten and a coral, and great numbers of a
large Gryphaea, very like, and, according to Professor E. Forbes,
probably identical with G. Orientalis, Forbes MS.,—a cretaceous
species (probably upper greensand) from Verdachellum, in Southern
India. These fossils seem to occupy nearly the same position with
those at the Puente del Inca,—namely, at the top of the
porphyritic conglomerate, and at the base of the gypseous
formation.
A little above the Hacienda of Pluclaro, I made a detour on the
northern side of the valley, to examine the superincumbent
gypseous strata, which I estimated at 6,000 feet in thickness.
The uppermost beds of the porphyritic conglomerate, on which the
gypseous strata conformably rest, are variously coloured, with
one very singular and beautiful stratum composed of purple
pebbles of various kinds of porphyry, embedded in white
calcareous spar, including cavities lined with bright-green
crystallised epidote. The whole pile of strata belonging to both
formations is inclined, apparently from the above-mentioned axis
of Pluclaro, at an angle of between 20 and 30 degrees to the
east. I will here give a section of the principal beds met with
in crossing the entire thickness of the gypseous strata.
Firstly: above the porphyritic conglomerate formation, there is a
fine- grained, red, crystalline sandstone.
Secondly: a thick mass of smooth-grained, calcareo-aluminous,
shaly rock, often marked with dendritic manganese, and having,
where most compact, the external appearance of honestone. It is
easily fusible. I shall for the future, for convenience' sake,
call this variety pseudo-honestone. Some of the varieties are
quite black when freshly broken, but all weather into a
yellowish-ash coloured, soft, earthy substance, precisely as is
the case with the compact shaly rocks of the Peuquenes range.
This stratum is of the same general nature with many of the beds
near Los Hornos in the Illapel section. In this second bed, or in
the underlying red sandstone (for the surface was partially
concealed by detritus), there was a thick mass of gypsum, having
the same mineralogical characters with the great beds described
in our sections across the Cordillera.
Thirdly: a thick stratum of fine-grained, red, sedimentary
matter, easily fusible into a white glass, like the basis of
claystone porphyry; but in parts jaspery, in parts brecciated,
and including crystalline specks of carbonate of lime. In some of
the jaspery layers, and in some of the black siliceous slaty
bands, there were irregular seams of imperfect pitchstone,
undoubtedly of metamorphic origin, and other seams of brown,
crystalline limestone. Here, also, were masses, externally
resembling ill-preserved silicified wood.
Fourthly and fifthly: calcareous pseudo-honestone; and a thick
stratum concealed by detritus.
Sixthly: a thinly stratified mass of bright green, compact,
smooth-grained, calcareo-argillaceous stone, easily fusible, and
emitting a strong aluminous odour: the whole has a highly
angulo-concretionary structure; and it resembles, to a certain
extent, some of the upper tufaceo-infusorial deposits of the
Patagonian tertiary formation. It is in its nature allied to our
pseudo-honestone, and it includes well characterised layers of
that variety; and other layers of a pale green, harder, and
brecciated variety; and others of red sedimentary matter, like
that of bed Three. Some pebbles of porphyries are embedded in the
upper part.
Seventhly: red sedimentary matter or sandstone like that of bed
One, several hundred feet in thickness, and including jaspery
layers, often having a finely brecciated structure.
Eighthly: white, much indurated, almost crystalline tuff, several
hundred feet in thickness, including rounded grains of quartz and
particles of green matter like that of bed Six. Parts pass into a
very pale green, semi- porcellanic stone.
Ninthly: red or brown coarse conglomerate, three or four hundred
feet thick, formed chiefly of pebbles of porphyries, with
volcanic particles, in an arenaceous, non-calcareous, fusible
basis: the upper two feet are arenaceous without any pebbles.
Tenthly: the last and uppermost stratum here exhibited, is a
compact, slate-coloured porphyry, with numerous elongated
crystals of glassy feldspar, from one hundred and fifty to two
hundred feet in thickness; it lies strictly conformably on the
underlying conglomerate, and is undoubtedly a submarine lava.
This great pile of strata has been broken up in several places by
intrusive hillocks of purple claystone porphyry, and by dikes of
porphyritic greenstone: it is said that a few poor metalliferous
veins have been discovered here. From the fusible nature and
general appearance of the finer-grained strata, they probably owe
their origin (like the allied beds of the Uspallata range, and of
the Upper Patagonian tertiary formations), to gentle volcanic
eruptions, and to the abrasion of volcanic rocks. Comparing these
beds with those in the mining district of Arqueros, we see at
both places rocks easily fusible, of the same peculiar bright
green and red colours, containing calcareous matter, often having
a finely brecciated structure, often passing into each other, and
often alternating together: hence I cannot doubt that the only
difference between them, lies in the Arqueros beds having been
more metamorphosed (in conformity with their more dislocated and
injected condition), and consequently in the calcareous matter,
oxide of iron and green colouring matter, having been segregated
under a more crystalline form.
The strata are inclined, as before stated, from 20 to 30 degrees
eastward, towards an irregular north and south chain of andesitic
porphyry and of porphyritic greenstone, where they are abruptly
cut off. In the valley of Coquimbo, near to the H. of
Gualliguaca, similar plutonic rocks are met with, apparently a
southern prolongation of the above chain; and eastward of it we
have an escarpment of the porphyritic conglomerate, with the
strata inclined at a small angle eastward, which makes the third
escarpment, including that nearest the coast. Proceeding up the
valley we come to another north and south line of granite,
andesite, and blackish porphyry, which seem to lie in an
irregular trough of the porphyritic conglomerate. Again, on the
south side of the R. Claro, there are some irregular granitic
hills, which have thrown off the strata of porphyritic
conglomerate to the N.W. by W.; but the stratification here has
been much disturbed. I did not proceed any farther up the valley,
and this point is about two-thirds of the distance between the
Pacific and the main Cordillera.
I will describe only one other section, namely, on the north side
of the R. Claro, which is interesting from containing fossils:
the strata are much dislocated by faults and dikes, and are
inclined to the north, towards a mountain of andesite and
porphyry, into which they appear to become almost blended. As the
beds approach this mountain, their inclination increases up to an
angle of 70 degrees, and in the upper part, the rocks become
highly metamorphosed. The lowest bed visible in this section, is
a purplish hard sandstone. Secondly, a bed two or three hundred
feet thick, of a white siliceous sandstone, with a calcareous
cement, containing seams of slaty sandstone, and of hard
yellowish-brown (dolomitic?) limestone; numerous, well-rounded,
little pebbles of quartz are included in the sandstone. Thirdly,
a dark coloured limestone with some quartz pebbles, from fifty to
sixty feet in thickness, containing numerous silicified shells,
presently to be enumerated. Fourthly, very compact, calcareous,
jaspery sandstone, passing into (fifthly) a great bed, several
hundred feet thick, of conglomerate, composed of pebbles of
white, red, and purple porphyries, of sandstone and quartz,
cemented by calcareous matter. I observed that some of the finer
parts of this conglomerate were much indurated within a foot of a
dike eight feet in width, and were rendered of a paler colour
with the calcareous matter segregated into white crystallised
particles; some parts were stained green from the colouring
matter of the dike. Sixthly, a thick mass, obscurely stratified,
of a red sedimentary stone or sandstone, full of crystalline
calcareous matter, imperfect crystals of oxide of iron, and I
believe of feldspar, and therefore closely resembling some of the
highly metamorphosed beds at Arqueros: this bed was capped by,
and appeared to pass in its upper part into, rocks similarly
coloured, containing calcareous matter, and abounding with minute
crystals, mostly elongated and glassy, of reddish albite.
Seventhly, a conformable stratum of fine reddish porphyry with
large crystals of (albitic?) feldspar; probably a submarine lava.
Eighthly, another conformable bed of green porphyry, with specks
of green earth and cream-coloured crystals of feldspar. I believe
that there are other superincumbent crystalline strata and
submarine lavas, but I had not time to examine them.
The upper beds in this section probably correspond with parts of
the great gypseous formation; and the lower beds of red sandstone
conglomerate and fossiliferous limestone no doubt are the
equivalents of the Hippurite stratum, seen in descending from
Arqueros to Pluclaro, which there lies conformably upon the
porphyritic conglomerate formation. The fossils found in the
third bed, consist of:—
Pecten Dufreynoyi, d'Orbigny, "Voyage, Part Pal." This species,
which occurs here in vast numbers, according to M. D'Orbigny,
resembles certain cretaceous forms.
Ostrea hemispherica, d'Orbigny, "Voyage" etc.
Also resembles, according to the same author, cretaceous forms.
Terebratula aenigma, d'Orbigny, "Voyage" etc. (Pl. 22 Figures
10-12.)
Is allied, according to M. d'Orbigny, to T. concinna from the
Forest Marble. A series of this species, collected in several
localities hereafter to be referred to, has been laid before
Professor Forbes; and he informs me that many of the specimens
are almost undistinguishable from our oolitic T. tetraedra, and
that the varieties amongst them are such as are found in that
variable species. Generally speaking, the American specimens of
T. aenigma may be distinguished from the British T. tetraedra, by
the surface having the ribs sharp and well-defined to the beak,
whilst in the British species they become obsolete and smoothed
down; but this difference is not constant. Professor Forbes adds,
that, possibly, internal characters may exist, which would
distinguish the American species from its European allies.
Spirifer linguiferoides, E. Forbes.
Professor Forbes states that this species is very near to S.
linguifera of Phillips (a carboniferous limestone fossil), but
probably distinct. M. d'Orbigny considers it as perhaps
indicating the Jurassic period.
Ammonites, imperfect impression of.
M. Domeyko has sent to France a collection of fossils, which, I
presume, from the description given, must have come from the
neighbourhood of Arqueros; they consist of:—
Pecten Dufreynoyi, d'Orbigny, "Voyage" Part Pal. Ostrea
hemispherica, d'Orbigny, "Voyage" Part Pal. Turritella Andii,
d'Orbigny, "Voyage" Part Pal. (Pleurotomaria Humboldtii of Von
Buch). Hippurites Chilensis, d'Orbigny, "Voyage" Part Pal.
The specimens of this Hippurite, as well as those I collected in
my descent from Arqueros, are very imperfect; but in M.
d'Orbigny's opinion they resemble, as does the Turritella Andii,
cretaceous (upper greensand) forms.
Nautilus Domeykus, d'Orbigny, "Voyage" Part Pal. Terebratula
aenigma, d'Orbigny, "Voyage" Part Pal. Terebratula ignaciana,
d'Orbigny, "Voyage" Part Pal.
This latter species was found by M. Domeyko in the same block of
limestone with the T. aenigma. According to M. d'Orbigny, it
comes near to T. ornithocephala from the Lias. A series of this
species collected at Guasco, has been examined by Professor E.
Forbes, and he states that it is difficult to distinguish between
some of the specimens and the T. hastata from the mountain
limestone; and that it is equally difficult to draw a line
between them and some Marlstone Terebratulae. Without a knowledge
of the internal structure, it is impossible at present to decide
on their identity with analogous European forms.
The remarks given on the several foregoing shells, show that, in
M. d'Orbigny's opinion, the Pecten, Ostrea, Turritella, and
Hippurite indicate the cretaceous period; and the Gryphaea
appears to Professor Forbes to be identical with a species,
associated in Southern India with unquestionably cretaceous
forms. On the other hand, the two Terebratulae and the Spirifer
point, in the opinion both of M. d'Orbigny and Professor Forbes,
to the oolitic series. Hence M. d'Orbigny, not having himself
examined this country, has concluded that there are here two
distinct formations; but the Spirifer and T. aenigma were
certainly included in the same bed with the Pecten and Ostrea,
whence I extracted them; and the geologist M. Domeyko sent home
the two Terebratulae with the other-named shells, from the same
locality, without specifying that they came from different beds.
Again, as we shall presently see, in a collection of shells given
me from Guasco, the same species, and others presenting analogous
differences, are mingled together, and are in the same condition;
and lastly, in three places in the valley of Copiapo, I found
some of these same species similarly grouped. Hence there cannot
be any doubt, highly curious though the fact be, that these
several fossils, namely, the Hippurites, Gryphaea, Ostrea,
Pecten, Turritella, Nautilus, two Terebratulae, and Spirifer all
belong to the same formation, which would appear to form a
passage between the oolitic and cretaceous systems of Europe.
Although aware how unusual the term must sound, I shall, for
convenience' sake, call this formation cretaceo- oolitic.
Comparing the sections in this valley of Coquimbo with those in
the Cordillera described in the last chapter, and bearing in mind
the character of the beds in the intermediate district of Los
Hornos, there is certainly a close general mineralogical
resemblance between them, both in the underlying porphyritic
conglomerate, and in the overlying gypseous formation.
Considering this resemblance, and that the fossils from the
Puente del Inca at the base of the gypseous formation, and
throughout the greater part of its entire thickness on the
Peuquenes range, indicate the Neocomian period,—that is, the dawn
of the cretaceous system, or, as some have believed, a passage
between this latter and the oolitic series—I conclude that
probably the gypseous and associated beds in all the sections
hitherto described, belong to the same great formation, which I
have denominated—cretaceo-oolitic. I may add, before leaving
Coquimbo, that M. Gay found in the neighbouring Cordillera, at
the height of 14,000 feet above the sea, a fossiliferous
formation, including a Trigonia and Pholadomya (D'Orbigny
"Voyage" Part Geolog. page 242.);—both of which genera occur at
the Puente del Inca.
COQUIMBO TO GUASCO.
The rocks near the coast, and some way inland, do not differ from
those described northwards of Valparaiso: we have much
greenstone, syenite, feldspathic and jaspery slate, and
grauwackes having a basis like that of claystone; there are some
large tracts of granite, in which the constituent minerals are
sometimes arranged in folia, thus composing an imperfect gneiss.
There are two large districts of mica-schists, passing into
glossy clay-slate, and resembling the great formation in the
Chonos Archipelago. In the valley of Guasco, an escarpment of
porphyritic conglomerate is first seen high up the valley, about
two leagues eastward of the town of Ballenar. I heard of a great
gypseous formation in the Cordillera; and a collection of shells
made there was given me. These shells are all in the same
condition, and appear to have come from the same bed: they
consist of:—
Turritella Andii, d'Orbigny, "Voyage" Part Pal. Pecten
Dufreynoyi, d'Orbigny, "Voyage" Part Pal. Terebatula ignaciana,
d'Orbigny, "Voyage" Part Pal.
The relations of these species have been given under the head of
Coquimbo.
Terebratula aenigma, d'Orbigny, "Voyage" Part Pal.
This shell M. d'Orbigny does not consider identical with his T.
aenigma, but near to T. obsoleta. Professor Forbes thinks that it
is certainly a variety of T. aenigma: we shall meet with this
variety again at Copiapo.
Spirifer Chilensis, E. Forbes.
Professor Forbes remarks that this fossil resembles several
carboniferous limestone Spirifers; and that it is also related to
some liassic species, as S. Wolcotii.
If these shells had been examined independently of the other
collections, they would probably have been considered, from the
characters of the two Terebratulae, and from the Spirifer, as
oolitic; but considering that the first species, and according to
Professor Forbes, the four first, are identical with those from
Coquimbo, the two formations no doubt are the same, and may, as I
have said, be provisionally called cretaceo-oolitic.
VALLEY OF COPIAPO.
The journey from Guasco to Copiapo, owing to the utterly desert
nature of the country, was necessarily so hurried, that I do not
consider my notes worth giving. In the valley of Copiapo some of
the sections are very interesting. From the sea to the town of
Copiapo, a distance estimated at thirty miles, the mountains are
composed of greenstone, granite, andesite, and blackish porphyry,
together with some dusky-green feldspathic rocks, which I believe
to be altered clay-slate: these mountains are crossed by many
brown-coloured dikes, running north and south. Above the town,
the main valley runs in a south-east and even more southerly
course towards the Cordillera, where it is divided into three
great ravines, by the northern one of which, called Jolquera, I
penetrated for a short distance. The section, Section 1/3 in
Plate 1, gives an eye-sketch of the structure and composition of
the mountains on both sides of this valley: a straight east and
west line from the town to the Cordillera is perhaps not more
than thirty miles, but along the valley the distance is much
greater. Wherever the valley trended very southerly, I have
endeavoured to contract the section into its true proportion.
This valley, I may add, rises much more gently than any other
valley which I saw in Chile.
To commence with our section, for a short distance above the town
we have hills of the granitic series, together with some of that
rock [A], which I suspect to be altered clay-slate, but which
Professor G. Rose, judging from specimens collected by Meyen at
P. Negro, states is serpentine passing into greenstone. We then
come suddenly to the great gypseous formation [B], without having
passed over, differently from, in all the sections hitherto
described, any of the porphyritic conglomerate. The strata are at
first either horizontal or gently inclined westward; then highly
inclined in various directions, and contorted by underlying
masses of intrusive rocks; and lastly, they have a regular
eastward dip, and form a tolerably well pronounced north and
south line of hills. This formation consists of thin strata, with
innumerable alternations, of black, calcareous slate-rock, of
calcareo-aluminous stones like those at Coquimbo, which I have
called pseudo-honestones of green jaspery layers, and of
pale-purplish, calcareous, soft rotten-stone, including seams and
veins of gypsum. These strata are conformably overlaid by a great
thickness of thinly stratified, compact limestone with included
crystals of carbonate of lime. At a place called Tierra Amarilla,
at the foot of a mountain thus composed there is a broad vein, or
perhaps stratum, of a beautiful and curious crystallised mixture,
composed, according to Professor G. Rose, of sulphate of iron
under two forms, and of the sulphates of copper and alumina
(Meyen's "Reise" etc. Th. 1, s. 394.): the section is so obscure
that I could not make out whether this vein or stratum occurred
in the gypseous formation, or more probably in some underlying
masses [A], which I believe are altered clay-slate.
SECOND AXIS OF ELEVATION.
After the gypseous masses [B], we come to a line of hills of
unstratified porphyry [C], which on their eastern side blend into
strata of great thickness of porphyritic conglomerate, dipping
eastward. This latter formation, however, here has not been
nearly so much metamorphosed as in most parts of Central Chile;
it is composed of beds of true purple claystone porphyry,
repeatedly alternating with thick beds of purplish-red
conglomerate with the well-rounded, large pebbles of various
porphyries, not blended together.
THIRD AXIS OF ELEVATION.
Near the ravine of Los Hornitos, there is a well-marked line of
elevation, extending for many miles in a N.N.E. and S.S.W.
direction, with the strata dipping in most parts (as in the
second axis) only in one direction, namely, eastward at an
average angle of between 30 and 40 degrees. Close to the mouth of
the valley, however, there is, as represented in the section, a
steep and high mountain [D], composed of various green and brown
intrusive porphyries enveloped with strata, apparently belonging
to the upper parts of the porphyritic conglomerate, and dipping
both eastward and westward. I will describe the section seen on
the eastern side of this mountain [D], beginning at the base with
the lowest bed visible in the porphyritic conglomerate, and
proceeding upwards through the gypseous formation. Bed 1 consists
of reddish and brownish porphyry varying in character, and in
many parts highly amygdaloidal with carbonate of lime, and with
bright green and brown bole. Its upper surface is throughout
clearly defined, but the lower surface is in most parts
indistinct, and towards the summit of the mountain [D] quite
blended into the intrusive porphyries. Bed 2, a pale lilac, hard
but not heavy stone, slightly laminated, including small
extraneous fragments, and imperfect as well as some perfect and
glassy crystals of feldspar; from one hundred and fifty to two
hundred feet in thickness. When examining it in situ, I thought
it was certainly a true porphyry, but my specimens now lead me to
suspect that it possibly may be a metamorphosed tuff. From its
colour it could be traced for a long distance, overlying in one
part, quite conformably to the porphyry of bed 1, and in another
not distant part, a very thick mass of conglomerate, composed of
pebbles of a porphyry chiefly like that of bed 1: this fact shows
how the nature of the bottom formerly varied in short horizontal
distances. Bed 3, white, much indurated tuff, containing minute
pebbles, broken crystals, and scales of mica, varies much in
thickness. This bed is remarkable from containing many globular
and pear-shaped, externally rusty balls, from the size of an
apple to a man's head, of very tough, slate-coloured porphyry,
with imperfect crystals of feldspar: in shape these balls do not
resemble pebbles, AND I BELIEVE THAT THEY ARE SUBAQUEOUS VOLCANIC
BOMBS; they differ from SUBAERIAL bombs only in not being
vesicular. Bed 4; a dull purplish-red, hard conglomerate, with
crystallised particles and veins of carbonate of lime, from three
hundred to four hundred feet in thickness. The pebbles are of
claystone porphyries of many varieties; they are tolerably well
rounded, and vary in size from a large apple to a man's head.
This bed includes three layers of coarse, black, calcareous,
somewhat slaty rock: the upper part passes into a compact red
sandstone.
In a formation so highly variable in mineralogical nature, any
division not founded on fossil remains, must be extremely
arbitrary: nevertheless, the beds below the last conglomerate
may, in accordance with all the sections hitherto described, be
considered as belonging to the porphyritic conglomerate, and
those above it to the gypseous formation, marked [E] in the
section. The part of the valley in which the following beds are
seen is near Potrero Seco. Bed 5, compact, fine-grained, pale
greenish-grey, non- calcareous, indurated mudstone, easily
fusible into a pale green and white glass. Bed 6, purplish,
coarse-grained, hard sandstone, with broken crystals of feldspar
and crystallised particles of carbonate of lime; it possesses a
slightly nodular structure. Bed 7, blackish-grey, much indurated,
calcareous mudstone, with extraneous particles of unequal size;
the whole being in parts finely brecciated. In this mass there is
a stratum, twenty feet in thickness, of impure gypsum. Bed 8, a
greenish mudstone, with several layers of gypsum. Bed 9, a highly
indurated, easily fusible, white tuff, thickly mottled with
ferruginous matter, and including some white semi-porcellanic
layers, which are interlaced with ferruginous veins. This stone
closely resembles some of the commonest varieties in the
Uspallata chain. Bed 10, a thick bed of rather bright green,
indurated mudstone or tuff, with a concretionary nodular
structure so strongly developed that the whole mass consists of
balls. I will not attempt to estimate the thickness of the strata
in the gypseous formation hitherto described, but it must
certainly be very many hundred feet. Bed 11 is at least 800 feet
in thickness: it consists of thin layers of whitish, greenish, or
more commonly brown, fine-grained, indurated tuffs, which crumble
into angular fragments: some of the layers are semi-porcellanic,
many of them highly ferruginous, and some are almost composed of
carbonate of lime and iron with drusy cavities lined with
quartzf-crystals. Bed 12, dull purplish or greenish or dark-grey,
very compact and much indurated mudstone: estimated at 1,500 feet
in thickness: in some parts this rock assumes the character of an
imperfect coarse clay-slate; but viewed under a lens, the basis
always has a mottled appearance, with the edges of the minute
component particles blending together. Parts are calcareous, and
there are numerous veins of highly crystalline carbonate of lime
charged with iron. The mass has a nodular structure, and is
divided by only a few planes of stratification: there are,
however, two layers, each about eighteen inches thick, of a dark
brown, finer-grained stone, having a conchoidal, semi-porcellanic
fracture, which can be followed with the eye for some miles
across the country.
I believe this last great bed is covered by other nearly similar
alternations; but the section is here obscured by a tilt from the
next porphyritic chain, presently to be described. I have given
this section in detail, as being illustrative of the general
character of the mountains in this neighbourhood; but it must not
be supposed that any one stratum long preserves the same
character. At a distance of between only two and three miles the
green mudstones and white indurated tuffs are to a great extent
replaced by red sandstone and black calcareous shaly rocks,
alternating together. The white indurated tuff, bed 11, here
contains little or no gypsum, whereas on the northern and
opposite side of the valley, it is of much greater thickness and
abounds with layers of gypsum, some of them alternating with thin
seams of crystalline carbonate of lime. The uppermost,
dark-coloured, hard mudstone, bed 12, is in this neighbourhood
the most constant stratum. The whole series differs to a
considerable extent, especially in its upper part, from that met
with at [BB], in the lower part of the valley; nevertheless, I do
not doubt that they are equivalents.
FOURTH AXIS OF ELEVATION (VALLEY OF COPIAPO).
This axis is formed of a chain of mountains [F], of which the
central masses (near La Punta) consist of andesite containing
green hornblende and coppery mica, and the outer masses of
greenish and black porphyries, together with some fine
lilac-coloured claystone porphyry; all these porphyries being
injected and broken up by small hummocks of andesite. The central
great mass of this latter rock, is covered on the eastern side by
a black, fine-grained, highly micaceous slate, which, together
with the succeeding mountains of porphyry, are traversed by
numerous white dikes, branching from the andesite, and some of
them extending in straight lines, to a distance of at least two
miles. The mountains of porphyry eastward of the micaceous schist
soon, but gradually, assume (as observed in so many other cases)
a stratified structure, and can then be recognised as a part of
the porphyritic conglomerate formation. These strata [G] are
inclined at a high angle to the S.E., and form a mass from
fifteen hundred to two thousand feet in thickness. The gypseous
masses to the west already described, dip directly towards this
axis, with the strata only in a few places (one of which is
represented in the section) thrown from it: hence this fourth
axis is mainly uniclinal towards the S.E., and just like our
third axis, only locally anticlinal.
The above strata of porphyritic conglomerate [G] with their
south-eastward dip, come abruptly up against beds of the gypseous
formation [H], which are gently, but irregularly, inclined
westward: so that there is here a synclinal axis and great fault.
Further up the valley, here running nearly north and south, the
gypseous formation is prolonged for some distance; but the
stratification is unintelligible, the whole being broken up by
faults, dikes, and metalliferous veins. The strata consist
chiefly of red calcareous sandstones, with numerous veins in the
place of layers, of gypsum; the sandstone is associated with some
black calcareous slate-rock, and with green pseudo-honestones,
passing into porcelain-jasper. Still further up the valley, near
Las Amolanas [I], the gypseous strata become more regular,
dipping at an angle of between 30 and 40 degrees to W.S.W., and
conformably overlying, near the mouth of the ravine of Jolquera,
strata [K] of porphyritic conglomerate. The whole series has been
tilted by a partially concealed axis [L], of granite, andesite,
and a granitic mixture of white feldspar, quartz, and oxide of
iron.
FIFTH AXIS OF ELEVATION (VALLEY OF COPIAPO, NEAR LOS AMOLANAS).
I will describe in some detail the beds [I] seen here, which, as
just stated, dip to W.S.W., at an angle of from 30 to 40 degrees.
I had not time to examine the underlying porphyritic
conglomerate, of which the lowest beds, as seen at the mouth of
the Jolquera, are highly compact, with crystals of red oxide of
iron; and I am not prepared to say whether they are chiefly of
volcanic or metamorphic origin. On these beds there rests a
coarse purplish conglomerate, very little metamorphosed, composed
of pebbles of porphyry, but remarkable from containing one pebble
of granite;- -of which fact no instance has occurred in the
sections hitherto described. Above this conglomerate, there is a
black siliceous claystone, and above it numerous alternations of
dark-purplish and green porphyries, which may be considered as
the uppermost limit of the porphyritic conglomerate formation.
Above these porphyries comes a coarse, arenaceous conglomerate,
the lower half white and the upper half of a pink colour,
composed chiefly of pebbles of various porphyries, but with some
of red sandstone and jaspery rocks. In some of the more
arenaceous parts of the conglomerate, there was an oblique or
current lamination; a circumstance which I did not elsewhere
observe. Above this conglomerate, there is a vast thickness of
thinly stratified, pale-yellowish, siliceous sandstone, passing
into a granular quartz-rock, used for grindstones (hence the name
of the place Las Amolanas), and certainly belonging to the
gypseous formation, as does probably the immediately underlying
conglomerate. In this yellowish sandstone there are layers of
white and pale-red siliceous conglomerate; other layers with
small, well-rounded pebbles of white quartz, like the bed at the
R. Claro at Coquimbo; others of a greenish, fine-grained, less
siliceous stone, somewhat resembling the pseudo-honestones lower
down the valley; and lastly, others of a black calcareous
shale-rock. In one of the layers of conglomerate, there was
embedded a fragment of mica-slate, of which this is the first
instance; hence perhaps, it is from a formation of mica-slate,
that the numerous small pebbles of quartz, both here and at
Coquimbo, have been derived. Not only does the siliceous
sandstone include layers of the black, thinly stratified, not
fissile, calcareous shale-rock, but in one place the whole mass,
especially the upper part, was, in a marvellously short
horizontal distance, after frequent alternations, replaced by it.
When this occurred, a mountain-mass, several thousand feet in
thickness was thus composed; the black calcareous shale-rock,
however, always included some layers of the pale-yellowish
siliceous sandstone, of the red conglomerate, and of the greenish
jaspery and pseudo-honestone varieties. It likewise included
three or four widely separated layers of a brown limestone,
abounding with shells immediately to be described. This pile of
strata was in parts traversed by many veins of gypsum. The
calcareous shale-rock, though when freshly broken quite black,
weathers into an ash- colour: in which respect and in general
appearance, it perfectly resembles those great fossiliferous beds
of the Peuquenes range, alternating with gypsum and red
sandstone, described in the last chapter.
The shells out of the layers of brown limestone, included in the
black calcareous shale-rock, which latter, as just stated,
replaces the white siliceous sandstone, consist of:—
Pecten Dufreynoyi, d'Orbigny, "Voyage" Part Pal. Turritella
Andii, d'Orbigny, "Voyage" Part Pal.
Astarte Darwinii, E. Forbes. Gryphaea Darwinii, E. Forbes.
An intermediate form between G. gigantea and G. incurva.
Gryphaea nov. spec.?, E. Forbes. Perna Americana, E. Forbes.
Avicula, nov. spec.
Considered by Mr. G.B. Sowerby as the A. echinata, by M.
d'Orbigny as certainly a new and distinct species, having a
Jurassic aspect. The specimen has been unfortunately lost.
Terebratula aenigma, d'Orbigny, (var. of do. E. Forbes.)
This is the same variety, with that from Guasco, considered by M.
D'Orbigny to be a distinct species from his T. aenigma, and
related to T. obsoleta.
Plagiostoma and Ammonites, fragments of.
The lower layers of the limestone contained thousands of the
Gryphaea; and the upper ones as many of the Turritella, with the
Gryphaea (nov. species) and Serpulae adhering to them; in all the
layers, the Terebratula and fragments of the Pecten were
included. It was evident, from the manner in which species were
grouped together, that they had lived where now embedded. Before
making any further remarks, I may state, that higher up this same
valley we shall again meet with a similar association of shells;
and in the great Despoblado Valley, which branches off near the
town from that of Copiapo, the Pecten Dufreynoyi, some Gryphites
(I believe G. Darwinii), and the TRUE Terebratula aenigma of
d'Orbigny were found together in an equivalent formation, as will
be hereafter seen. A specimen also, I may add, of the true T.
aenigma, was given me from the neighbourhood of the famous silver
mines of Chanuncillo, a little south of the valley of the
Copiapo, and these mines, from their position, I have no doubt,
lie within the great gypseous formation: the rocks close to one
of the silver veins, judging from fragments shown me, resemble
those singular metamorphosed deposits from the mining district of
Arqueros near Coquimbo.
I will reiterate the evidence on the association of these several
shells in the several localities.
COQUIMBO.
In the same bed, Rio Claro: Pecten Dufreynoyi. Ostrea
hemispherica. Terebratula aenigma. Spirifer linguiferoides.
Same bed, near Arqueros: Hippurites Chilensis. Gryphaea
orientalis.
Collected by M. Domeyko from the same locality, apparently near
Arqueros: Terebratula aenigma and Terebratula ignaciana, in same
block of limestone: Pecten Dufreynoyi. Ostrea hemispherica.
Hippurites Chilensis. Turritella Andii. Nautilus Domeykus.
GUASCO.
In a collection from the Cordillera, given me: the specimens all
in the same condition: Pecten Dufreynoyi. Turritella Andii.
Terebratula ignaciana. Terebratula aenigma, var. Spirifer
Chilensis.
COPIAPO.
Mingled together in alternating beds in the main valley of
Copiapo near Las Amolanas, and likewise higher up the valley:
Pecten Dufreynoyi. Turritella Andii. Terebratula aenigma, var. as
at Guasco. Astarte Darwinii. Gryphaea Darwinii. Gryphaea nov.
species? Perna Americana. Avicula, nov. species.
Main valley of Copiapo, apparently same formation with that of
Amolanas: Terebratula aenigma (true).
In the same bed, high up the great lateral valley of the
Despoblado, in the ravine of Maricongo: Terebratula aenigma
(true). Pecten Dufreynoyi. Gryphaea Darwinii?
Considering this table, I think it is impossible to doubt that
all these fossils belong to the same formation. If, however, the
species from Las Amolanas, in the Valley of Copiapo, had, as in
the case of those from Guasco, been separately examined, they
would probably have been ranked as oolitic; for, although no
Spirifers were found here, all the other species, with the
exception of the Pecten, Turritella, and Astarte, have a more
ancient aspect than cretaceous forms. On the other hand, taking
into account the evidence derived from the cretaceous character
of these three shells, and of the Hippurites, Gryphaea
orientalis, and Ostrea, from Coquimbo, we are driven back to the
provisional name already used of cretaceo-oolitic. From
geological evidence, I believe this formation to be the
equivalent of the Neocomian beds of the Cordillera of Central
Chile.
To return to our section near Las Amolanas:—Above the yellow
siliceous sandstone, or the equivalent calcareous slate-rock,
with its bands of fossil-shells, according as the one or other
prevails, there is a pile of strata, which cannot be less than
from two to three thousand feet in thickness, in main part
composed of a coarse, bright red conglomerate, with many
intercalated beds of red sandstone, and some of green and other
coloured porcelain-jaspery layers. The included pebbles are
well-rounded, varying from the size of an egg to that of a
cricket-ball, with a few larger; and they consist chiefly of
porphyries. The basis of the conglomerate, as well as some of the
alternating thin beds, are formed of a red, rather harsh, easily
fusible sandstone, with crystalline calcareous particles. This
whole great pile is remarkable from the thousands of huge,
embedded, silicified trunks of trees, one of which was eight feet
long, and another eighteen feet in circumference: how marvellous
it is, that every vessel in so thick a mass of wood should have
been converted into silex! I brought home many specimens, and all
of them, according to Mr. R. Brown, present a coniferous
structure.
Above this great conglomerate, we have from two to three hundred
feet in thickness of red sandstone; and above this, a stratum of
black calcareous slate-rock, like that which alternates with and
replaces the underlying yellowish-white, siliceous sandstone.
Close to the junction between this upper black slate-rock and the
upper red sandstone, I found the Gryphaea Darwinii, the
Turritella Andii, and vast numbers of a bivalve, too imperfect to
be recognised. Hence we see that, as far as the evidence of these
two shells serves—and the Turritella is an eminently
characteristic species—the whole thickness of this vast pile of
strata belongs to the same age. Again, above the last-mentioned
upper red sandstone, there were several alternations of the
black, calcareous slate-rock; but I was unable to ascend to them.
All these uppermost strata, like the lower ones, vary extremely
in character in short horizontal distances. The gypseous
formation, as here seen, has a coarser, more mechanical texture,
and contains much more siliceous matter than the corresponding
beds lower down the valley. Its total thickness, together with
the upper beds of the porphyritic conglomerate, I estimated at
least at 8,000 feet; and only a small portion of the porphyritic
conglomerate, which on the eastern flank of the fourth axis of
elevation appeared to be from fifteen hundred to two thousand
feet thick, is here included. As corroborative of the great
thickness of the gypseous formation, I may mention that in the
Despoblado Valley (which branches from the main valley a little
above the town of Copiapo) I found a corresponding pile of red
and white sandstones, and of dark, calcareous, semi-jaspery
mudstones, rising from a nearly level surface and thrown into an
absolutely vertical position; so that, by pacing, I ascertained
their thickness to be nearly two thousand seven hundred feet;
taking this as a standard of comparison, I estimated the
thickness of the strata ABOVE the porphyritic conglomerate at
7,000 feet.
The fossils before enumerated, from the limestone-layers in the
whitish siliceous sandstone, are now covered, on the least
computation, by strata from 5,000 to 6,000 feet in thickness.
Professor E. Forbes thinks that these shells probably lived at a
depth of from about 30 to 40 fathoms, that is from 180 to 240
feet; anyhow, it is impossible that they could have lived at the
depth of from 5,000 to 6,000 feet. Hence in this case, as in that
of the Puente del Inca, we may safely conclude that the bottom of
the sea on which the shells lived, subsided, so as to receive the
superincumbent submarine strata: and this subsidence must have
taken place during the existence of these shells; for, as I have
shown, some of them occur high up as well as low down in the
series. That the bottom of the sea subsided, is in harmony with
the presence of the layers of coarse, well- rounded pebbles
included throughout this whole pile of strata, as well as of the
great upper mass of conglomerate from 2,000 to 3,000 feet thick;
for coarse gravel could hardly have been formed or spread out at
the profound depths indicated by the thickness of the strata. The
subsidence, also, must have been slow to have allowed of this
often-recurrent spreading out of the pebbles. Moreover, we shall
presently see that the surfaces of some of the streams of
porphyritic lava beneath the gypseous formation, are so highly
amygdaloidal that it is scarcely possible to believe that they
flowed under the vast pressure of a deep ocean. The conclusion of
a great subsidence during the existence of these cretaceo-oolitic
fossils, may, I believe, be extended to the district of Coquimbo,
although owing to the fossiliferous beds there not being directly
covered by the upper gypseous strata, which in the section north
of the valley are about 6,000 feet in thickness, I did not there
insist on this conclusion.
The pebbles in the above conglomerates, both in the upper and
lower beds, are all well rounded, and, though chiefly composed of
various porphyries, there are some of red sandstone and of a
jaspery stone, both like the rocks intercalated in layers in this
same gypseous formation; there was one pebble of mica-slate and
some of quartz, together with many particles of quartz. In these
respects there is a wide difference between the gypseous
conglomerates and those of the porphyritic-conglomerate
formation, in which latter, angular and rounded fragments, almost
exclusively composed of porphyries, are mingled together, and
which, as already often remarked, probably were ejected from
craters deep under the sea. From these facts I conclude, that
during the formation of the conglomerates, land existed in the
neighbourhood, on the shores of which the innumerable pebbles
were rounded and thence dispersed, and on which the coniferous
forests flourished—for it is improbable that so many thousand
logs of wood should have drifted from any great distance. This
land, probably islands, must have been mainly formed of
porphyries, with some mica-slate, whence the quartz was derived,
and with some red sandstone and jaspery rocks. This latter fact
is important, as it shows that in this district, even previously
to the deposition of the lower gypseous or cretaceo-oolitic beds,
strata of an analogous nature had elsewhere, no doubt in the more
central ranges of the Cordillera, been elevated; thus recalling
to our minds the relations of the Cumbre and Uspallata chains.
Having already referred to the great lateral valley of the
Despoblado, I may mention that above the 2,700 feet of red and
white sandstone and dark mudstone, there is a vast mass of
coarse, hard, red conglomerate, some thousand feet in thickness,
which contains much silicified wood, and evidently corresponds
with the great upper conglomerate at Las Amolanas: here, however,
the conglomerate consists almost exclusively of pebbles of
granite, and of disintegrated crystals of reddish feldspar and
quartz firmly recemented together. In this case, we may conclude
that the land whence the pebbles were derived, and on which the
now silicified trees once flourished, was formed of granite.
The mountains near Las Amolanas, composed of the cretaceo-oolitic
strata, are interlaced with dikes like a spider's web, to an
extent which I have never seen equalled, except in the denuded
interior of a volcanic crater: north and south lines, however,
predominate. These dikes are composed of green, white, and
blackish rocks, all porphyritic with feldspar, and often with
large crystals of hornblende. The white varieties approach
closely in character to andesite, which composes as we have seen,
the injected axes of so many of the lines of elevation. Some of
the green varieties are finely laminated, parallel to the walls
of the dikes.
SIXTH AXIS OF ELEVATION (VALLEY OF COPIAPO).
This axis consists of a broad mountainous mass [O] of andesite,
composed of albite, brown mica, and chlorite, passing into
andesitic granite, with quartz: on its western side it has thrown
off, at a considerable angle, a thick mass of stratified
porphyries, including much epidote [NN], and remarkable only from
being divided into very thin beds, as highly amygdaloidal on
their surfaces as subaerial lava-streams are often vesicular.
This porphyritic formation is conformably covered, as seen some
way up the ravine of Jolquera, by a mere remnant of the lower
part of the cretaceo-oolitic formation [MM], which in one part
encases, as represented in the coloured section, the foot of the
andesitic axis [L], of the already described fifth line, and in
another part entirely conceals it: in this latter case, the
gypseous or cretaceo-oolitic strata falsely appeared to dip under
the porphyritic conglomerate of the fifth axis. The lowest bed of
the gypseous formation, as seen here [M], is of yellowish
siliceous sandstone, precisely like that of Amolanas, interlaced
in parts with veins of gypsum, and including layers of the black,
calcareous, non-fissile slate-rock: the Turritella Andii, Pecten
Dufreynoyi, Terebratula aenigma, var., and some Gryphites were
embedded in these layers. The sandstone varies in thickness from
only twenty to eighty feet; and this variation is caused by the
inequalities in the upper surface of an underlying stream of
purple claystone porphyry. Hence the above fossils here lie at
the very base of the gypseous or cretaceo-oolitic formation, and
hence they were probably once covered up by strata about seven
thousand feet in thickness: it is, however, possible, though from
the nature of all the other sections in this district not
probable, that the porphyritic claystone lava may in this case
have invaded a higher level in the series. Above the sandstone
there is a considerable mass of much indurated, purplish-black,
calcareous claystone, allied in nature to the often-mentioned
black calcareous slate- rock. Eastward of the broad andesitic
axis of this sixth line, and penetrated by many dikes from it,
there is a great formation [P] of mica-schist, with its usual
variations, and passing in one part into a ferruginous
quartz-rock. The folia are curved and highly inclined, generally
dipping eastward. It is probable that this mica-schist is an old
formation, connected with the granitic rocks and metamorphic
schists near the coast; and that the one fragment of mica-slate,
and the pebbles of quartz low down in the gypseous formation at
Las Amolanas, have been derived from it. The mica-schist is
succeeded by stratified porphyritic conglomerate [Q] of great
thickness, dipping eastward with a high inclination: I have
included this latter mountain-mass in the same anticlinal axis
with the porphyritic streams [NN]; but I am far from sure that
the two masses may not have been independently upheaved.
SEVENTH AXIS OF ELEVATION.
Proceeding up the ravine, we come to another mass [R] of
andesite; and beyond this, we again have a very thick, stratified
porphyritic formation [S], dipping at a small angle eastward, and
forming the basal part of the main Cordillera. I did not ascend
the ravine any higher; but here, near Castano, I examined several
sections, of which I will not give the details, only observing,
that the porphyritic beds, or submarine lavas, preponderate
greatly in bulk over the alternating sedimentary layers, which
have been but little metamorphosed: these latter consist of
fine-grained red tuffs and of whitish volcanic grit-stones,
together with much of a singular, compact rock, having an almost
crystalline basis, finely brecciated with red and green
fragments, and occasionally including a few large pebbles. The
porphyritic lavas are highly amygdaloidal, both on their upper
and lower surfaces; they consist chiefly of claystone porphyry,
but with one common variety, like some of the streams at the
Puente del Inca, having a grey mottled basis, abounding with
crystals of red hydrous oxide of iron, green ones apparently of
epidote, and a few glassy ones of feldspar. This pile of strata
differs considerably from the basal strata of the Cordillera in
Central Chile, and may possibly belong to the upper and gypseous
series: I saw, however, in the bed of the valley, one fragment of
porphyritic breccia-conglomerate, exactly like those great masses
met with in the more southern parts of Chile.
Finally, I must observe, that though I have described between the
town of Copiapo and the western flank of the main Cordillera
seven or eight axes of elevation, extending nearly north and
south, it must not be supposed that they all run continuously for
great distances. As was stated to be the case in our sections
across the Cordillera of Central Chile, so here most of the lines
of elevation, with the exception of the first, third, and fifth,
are very short. The stratification is everywhere disturbed and
intricate; nowhere have I seen more numerous faults and dikes.
The whole district, from the sea to the Cordillera, is more or
less metalliferous; and I heard of gold, silver, copper, lead,
mercury, and iron veins. The metamorphic action, even in the
lower strata, has certainly been far less here than in Central
Chile.
VALLEY OF THE DESPOBLADO.
This great barren valley, which has already been alluded to,
enters the main valley of Copiapo a little above the town: it
runs at first northerly, then N.E., and more easterly into the
Cordillera; I followed its dreary course to the foot of the first
main ridge. I will not give a detailed section, because it would
be essentially similar to that already given, and because the
stratification is exceedingly complicated. After leaving the
plutonic hills near the town, I met first, as in the main valley,
with the gypseous formation, having the same diversified
character as before, and soon afterwards with masses of
porphyritic conglomerate, about one thousand feet in thickness.
In the lower part of this formation there were very thick beds
composed of fragments of claystone porphyries, both angular and
rounded, with the smaller ones partially blended together and the
basis rendered porphyritic; these beds separated distinct
streams, from sixty to eighty feet in thickness, of claystone
lavas. Near Paipote, also, there was much true porphyritic
breccia-conglomerate: nevertheless, few of these masses were
metamorphosed to the same degree with the corresponding formation
in Central Chile. I did not meet in this valley with any true
andesite, but only with imperfect andesitic porphyry, including
large crystals of hornblende: numerous as have been the varieties
of intrusive porphyries already mentioned, there were here
mountains composed of a new kind, having a compact, smooth,
cream-coloured basis, including only a few crystals of feldspar,
and mottled with dendritic spots of oxide of iron. There were
also some mountains of a porphyry with a brick-red basis,
containing irregular, often lens-shaped, patches of compact
feldspar, and crystals of feldspar, which latter to my surprise I
find to be orthite.
At the foot of the first ridge of the main Cordillera, in the
ravine of Maricongo, and at an elevation which, from the extreme
coldness and appearance of the vegetation, I estimated at about
ten thousand feet, I found beds of white sandstone and of
limestone including the Pecten Dufreynoyi, Terebratula aenigma,
and some Gryphites. This ridge throws the water on the one hand
into the Pacific, and on the other, as I was informed, into a
great gravel-covered, basin-like plain, including a salt- lake,
and without any drainage-exit. In crossing the Cordillera by this
Pass, it is said that three principal ridges must be traversed,
instead of two, or only one as in Central Chile.
The crest of this first main ridge and the surrounding mountains,
with the exception of a few lofty pinnacles, are capped by a
great thickness of a horizontally stratified, tufaceous deposit.
The lowest bed is of a pale purple colour, hard, fine-grained,
and full of broken crystals of feldspar and scales of mica. The
middle bed is coarser, and less hard, and hence weathers into
very sharp pinnacles; it includes very small fragments of
granite, and innumerable ones of all sizes of grey vesicular
trachyte, some of which were distinctly rounded. The uppermost
bed is about two hundred feet in thickness, of a darker colour
and apparently hard: but I had not time to ascend to it. These
three horizontal beds may be seen for the distance of many
leagues, especially westward or in the direction of the Pacific,
capping the summits of the mountains, and standing on the
opposite sides of the immense valleys at exactly corresponding
heights. If united they would form a plain, inclined very
slightly towards the Pacific; the beds become thinner in this
direction, and the tuff (judging from one point to which I
ascended, some way down the valley) finer-grained and of less
specific gravity, though still compact and sonorous under the
hammer. The gently inclined, almost horizontal stratification,
the presence of some rounded pebbles, and the compactness of the
lowest bed, though rendering it probable, would not have
convinced me that this mass had been of subaqueous origin, for it
is known that volcanic ashes falling on land and moistened by
rain often become hard and stratified; but beds thus originating,
and owing their consolidation to atmospheric moisture, would have
covered almost equally every neighbouring summit, high and low,
and would not have left those above a certain exact level
absolutely bare; this circumstance seems to me to prove that the
volcanic ejections were arrested at their present, widely
extended, equable level, and there consolidated by some other
means than simple atmospheric moisture; and this no doubt must
have been a sheet of water. A lake at this great height, and
without a barrier on any one side, is out of the question;
consequently we must conclude that the tufaceous matter was
anciently deposited beneath the sea. It was certainly deposited
before the excavation of the valleys, or at least before their
final enlargement (I have endeavoured to show in my "Journal"
etc. (2nd edition) page 355, that this arid valley was left by
the retreating sea, as the land slowly rose, in the state in
which we now see it.); and I may add, that Mr. Lambert, a
gentleman well acquainted with this country, informs me, that in
ascending the ravine of Santandres (which branches off from the
Despoblado) he met with streams of lava and much erupted matter
capping all the hills of granite and porphyry, with the exception
of some projecting points; he also remarked that the valleys had
been excavated subsequently to these eruptions.
This volcanic formation, which I am informed by Mr. Lambert
extends far northward, is of interest, as typifying what has
taken place on a grander scale on the corresponding western side
of the Cordillera of Peru. Under another point of view, however,
it possesses a far higher interest, as confirming that conclusion
drawn from the structure of the fringes of stratified shingle
which are prolonged from the plains at the foot of the Cordillera
far up the valleys,—namely, that this great range has been
elevated in mass to a height of between eight and nine thousand
feet (I may here mention that on the south side of the main
valley of Copiapo, near Potrero Seco, the mountains are capped by
a thick mass of horizontally stratified shingle, at a height
which I estimated at between fifteen hundred and two thousand
feet above the bed of the valley. This shingle, I believe, forms
the edge of a wide plain, which stretches southwards between two
mountain ranges.); and now, judging from this tufaceous deposit,
we may conclude that the horizontal elevation has been in the
district of Copiapo about ten thousand feet.
(FIGURE 24.)
In the valley of the Despoblado, the stratification, as before
remarked has been much disturbed, and in some points to a greater
degree than I have anywhere else seen. I will give two cases: a
very thick mass of thinly stratified red sandstone, including
beds of conglomerate, has been crushed together (as represented
in Figure 24) into a yoke or urn-formed trough, so that the
strata on both sides have been folded inwards: on the right hand
the properly underlying porphyritic claystone conglomerate is
seen overlying the sandstone, but it soon becomes vertical, and
then is inclined towards the trough, so that the beds radiate
like the spokes of a wheel: on the left hand, the inverted
porphyritic conglomerate also assumes a dip towards the trough,
not gradually, as on the right hand, but by means of a vertical
fault and synclinal break; and a little still further on towards
the left, there is a second great oblique fault (both shown by
the arrow- lines), with the strata dipping to a directly opposite
point; these mountains are intersected by infinitely numerous
dikes, some of which can be seen to rise from hummocks of
greenstone, and can be traced for thousands of feet. In the
second case, two low ridges trend together and unite at the head
of a little wedge-shaped valley: throughout the right- hand
ridge, the strata dip at 45 degrees to the east; in the left-hand
ridge, we have the very same strata and at first with exactly the
same dip; but in following this ridge up the valley, the strata
are seen very regularly to become more and more inclined until
they stand vertical, they then gradually fall over (the basset
edges forming symmetrical serpentine lines along the crest), till
at the very head of the valley they are reversed at an angle of
45 degrees: so that at this point the beds have been turned
through an angle of 135 degrees; and here there is a kind of
anticlinal axis, with the strata on both sides dipping to
opposite points at an angle of 45 degrees, but those on the left
hand upside down.
ON THE ERUPTIVE SOURCES OF THE PORPHYRITIC CLAYSTONE AND
GREENSTONE LAVAS.
In Central Chile, from the extreme metamorphic action, it is in
most parts difficult to distinguish between the streams of
porphyritic lava and the porphyritic breccia-conglomerate, but
here, at Copiapo, they are generally perfectly distinct, and in
the Despoblado, I saw for the first time, two great strata of
purple claystone porphyry, after having been for a considerable
space closely united together, one above the other, become
separated by a mass of fragmentary matter, and then both thin
out;—the lower one more rapidly than the upper and greater
stream. Considering the number and thickness of the streams of
porphyritic lava, and the great thickness of the beds of
breccia-conglomerate, there can be little doubt that the sources
of eruption must originally have been numerous: nevertheless, it
is now most difficult even to conjecture the precise point of any
one of the ancient submarine craters. I have repeatedly observed
mountains of porphyries, more or less distinctly stratified
towards their summits or on their flanks, without a trace of
stratification in their central and basal parts: in most cases, I
believe this is simply due either to the obliterating effects of
metamorphic action, or to such parts having been mainly formed of
intrusive porphyries, or to both causes conjoined; in some
instances, however, it appeared to me very probable that the
great central unstratified masses of porphyry were the now
partially denuded nuclei of the old submarine volcanoes, and that
the stratified parts marked the points whence the streams flowed.
In one case alone, and it was in this Valley of the Despoblado, I
was able actually to trace a thick stratum of purplish porphyry,
which for a space of some miles conformably overlay the usual
alternating beds of breccia-conglomerates and claystone lavas,
until it became united with, and blended into, a mountainous mass
of various unstratified porphyries.
The difficulty of tracing the streams of porphyries to their
ancient and doubtless numerous eruptive sources, may be partly
explained by the very general disturbance which the Cordillera in
most parts has suffered; but I strongly suspect that there is a
more specific cause, namely, THAT THE ORIGINAL POINTS OF ERUPTION
TEND TO BECOME THE POINTS OF INJECTION. This in itself does not
seem improbable; for where the earth's crust has once yielded, it
would be liable to yield again, though the liquified intrusive
matter might not be any longer enabled to reach the submarine
surface and flow as lava. I have been led to this conclusion,
from having so frequently observed that, where part of an
unstratified mountain-mass resembled in mineralogical character
the adjoining streams or strata, there were several other kinds
of intrusive porphyries and andesitic rocks injected into the
same point. As these intrusive mountain-masses form most of the
axes-lines in the Cordillera, whether anticlinal, uniclinal, or
synclinal, and as the main valleys have generally been hollowed
out along these lines, the intrusive masses have generally
suffered much denudation. Hence they are apt to stand in some
degree isolated, and to be situated at the points where the
valleys abruptly bend, or where the main tributaries enter. On
this view of there being a tendency in the old points of eruption
to become the points of subsequent injection and disturbance, and
consequently of denudation, it ceases to be surprising that the
streams of lava in the porphyritic claystone conglomerate
formation, and in other analogous cases, should most rarely be
traceable to their actual sources.
IQUIQUE, SOUTHERN PERU.
Differently from what we have seen throughout Chile, the coast
here is formed not by the granitic series, but by an escarpment
of the porphyritic conglomerate formation, between two and three
thousand feet in height. (The lowest point, where the road
crosses the coast-escarpment, is 1,900 feet by the barometer
above the level of the sea.) I had time only for a very short
examination; the chief part of the escarpment appears to be
composed of various reddish and purple, sometimes laminated,
porphyries, resembling those of Chile; and I saw some of the
porphyritic breccia-conglomerate; the stratification appeared but
little inclined. The uppermost part, judging from the rocks near
the famous silver mine of Huantajaya, consists of laminated,
impure, argillaceous, purplish-grey limestone, associated, I
believe, with some purple sandstone. (Mr. Bollaert has described
"Geological Proceedings" volume 2 page 598, a singular mass of
stratified detritus, gravel, and sand, eighty-one yards in
thickness, overlying the limestone, and abounding with loose
masses of silver ore. The miners believe that they can attribute
these masses to their proper veins.) In the limestone shells are
found: the three following species were given me:—
Lucina Americana, E. Forbes. Terebratula inca, E. Forbes.
Terebratula aenigma, D'Orbigny.
This latter species we have seen associated with the fossils of
which lists have been given in this chapter, in two places in the
valley of Coquimbo, and in the ravine of Maricongo at Copiapo.
Considering this fact, and the superposition of these beds on the
porphyritic conglomerate formation; and, as we shall immediately
see, from their containing much gypsum, and from their otherwise
close general resemblance in mineralogical nature with the strata
described in the valley of Copiapo, I have little doubt that
these fossiliferous beds of Iquique belong to the great
cretaceo-oolitic formation of Northern Chile. Iquique is situated
seven degrees latitude north of Copiapo; and I may here mention,
that an Ammonites, nov. species, and an Astarte, nov. species,
were given me from the Cerro Pasco, about ten degrees of latitude
north of Iquique, and M. D'Orbigny thinks that they probably
indicate a Neocomian formation. Again, fifteen degrees of
latitude northward, in Colombia, there is a grand fossiliferous
deposit, now well known from the labours of Von Buch, Lea,
d'Orbigny, and Forbes, which belongs to the earlier stages of the
cretaceous system. Hence, bearing in mind the character of the
few fossils from Tierra del Fuego, there is some evidence that a
great portion of the stratified deposits of the whole vast range
of the South American Cordillera belongs to about the same
geological epoch.
Proceeding from the coast escarpment inwards, I crossed, in a
space of about thirty miles, an elevated undulatory district,
with the beds dipping in various directions. The rocks are of
many kinds,—white laminated, sometimes siliceous
sandstone,—purple and red sandstone, sometimes so highly
calcareous as to have a crystalline fracture,—argillaceous
limestone,—black calcareous slate-rock, like that so often
described at Copiapo and other places,—thinly laminated,
fine-grained, greenish, indurated, sedimentary, fusible rocks,
approaching in character to the so- called pseudo-honestone of
Chile, including thin contemporaneous veins of gypsum,—and
lastly, much calcareous, laminated porcelain jasper, of a green
colour, with red spots, and of extremely easy fusibility: I
noticed one conformable stratum of a freckled-brown, feldspathic
lava. I may here mention that I heard of great beds of gypsum in
the Cordillera. The only novel point in this formation, is the
presence of innumerable thin layers of rock-salt, alternating
with the laminated and hard, but sometimes earthy, yellowish, or
bright red and ferruginous sandstones. The thickest layer of salt
was only two inches, and it thinned out at both ends. On one of
these saliferous masses I noticed a stratum about twelve feet
thick, of dark-brown, hard brecciated, easily fusible rock,
containing grains of quartz and of black oxide of iron, together
with numerous imperfect fragments of shells. The problem of the
origin of salt is so obscure, that every fact, even geographical
position, is worth recording. (It is well known that stratified
salt is found in several places on the shores of Peru. The island
of San Lorenzo, off Lima, is composed of a pile of thin strata,
about eight hundred feet in thickness, composed of yellowish and
purplish, hard siliceous, or earthy sandstones, alternating with
thin layers of shale, which in places passes into a greenish,
semi-porcellanic, fusible rock. There are some thin beds of
reddish mudstone, and soft ferruginous rotten-stones, with layers
of gypsum. In nearly all these varieties, especially in the
softer sandstones, there are numerous thin seams of rock-salt: I
was informed that one layer has been found two inches in
thickness. The manner in which the minutest fissures of the
dislocated beds have been penetrated by the salt, apparently by
subsequent infiltration, is very curious. On the south side of
the island, layers of coal and of impure limestone have been
discovered. Hence we here have salt, gypsum, and coal associated
together. The strata include veins of quartz, carbonate of lime,
and iron pyrites; they have been dislocated by an injected mass
of greenish-brown feldspathic trap. Not only is salt abundant on
the extreme western limits of the district between the Cordillera
and the Pacific, but, according to Helms, it is found in the
outlying low hills on the eastern flank of the Cordillera. These
facts appear to me opposed to the theory, that rock-salt is due
to the sinking of water, charged with salt, in mediterranean
spaces of the ocean. The general character of the geology of
these countries would rather lead to the opinion, that its origin
is in some way connected with volcanic heat at the bottom of the
sea: see on this subject Sir R. Murchison "Anniversary Address to
the Geological Society" 1843 page 65.) With the exception of
these saliferous beds, most of the rocks as already remarked,
present a striking general resemblance with the upper parts of
the gypseous or cretaceo-oolitic formation of Chile.
METALLIFEROUS VEINS.
I have only a few remarks to make on this subject: in nine mining
districts, some of them of considerable extent, which I visited
in CENTRAL Chile, I found the PRINCIPAL veins running from
between [N. and N.W.] to [S. and S.E.] (These mining districts
are Yaquil near Nancagua, where the direction of the chief veins,
to which only in all cases I refer, is north and south; in the
Uspallata range, the prevailing line is N.N.W. and S.S.E.; in the
C. de Prado, it is N.N.W. and S.S.E.; near Illapel, it is N. by
W. and S. by E.; at Los Hornos the direction varies from between
[N. and N.W.] to [S. and S.E.]; at the C. de los Hornos (further
northward), it is N.N.W. and S.S.E.; at Panuncillo, it is N.N.W.
and S.S.E.; and, lastly, at Arqueros, the direction is N.W. and
S.E.): in some other places, however, their courses appeared
quite irregular, as is said to be generally the case in the whole
valley of Copiapo: at Tambillos, south of Coquimbo, I saw one
large copper vein extending east and west. It is worthy of
notice, that the foliation of the gneiss and mica-slate, where
such rocks occur, certainly tend to run like the metalliferous
veins, though often irregularly, in a direction a little westward
of north. At Yaquil, I observed that the principal auriferous
veins ran nearly parallel to the grain or imperfect cleavage of
the surrounding GRANITIC rocks. With respect to the distribution
of the different metals, copper, gold, and iron are generally
associated together, and are most frequently found (but with many
exceptions, as we shall presently see) in the rocks of the lower
series, between the Cordillera and the Pacific, namely, in
granite, syenite, altered feldspathic clay-slate, gneiss, and as
near Guasco mica-schist. The copper-ores consist of sulphurets,
oxides, and carbonates, sometimes with laminae of native metal: I
was assured that in some cases (as at Panuncillo S.E. of
Coquimbo), the upper part of the same vein contains oxides, and
the lower part sulphurets of copper. (The same fact has been
observed by Mr. Taylor in Cuba: "London Philosophical Journal"
volume 11 page 21.) Gold occurs in its native form; it is
believed that, in many cases, the upper part of the vein is the
most productive part: this fact probably is connected with the
abundance of this metal in the stratified detritus of Chile,
which must have been chiefly derived from the degradation of the
upper portions of the rocks. These superficial beds of
well-rounded gravel and sand, containing gold, appeared to me to
have been formed under the sea close to the beach, during the
slow elevation of the land: Schmidtmeyer remarks that in Chile
gold is sought for in shelving banks at the height of some feet
on the sides of the streams, and not in their beds, as would have
been the case had this metal been deposited by common alluvial
action. ("Travels in Chile" page 29.) Very frequently the
copper-ores, including some gold, are associated with abundant
micaceous specular iron. Gold is often found in iron-pyrites: at
two gold mines at Yaquil (near Nancagua), I was informed by the
proprietor that in one the gold was always associated with
copper-pyrites, and in the other with iron-pyrites: in this
latter case, it is said that if the vein ceases to contain
iron-pyrites, it is yet worth while to continue the search, but
if the iron-pyrites, when it reappears, is not auriferous, it is
better at once to give up working the vein. Although I believe
copper and gold are most frequently found in the lower granitic
and metamorphic schistose series, yet these metals occur both in
the porphyritic conglomerate formation (as on the flanks of the
Bell of Quillota and at Jajuel), and in the superincumbent
strata. At Jajuel I was informed that the copper-ore, with some
gold, is found only in the greenstones and altered feldspathic
clay-slate, which alternate with the purple porphyritic
conglomerate. Several gold veins and some of copper- ore are
worked in several parts of the Uspallata range, both in the
metamorphosed strata, which have been shown to have been of
probably subsequent origin to the Neocomian or gypseous formation
of the main Cordillera, and in the intrusive andesitic rocks of
that range. At Los Hornos (N.E. of Illapel), likewise, there are
numerous veins of copper- pyrites and of gold, both in the strata
of the gypseous formation and in the injected hills of andesite
and various porphyries.
Silver, in the form of a chloride, sulphuret, or an amalgam, or
in its native state, and associated with lead and other metals,
and at Arqueros with pure native copper, occurs chiefly in the
upper great gypseous or cretaceo-oolitic formation which forms
probably the richest mass in Chile. We may instance the mining
districts of Arqueros near Coquimbo, and of nearly the whole
valley of Copiapo, and of Iquique (where the principal veins run
N.E. by E. and S.W. by W.), in Peru. Hence comes Molina's remark,
that silver is born in the cold and solitary deserts of the Upper
Cordillera. There are, however, exceptions to this rule: at Paral
(S.E. of Coquimbo) silver is found in the porphyritic
conglomerate formation; as I suspect is likewise the case at S.
Pedro de Nolasko in the Peuquenes Pass. Rich argentiferous lead
is found in the clay-slate of the Uspallata range; and I saw an
old silver-mine in a hill of syenite at the foot of the Bell of
Quillota: I was also assured that silver has been found in the
andesitic and porphyritic region between the town of Copiapo and
the Pacific. I have stated in a previous part of this chapter,
that in two neighbouring mines at Arqueros the veins in one were
productive when they traversed the singular green sedimentary
beds, and unproductive when crossing the reddish beds; whereas at
the other mine exactly the reverse takes place; I have also
described the singular and rare case of numerous particles of
native silver and of the chloride being disseminated in the green
rock at the distance of a yard from the vein. Mercury occurs with
silver both at Arqueros and at Copiapo: at the base of C. de los
Hornos (S.E. of Coquimbo, a different place from Los Hornos,
before mentioned) I saw in a syenitic rock numerous quartzose
veins, containing a little cinnabar in nests: there were here
other parallel veins of copper and of a ferrugino-auriferous ore.
I believe tin has never been found in Chile.
From information given me by Mr. Nixon of Yaquil (At the Durazno
mine, the gold is associated with copper-pyrites, and the veins
contain large prisms of plumbago. Crystallised carbonate of lime
is one of the commonest minerals in the matrix of the Chilean
veins.), and by others, it appears that in Chile those veins are
generally most permanently productive, which, consisting of
various minerals (sometimes differing but slightly from the
surrounding rocks), include parallel strings RICH in metals; such
a vein is called a veta real. More commonly the mines are worked
only where one, two, or more thin veins or strings running in a
different direction, intersect a POOR "veta real:" it is
unanimously believed that at such points of intersection
(cruceros), the quantity of metal is much greater than that
contained in other parts of the intersecting veins. In some
cruceros or points of intersection, the metals extend even beyond
the walls of the main, broad, stony vein. It is said that the
greater the angle of intersection, the greater the produce; and
that nearly parallel strings attract each other; in the Uspallata
range, I observed that numerous thin auri-ferruginous veins
repeatedly ran into knots, and then branched out again. I have
already described the remarkable manner in which rocks of the
Uspallata range are indurated and blackened (as if by a blast of
gunpowder) to a considerable distance from the metallic veins.
Finally, I may observe, that the presence of metallic veins seems
obviously connected with the presence of intrusive rocks, and
with the degree of metamorphic action which the different
districts of Chile have undergone. (Sir R. Murchison and his
fellow travellers have given some striking facts on this subject
in their account of the Ural Mountains ("Geological Proceedings"
volume 3 page 748.) Such metamorphosed areas are generally
accompanied by numerous dikes and injected masses of andesite and
various porphyries: I have in several places traced the
metalliferous veins from the intrusive masses into the encasing
strata. Knowing that the porphyritic conglomerate formation
consists of alternate streams of submarine lavas and of the
debris of anciently erupted rocks, and that the strata of the
upper gypseous formation sometimes include submarine lavas, and
are composed of tuffs, mudstones, and mineral substances,
probably due to volcanic exhalations,—the richness of these
strata is highly remarkable when compared with the erupted beds,
often of submarine origin, but NOT METAMORPHOSED, which compose
the numerous islands in the Pacific, Indian, and Atlantic Oceans;
for in these islands metals are entirely absent, and their nature
even unknown to the aborigines.
A SUMMARY OF THE GEOLOGICAL HISTORY OF THE CHILEAN CORDILLERA,
AND OF THE SOUTHERN PARTS OF SOUTH AMERICA.
We have seen that the shores of the Pacific, for a space of 1,200
miles from Tres Montes to Copiapo, and I believe for a very much
greater distance, are composed, with the exception of the
tertiary basins, of metamorphic schists, plutonic rocks, and more
or less altered clay-slate. On the floor of the ocean thus
constituted, vast streams of various purplish claystone and
greenstone porphyries were poured forth, together with great
alternating piles of angular and rounded fragments of similar
rocks ejected from the submarine craters. From the compactness of
the streams and fragments, it is probable that, with the
exception of some districts in Northern Chile, the eruptions took
place in profoundly deep water. The orifices of eruption appear
to have been studded over a breadth, with some outliers, of from
fifty to one hundred miles: and closely enough together, both
north and south, and east and west, for the ejected matter to
form a continuous mass, which in Central Chile is more than a
mile in thickness. I traced this mould-like mass, for only 450
miles; but judging from what I saw at Iquique, from specimens,
and from published accounts, it appears to have a manifold
greater length. In the basal parts of the series, and especially
towards the flanks of the range, mud, since converted into a
feldspathic slaty rock, and sometimes into greenstone, was
occasionally deposited between the beds of erupted matter: with
this exception the uniformity of the porphyritic rocks is very
remarkable.
At the period when the claystone and greenstone porphyries nearly
or quite ceased being erupted, that great pile of strata which,
from often abounding with gypsum, I have generally called the
gypseous formation was deposited, and feldspathic lavas, together
with other singular volcanic rocks, were occasionally poured
forth: I am far from pretending that any distinct line of
demarcation can be drawn between this formation and the
underlying porphyries and porphyritic conglomerate, but in a mass
of such great thickness, and between beds of such widely
different mineralogical nature, some division was necessary. At
about the commencement of the gypseous period, the bottom of the
sea here seems first to have been peopled by shells, not many in
kind, but abounding in individuals. At the P. del Inca the
fossils are embedded near the base of the formation; in the
Peuquenes range, at different levels, halfway up, and even higher
in the series; hence, in these sections, the whole pile of strata
belongs to the same period: the same remark is applicable to the
beds at Copiapo, which attain a thickness of between seven and
eight thousand feet. The fossil shells in the Cordillera of
Central Chile, in the opinion of all the palaeontologists who
have examined them, belong to the earlier stages of the
cretaceous system; whilst in Northern Chile there is a most
singular mixture of cretaceous and oolitic forms: from the
geological relations, however, of these two districts, I cannot
but think that they all belong to nearly the same epoch, which I
have provisionally called cretaceo-oolitic.
The strata in this formation, composed of black calcareous
shaly-rocks of red and white, and sometimes siliceous sandstone,
of coarse conglomerates, limestones, tuffs, dark mudstones, and
those singular fine-grained rocks which I have called
pseudo-honestones, vast beds of gypsum, and many other jaspery
and scarcely describable varieties, vary and replace each other
in short horizontal distances, to an extent, I believe,
unequalled even in any tertiary basin. Most of these substances
are easily fusible, and have apparently been derived either from
volcanoes still in quiet action, or from the attrition of
volcanic products. If we picture to ourselves the bottom of the
sea, rendered uneven in an extreme degree, with numerous craters,
some few occasionally in eruption, but the greater number in the
state of solfataras, discharging calcareous, siliceous,
ferruginous matters, and gypsum or sulphuric acid to an amount
surpassing, perhaps, even the existing sulphureous volcanoes of
Java (Von Buch's "Description Physique des Iles Canaries" page
428.), we shall probably understand the circumstances under which
this singular pile of varying strata was accumulated. The shells
appear to have lived at the quiescent periods when only limestone
or calcareo-argillaceous matter was depositing. From Dr. Gillies'
account, this gypseous or cretaceo-oolitic formation extends as
far south as the Pass of Planchon, and I followed it northward at
intervals for 500 miles: judging from the character of the beds
with the Terebratula aenigma, at Iquique, it extends from four to
five hundred miles further: and perhaps even for ten degrees of
latitude north of Iquique to the Cerro Pasco, not far from Lima:
again, we know that a cretaceous formation, abounding with
fossils, is largely developed north of the equator, in Colombia:
in Tierra del Fuego, at about this same period, a wide district
of clay-slate was deposited, which in its mineralogical
characters and external features, might be compared to the
Silurian regions of North Wales. The gypseous formation, like
that of the porphyritic breccia- conglomerate on which it rests,
is of inconsiderable breadth; though of greater breadth in
Northern than in Central Chile.
As the fossil shells in this formation are covered, in the
Peuquenes ridge, by a great thickness of strata; at the Puente
del Inca, by at least five thousand feet; at Coquimbo, though the
superposition there is less plainly seen, by about six thousand
feet; and at Copiapo, certainly by five or six thousand, and
probably by seven thousand feet (the same species there recurring
in the upper and lower parts of the series), we may feel
confident that the bottom of the sea subsided during this
cretaceo-oolitic period, so as to allow of the accumulation of
the superincumbent submarine strata. This conclusion is confirmed
by, or perhaps rather explains, the presence of the many beds at
many levels of coarse conglomerate, the well- rounded pebbles in
which we cannot believe were transported in very deep water. Even
the underlying porphyries at Copiapo. with their highly
amygdaloidal surfaces, do not appear to have flowed under great
pressure. The great sinking movement thus plainly indicated, must
have extended in a north and south line for at least four hundred
miles, and probably was co- extensive with the gypseous
formation.
The beds of conglomerate just referred to, and the
extraordinarily numerous silicified trunks of fir-trees at Los
Hornos, perhaps at Coquimbo and at two distant points in the
valley of Copiapo, indicate that land existed at this period in
the neighbourhood. This land, or islands, in the northern part of
the district of Copiapo, must have been almost exclusively
composed, judging from the nature of the pebbles of granite: in
the southern parts of Copiapo, it must have been mainly formed of
claystone porphyries, with some mica-schist, and with much
sandstone and jaspery rocks exactly like the rocks in the
gypseous formation, and no doubt belonging to its basal series.
In several other places also, during the accumulation of the
gypseous formation, its basal parts and the underlying
porphyritic conglomerate must likewise have been already
partially upheaved and exposed to wear and tear; near the Puente
del Inca and at Coquimbo, there must have existed masses of
mica-schist or some such rock, whence were derived the many small
pebbles of opaque quartz. It follows from these facts, that in
some parts of the Cordillera the upper beds of the gypseous
formation must lie unconformably on the lower beds; and the whole
gypseous formation, in parts, unconformably on the porphyritic
conglomerate; although I saw no such cases, yet in many places
the gypseous formation is entirely absent; and this, although no
doubt generally caused by quite subsequent denudation, may in
others be due to the underlying porphyritic conglomerate having
been locally upheaved before the deposition of the gypseous
strata, and thus having become the source of the pebbles of
porphyry embedded in them. In the porphyritic conglomerate
formation, in its lower and middle parts, there is very rarely
any evidence, with the exception of the small quartz pebbles at
Jajuel near Aconcagua, and of the single pebble of granite at
Copiapo, of the existence of neighbouring land: in the upper
parts, however, and especially in the district of Copiapo, the
number of thoroughly well-rounded pebbles of compact porphyries
make me believe, that, as during the prolonged accumulation of
the gypseous formation the lower beds had already been locally
upheaved and exposed to wear and tear, so it was with the
porphyritic conglomerate. Hence in following thus far the
geological history of the Cordillera, it may be inferred that the
bed of a deep and open, or nearly open, ocean was filled up by
porphyritic eruptions, aided probably by some general and some
local elevations, to that comparatively shallow level at which
the cretaceo- oolitic shells first lived. At this period, the
submarine craters yielded at intervals a prodigious supply of
gypsum and other mineral exhalations, and occasionally, in
certain places poured forth lavas, chiefly of a feldspathic
nature: at this period, islands clothed with fir-trees and
composed of porphyries, primary rocks, and the lower gypseous
strata had already been locally upheaved, and exposed to the
action of the waves;—the general movement, however, at this time
having been over a very wide area, one of slow subsidence,
prolonged till the bed of the sea sank several thousand feet.
In Central Chile, after the deposition of a great thickness of
the gypseous strata, and after their upheaval, by which the
Cumbre and adjoining ranges were formed, a vast pile of tufaceous
matter and submarine lava was accumulated, where the Uspallata
chain now stands; also after the deposition and upheaval of the
equivalent gypseous strata of the Peuquenes range, the great
thick mass of conglomerate in the valley of Tenuyan was
accumulated: during the deposition of the Uspallata strata, we
know absolutely, from the buried vertical trees, that there was a
subsidence of some thousand feet; and we may infer from the
nature of the conglomerate in the valley of Tenuyan, that a
similar and perhaps contemporaneous movement there took place. We
have, then, evidence of a second great period of subsidence; and,
as in the case of the subsidence which accompanied the
accumulation of the cretaceo-oolitic strata, so this latter
subsidence appears to have been complicated by alternate or local
elevatory movement— for the vertical trees, buried in the midst
of the Uspallata strata, must have grown on dry land, formed by
the upheaval of the lower submarine beds. Presently I shall have
to recapitulate the facts, showing that at a still later period,
namely, at nearly the commencement of the old tertiary deposits
of Patagonia and of Chile, the continent stood at nearly its
present level, and then, for the third time, slowly subsided to
the amount of several hundred feet, and was afterwards slowly
re-uplifted to its present level.
The highest peaks of the Cordillera appear to consist of active
or more commonly dormant volcanoes,—such as Tupungato, Maypu, and
Aconcagua, which latter stands 23,000 feet above the level of the
sea, and many others. The next highest peaks are formed of the
gypseous and porphyritic strata, thrown into vertical or highly
inclined positions. Besides the elevation thus gained by angular
displacements, I infer, without any hesitation—from the
stratified gravel-fringes which gently slope up the valleys of
the Cordillera from the gravel-capped plains at their base, which
latter are connected with the plains, still covered with recent
shells on the coast— that this great range has been upheaved in
mass by a slow movement, to an amount of at least 8,000 feet. In
the Despoblado Valley, north of Copiapo, the horizontal
elevation, judging from the compact, stratified tufaceous
deposit, capping the distant mountains at corresponding heights,
was about ten thousand feet. It is very possible, or rather
probable, that this elevation in mass may not have been strictly
horizontal, but more energetic under the Cordillera, than towards
the coast on either side; nevertheless, movements of this kind
may be conveniently distinguished from those by which strata have
been abruptly broken and upturned. When viewing the Cordillera,
before having read Mr. Hopkins's profound "Researches on Physical
Geology," the conviction was impressed on me, that the angular
dislocations, however violent, were quite subordinate in
importance to the great upward movement in mass, and that they
had been caused by the edges of the wide fissures, which
necessarily resulted from the tension of the elevated area,
having yielded to the inward rush of fluidified rock, and having
thus been upturned.
The ridges formed by the angularly upheaved strata are seldom of
great length: in the central parts of the Cordillera they are
generally parallel to each other, and run in north and south
lines; but towards the flanks they often extend more or less
obliquely. The angular displacement has been much more violent in
the central than in the exterior MAIN lines; but it has likewise
been violent in some of the MINOR lines on the extreme flanks.
The violence has been very unequal on the same short lines; the
crust having apparently tended to yield on certain points along
the lines of fissures. These points, I have endeavoured to show,
were probably first foci of eruption, and afterwards of injected
masses of porphyry and andesite. (Sir R. Murchison and his
companions state "Geological Proceedings" volume 3 page 747, that
no true granite appears in the higher Ural Mountains; but that
syenitic greenstone—a rock closely analogous to our andesite—is
far the most abundant of the intrusive masses.) The close
similarity of the andesitic granites and porphyries, throughout
Chile, Tierra del Fuego, and even in Peru, is very remarkable.
The prevalence of feldspar cleaving like albite, is common not
only to the andesites, but (as I infer from the high authority of
Professor G. Rose, as well as from my own measurements) to the
various claystone and greenstone porphyries, and to the trachytic
lavas of the Cordillera. The andesitic rocks have in most cases
been the last injected ones, and they probably form a continuous
dome under this great range: they stand in intimate relationship
with the modern lavas; and they seem to have been the immediate
agent in metamorphosing the porphyritic conglomerate formation,
and often likewise the gypseous strata, to the extraordinary
extent to which they have suffered.
With respect to the age at which the several parallel ridges
composing the Cordillera were upthrown, I have little evidence.
Many of them may have been contemporaneously elevated and
injected in the same manner as in volcanic archipelagoes lavas
are contemporaneously ejected on the parallel lines of fissure.
("Volcanic Islands" etc.) But the pebbles apparently derived from
the wear and tear of the porphyritic conglomerate formation,
which are occasionally present in the upper parts of this same
formation, and are often present in the gypseous formation,
together with the pebbles from the basal parts of the latter
formation in its upper strata, render it almost certain that
portions, we may infer ridges, of these two formations were
successively upheaved. In the case of the gigantic Portillo
range, we may feel almost certain that a preexisting granitic
line was upraised (not by a single blow, as shown by the highly
inclined basaltic streams in the valley on its eastern flank) at
a period long subsequent to the upheavement of the parallel
Peuquenes range. (I have endeavoured to show in my "Journal" 2nd
edition page 321, that the singular fact of the river, which
drains the valley between these two ranges, passing through the
Portillo and higher line, is explained by its slow and subsequent
elevation. There are many analogous cases in the drainage of
rivers: see "Edinburgh New Philosophical Journal" volume 28 pages
33 and 44.) Again, subsequently to the upheavement of the Cumbre
chain, that of Uspallata was formed and elevated; and afterwards,
I may add, in the plain of Uspallata, beds of sand and gravel
were violently upthrown. The manner in which the various kinds of
porphyries and andesites have been injected one into the other,
and in which the infinitely numerous dikes of various composition
intersect each other, plainly show that the stratified crust has
been stretched and yielded many times over the same points. With
respect to the age of the axes of elevation between the Pacific
and the Cordillera, I know little: but there are some lines which
must—namely, those running north and south in Chiloe, those eight
or nine east and west, parallel, far-extended, most symmetrical
uniclinal lines at P. Rumena, and the short N.W.-S.E. and N.E.-
S.W. lines at Concepcion—have been upheaved long after the
formation of the Cordillera. Even during the earthquake of 1835,
when the linear north and south islet of St. Mary was uplifted
several feet above the surrounding area, we perhaps see one
feeble step in the formation of a subordinate mountain-axis. In
some cases, moreover, for instance, near the baths of Cauquenes,
I was forcibly struck with the small size of the breaches cut
through the exterior mountain-ranges, compared with the size of
the same valleys higher up where entering the Cordillera; and
this circumstance appeared to me scarcely explicable, except on
the idea of the exterior lines having been subsequently upthrown,
and therefore having been exposed to a less amount of denudation.
From the manner in which the fringes of gravel are prolonged in
unbroken slopes up the valleys of the Cordillera, I infer that
most of the greater dislocations took place during the earlier
parts of the great elevation in mass: I have, however, elsewhere
given a case, and M. de Tschudi has given another, of a ridge
thrown up in Peru across the bed of a river, and consequently
after the final elevation of the country above the level of the
sea. ("Reise in Peru" Band 2 s.8: Author's "Journal" 2nd edition
page 359.)
Ascending to the older tertiary formations, I will not again
recapitulate the remarks already given at the end of the Fifth
Chapter,—on their great extent, especially along the shores of
the Atlantic—on their antiquity, perhaps corresponding with that
of the eocene deposits of Europe,—on the almost entire
dissimilarity, though the formations are apparently
contemporaneous, of the fossils from the eastern and western
coasts, as is likewise the case, even in a still more marked
degree, with the shells now living in these opposite though
approximate seas,—on the climate of this period not having been
more tropical than what might have been expected from the
latitudes of the places under which the deposits occur; a
circumstance rendered well worthy of notice, from the contrast
with what is known to have been the case during the older
tertiary periods of Europe, and likewise from the fact of the
southern hemisphere having suffered at a much later period,
apparently at the same time with the northern hemisphere, a
colder or more equable temperature, as shown by the zones
formerly affected by ice-action. Nor will I recapitulate the
proofs of the bottom of the sea, both on the eastern and western
coast, having subsided seven or eight hundred feet during this
tertiary period; the movement having apparently been
co-extensive, or nearly co-extensive, with the deposits of this
age. Nor will I again give the facts and reasoning on which the
proposition was founded, that when the bed of the sea is either
stationary or rising, circumstances are far less favourable than
when its level is sinking, to the accumulation of conchiferous
deposits of sufficient thickness, extension, and hardness to
resist, when upheaved, the ordinary vast amount of denudation. We
have seen that the highly remarkable fact of the absence of any
EXTENSIVE formations containing recent shells, either on the
eastern or western coasts of the continent,—though these coasts
now abound with living mollusca,—though they are, and apparently
have always been, as favourable for the deposition of sediment as
they were when the tertiary formations were copiously
deposited,—and though they have been upheaved to an amount quite
sufficient to bring up strata from the depths the most fertile
for animal life—can be explained in accordance with the above
proposition. As a deduction, it was also attempted to be shown,
first, that the want of close sequence in the fossils of
successive formations, and of successive stages in the same
formation, would follow from the improbability of the same area
continuing slowly to subside from one whole period to another, or
even during a single entire period; and secondly, that certain
epochs having been favourable at distant points, in the same
quarter of the world for the synchronous accumulation of
fossiliferous strata, would follow from movements of subsidence
having apparently, like those of elevation, contemporaneously
affected very large areas.
There is another point which deserves some notice, namely, the
analogy between the upper parts of the Patagonian tertiary
formation, as well as of the upper possibly contemporaneous beds
at Chiloe and Concepcion, with the great gypseous formation of
Cordillera; for in both formations, the rocks, in their fusible
nature, in their containing gypsum, and in many other characters,
show a connection, either intimate or remote, with volcanic
action; and as the strata in both were accumulated during
subsidence, it appears at first natural to connect this sinking
movement with a state of high activity in the neighbouring
volcanoes. During the cretaceo-oolitic period this certainly
appears to have been the case at the Puente del Inca, judging
from the number of intercalated lava-streams in the lower 3,000
feet of strata; but generally, the volcanic orifices seem at this
time to have existed as submarine solfataras, and were certainly
quiescent compared with their state during the accumulation of
the porphyritic conglomerate formation. During the deposition of
the tertiary strata we know that at S. Cruz, deluges of basaltic
lava were poured forth; but as these lie in the upper part of the
series, it is possible that the subsidence may at that time have
ceased: at Chiloe, I was unable to ascertain to what part of the
series the pile of lavas belonged. The Uspallata tuffs and great
streams of submarine lavas, were probably intermediate in age
between the cretaceo- oolitic and older tertiary formations, and
we know from the buried trees that there was a great subsidence
during their accumulation; but even in this case, the subsidence
may not have been strictly contemporaneous with the great
volcanic eruptions, for we must believe in at least one
intercalated period of elevation, during which the ground was
upraised on which the now buried trees grew. I have been led to
make these remarks, and to throw some doubt on the strict
contemporaneousness of high volcanic activity and movements of
subsidence, from the conviction impressed on my mind by the study
of coral formations, that these two actions do not generally go
on synchronously;—on the contrary, that in volcanic districts,
subsidence ceases as soon as the orifices burst forth into
renewed action, and only recommences when they again have become
dormant. ("The Structure and Distribution of Coral Reefs.")
At a later period, the Pampean mud, of estuary origin, was
deposited over a wide area,—in one district conformably on the
underlying old tertiary strata, and in another district
unconformably on them, after their upheaval and denudation.
During and before the accumulation, however, of these old
tertiary strata, and, therefore, at a very remote period,
sediment, strikingly resembling that of the Pampas, was
deposited; showing during how long a time in this case the same
agencies were at work in the same area. The deposition of the
Pampean estuary mud was accompanied, at least in the southern
parts of the Pampas, by an elevatory movement, so that the M.
Hermoso beds probably were accumulated after the upheaval of
those round the S. Ventana; and those at P. Alta after the
upheaval of the M. Hermoso strata; but there is some reason to
suspect that one period of subsidence intervened, during which
mud was deposited over the coarse sand of the Barrancas de S.
Gregorio, and on the higher parts of Banda Oriental. The
mammiferous animals characteristic of this formation, many of
which differ as much from the present inhabitants of South
America, as do the eocene mammals of Europe from the present ones
of that quarter of the globe, certainly co-existed at B. Blanca
with twenty species of mollusca, one balanus, and two corals, all
now living in the adjoining sea: this is likewise the case in
Patagonia with the Macrauchenia, which co-existed with eight
shells, still the commonest kinds on that coast. I will not
repeat what I have elsewhere said, on the place of habitation,
food, wide range, and extinction of the numerous gigantic
mammifers, which at this late period inhabited the two Americas.
The nature and grouping of the shells embedded in the old
tertiary formations of Patagonia and Chile show us, that the
continent at that period must have stood only a few fathoms below
its present level, and that afterwards it subsided over a wide
area, seven or eight hundred feet. The manner in which it has
since been rebrought up to its actual level, was described in
detail in the First and Second Chapters. It was there shown that
recent shells are found on the shores of the Atlantic, from
Tierra del Fuego northward for a space of at least 1,180 nautical
miles, and at the height of about 100 feet in La Plata, and of
400 feet in Patagonia. The elevatory movements on this side of
the continent have been slow; and the coast of Patagonia, up to
the height in one part of 950 feet and in another of 1,200 feet,
is modelled into eight great, step-like, gravel-capped plains,
extending for hundreds of miles with the same heights; this fact
shows that the periods of denudation (which, judging from the
amount of matter removed, must have been long continued) and of
elevation were synchronous over surprisingly great lengths of
coasts. On the shores of the Pacific, upraised shells of recent
species, generally, though not always, in the same proportional
numbers as in the adjoining sea, have actually been found over a
north and south space of 2,075 miles, and there is reason to
believe that they occur over a space of 2,480 miles. The
elevation on this western side of the continent has not been
equable; at Valparaiso, within the period during which upraised
shells have remained undecayed on the surface, it has been 1,300
feet, whilst at Coquimbo, 200 miles northward, it has been within
this same period only 252 feet. At Lima, the land has been
uplifted at least 80 feet since Indian man inhabited that
district; but the level within historical times apparently has
subsided. At Coquimbo, in a height of 364 feet, the elevation has
been interrupted by five periods of comparative rest. At several
places the land has been lately, or still is, rising both
insensibly and by sudden starts of a few feet during
earthquake-shocks; this shows that these two kinds of upward
movement are intimately connected together. For a space of 775
miles, upraised recent shells are found on the two opposite sides
of the continent; and in the southern half of this space, it may
be safely inferred from the slope of the land up to the
Cordillera, and from the shells found in the central part of
Tierra del Fuego, and high up the River Santa Cruz, that the
entire breadth of the continent has been uplifted. From the
general occurrence on both coasts of successive lines of
escarpments, of sand-dunes and marks of erosion, we must conclude
that the elevatory movement has been normally interrupted by
periods, when the land either was stationary, or when it rose at
so slow a rate as not to resist the average denuding power of the
waves, or when it subsided. In the case of the present high
sea-cliffs of Patagonia and in other analogous instances, we have
seen that the difficulty in understanding how strata can be
removed at those depths under the sea, at which the currents and
oscillations of the water are depositing a smooth surface of mud,
sand, and sifted pebbles, leads to the suspicion that the
formation or denudation of such cliffs has been accompanied by a
sinking movement.
In South America, everything has taken place on a grand scale,
and all geological phenomena are still in active operation. We
know how violent at the present day the earthquakes are, we have
seen how great an area is now rising, and the plains of tertiary
origin are of vast dimensions; an almost straight line can be
drawn from Tierra del Fuego for 1,600 miles northward, and
probably for a much greater distance, which shall intersect no
formation older than the Patagonian deposits; so equable has been
the upheaval of the beds, that throughout this long line, not a
fault in the stratification or abrupt dislocation was anywhere
observable. Looking to the basal, metamorphic, and plutonic rocks
of the continent, the areas formed of them are likewise vast; and
their planes of cleavage and foliation strike over surprisingly
great spaces in uniform directions. The Cordillera, with its
pinnacles here and there rising upwards of twenty thousand feet
above the level of the sea, ranges in an unbroken line from
Tierra del Fuego, apparently to the Arctic circle. This grand
range has suffered both the most violent dislocations, and slow,
though grand, upward and downward movements in mass; I know not
whether the spectacle of its immense valleys, with
mountain-masses of once liquified and intrusive rocks now bared
and intersected, or whether the view of those plains, composed of
shingle and sediment hence derived, which stretch to the borders
of the Atlantic Ocean, is best adapted to excite our astonishment
at the amount of wear and tear which these mountains have
undergone.
The Cordillera from Tierra del Fuego to Mexico, is penetrated by
volcanic orifices, and those now in action are connected in great
trains. The intimate relation between their recent eruptions and
the slow elevation of the continent in mass, appears to me highly
important, for no explanation of the one phenomenon can be
considered as satisfactory which is not applicable to the other.
(On the Connection of certain Volcanic Phenomena in South
America: "Geological Transactions" volume 5 page 609.) The
permanence of the volcanic action on this chain of mountains is,
also, a striking fact; first, we have the deluges of submarine
lavas alternating with the porphyritic conglomerate strata, then
occasionally feldspathic streams and abundant mineral exhalations
during the gypseous or cretaceo- oolitic period: then the
eruptions of the Uspallata range, and at an ancient but unknown
period, when the sea came up to the eastern foot of the
Cordillera, streams of basaltic lava at the foot of the Portillo
range; then the old tertiary eruptions; and lastly, there are
here and there amongst the mountains, much worn and apparently
very ancient volcanic formations without any craters; there are,
also, craters quite extinct, and others in the condition of
solfataras, and others occasionally or habitually in fierce
action. Hence it would appear that the Cordillera has been,
probably with some quiescent periods, a source of volcanic matter
from an epoch anterior to our cretaceo-oolitic formation to the
present day; and now the earthquakes, daily recurrent on some
part of the western coast, give little hope that the subterranean
energy is expended.
Recurring to the evidence by which it was shown that some at
least of the parallel ridges, which together compose the
Cordillera, were successively and slowly upthrown at widely
different periods; and that the whole range certainly once, and
almost certainly twice, subsided some thousand feet, and being
then brought up by a slow movement in mass, again, during the old
tertiary formations, subsided several hundred feet, and again was
brought up to its present level by a slow and often interrupted
movement; we see how opposed is this complicated history of
changes slowly effected, to the views of those geologists who
believe that this great mountain-chain was formed in late times
by a single blow. I have endeavoured elsewhere to show, that the
excessively disturbed condition of the strata in the Cordillera,
so far from indicating single periods of extreme violence,
presents insuperable difficulties, except on the admission that
the masses of once liquified rocks of the axes were repeatedly
injected with intervals sufficiently long for their successive
cooling and consolidation. ("Geological Transactions" volume 5
page 626.) Finally, if we look to the analogies drawn from the
changes now in progress in the earth's crust, whether to the
manner in which volcanic matter is erupted, or to the manner in
which the land is historically known to have risen and sunk: or
again, if we look to the vast amount of denudation which every
part of the Cordillera has obviously suffered, the changes
through which it has been brought into its present condition,
will appear neither to have been too slowly effected, nor to have
been too complicated.
NOTE.
As, both in France and England, translations of a passage in
Professor Ehrenberg's Memoir, often referred to in the Fourth
Chapter of this volume, have appeared, implying that Professor
Ehrenberg believes, from the character of the infusoria, that the
Pampean formation was deposited by a sea-debacle rushing over the
land, I may state, on the authority of a letter to me, that these
translations are incorrect. The following is the passage in
question:—
"Durch Beachtung der mikroscopischen Formen hat sich nun
feststellen lassen, das die Mastodonten-Lager am La Plata und die
Knochen-Lager am Monte Hermoso, who wie die der
Riesen-Gurtelthiere in den Dunenhugeln bei Bahia Blanca, beides
in Patagonien, unveranderte brakische Susswasserbildungen sind,
die einst wohl sammtlich zum obersten Fluthgebiethe des Meeres im
tieferen Festlande gehorten."—"Monatsberichten der konigl. Akad.
etc." zu Berlin vom April 1845.
INDEX.
Abich, on a new variety of feldspar.
Abrolhos islands.
Absence of recent formations on the S. American coasts.
Aguerros on elevation of Imperial.
Albite, constituent mineral in andesite. —in rocks of Tierra del
Fuego. —in porphyries. —crystals of, with orthite.
Alison, Mr., on elevation of Valparaiso.
Alumina, sulphate of.
Ammonites from Concepcion.
Amolanas, Las.
Amygdaloid, curious varieties of.
Amygdaloids of the Uspallata range. —of Copiapo.
Andesite of Chile. —in the valley of Maypu. —of the Cumbre pass.
—of the Uspallata range. —of Los Hornos. —of Copiapo.
Anhydrite, concretions of.
Araucaria, silicified wood of. Arica, elevation of.
Arqueros, mines of.
Ascension, gypsum deposited on. —laminated volcanic rocks of.
Augite in fragments, in gneiss. —with albite, in lava.
Austin, Mr. R.A.C., on bent cleavage lamina.
Austin, Captain, on sea-bottom.
Australia, foliated rocks of.
Azara labiata, beds of, at San Pedro.
Baculites vagina.
Bahia Blanca, elevation of. —formations near. —character of
living shells of.
Bahia (Brazil), elevation near. —crystalline rocks of.
Ballard, M., on the precipitation of sulphate of soda.
Banda Oriental, tertiary formations of. —crystalline rocks of.
Barnacles above sea-level. —adhering to upraised shells.
Basalt of S. Cruz. —streams of, in the Portillo range. —in the
Uspallata range.
Basin chains of Chile.
Beagle Channel.
Beaumont, Elie de, on inclination of lava-streams. —on viscid
quartz-rocks.
Beech-tree, leaves of fossil.
Beechey, Captain, on sea-bottom.
Belcher, Lieutenant, on elevated shells from Concepcion.
Bella Vista, plain of.
Benza, Dr., on decomposed granite.
Bettington, Mr., on quadrupeds transported by rivers.
Blake, Mr., on the decay of elevated shells near Iquique. —on
nitrate of soda.
Bole.
Bollaert, Mr., on mines of Iquique.
Bones, silicified. —fossil, fresh condition of.
Bottom of sea off Patagonia.
Bougainville, on elevation of the Falkland islands.
Boulder formation of S. Cruz. —of Falkland islands. —anterior to
certain extinct quadrupeds. —of Tierra del Fuego.
Boulders in the Cordillera. —transported by earthquake-waves. —in
fine-grained tertiary deposits.
Brande, Mr., on a mineral spring.
Bravais, M., on elevation of Scandinavia.
Brazil, elevation of. —crystalline rocks of.
Broderip, Mr., on elevated shells from Concepcion.
Brown, Mr. R., on silicified wood of Uspallata range.
Brown, on silicified wood.
Bucalema, elevated shells near.
Buch, Von, on cleavage. —on cretaceous fossils of the Cordillera.
—on the sulphureous volcanoes of Java.
Buenos Ayres.
Burchell, Mr., on elevated shells of Brazil.
Byron, on elevated shells.
Cachapual, boulders in valley of.
Caldcleugh, Mr., on elevation of Coquimbo. —on rocks of the
Portillo range.
Callao, elevation near. —old town of.
Cape of Good Hope, metamorphic rocks of.
Carcharias megalodon.
Carpenter, Dr., on microscopic organisms.
Castro (Chiloe), beds near.
Cauquenes Baths, boulders near. —pebbles in porphyry near.
—volcanic formation near. —stratification near.
Caves above sea-level.
Cervus pumilus, fossil-horns of.
Chevalier, M., on elevation near Lima.
Chile, structure of country between the Cordillera and the
Pacific. —tertiary formations of. —crystalline rocks in.
—central, geology of. —northern, geology of.
Chiloe, gravel on coast. —elevation of. —tertiary formation of.
—crystalline rocks of.
Chlorite-schist, near M. Video.
Chonos archipelago, tertiary formations of. —crystalline rocks
of.
Chupat, Rio, scoriae transported by.
Claro, Rio, fossiliferous beds of.
Clay-shale of Los Hornos.
Clay-slate, formation of, Tierra del Fuego. —of Concepcion.
—feldspathic, of Chile. — —of the Uspallata range. —black
siliceous, band of, in porphyritic formations of Chile.
Claystone porphyry, formation of, in Chile. —origin of. —eruptive
sources of.
Cleavage, definition of. —at Bahia. —Rio de Janeiro. —Maldonado.
—Monte Video. —S. Guitru-gueyu. —Falkland I. —Tierra del Fuego.
—Chonos I. —Chiloe. —Concepcion. —Chile. —discussion on.
Cleavage-laminae superficially bent.
Cliffs, formation of.
Climate, late changes in. —of Chile during tertiary period.
Coal of Concepcion. —S. Lorenzo.
Coast-denudation of St. Helena.
Cobija, elevation of.
Colombia, cretaceous formation of.
Colonia del Sacramiento, elevation of. —Pampean formation near
Colorado, Rio, gravel of. —sand-dunes of. —Pampean formation
near.
Combarbala.
Concepcion, elevation of. —deposits of. —crystalline rocks of.
Conchalee, gravel-terraces of.
Concretions of gypsum, at Iquique. —in sandstone at S. Cruz. —in
tufaceous tuff of Chiloe. —in gneiss. —in claystone-porphyry at
Port Desire. —in gneiss at Valparaiso. —in metamorphic rocks. —of
anhydrite. —relations of, to veins.
Conglomerate claystone of Chile. —of Tenuyan. —of the Cumbre
Pass. —of Rio Claro. —of Copiapo.
Cook, Captain, on form of sea-bottom.
Copiapo, elevation of. —tertiary formations of. —secondary
formations of.
Copper, sulphate of. —native, at Arqueros. —mines of, at
Panuncillo. —veins, distribution of.
Coquimbo, elevation and terraces of. —tertiary formations of.
—secondary formations of.
Corallines living on pebbles.
Cordillera, valleys bordered by gravel fringes. —basal strata of.
—fossils of. —elevation of. —gypseous formations of.
—claystone-porphyries of. —andesitic rocks of. —volcanoes of.
Coste, M., on elevation of Lemus.
Coy inlet, tertiary formation of.
Crassatella Lyellii.
Cruickshanks, Mr., on elevation near Lima.
Crystals of feldspar, gradual formation of, at Port Desire.
Cumbre, Pass of, in Cordillera.
Cuming, Mr., on habits of the Mesodesma. —on range of living
shells on west coast.
Dana, Mr., on foliated rocks. —on amygdaloids.
Darwin, Mount.
D'Aubuisson, on concretions. —on foliated rocks. Decay, gradual,
of upraised shells.
Decomposition of granite rocks.
De la Beche, Sir H., his theoretical researches in geology. —on
the action of salt on calcareous rocks. —on bent
cleavage-laminae.
Denudation on coast of Patagonia. —great powers of. —of the
Portillo range.
Deposits, saline.
Despoblado, valley of.
Detritus, nature of, in Cordillera.
Devonshire, bent cleavage in.
Dikes, in gneiss of Brazil. —near Rio de Janeiro. —pseudo, at
Port Desire. —in Tierra del Fuego. —in Chonos archipelago,
containing quartz. —near Concepcion, with quartz.
—granitic-porphyritic, at Valparaiso. —rarely vesicular in
Cordillera. —absent in the central ridges of the Portillo pass.
—of the Portillo range, with grains of quartz. —intersecting each
other often. —numerous at Copiapo.
Domeyko, M., on the silver mines of Coquimbo. on the fossils of
Coquimbo.
D'Orbigny, M. A., on upraised shells of Monte Video. —on elevated
shells at St. Pedro. —on elevated shells near B. Ayres. —on
elevation of S. Blas. —on the sudden elevation of La Plata. —on
elevated shells near Cobija. —on elevated shells near Arica. —on
the climate of Peru. —on salt deposits of Cobija. —on crystals of
gypsum in salt-lakes. —on absence of gypsum in the Pampean
formation. —on fossil remains from Bahia Blanca. —on fossil
remains from the banks of the Parana. —on the geology of St. Fe.
—on the age of Pampean formation. —on the Mastodon Andium. —on
the geology of the Rio Negro. —on the character of the Patagonian
fossils. —on fossils from Concepcion. — —from Coquimbo. — —from
Payta. —on fossil tertiary shells of Chile. —on cretaceous
fossils of Tierra del Fuego. — —from the Cordillera of Chile.
Earth, marine origin of.
Earthenware, fossil.
Earthquake, effect of, at S. Maria. —elevation during, at Lemus.
—of 1822, at Valparaiso. —effects of, in shattering surface.
—fissures made by. —probable effects on cleavage.
Earthquakes in Pampas.
Earthquake-waves, power of, in throwing up shells. —effects of,
near Lima. —power of, in transporting boulders.
Edmonston, Mr., on depths at which shells live at Valparaiso.
Ehrenberg, Professor, on infusoria in the Pampean formation. —on
infusoria in the Patagonian formation.
Elevation of La Plata. —Brazil. —Bahia Blanca. —San Blas.
—Patagonia. —Tierra del Fuego. —Falkland islands. —Pampas.
—Chonos archipelago. —Chiloe. —Chile. —Valparaiso. —Coquimbo.
—Guasco. —Iquique. —Cobija. —Lima. —sudden, at S. Maria. — —at
Lemus. —insensible, at Chiloe. — —at Valparaiso. — —at Coquimbo.
—axes of, at Chiloe. — —at P. Rumena. —at Concepcion.
—unfavourable for the accumulation of permanent deposits. —lines
of, parallel to cleavage and foliation. —lines of, oblique to
foliation. —areas of, causing lines of elevation and cleavage.
—lines of, in the Cordillera. —slow, in the Portillo range. —two
periods of, in Cordillera of Central Chile. —of the Uspallata
range. —two periods of, in Cumbre Pass. —horizontal, in the
Cordillera of Copiapo. —axes of, coincident with volcanic
orifices. —of the Cordillera, summary on.
Elliott, Captain, on human remains.
Ensenada, elevated shells of.
Entre Rios, geology of.
Equus curvidens.
Epidote in Tierra del Fuego. —in gneiss. —frequent in Chile. —in
the Uspallata range. —in porphyry of Coquimbo.
Erman, M., on andesite. Escarpments, recent, of Patagonia.
Extinction of fossil mammifers.
Falkland islands, elevation of. —pebbles on coast. —geology of.
Falkner, on saline incrustations.
Faults, great, in Cordillera.
Feldspar, earthy, metamorphosis of, at Port Desire. —albitic.
—crystals of, with albite. —orthitic, in conglomerate of Tenuyan.
—in granite of Portillo range. —in porphyries in the Cumbre Pass.
Feuillee on sea-level at Coquimbo.
Fissures, relations of, to concretions. —upfilled, at Port
Desire. —in clay-slate.
Fitton, Dr., on the geology of Tierra del Fuego.
Fitzroy, Captain, on the elevation of the Falkland islands. —on
the elevation of Concepcion.
Foliation, definition of. —of rocks at Bahia. —Rio de Janeiro.
—Maldonado. —Monte Video. —S. Guitru-gueyu. —Falkland I. —Tierra
del Fuego. —Chonos archipelago. —Chiloe. —Concepcion. —Chile.
—discussion on.
Forbes, Professor E., on cretaceous fossils of Concepcion. —on
cretaceous fossils and subsidence in Cumbre Pass. —on fossils
from Guasco. — —from Coquimbo. — —from Copiapo. —on depths at
which shells live.
Formation, Pampean. — —area of. — —estuary origin. —tertiary of
Entre Rios. —of Banda Oriental. —volcanic, in Banda Oriental. —of
Patagonia. —summary on. —tertiary of Tierra del Fuego. — —of the
Chonos archipelago. — —of Chiloe. — —of Chile. — —of Concepcion.
— —of Navidad. — —of Coquimbo. — —of Peru. — —subsidence during.
—volcanic, of Tres Montes. — —of Chiloe. — —old, near Maldonado.
— —with laminar structure. — —ancient, in Tierra del Fuego.
—recent, absent on S. American coast. —metamorphic, of
claystone-porphyry of Patagonia. —foliation of. —plutonic, with
laminar structure. —palaeozoic, of the Falkland I. —claystone, at
Concepcion. —Jurassic, of Cordillera. —Neocomian, of the Portillo
Pass. —volcanic, of Cumbre Pass. —gypseous, of Los Hornos. — —of
Coquimbo. — —of Guasco. — —of Copiapo. — —of Iquique.
—cretaceo-oolitic, of Coquimbo. — —of Guasco. — —of Copiapo. —
—of Iquique.
Fossils, Neocomian, of Portillo Pass. — —of Cumbre Pass.
—secondary, of Coquimbo. — —of Guasco. — —of Copiapo. — —of
Iquique. —palaeozoic, from the Falklands.
Fragments of hornblende-rock in gneiss. —of gneiss in gneiss.
Freyer, Lieutenant, on elevated shells of Arica.
Frezier on sea-level at Coquimbo.
Galapagos archipelago, pseudo-dikes of.
Gallegos, Port, tertiary formation of.
Garnets in gneiss. —in mica-slate. —at Panuncillo.
Gardichaud, M., on granites of Brazil.
Gay, M., on elevated shells. —on boulders in the Cordillera. —on
fossils from Cordillera of Coquimbo.
Gill, Mr., on brickwork transported by an earthquake-wave.
Gillies, Dr., on heights in the Cordillera. —on extension of the
Portillo range.
Glen Roy, parallel roads of. —sloping terraces of.
Gneiss, near Bahia. —of Rio de Janeiro. —decomposition of.
Gold, distribution of.
Gorodona, formations near. Granite, axis of oblique, to
foliation. —andesitic. —of Portillo range. —veins of, quartzose.
—pebble of, in porphyritic conglomerate. —conglomerate.
Grauwacke of Uspallata range.
Gravel at bottom of sea. —formation of, in Patagonia. —means of
transportation of. —strata of, inclined.
Gravel-terraces in Cordillera.
Greenough, Mr., on quartz veins.
Greenstone, resulting from metamorphose hornblende-rock. —of
Tierra del Fuego. —on the summit of the Campana of Quillota.
—porphyry. —relation of, to clay-slate.
Gryphaea orientalis.
Guasco, elevation of. —secondary formation of.
Guitru-gueyu, Sierra.
Guyana, gneissic rocks of.
Gypsum, nodules of, in gravel at Rio Negro. —deposited from
sea-water. —deposits of, at Iquique. —crystals of, in salt lakes.
—in Pampean formation. —in tertiary formation of Patagonia.
—great formation of, in the Portillo Pass. — —in the Cumbre Pass.
— —near Los Hornos. — —at Coquimbo. — —at Copiapo. — —near
Iquique. —of San Lorenzo.
Hall, Captain, on terraces at Coquimbo.
Hamilton, Mr., on elevation near Tacna.
Harlan, Dr., on human remains.
Hayes, Mr. A., on nitrate of soda.
Henslow, Professor, on concretions.
Herbert, Captain, on valleys in the Himalaya.
Herradura Bay, elevated shells of. —tertiary formations of.
Himalaya, valleys in.
Hippurites Chilensis.
Hitchcock, Professor, on dikes.
Honestones, pseudo, of Coquimbo. —of Copiapo.
Hooker, Dr. J.D., on fossil beech-leaves.
Hopkins, Mr., on axes of elevation oblique to foliation. —on
origin of lines of elevation.
Hornblende-rock, fragments of, in gneiss.
Hornblende-schist, near M. Video.
Hornos, Los, section near.
Hornstone, dike of.
Horse, fossil tooth of.
Huafo island. —subsidence at.
Huantajaya, mines of.
Humboldt, on saline incrustations. —on foliations of gneiss. —on
concretions in gneiss.
Icebergs, action on cleavage.
Illapel, section near.
Imperial, beds of shells near.
Incrustations, saline.
Infusoria in Pampean formation. —in Patagonian formation.
Iodine, salts of.
Iquique, elevation of. —saliferous deposits of. —cretaceo-oolitic
formation of.
Iron, oxide of, in lavas. —in sedimentary beds. —tendency in, to
produce hollow concretions. —sulphate of.
Isabelle, M., on volcanic rocks of Banda Oriental.
Joints in clay-slate.
Jukes, Mr., on cleavage in Newfoundland.
Kamtschatka, andesite of.
Kane, Dr., on the production of carbonate of soda.
King George's sound, calcareous beds of.
Lakes, origin of. —fresh-water, near salt lakes.
Lava, basaltic, of S. Cruz. —claystone-porphyry, at Chiloe. —
—ancient submarine. —basaltic, of the Portillo range.
—feldspathic, of the Cumbre Pass. —submarine, of the Uspallata
range. —basaltic, of the Uspallata range. —submarine, of
Coquimbo. —of Copiapo.
Lemus island.
Lemuy islet.
Lignite of Chiloe. —of Concepcion.
Lima, elevation of.
Lime, muriate of.
Limestone of Cumbre Pass. —of Coquimbo. —of Copiapo.
Lund and Clausen on remains of caves in Brazil.
Lund, M., on granites of Brazil.
Lyell, M., on upraised shells retaining their colours. —on
terraces at Coquimbo. —on elevation near Lima. —on fossil horse's
tooth. —on the boulder-formation being anterior to the extinction
of North American mammifers. —on quadrupeds washed down by
floods. —on age of American fossil mammifers. —on changes of
climate. —on denudation. —on foliation.
MacCulloch, Dr., on concretions. —on beds of marble.
Maclaren, Mr., letter to, on coral-formations.
Macrauchenia Patachonica.
Madeira, subsidence of.
Magellan, Strait, elevation near, of.
Magnesia, sulphate of, in veins.
Malcolmson, Dr., on trees carried out to sea.
Maldonado, elevation of. —Pampean formation of. —crystalline
rocks of.
Mammalia, fossil, of Bahia Blanca. — —near St. Fe. — —of Banda
Oriental. — —of St. Julian. — —at Port Gallegos. —washed down by
floods. —number of remains of, and range of, in Pampas.
Man, skeletons of (Brazil). —remains of, near Lima. —Indian,
antiquity of.
Marble, beds of.
Maricongo, ravine of.
Marsden, on elevation of Sumatra.
Mastodon Andium, remains of. —range of.
Maypu, Rio, mouth of, with upraised shells. —gravel fringes of.
—debouchement from the Cordillera.
Megalonyx, range of.
Megatherium, range of.
Miers, Mr., on elevated shells. —on the height of the Uspallata
plain.
Minas, Las.
Mocha Island, elevation of. —tertiary form of. —subsidence at.
Molina, on a great flood.
Monte Hermoso, elevation of. —fossils of.
Monte Video, elevation of. —Pampean formation of. —crystalline
rocks of.
Morris and Sharpe, Messrs., on the palaeozoic fossils of the
Falklands.
Mud, Pampean. —long deposited on the same area.
Murchison, Sir R., on cleavage. —on waves transporting gravel.
—on origin of salt formations. —on the relations of metalliferous
veins and intrusive rocks. —on the absence of granite in the
Ural.
Nautilus d'Orbignyanus.
Navidad, tertiary formations of, subsidence of.
Negro, Rio, pumice of pebbles of. —gravel of. —salt lakes of.
—tertiary strata of.
North America, fossil remains of.
North Wales, sloping terraces absent in. —bent cleavage of.
Neuvo Gulf, plains of. —tertiary formation of.
Owen, Professor, on fossil mammiferous remains.
Palmer, Mr., on transportation of gravel.
Pampas, elevation of. —earthquakes of. —formation of. —localities
in which fossil mammifers have been found.
Panuncillo, mines of.
Parana, Rio, on saline incrustations. —Pampean formations near.
—on the S. Tandil.
Parish, Sir W., on elevated shells near Buenos Ayres. —on
earthquakes in the Pampas. —on fresh-water near salt lakes. —on
origin of Pampean formation.
Patagonia, elevation and plains of. —denudation of.
—gravel-formation of. —sea-cliffs of. —subsidence during tertiary
period. —crystalline rocks of.
Payta, tertiary formations of.
Pebbles of pumice. —decrease in size on the coast of Patagonia.
—means of transportation. —encrusted with living corallines.
—distribution of, at the eastern foot of Cordillera. —dispersal
of, in the Pampas. —zoned with colour.
Pentland, Mr., on heights in the Cordillera. —on fossils of the
Cordillera.
Pernambuco.
Peru, tertiary formations of.
Peuquenes, Pass of, in the Cordillera. —ridge of.
Pholas, elevated shells of.
Pitchstone of Chiloe. —of Port Desire. —near Cauquenes. —layers
of, in the Uspallata range. —of Los Hornos. —of Coquimbo.
Plains of Patagonia. —of Chiloe. —of Chile. —of Uspallata. —on
eastern foot of Cordillera. —of Iquique.
Plata, La, elevation of. —tertiary formation of. —crystalline
rocks of.
Playfair, Professor, on the transportation of gravel.
Pluclaro, axis of.
Pondicherry, fossils of.
Porcelain rocks of Port Desire. —of the Uspallata range.
Porphyry, pebbles of, strewed over Patagonia.
Porphyry, claystone, of Chiloe, — —of Patagonia. — —of Chile.
—greenstone, of Chile. —doubly columnar. —claystone, rare, on the
eastern side of the Portillo Pass. —brick-red and orthitic, of
Cumbre Pass. —intrusive, repeatedly injected. —claystone of the
Uspallata range. — —of Copiapo. — —eruptive sources of.
Port Desire, elevation and plains of. —tertiary formation of.
—porphyries of.
Portillo Pass in the Cordillera.
Portillo chain. —compared with that of the Uspallata.
Prefil or sea-wall of Valparaiso.
Puente del Inca, section of.
Pumice, pebbles of. —conglomerate of R. Negro. —hills of, in the
Cordillera.
Punta Alta, elevation of. —beds of.
Quartz-rock of the S. Ventana. —C. Blanco. —Falkland islands.
—Portillo range. —viscidity of. —veins of, near Monte Video. —
—in dike of greenstone. —grains of, in mica slate. — —in dikes.
—veins of, relations to cleavage.
Quillota, Campana of.
Quintero, elevation of.
Quiriquina, elevation of. —deposits of.
Rancagua, plain of.
Rapel, R. elevation near.
Reeks, Mr. T., his analysis of decomposed shells. —his analysis
of salts.
Remains, human.
Rio de Janeiro, elevation near. —crystalline rocks of.
Rivers, small power of transporting pebbles. —small power of, in
forming valleys. —drainage of, in the Cordillera.
Roads, parallel, of Glen Roy.
Rocks, volcanic, of Banda Oriental. —Tres Montes. —Chiloe.
—Tierra del Fuego. —with laminar structure.
Rodents, fossil, remains of.
Rogers, Professor, address to Association of American Geologists.
Rose, Professor G., on sulphate of iron at Copiapo.
S. Blas, elevation of.
S. Cruz, elevation and plains of. —valley of. —nature of gravel
in valley of. —boulder formation of. —tertiary formation of.
—subsidence at.
S. Fe Bajada, formations of.
S. George's bay, plains of.
S. Helena island, sea-cliffs, and subsidence of.
S. Josef, elevation of. —tertiary formation of.
S. Juan, elevation near.
S. Julian, elevation and plains of. —salt lake of. —earthy
deposit with mammiferous remains. —tertiary formations of.
—subsidence at.
S. Lorenzo, elevation of. —old salt formation of.
S. Mary, island of, elevation of.
S. Pedro, elevation of.
Salado, R., elevated shells of. —Pampean formation of.
Salines.
Salt, with upraised shell. —lakes of. —purity of, in salt lakes.
—deliquescent, necessary for the preservation of meat. —ancient
formation of, at Iquique. — —at S. Lorenzo. —strata of, origin
of.
Salts, superficial deposits of.
Sand-dunes of the Uruguay. —of the Pampas. —near Bahia Blanca.
—of the Colorado. —of S. Cruz. —of Arica.
Sarmiento, Mount.
Schmidtmeyer on auriferous detritus.
Schomburghk, Sir R., on sea-bottom. —on the rocks of Guyana.
Scotland, sloping terraces of.
Sea, nature of bottom of, off Patagonia. —power of, in forming
valleys.
Sea cliffs, formation of.
Seale, Mr., model of St. Helena.
Sebastian Bay, tertiary formation of.
Sedgwick, Professor, on cleavage.
Serpentine of Copiapo.
Serpulae, on upraised rocks.
Shale-rock, of the Portillo Pass. —of Copiapo.
Shells, upraised state of, in Patagonia. —elevated, too small for
human food. —transported far inland, for food. —upraised,
proportional numbers varying. — —gradual decay of. — —absent on
high plains of Chile. — —near Bahia Blanca. —preserved in
concretions. —living and fossil range of, on west coast. —living,
different on the east and west coast.
Shingle of Patagonia.
Siau, M., on sea-bottom.
Silver mines of Arqueros. —of Chanuncillo. —of Iquique.
—distribution of.
Slip, great, at S. Cruz.
Smith, Mr., of Jordan Hill, on upraised shells retaining their
colours. —on Madeira. —on elevated seaweed. —on inclined gravel
beds.
Soda, nitrate of. —sulphate of, near Bahia Blanca. —carbonate of.
Soundings off Patagonia. —in Tierra del Fuego.
Spirifers.
Spix and Martius on Brazil. Sprengel on the production of
carbonate of soda.
Springs, mineral, in the Cumbre Pass.
Stratification of sandstone in metamorphic rocks. —of clay-slate
in Tierra del Fuego. —of the Cordillera of Central Chile. —little
disturbed in Cumbre Pass. —disturbance of, near Copiapo.
Streams of lava at S. Cruz, inclination of. —in the Portillo
range.
String of cotton with fossil-shells.
Struthiolaria ornata.
Studer, M., on metamorphic rocks.
Subsidence during formation of sea-cliffs. —near Lima. —probable,
during Pampean formation. —necessary for the accumulation of
permanent deposits. —during the tertiary formations of Chile and
Patagonia. —probable during the Neocomian formation of the
Portillo Pass. —probable during the formation of conglomerate of
Tenuyan. —during the Neocomian formation of the Cumbre Pass. —of
the Uspallata range. —great, at Copiapo. — —during the formation
of the Cordillera.
Sulphur, volcanic exhalations of.
Sumatra, promontories of.
A Summary on the recent elevatory movements. —on the Pampean
formation. —on the tertiary formations of Patagonia and Chile.
—on the Chilean Cordillera. —on the cretaceo-oolitic formation.
—on the subsidences of the Cordillera. —on the elevation of the
Cordillera.
Tacna, elevation of.
Tampico, elevated shells near.
Tandil, crystalline rocks of.
Tapalguen, Pampean formation of. —crystalline rocks of.
Taylor, Mr., on copper veins of Cuba.
Temperature of Chile during the tertiary period.
Tension, lines of, origin of, axes of elevation and of cleavage.
Tenuy Point, singular section of.
Tenuyan, valley of.
Terraces of the valley of S. Cruz. —of equable heights throughout
Patagonia. —of Patagonia, formation of. —of Chiloe. —at
Conchalee. —of Coquimbo. —not horizontal at Coquimbo. —of Guasco.
—of S. Lorenzo. —of gravel within the Cordillera.
Theories on the origin of the Pampean formation.
Tierra Amarilla.
Tierra del Fuego, form of sea-bottom. —tertiary formations of.
—clay-slate formation of. —cretaceous formation of. —crystalline
rocks of. —cleavage of clay-slate.
Tosca rock.
Trachyte of Chiloe. —of Port Desire. —in the Cordillera.
Traditions of promontories having been islands. —on changes of
level near Lima.
Trees buried in plain of Iquique. —silicified, vertical, of the
Uspallata range.
Tres Montes, elevation of. —volcanic rocks of.
Trigonocelia insolita.
Tristan Arroyo, elevated shells of.
Tschudi, Mr., on subsidence near Lima.
Tuff, calcareous, at Coquimbo. —on basin-plain near St. Jago.
—structure of, in Pampas. —origin of, in Pampas. —pumiceous, of
R. Negro. —Nuevo Gulf. —Port Desire. —S. Cruz. —Patagonia,
summary on Chiloe. —formation of, in Portillo chain. —great
deposit of, at Copiapo.
Tuffs, volcanic, metamorphic, of Uspallata. —of Coquimbo.
Ulloa, on rain in Peru. —on elevation near Lima.
Uruguay, Rio, elevation of country near.
Uspallata, plain of. —pass of. —range of. —concluding remarks on.
Valdivia, tertiary beds of. —mica-slate of.
Valley of S. Cruz, structure of. —Coquimbo. —Guasco, structure
of. —Copiapo, structure of. —S. Cruz, tertiary formations of.
—Coquimbo, geology of. —Guasco, secondary formations of.
—Copiapo, secondary formations of. —Despoblado.
Valleys in the Cordillera bordered by gravel fringes. —formation
of. —in the Cordillera.
Valparaiso, elevation of. —gneiss of.
Vein of quartz near Monte Video. —in mica-slate. —relations of,
to cleavage. —in a trap dike. —of granite, quartzose.
—remarkable, in gneiss, near Valparaiso.
Veins, relations of, to concretions. —metalliferous, of the
Uspallata range. —metalliferous, discussion on.
Venezuela, gneissic rocks of.
Ventana, Sierra, Pampean formation near. —quartz-rock of.
Villa Vincencio Pass.
Volcan, Rio, mouth of. —fossils of.
Volcanoes of the Cordillera. —absent, except near bodies of
water. —ancient submarine, in Cordillera. —action of, in relation
to changes of level. —long action of, in the Cordillera.
Wafer on elevated shells.
Waves caused by earthquakes, power of, in transporting boulders.
—power of, in throwing up shells.
Weaver, Mr., on elevated shells.
White, Martin., on sea-bottom.
Wood, silicified, of Entre Rios. —S. Cruz. —Chiloe. —Uspallata
range. —Los Hornos. —Copiapo.
Yeso, Rio, and plain of.
Ypun Island, tertiary formation of.
Zeagonite.
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