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Title: Illustrations of the Huttonian Theory of the Earth
Author: Playfair, John
Language: English
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HUTTONIAN THEORY OF THE EARTH ***
Transcriber Note
Since § 134 appeared on page 143 and again on page 147, the Section
number on pages 147 and all succeeding were incremented. The Table of
Contents was also incremented respectively. Text emphasis denoted by
_Italics_. List of ERRATA below has been applied to the text.
ERRATA.
Page 44. line 4. from the bottom, _for_ that _read_ as
" 189. " 6. " _for_ appearenes _read_ appearances
" 464. " 4. " _for_ D'AUBENTON _read_ DAUBENTON
" 482. " 12. " _for_ adversaries _read_ adversary
ILLUSTRATIONS
OF THE
HUTTONIAN THEORY
OF THE EARTH
BY JOHN PLAYFAIR
F. R. S. EDIN. AND PROFESSOR OF MATHEMATICS
IN THE UNIVERSITY OF EDINBURGH.
=========================================================
Nunc naturalem causam quærimus et assiduam,
non raram et foriuitam.
SENECA.
=========================================================
EDINBURGH:
PRINTED FOR CADELL AND DAVIES, LONDON, AND
WILLIAM CREECH, EDINBURGH.
======
1802.
Entered in Stationers Hall.
NEILL & CO. }
Printers, Edinburgh }
ADVERTISEMENT.
THE Treatise here offered to the Public, was drawn up with a view of
explaining Dr Hutton's Theory of the Earth in a manner more popular and
perspicuous than is done in his own writings. The obscurity of these
has been often complained of; and thence, no doubt, it has arisen,
that so little attention has been paid to the ingenious and original
speculations which they contain.
THE simplest way of accomplishing the object proposed, seemed to be, to
present a General Outline of the System, in one continued Discourse;
and to introduce afterwards, in the form of Notes, what farther
elucidation any particular subject was thought to demand. Through the
whole, I have aimed at little more than a clear exposition of facts,
and a plain deduction of the conclusions grounded on them; nor shall
I claim any merit to myself, if, in the order which I have found it
necessary to adopt, some arguments may have taken a new form, and some
additions may have been made to a system naturally rich in the number
and variety of its illustrations.
OF the qualifications which this undertaking requires, there is one
that I may safely suppose my self to possess. Having been instructed by
Di Hutton himself in his theory of the earth; having lived in intimate
friendship with that excellent man for several years, and almost in the
daily habit of discussing the questions here treated of; I have had the
best opportunity of understanding his views, and becoming acquainted
with his peculiarities, whether of expression or of thought. In the
other qualifications necessary for the illustration o a system so
extensive and various, I am abundantly sensible of my deficiency, and
shall therefore with great deference, and considerable anxiety wait
that decision from which there is no appeal.
EDINBURGH COLLEGE, }
1st March 1802. }
TABLE
OF
CONTENTS.
INTRODUCTION. Object of a Theory of the Earth. Division of minerals
into _Stratified_ and _Unstratified_.
Page 1
SECTION I.
PHENOMENA PECULIAR TO STRATIFIED BODIES.
1. Materials of the Strata.
Page 4
Present strata composed of the remains of more ancient rocks, § 1.
Proofs from calcareous strata, § 2.--from siliceous, § 3.--from
argillaceous, § 4.--from bituminous, § 5, 6. Absence of organized
remains from the strata called _primitive_, not universally true,
§ 8, 9.--Term Primary substituted for Primitive. Composition from
the materials of more ancient rocks, § 10.
2. Consolidation of the Strata.
p. 15
Consolidation, what, § 11. Objections to aqueous consolidation, §
12, 13, 14. Compression affects the action of fire on bodies, §
15, 16, 17.--Igneous consolidation of minerals prove from fossil
wood, § 19.--From the flints in chalk, § 20.--from sandstone,
§ 21.--from the calcareous strata, § 23, 24, 25.--from the
argillaceous, § 26, 27.--from the bituminous, § 28, 29.--from the
saline, § 32. Salt mines in Cheshire. Trona of Africa, § 54, 35.
3. Position of the Strata.
p. 40
Strata formed at the bottom of the sea, § 36. Apparent elevation
not produced by the retreat of the sea, § 37.--Strata,
horizontal, when formed, § 38, 39. Disturbance of the strata
proved from their inclined position § 40, 41, 42.--from shifts,
&c. § 42. Shifts of different dates, _ib._ Disturbance of the
primitive strata visible at their junction with the secondary, §
43, 44. This disturbance produced by a force directed upward, §
45, 46. This force the effect of subterraneous heat, § 47, 48.
SECTION II.
PHENOMENA PECULIAR TO UNSTRATIFIED BODIES.
1. Metallic Veins.
p. 57
Veins defined. They contain substances that were once in fusion, §
49, 50. Metallic veins, native metals, &c. § 51. Native copper, §
52. Manganese, § 53. Fragments of rocks included in veins, § 55.
Shifting and heaving of veins, § 56, 57. Veins of different dates,
§ 58. Stratification not found in veins. Coating of the sides,
what, § 59. Metallic veins most common in primitive strata; but
not confined to them, § 60.
2. Whinstone.
p. 66
Enumeration of stones of this genus, § 61. Whin, whether in veins
or in masses, resembles lava, § 62. Is a subterraneous lava,
§ 63. Columnar structure an argument for fusion, § 64. Not
produced by drying, § 65. Whinstone penetrated by pyrites, § 66.
Induration of the strata in contact with whin, § 67 Coal charred
by whinstone veins, § 68. Disturbance of the strata by whinstone
veins, § 69. Phenomena of whin interposed between strata, § 70,
71. Transition from whin to strata not gradual. § 72. Agates and
chalcedonies in whinstone, § 74. This stone melted and reproduced
from fusion by Sir James Hall, § 75. Mineral alkali found in it
by Dr Kennedy, _ib._ Whinstones of different formation, § 76.
Porphyry a species or variety of whinstone, §77.
3. Granite.
p. 82
Granite defined. Exists in masses and veins, § 77. The basis
of other rocks, § 78. Its original fluidity inferred from
the crystallization of its parts, § 79. Its fusion, from the
structure of the Portsoy granite, § 80, 81.--from granite veins,
§ 82. General conclusion as to the igneous origin of minerals, §
83, 84, 85. Actual existence of subterraneous heat known from hot
springs, volcanoes, earthquakes, § 86. Volcanic fire seated deep
under the surface, § 87. Subterraneous heat not accompanied by
burning, § 88, 89. Transmission of subterraneous heat, so as to
produce hot springs, &c. § 90, 91.
SECTION III
PHENOMENA COMMON TO STRATIFIED
AND UNSTRATIFIED BODIES.
p. 97
Chemical agents which produce the decomposition of mineral
substances at the surface, § 92, 93. Mechanical agents, § 95, 96.
Proofs of wearing from the sea shore, § 97, 98. Rivers, § 99,
100. Defiles among mountains, § 102. Supply of the soil from the
decomposition of rocks, § 103. Gravel in the soil, § 104, 105.
Gold found in the soil, § 106. Tin, § 107. Proofs of waste from
mountainous countries, § 108, 109. Structure of Valleys, § 111.
Transportation of stones, § 112. Nearest measure of the waste,
§ 113. General remarks, § 114, 115. No production of minerals
on the surface, § 116. Reproduction at the bottom of the sea, §
117. Continued system of decay and renovation, § 118. Defence
against the charge of impiety, 119. Antiquity and order of the
revolutions of the globe, § 120, 121, 122, 123, 124. Consistency
with the Sacred Writings, § 125. Scope of this theory of the
earth distinguishes it from others; beauty and extent of its
views, § 126. New facts, § 127. Comparison of this theory with
that of Buffon, § 129. Of Lazzaro Moro, § 130. _Plutonic_ system,
§ 131. Distinguished by the principle of compression, § 132.
Explains the oblate figure of the earth, _ib._ Prejudices against
this system, § 133. What may be expected from the progress of
science, § 134.
NOTES AND ADDITIONS.
NOTE I.--Origin of Calcareous Earth.
p. 143
Dr Hutton's opinion on this subject accurately stated, § 135.
Misrepresented by Kirwan, § 136.
NOTE II.--Origin of Coal.
p. 147
Vegetable origin of coal. Opinion of Buffon, § 137.--of Arduino,
_ib._--of Lehman, § 138. Distinction attempted between wood coal
and mineral coal, § 139. Not of different origin, but gradually
pass one into the other, § 140. Bovey coal, § 141. Kirwan derives
the matter of mineral coal from the decomposition of hornblende,
&c. 143. Absurdity of this supposition, § 144, 145, 146, 147.
NOTE III.--Primitive Mountains.
p. 160
Lehman introduced the term Primitive mountains, § 149. Supposed
more ancient than organized bodies, § 150. Stratification of
primitive mountains denied by Pini, and maintained by Saussure, §
151.
NOTE IV.--Primary Strata not Primitive.
p. 163
Shells found in primary strata, § 152. Sandstone in primary
mountains, § 153. Quartzy sand in the schistus of the Grampians,
_ib._ Rocks distinguished by Werner into three orders, § 154.
Objections to this arrangement, § 155.
NOTE V.--Transportation of the Materials
of the Strata.
p. 171
The transportation of materials, objected to by the Neptunists, is
implied in their own system, § 156, 157, 158. Proofs of great
transportation from the animal and vegetable remains, found in
rocks, § 160.
NOTE VI.--Kirwan's Notion of Precipitation.
p. 180
Difficulty, of precipitating the materials dissolved in the chaotic
fluid, § 162. Insufficiency of the explanation attempted, _ib._
NOTE VII.--Compression in the Mineral Regions.
p. 181
Effects ascribed to compression by Newton, compared will the
effects ascribed to it in this theory, § 163. Fallacy of Kirwan's
argument concerning the fusion of Carrara marble, § 165, 166.
Heat of the mineral region may be supported without fuel, § 167.
Quotation from Newton's Optics, _ib._ General remarks, § 169.
NOTE VIII.--Sparry Structure of Calcareous
Petrifactions.
p. 190
Sparry and organic structure co-exist in certain fossils, § 171.
Sparry and stratified structure co-exist in gneiss, marble, &c. §
172.
NOTE IX.--Petroleum, &c.
p. 194
Petroleum, &c. from the distillation of coal, § 173. Gradation from
petroleum to coal often met with, § 174. Connection of amber and
coal, § 175. Why mines of blind coal have not always petroleum
mines near them, § 176.
NOTE X.--The Height above the Level of the Sea,
at which Marks of
Aqueous Deposition are now found.
p. 199
These marks consist either in stratification or in marine objects,
§ 177. The marks of stratification observed, 14739 feet above
the sea, § 178. Shells in Peru, 14190, § 179. Kirwan's mistake
concerning these shells, § 180. His error similar to VOLTAIRE'S,
§ 181.
NOTE XI.--Fracture and Dislocation
of the Strata.
p. 204
Slips, § 182. Rib of limestone in a slip near Huddersfield, § 183,
184. Singular fracture of pudding-stones at Oban in Argyleshire,
§ 185. Similar phenomena observed by Saussure between Nice and
Genoa, _ib._ Remarks on it, § 186.
NOTE XII.--Elevation and Inflection
of the Strata.
p. 209
Junction of primary and secondary strata, § 187. Breccia
incumbent on the primary, § 188. Junction of the primary and
secondary strata: At Torbay in Devonshire, § 190,--coast of
Berwickshire, § 191,--Cullen in Banffshire, § 193,--Ardencaple
in Dunbartonshire, Arran, &c. § 194,--Pembrokeshire,
§ 195,--Jedburgh, § 196,--Ingleborough in Yorkshire, §
197,--Cumberland, § 198. Inflection of the strata, § 199.
Remarkable instances in the Alps and Pyrenees, § 200, 201,--on
Ben-Lawers in Perthshire, § 202,--coast of Berwickshire,
_ib._--Plymouth, § 203. Strata suffering such inflections
have been soft and ductile, § 204. General property of these
inflections, § 205, 206. uniform stretch of the primary strata,
§ 207. Inferences as to the nature of the elevating force, §
208. Imperfection of other theories. Crystallisation, _ib._
Marks of undulæ in the schistus, § 209. Elevation of the strata
a stronghold of the Huttonian theory, § 210. Elevation of the
strata enables us to see far into the interior of the earth, §
211.
NOTE XIII.--Metallic Veins.
p. 239
Specimens of native iron, § 212, 213. Margraaf's specimen, § 214.
Kirwan's hypothesis, § 215. Increase of the specific gravity of
native gold by fusion, no argument against its igneous origin,
§ 216. Specimens of gold and silver shooting through quartz, an
argument in favour of the Huttonian theory, § 218, 219. Proof in
favour of the same from chalcedony including calcareous spar,
§ 220. Matter that fills veins not from above or from either
side, § 221. Opinion of the Neptunists, § 222. Supposed fact
that veins are less rich as the depth increases, § 223. No marks
of horizontal deposition in veins; their coating differs from
stratification, § 224. Neptunists appear to be misled by the term
Stratification, § 225, 226. Veins heave or shift one another,
§ 227. Vast force employed for that purpose, § 228. Veins of
different formation, § 231. Pieces of rock insulated in veins, §
232. Supposition that veins have been filled by infiltrations,
absurd, 233. _Lenticular_ veins, and Pipe veins, § 234.
NOTE XIV.--On Whinstone.
p. 260
Whinstone, neither of volcanic nor aqueous formation, § 235.
Zeolite and carbonate of lime included in whinstone, but not in
lava, _ib._ Not introduced by infiltration, § 236. Disposition
of whinstone mountains differs from that of streams of lava,
§ 237. This argument first employed by Mr Strange, § 238. His
general views of this subject, § 239. Explanation of the regular
structure of whinstone hills, according to Dr Hutton's theory, §
240. Many hills supposed to be extinguished volcanoes, are rocks
of real whinstone that has flowed deep under the surface. Vein
of whinstone mistaken for a stream of lava by Faujas, § 241,
242. Submarine volcanoes of Dolomieu, § 243. Objections to this
theory, § 244, 245. Dolomieu in another place contends for the
aqueous formation of basaltes, § 246. His arguments answered;
also those of Bergman, § 248, 249. Argument of WERNER for the
aqueous origin of basaltes, § 250. Remarks on the supposed
gradual transition of basaltes to argillaceous schistus, §
251, 252. Of the shells said to be found in basaltes, § 253.
Instances from Portrush in Ireland, and from Cerigo on the
Coast of Greece, _ib._ and 254,--from the Veronese, § 255.
Objections to the Neptunian formation of whinstone, founded on
the difference between it and the contiguous stratified rocks,
§ 256. On the resemblance of the strata below and above certain
masses of whinstone, § 257. On the irregularity of the thickness
of those masses, § 258. Wedge form masses of whinstone included
between strata, § 259. Consequences of this wedge form, § 260.
Sandstone fragments included in whin, § 261, 262. Bending of
the strata contiguous to whinstone, § 264. Induration, § 265,
266. Charring of coal by whin, § 267. Of the manner in which the
bitumen may have been driven off by heat, § 268. Two kinds of
fossil coke, § 269. Graduation into plumbago, _ib._ and 270. Only
remaining objection obviated by Sir James Hall's experiments, §
271.
NOTE XV.--On Granite.
p. 307
Granite veins of two kinds, § 274. Veins Of which the communication
with large masses of the same stone is not visible: At the Isle
of Coll in the Hebrides, § 275,--at Portsoy, § 276,--in Cornwall,
§ 277,--in Glentilt, § 278. Veins visibly connected with larger
masses. Argument furnished by them in favour of this theory, §
279, 280. Impossibility of their being formed by infiltration, §
281. Veins of this kind in Arran, § 282,--Galloway, § 283,--sides
of Loch Chloney, Invernessshire, §284,--St Michael's Mount,
Cornwall § 285. Fragments of schistus contained in granite, § 287.
2. Granite of Portsoy.
p. 320
Description of this granite, § 288. Pierre graphique of M. Patrin
not perfectly the same with that of Portsoy, § 289. Quartz
crystallized in the pierre graphique, § 290. Instances of quartz
crystallized in other granites. In that of St Agnes in Cornwall,
§ 291. Whether this crystallization is only found in secondary
granites, § 292.
3. Stratification of Granite.
p. 326
Question stated concerning the stratification of granite, § 293.
Remarkable examples of stratified granite at Chorley Forest,
Leicestershire, and at Fassnet _Water_ in Berwickshire, § 295.
Stratification of Mont Blanc, and the Aiguilles of Chamouni
maintained by Saussure, § 296, 297. Seems nevertheless doubtful,
§ 298. In the granite mountains of Arran doubtful; § 300.
Explanation of the stratification of granite in this theory, §
301. If granitic veins were found proceeding from real granitic
strata, they could not be explained on the principles here laid
down, § 302. No such veins have been discovered, § 303. Answer to
an objection made to the igneous origin of granitic mountains, §
304, 305. Of the proportion of the earth's surface occupied by
granite rocks, § 306. Not exceeding a ninetieth part, § 307, 311.
Extent of granite in Scotland erroneously estimated by Dr Hutton,
§ 312. Amounts perhaps to a twenty-fourth of the whole surface, §
313. Observations on Mr Kirwan's opinion, § 314.
NOTE XVI.--Rivers and Lakes.
p. 350
The rivers have hollowed out the valleys, § 315. Illustration
from the course of the Danube, § 316. Courses of many rivers
retain marks of having consisted of a series of lakes, § 317,
318. Filling up and draining of lakes, § 320. Instances from
the lakes in Cumberland, § 321. Lake of Geneva, § 322. Lakes in
North America, _ib._ Cataracts, § 323. Difficulties in explaining
the generation and continuance of lakes exemplified in that of
Geneva, § 324. Attempt to resolve these difficulties, § 325, 326,
327, 328. All lakes not equally subject to them, § 329. Wasting
of the land by the rivers, proved from the mouths of rivers on
bold coasts, § 330. Examples from Cornwall, § 331.
NOTE XVII.--Remains of Decomposed Rocks.
p. 371
Plain of Crau, § 334. Its gravel from the decomposition of
pudding-stone, § 385. Same true of much of the gravel in this
island, § 337, 338. Mount Rigi in Switzerland the remains of a
body of pudding-stone, § 339. Measure of the destruction in the
stratified rocks sometimes, afforded by the unstratified, § 340,
341. Rate at which the elevation of mountains has been supposed
to decrease, § 342.
NOTE XVIII.--Transportation of Stones, &c.
p. 381
Gravel smaller and rounder the farther from its native place, §
343. Different sources of _caillou roulés_, § 344, 345. Stones
that have begun their migration before the cutting out of the
present valleys, § 346. Declivity necessary to enable stones
to travel from the top of Mont Blanc to the top of Mont Jura,
§ 347. Granite from Mont Blanc found eastward in the valley of
the Drance, § 348. Machinery employed by nature in transporting
rocks, § 349, 350. Instances of transported stones of great
size,--from the vicinity of Geneva, § 351, 352,--from the Isle
of Arran, 353. How gravity may contribute to the moving of large
stones, even when the declivity is small, § 354. Rocking-stones,
§ 355. Stone in Borrowdale,--in the valley of Urseren, § 356.
Large stones are sometimes the remains of veins, § 357. Of
the hypothesis of a _debacle_, § 358. Structure of valleys
unfavourable to this hypothesis, § 359, 360. Particularly of
valleys close at the ends, § 361, 362. Whether the supposition of
a _debacle_ is necessary to explain the moving of large masses
of rock, § 364. Whether the abrupt faces of hills indicate the
existence of any sudden torrent, &c. § 365. Fact concerning the
steep faces of the mountains in the south of Africa, § 366. A
fact stated that would lead necessarily to belief in a _debacle_;
no example of it has yet occurred, § 367.
NOTE XIX.--Transportation of Materials
by the Sea.
p. 413
Of the manner in which the _detritus_ of the land is spread out
over the bottom of the sea, § 369, 370. Seas rendered shallower,
§ 371. Sand banks, § 372. Great system of currents traced in the
Atlantic, § 373, 374. How far this transportation of materials
may affect the earth's diurnal motion, § 376, 377. Kirwan's
misapprehension of Frisi, and of Major Rennell, § 378, 379. His
mistake about the tides, § 380, 381,--and about the formation of
sand banks, § 384.
NOTE XX.--Inequalities of the
Planetary Motions.
p. 437
These inequalities all periodical, § 385. Circumstances on which
this depends, § 386. Affinity of this conclusion to that which Dr
Hutton has established with respect to the changes at the surface
of the earth, § 387.
NOTE XXI.--Changes in the Apparent
Level of the Sea.
p. 441
Relative level of the sea and land subject to change, § 388. Proofs
that it has sunk, on the shores of this island, § 389,--on the
coasts of France and Flanders, § 390, 391,--on the shores of the
Baltic, § 392. This has not arisen from the depression of the
sea, but from the elevation of the land, § 393, 394. The surface
of the Hadriatic higher now than formerly, § 395, 396. Also of
the Mediterranean, § 398. Irregularities in these changes, § 399,
400. Hypothesis of Frisi, that towards the equator the sea is
every where rising, § 401. Disproved, _ib._ Conclusion, § 402.
NOTE XXII.--Fossil Bones.
p. 458
Vegetable and animal remains contained in the fossil kingdom, §
403. Of those that are enveloped or penetrated with calcareous
earth, § 405, 406. Of the bones buried in the loose earth,
§ 407. Bones in Siberia referred to the rhinoceros and the
elephant, § 408. Those on the Ohio doubtful, § 408, 409. Opinion
of Camper, § 410.--of Cuvier, § 411. Objections to the latter,
_ib._ Enumeration of five _species_ of animals now extinct, §
412. Change in the animal, and vegetable kingdom may account for
bones found in countries where no analogous species now exists,
§ 414. Proofs that the animals whose bones are found in Siberia
inhabited that country, § 415-417.
NOTE XXIII.--Geology of Kirwan and De Luc.
p. 477
These authors have improperly drawn religion into their quarrel
with Dr Hutton, § 419. De Luc writes a history of what befel the
earth before the creation of the sun, § 420. Remarks on Kirwan's
geological writings, § 422, 423, 424, 425.
NOTE XXIV.--System of Buffon.
p. 483
In what Buffon's theory of the earth and Dr Hutton's agree,
and in what they differ, § 426, 427. Great merit of Buffon,
notwithstanding his errors, § 428.
NOTE XXV.--Figure of the Earth.
p. 488
Physical cause of the earth's oblate figure not obvious from its
present condition, § 429. How explained by the Neptunists, §
430. Examination of their solution, § 431, 432. Contradiction
implied in it, § 433. Insufficiency of Buffon's explanation, §
434. Of the principle on which the oblateness of the earth may
be accounted for in Dr Hutton's theory, § 435. Of the changes
that would happen in the figure of a terraqueous body like the
earth, supposing it ever so irregular, § 436, 437, 438. Two
different causes of change, _ib._ Ultimate figure, that by which
the causes of change are best resisted, § 439. Spheroidal figure,
never perfectly acquired, § 440. Agreement of this theory with
observation, § 441. Probable extension of the system of waste,
and renovation to the other planets, § 442, 443. Confirmation
from the system of Saturn, § 444, 445.
NOTE XXVI.--Prejudices relating to
the Theory of the Earth.
p. 510
Alleged by some that a theory of the earth ought not at present to
be attempted, § 446. The quick succession of geological theories
has partly arisen from their object being misunderstood, § 447. A
succession of theories is often a continued approximation to the
truth, § 448. The more various the phenomena of any class, the
greater the chance of discovering their true cause, § 449. Reason
to think that the leading facts in geology are now known, § 450,
451. A tendency may be observed in geological systems to approach
to one another, and to the Huttonian, § 452. Example from that
of Saussure, § 453,--of Dolomieu, § 454, 455. The discoveries of
Dr Black were necessary for understanding the true theory of the
earth, § 457. Use of theory in matters of observation, § 458, &c.
ILLUSTRATIONS, &c.
A VERY little attention to the phenomena of the mineral kingdom, is
sufficient to convince us, that the condition of the earth's surface
has not been the same at all times that it is at the present moment.
When we observe the impressions of plants in the heart of the hardest
rocks; when we discover trees converted into flint, and entire beds of
limestone or of marble composed of shells and corals; we see the same
individual in two states, the most widely different from one another;
and, in the latter instance, have a clear proof, that the present land
was once deep immersed under the waters of the ocean. If to this we
add, that many masses of rock, the most solid and compact, consist of
no other materials but sand and gravel; that, on the other hand, loose
gravel, such as is formed only in beds of rivers, or on the sea shore,
now abounds in places remote from both: if we reflect, at the same
time, on the irregular and broken figure of our continents, and the
identity of the mineral strata on opposite sides of the same valley, or
the same inlet of the sea; we shall see abundant reason to conclude,
that the earth has been the theatre of many great revolutions, and that
nothing on its surface has been exempted from their effects.
To trace the series of these revolutions, to explain their causes, and
thus to connect together all the indications of change that are found
in the mineral kingdom, is the proper object of a THEORY OF THE EARTH.
But, though the attention of men may be turned to the theory of the
earth by a very superficial acquaintance with the phenomena of geology,
the formation of such a theory requires an accurate and extensive
examination of those phenomena, and is inconsistent with any but a
very advanced state of the physical sciences. There is, perhaps, in
those sciences, no research more arduous than this; none certainly
where the subject is so complex; where the appearances are so extremely
diversified, or so widely scattered, and where the causes that have
operated are so remote from the sphere of ordinary observation. Hence
the attempt! to form a theory of the earth are of very modern origin,
and as, from the simplicity of its subject astronomy is the eldest, so,
on account of the complexness of its subject, geology is the youngest
of the sciences.
It is foreign from the present purpose to enter on any history of
the systems that, since the rise of this branch of science, have
been invented to explain the phenomena of the mineral kingdom. It is
sufficient to remark, that these systems are usually reduced to two
classes, according as they refer the origin of terrestrial bodies
to FIRE or to WATER; and that, conformably to this division, their
followers have of late been distinguished by the fanciful names of
_Vulcanists_ and _Neptunists_. To the former of these Dr Hutton belongs
much more than to the latter; though, as he employs the agency both of
fire and of water in his system, he cannot, in strict propriety, be
arranged with either.
In the succinct account which I am now about to give of this system, I
shall consider the mineral kingdom as divided into two parts, namely,
stratified and unstratified substances I shall treat, first, of the
phenomena peculiar to the stratified; next, of those peculiar to the
unstratified; and, lastly, of the phenomena common to both. Beginning,
then, with the first, the subject naturally divides itself into three
branches; viz. the _materials_ the _consolidation_, and the _position_
of the strata.
SECTION I.
OF THE PHENOMENA PECULIAR TO STRATIFIED BODIES.
1. _Materials of the Strata._
1. IT is well known that, on removing the loose earth which forms the
immediate surface of the land, we come to the solid rock, of which a
great proportion is found to be regularly disposed in strata, or beds
of determinate thickness, inclined at different angles to the horizon,
but separated from one another by equidistant superficies, that often
maintain their parallelism to a great extent. These strata bear such
evident marks of being deposited by water, that they are universally
acknowledged to have had their origin at the bottom of the sea; and
it is also admitted, that the materials which they consist of, were
then either soft, or in such a state of comminution and separation,
as renders them capable of arrangement by the action of the water in
which they were immersed. Thus far most of the theories of the earth
agree; but from this point they begin to diverge, and each to assume
a character and direction peculiar to itself. Dr Hutton's does so, by
laying down this fundamental proposition, That in all the strata we
discover proofs of the materials having existed as elements of bodies,
which must have been destroyed before the formation of those of which
these materials now actually make a part.[1]
[Footnote 1: Hutton's Theory, vol. I p. 20, &c.]
2. The calcareous strata are the portion of the mineral kingdom that
gives the clearest testimony to the truth of this assertion. They often
contain shells, corals, and other exuviæ of marine animals in so great
abundance, that they appear to be composed of no other materials.
Though these remains of organized bodies are now converted into stone
or into spar, their shape and interior structure are often so well
preserved, that the species of animal or plant of which they once made
a part, can still be distinguished and pointed out among the living
inhabitants of the ocean.
Others of the calcareous strata appear to be composed of fragments of
some ancient rocks, which, after having been broken, have been again
united into a compact stone. In these we find pieces clearly marked
as having been once continuous but now placed at a distance from one
another, and exhibiting exactly the same appearances as if they floated
in a fluid of the same specific gravity with themselves.
From these, therefore, and a variety of similar appearances, Dr Hutton
concludes, that the materials of all the calcareous strata have been
furnished, either from the dissolution of former strata, or from the
remains of organized bodies. But, though this conclusion is meant to be
extended to all the calcareous strata, it is not asserted that every
cubic inch of marble or of limestone contains in it the characters of
its former condition, and of the changes through which it has passed.
It may, however, be safely affirmed, that there is scarce any entire
stratum where such characters are not to be found. These must be
held as decisive with respect to the whole system of strata to which
they belong; they prove the existence of calcareous rocks before the
formation of the present; and, as the destruction of those is evidently
adequate to the supply of the materials of these that we now see, to
look for any other supply were superfluous, and could only embarrass
our reasonings by the introduction of unnecessary hypotheses.[2]
[Footnote 2: NOTE I.]
3. The same conclusions result from an examination of the siliceous
strata; under which we may comprehend the common sandstone, and also
those pudding-stones or breccias where the gravel consists of quartz.
In all these instances, it is plain, that the sand or gravel existed in
a state quite loose and unconnected, at the bottom of the sea, previous
to its consolidation into stone. But such bodies of gravel or sand
could only be formed from the attrition of large masses of quartz, or
from the dissolution of such sandstone strata as exist at present; for
it will hardly be alleged, that sand is a crystallization of quartz,
formed from that substance, when it passes from a fluid to a solid
state.
Those pudding-stones in which the gravel is round and polished, carry
the conclusion still farther, as such gravel can only be formed in the
beds of rivers or on the shores of the sea; for, in the depths of the
ocean, though currents are known to exist, yet there can be no motion
of the water sufficiently rapid to produce the attrition required to
give a round figure and smooth surface to hard and irregular pieces
of stone. There must have existed, therefore, not only a sea, but
continents, previously to the formation of the present strata.
The same thing is clearly shown by those petrifactions of wood, where,
though the vegetable structure is perfectly preserved, the whole mass
is siliceous, and has, perhaps, been found in the heart of some
mountain, deep imbedded in the solid rock.
4. Characters of the same import are also found among the argillaceous
strata, though perhaps more rarely than among the calcareous or
siliceous. Such are the impressions of the leaves and stems of
vegetables; also the bodies of fish and amphibious animals, found very
often in the different kinds of argillaceous schistus, and in most
instances having the figure accurately preserved, but the substance
of the animal replaced by clay or pyrites. These are all remains
of ancient seas or continents; the latter of which have long since
disappeared from the surface of the earth, but have still their memory
preserved in those archives, where nature has recorded the revolutions
of the globe.
5. Among bituminous bodies, pit-coal is the only one which constitutes
regular and extensive strata; and no fossil has its origin from the
waste of former continents, marked by stronger and more distinct
characters. Not to mention that the coal strata are alternated with
those that have been already enumerated, and that they often contain
shells and corals, perfectly mineralized, it is sufficient to remark,
that there are entire beds of this fossil, which appear to consist
wholly of wood, and in which the fibrous structure is perfectly
preserved. From these instances, the appearances of vegetable
structure may be traced through all possible gradations, down to an
evanescent state. This last state is undoubtedly the most common; and
though coal does not then, on bare inspection, make known its vegetable
origin, yet, if we take it in connection with the other terms of the
series, as we may call them; if we consider that the two extremes, viz.
coal, with the vegetable structure perfect, and coal without any such
structure visible, are often found in the same or in contiguous beds;
and, if we remark, that through all these gradations coal contains
nearly the same chemical elements, and yields, on analysis, bitumen and
charcoal, combined with a greater or less proportion of earth: if we
take all these circumstances into account, we cannot doubt that this
fossil is every where the same, and derives its origin from the trees
and plants that grew on the surface of the earth before the formation
of the present land.
6. Dr Hutton has further observed, that if those ancient continents
were at all similar to the present, we can be at no loss to account for
the want of any distinct mark of vegetable organization in the greater
part of the coal strata. It is plain, that the daily waste of animal
and vegetable substances on the surface of the earth, must disengage
a great quantity of oily as well as carbonic matter, which, with
whatever element it is at first combined, is ultimately delivered into
the ocean. Thus, the oily or fuliginous parts of animal and vegetable
substances, let loose by burning, first ascend into the atmosphere, but
are at length precipitated, and either fall immediately into the sea,
or are, in part at least, washed down into it from the land. From other
causes also, much vegetable matter is carried down by the rivers; and
the whole quantity of animal and vegetable substances thus delivered
into the sea, must be very considerable, amounting annually to the
whole residuum of those substances, not employed in the maintenance or
reproduction of animal and vegetable bodies. Whether chemically united
to the waters of the ocean, or simply suspended in them, this matter is
at last precipitated, and, mingling with earthy substances, is formed
into strata, the place of which will be determined by the currents, the
position of the present continents, and many other circumstances not
easily enumerated.
If, then, an order of things similar to what we now see, existed before
the formation of the present strata, it would necessarily happen, that
the animal and vegetable substances, diffused through the ocean, being
separated from the water, would be deposited at the bottom of the
sea, and, in the course of ages, would form beds, less or more pure,
according to the quantity of earth and other substances deposited
at the same time. These beds being consolidated and mineralized by
operations that are afterwards to be considered, have been converted
into pit-coal, the parts of which are impalpable, and retain nothing of
their primitive structure.[3]
[Footnote 3: NOTE II.]
If, then, the formation of coal from animal and vegetable bodies be
admitted, the general position which derives the origin of the strata
from the waste of former land, as it is applicable to all the kinds
already enumerated, and of course to all those with which they are
alternated, comprehends a very large portion of the earth's surface.
It comprehends, indeed, all the strata usually distinguished by the
name of _Secondary_; but there is another great division of the mineral
kingdom, viz. the rocks, called _Primitive_, which, as they are never
alternated with the secondary, but are always inferior to them, must
be further examined, before we can decide whether the same conclusion
extends to them or not.
7. Here it must be carefully observed, that, among the primary rocks,
the granite is not meant to be included, except where that stone is
stratified, and either coincides with veined granite or with gneiss.
The primitive strata, in Dr Hutton's theory, comprehend, besides
gneiss, the micaceous, chlorite, hornblende, and siliceous schistus,
together with slate, and some other kinds of argillite; to which we
must add, serpentine, micaceous limestone, and the greater part of
marbles. These are mostly distinguished by their laminated structure,
by having their planes much elevated with respect to the horizon,
and by belonging more to the mountainous than the level parts of the
earth's surface. They rarely contain vestiges of organized bodies;
so rarely, indeed, that they were called primitive by the geologists
who first distinguished them from other rocks, on the supposition of
their being part of the primeval nucleus of the globe, which had never
undergone any change whatsoever; but this, I believe, has now almost
ceased to be the opinion of any geologist.[4] The Neptunists hold the
rocks, here enumerated, and also granite, to be produced by aqueous
deposition; but maintain them to be in the strictest sense primeval,
and of a formation antecedent to all organized bodies.
[Footnote 4: NOTE III.]
8. In opposition to this, Dr Hutton maintained, that the primary
schistus, like all the other strata, was formed of materials deposited
at the bottom of the sea, and collected from the waste of rocks still
more ancient. When, therefore, he conformed to the received language
of mineralogists, by calling these strata primitive, he only meant
to describe them as more ancient than any other strata now existing,
but not as more ancient than any that ever had existed. They are
distinguished, in his system, by the name of _Primary_, rather than of
_Primitive_ strata.
That the account now given of their origin is well founded, may be
proved by unquestionable facts. For, first, though, agreeably to the
observation just made, the ancient strata do but rarely contain any
remains of organized bodies, they are not entirely destitute of them.
Different places in this island have been pointed out by Dr Hutton,
where marine objects have been discovered in primary limestone, either
by himself or others, and it would not be difficult to add more
instances of the same kind.[5] In Dauphine, coal, which is certainly a
derivative substance, has been found among mountains which have a title
to the character of primitive, such as no one will dispute. These facts
put the composition of such rocks from loose materials, beyond all
doubt, and also prove their formation to be posterior to the existence
of an animal and vegetable system. They do indeed prove this in the
strictest sense, only of the particular beds in which they are found;
but as these beds are in all other respects as much to be accounted
primary as any part of the mineral kingdom, it is evident that the
negative instances are here of no force, and that nothing can be gained
to the adversaries of this opinion by denying it in general, if they
are obliged to admit it in a single case.
[Footnote 5: NOTE IV.]
9. Again, it is certain, as Dr Hutton remarks, that there are few
considerable bodies of schistus, even the most decidedly primitive,
where sand and gravel may not in some parts be observed. Indeed, it is
not only true that they are to be found in some parts of them; but,
in fact, among many of the primitive mountains, we find large tracts,
composed entirely of a schistose and much indurated sandstone, in beds
highly inclined, sometimes alone, sometimes alternated with other
schisti. In many of them, the sand of which they consist appears to be
entirely of granite, from the detritus of which rock it should seem
that they were chiefly formed.
10. Thus we conclude, that the strata both primary and secondary,
both those of ancient and those of more recent origin, have had their
materials furnished from the ruins of former continents, from the
dissolution of rocks, or the destruction of animal or vegetable
bodies, similar, at least in some respects, to those that now occupy
the surface of the earth. This conclusion is not indeed proved of every
individual portion of rock, but it is demonstrated of many and large
parts, and those scattered indifferently through all the varieties of
the strata; and therefore, from the rules of the strictest reasoning,
we must infer, that the whole is derived from the same origin.[6]
[Footnote 6: NOTE V.]
Thus far concerning the materials of the strata; and, as these were
originally loose and unconnected, we must next consider by what means
they were consolidated into stone.
2. _Consolidation of the Strata_.
11. Though Dr Hutton has no where defined the meaning of the term
consolidation, he has been scrupulously exact in using it constantly
in the same sense. He understands by it, not merely that quality in a
hard body, by which its parts cohere together, but also that by which
it fills up the space comprehended within its surface, being to sense
without porosity, and impervious to air and moisture.
Now, a porous mass of unconnected materials, such as the strata appear
originally to have been, can acquire hardness and solidity only in two
ways, that is, either when it is first reduced by heat into a state of
fusion, or at least of softness, and afterwards permitted to cool; or
when matter that is dissolved in some fluid menstruum, is introduced
along with that menstruum into the porous mass, and, being deposited,
forms a cement by which the whole is rendered firm and compact. Fire
and water, therefore, are the only two physical agents to which we can
ascribe the consolidation of the strata; and, in order to determine to
which of them that effect is to be attributed, we must inquire whether
there are any certain characters that distinguish the action of the one
from that of the other, and which may be compared with the phenomena
actually observed among mineral substances.
12. First, then, it is evident, that the consolidation produced by the
action of water, or of another fluid menstruum, in the manner just
referred to, must necessarily be imperfect, and can never entirely
banish the porosity of the mass. For the bulk of the solvent, and of
the matter it contained in solution, being greater than the bulk of
either taken singly, when the latter was deposited, the former would
have sufficient room left, and would continue to occupy a certain
space in the interior of the strata. A liquid solvent, therefore, could
never shut up the pores of a body to the entire exclusion of itself;
and, had mineral substances been consolidated, as here supposed, the
solvent ought either to remain within them in a liquid state, or if
evaporated, should have left the pores empty and the body pervious to
water. Neither of these however, is the fact; many stratified bodies
are perfectly impervious to water, and few mineral substances contain
water in a liquid state. That they sometimes contain it, chemically
united to them, is no proof of their solidity having been brought
about by that fluid; for such chemical union is as consistent with the
supposition of igneous as of aqueous consolidation, since the region in
which the fire was applied, on every hypothesis must have abounded with
humidity.
13. Again, if water was the solvent by which the consolidating matter
was introduced into the interstices of the strata, that matter could
consist only of such substances as are soluble in water, whereas
it consists of a vast variety of substances, altogether insoluble
either in it, or in any single menstruum whatsoever. The strata are
consolidated, for example, by quartz, by fluor, by feldspar, and by all
the metals, in their endless combinations with sulphureous bodies. To
affirm that water was ever capable of dissolving these substances, is
to ascribe to it powers which it confessedly has not at present; and,
therefore, it is to introduce an hypothesis, not merely gratuitous, but
one which, physically speaking, is absurd and impossible.
This is not all, however; for, even if this difficulty were to be
passed over, it would still be required to explain, how the water,
which, together with the matter which it held in solution, had
insinuated itself into the pores of the strata, became suddenly
disposed to deposit that matter, and to allow it, by crystallization
or concretion, to assume a solid form.[7] The Neptunists must either
assign a sufficient reason for this great and universal change, or must
expect to see their system treated as an inartificial accumulation
of hypotheses which assigns opposite virtues to the same subject,
and is alike at variance with nature and with itself; in a word, a
system that might pass for the invention of an age, when as yet sound
philosophy had not alighted on the earth, nor taught man that he is
but the minister and interpreter of nature, and can neither extend his
power nor his knowledge a hair's-breadth beyond his experience and
observation of the present order of things.[8]
[Footnote 7: NOTE VI.]
[Footnote 8: Homo naturæ minister, et interpres tantùm facit et
intelligit, quantùm de naturæ ordine re, vel mente, observaverit: nec
amplius scit, aut potest.--Nov. Org. lib. i. aph. 1.]
14. Such are the more obvious, but I think unanswerable objections,
that may be urged against the aqueous consolidation of the strata.
It is true, that stony concretions, some of them much indurated, are
formed in the humid way under our eyes. Very particular conditions,
however, are required for that purpose, and conditions such as can
hardly have existed at the bottom of the sea. First. The water must
dissolve the substance of which the concretion is to be formed, as it
actually does in the case of calcareous, and in certain circumstances,
in that of siliceous, earth. Secondly. It must be separated from
that substance, as by evaporation, or by a combination of the matter
dissolved with some third substance, to which it has a greater affinity
than to water, so as to form with it an insoluble compound. Lastly,
the water that is deprived of its solution must be carried off, and
more of that which contains the solution must be supplied, as sometimes
happens where water runs in a stream, or drops from the roof of a
cavern. The two last conditions are peculiarly inapplicable to the
bottom of the sea, where the state of the surrounding fluid would
neither permit the water that was deprived of its solution from being
drawn off, nor that which contained the solution from succeeding it.
It is further to be observed, that the consolidation of stalactitical
concretions, that is, the filling up of their pores, is always
imperfect, and is brought about by the repeated action of the fluid
running through the porous mass, and continuing to deposit there
some of the matter it holds in solution. This, which is properly
infiltration, is incompatible with the nature of a fluid, either
nearly, or altogether quiescent.
15. In order to judge whether objections of equal weight can be opposed
to the hypothesis of igneous consolidation, we must attend to a very
important remark, first made by Dr Hutton, and applied with wonderful
success to explain the most mysterious phenomena of the mineral kingdom.
It is certain, that the effects of fire on bodies vary with the
circumstances under which it is applied to them, and, therefore, a
considerable allowance must be made, if we would compare the operation
of that element when it consolidated the strata, with the results of
our daily experience. The materials of the strata were disposed, as we
have already seen, loose and unconnected, at the bottom of the sea;
that is, even on the most moderate estimation, at the depth of several
miles under its surface. At this depth, and under the pressure of a
column of water of so great a height, the action of heat would differ
much from that which we observe here upon the surface; and, though
our experience does not enable us to compute with accuracy the amount
of this difference, it nevertheless points out the direction in which
it must lie, and even marks certain limits to which it would probably
extend.
The tendency of an increased pressure on the bodies to which heat is
applied, is to restrain the volatility of those parts which otherwise
would make their escape, and to force them to endure a more intense
action of heat. At a certain depth under the surface of the sea, the
power even of a very intense heat might therefore be unable to drive
off the oily or bituminous parts from the inflammable matter there
deposited, so that, when the heat was withdrawn, these principles
might be found still united to the earthy and carbonic parts, forming
a substance very unlike the residuum obtained after combustion under
a pressure no greater than the weight of the atmosphere. It is in
like manner reasonable to believe, that, on the application of heat
to calcareous bodies under great compression, the carbonic gas would
be forced to remain; the generation of quicklime would be prevented,
and the whole might be softened, or even completely melted; which last
effect, though not directly deducible from any experiment yet made, is
rendered very probable, from the analogy of certain chemical phenomena.
16. An analogy of this kind, derived from a property of the barytic
earth, was suggested by that excellent chemist and philosopher, the
late Dr BLACK. The barytic earth, as is well known, has a stronger
attraction for fixed air than common calcareous earth has, so that the
carbonate of barytes is able to endure a great degree of heat before
its fixed air is expelled. Accordingly, when exposed to an increasing
heat, at a certain temperature, it is brought into fusion, the fixed
air still remaining united to it: if the heat be further increased,
the air is driven off, the earth loses its fluidity, and appears in
a caustic state. Here, it is plain, that the barytic earth, which is
infusible, or very refractory, _per se_, as well as the calcareous,
owes its fusibility to the presence of the fixed air; and it is
therefore probable, that the same thing would happen to the calcareous
earth, if by any means the fixed air were prevented from escaping when
great heat is applied to it. This escape of the fixed air is exactly
what the compression in the subterraneous regions is calculated to
prevent, and therefore we are not to wonder if, among the calcareous
strata, we find marks of actual fusion having taken place.[9]
[Footnote 9: NOTE VII.]
17. These effects of pressure to resist the decomposition, and augment
the fusibility of bodies, being once supposed, we shall find little
difficulty in conceiving the consolidation of the strata by heat, since
the intervals between the loose materials of which they originally
consisted may have been closed, either by the softening of those
materials, or by the introduction of foreign matter among them, in
the state of a fluid, or of an elastic vapour. No objection to this
hypothesis can arise from the considerations stated in the preceding
case; the solvent here employed would want no pores to lodge in after
its work was completed, nor would it find any difficulty in making its
retreat through the densest and most solid substances in the mineral
kingdom. Neither can its incapacity to dissolve the bodies submitted
to its action be alleged. Heat is the most powerful and most general
of all solvents; and, though some bodies, such as the calcareous, are
able to resist its force on the surface of the earth, yet, as has just
been shown, it is perfectly agreeable to analogy to suppose, that,
under great pressure, the carbonic state of the lime being preserved,
the purest limestone or marble might be softened, or even melted.
With respect to other substances, less doubt of their fusibility is
entertained; and though, in our experiments, the refractory nature of
siliceous earth has not been completely subdued, a degree of softness
and an incipient fusion have nevertheless been induced.
Thus it appears, in general, that the same difficulties do not press
against the two theories of aqueous and of igneous consolidation; and,
that the latter employs an agent incomparably more powerful than the
former, of more general activity, and, what is of infinite importance
in a philosophical theory, vastly more definite in the laws of its
operation.
18. A more particular examination of the different kinds of fossils
will confirm this conclusion, and will show, that, wherever they bear
marks of having been fluid, these marks are such as characterize the
fluidity of fusion, and distinguish it from that which is produced
by solution in a menstruum. Dr Hutton has enumerated many of these
discovered in the course of that careful and accurate examination of
fossils, in which he probably never was excelled by any mineralogist.
It will be sufficient here to point out a few of the most remarkable
examples.
19. Fossil wood, penetrated by siliceous matter, is a substance well
known to mineralogists; it is found in great abundance in various
situations, and frequently in the heart of great bodies of rock. On
examination, the siliceous matter is often observed to have penetrated
the wood very unequally, so that the vegetable structure remains in
some places entire; and in other places is lost in a homogeneous mass
of agate or jasper. Where this happens, it may be remarked, that the
line which separates these two parts is quite sharp and distinct,
altogether different from what must have taken place, had the flinty
matter been introduced into the body of the wood, by any fluid in which
it was dissolved, as it would then have pervaded the whole, if not
uniformly, yet with a regular gradation. In those specimens of fossil
wood that are partly penetrated by agate, and partly not penetrated
at all, the same sharpness of termination may be remarked, and is an
appearance highly characteristic of the fluidity produced by fusion.
20. The round nodules of flint that are found in chalk, quite insulated
and separate from one another, afford an argument of the same kind;
since the flinty matter, if it had been carried into the chalk by any
solvent, must have been deposited with a certain degree of uniformity,
and would not now appear collected into separate masses, without any
trace of its existence in the intermediate parts. On the other hand, if
we conceive the melted flint to have been forcibly injected among the
chalk, and to have penetrated it, somewhat as mercury may, by pressure,
be made to penetrate through the pores of wood, it might, on cooling,
exhibit the same appearances that the chalk-beds of England do actually
present us with.
The siliceous pudding-stone is an instance closely connected with the
two last; in it we find both the pebbles, and the cement which unites
them, consisting of flint equally hard and consolidated; and this
circumstance, for which it is impossible to account by infiltration, or
the insinuation of an aqueous solvent, is perfectly consistent with the
supposition, that a stream of melted flint has been forcibly injected
among a mass of loose gravel.
21. The common grit, or sandstone, though it certainly gives no
indication of having possessed fluidity, is strongly expressive of the
effects of heat. It is so, especially in those instances where the
particles of quartzy sand, of which it is composed, are firmly and
closely united, without the help of any cementing substance whatsoever.
This appearance, which is very common, seems to be quite inconsistent
with every idea of consolidation, except an incipient fusion, which,
with the assistance of a suitable compression, has enabled the
particles of quartz to unite into stone.
It has indeed been asserted, that the mere apposition of stony
particles, so as to permit their corpuscular attraction to take place,
was sufficient to form them into stone. To this Dr Hutton has very well
replied, that, admitting the possibility of a hard and firm body being
produced in this way of which, however, we have no proof, the close
and compact texture, the perfect consolidation of the stones we are
now speaking of, would still remain to be explained, and of this it is
evident that the mere apposition of particles, and the force of their
mutual attraction, can afford no solution.
22. These proofs that the strata must have endured the action of
intense heat, though immediately deduced from those of the siliceous
genus only, extend in reality to all the strata, of every kind, with
which they are found alternated. It is impossible that heat, of the
intensity here supposed, can have acted on a particular stratum, and
not on those that are contiguous to it; and, as there are no strata of
any kind with which the quartzy and siliceous are not intermixed, so
there are none of which the igneous consolidation is not thus rendered
probable. We need rest nothing, however, on this argument, as the
fossils of every genus may be shown to speak distinctly for themselves.
23. Those of the calcareous genus do so perhaps more sparingly than
the rest; yet even among them there are many facts, that, though taken
unconnected with all others, are sufficient to establish the action of
subterraneous fire. Such, for example, are the calcareous breccias,
composed of fragments of marble or limestone, and not only adapted to
each other's shape, but indented into one another, in a manner not
a little resembling the _sutures_ of the human _cranium_. From such
instances, it is impossible not to infer the softness of the calcareous
fragments when they were consolidated into one mass. Now, this softness
could be induced only by heat; for it must be acknowledged, that the
action of any other solvent is quite inadequate to the softening of
large fragments of stone, without dissolving them altogether.
24. In many other instances it appears certain, that the stones
of the calcareous genus have been reduced by heat into a state of
fluidity much more perfect. Thus, the saline or finer kinds of marble,
and many others that have a structure highly crystallized, must
have been softened to a degree little short of fusion, before this
crystallization could take place. Even the petrifactions which abound
so much in limestones tend to establish the same fact; for they possess
a sparry structure, and must have acquired that structure in their
transition from a fluid to a solid state.[10]
[Footnote 10: NOTE VIII.]
25. In accounting, by the operation of heat, for these appearances
of fluidity, Dr Hutton has proceeded on the principle already laid
down, as conformable to analogy, that calcareous earth, under great
compression, may have its fixed air retained in it, notwithstanding the
action of intense heat, and may, by that means, be reduced into fusion,
or into a state approaching to it. In all this I do not think that he
has departed from the strictest rules of philosophical investigation.
The facts just stated prove, that limestone was once soft, its
fragments retaining at the same time their peculiar form, an effect to
which we know of none similar but those of fire; and, therefore, though
we could not conjecture how heat might be applied to limestone so as
to melt it, instead of reducing it to a calx, we should, nevertheless,
have been forced to suppose, that this had actually taken place in the
bowels of the earth; and was a fact which, though we were not able to
explain it, we were not entitled to deny. The principle just mentioned
relieves us therefore from a difficulty, that would have embarrassed,
but could not have overturned, this theory of the earth.
26. From the arguments which the argillaceous strata afford for the
igneous consolidation of fossils, I shall select one on which Dr Hutton
used to lay considerable stress, and which some of the adversaries of
his system have endeavoured to refute. This argument is founded on the
structure of certain ironstones called _septaria_, often met with among
the argillaceous schistus, particularly in the vicinity of coal. These
stones are usually of a lenticular or spheroidal form, and are divided
in their interior into distinct _septa_, by veins of calcareous spar,
of which one set are circular and concentric, the other rectilineal;
diverging from the centre of the former, and diminishing in size as
they recede from it. Now, what is chiefly to be remarked is, that
these veins terminate before they reach the surface of the stone; so
that the matter with which they are filled cannot have been introduced
from without by infiltration, or in any other way whatsoever. The only
other supposition, therefore, that is left for explaining the singular
structure of this fossil, is, that the whole mass was originally
fluid, and that, in cooling, the calcareous part separated from the
rest, and afterwards crystallized.
27. It has been urged against this theory of the septaria, that these
stones are sometimes found with the calcareous veins extending all
the way to the circumference, and of course communicating with the
outside. But it must be observed, that this fact does not affect the
argument drawn from specimens in which no such communication takes
place. It is at best only an ambiguous instance, that may be explained
by two opposite theories, and may be reconciled either to the notion
of igneous or of aqueous consolidation: but if there is a single close
septarium in nature, it can, of course, be explained only by one of
these theories, and the other must, of necessity, be rejected. Besides,
it is plain, that a close septarium can never have been open, though an
open septarium may very well have been close; and indeed, as this stone
is, in certain circumstances, subject to perpetual exfoliation, it
would be wonderful if no one was ever found with the calcareous veins
reaching to the surface. With regard to the light, therefore, that they
give into their own history, these two kinds of septaria are by no
means on an equal footing; and this may serve to show, how necessary
it is, in all inductive reasoning, and particularly in a subject so
complex as geology, to separate with care such phenomena as admit of
two solutions, from such as admit only of one.
28. The bituminous strata come next to be considered; and they are of
great consequence in the present argument, because their dissimilarity
in so many particulars to all other mineral substances, renders them
what Lord Bacon calls an _instantia singularis_, having the first
rank among facts subservient to inductive investigation. But though
unlike in substance to other fossils, and composed, as has been shown,
of materials that belonged not originally to the mineral kingdom,
they agree in many material circumstances with the strata already
enumerated. Their beds are disposed in the same manner, and are
alternated indiscriminately with those of all the secondary rocks,
and, being formed in the same region, must have been subject to the
same accidents, and have endured the operation of the same causes.
They are traversed too like the other strata, by veins of the metals,
of spars, of basaltes, and of other substances; and, whatever argument
may hereafter be derived from this to prove the action of fire on the
strata so traversed, is as much applicable to coal as to any other
mineral. The coal strata also contain pyrites in great abundance, a
substance that is perhaps, more than any other, the decided progeny of
fire. This compound of metal and sulphur, which is found in mineral
bodies of every kind, I believe, without any exception, is destroyed
by the contact of moisture, and resolved into a vitriolic salt. At
the same time it is found in the strata, not traversing them in
veins, which may be supposed of more recent formation than the strata
themselves; but existing in the heart of the most solid rocks, often
nicely crystallized, and completely inclosed, on all sides, without the
most minute vacuity. The pyrites must have been present, therefore,
when the strata were consolidated, and it is inconceivable, if their
consolidation was brought about in the wet way, that a substance
should be so generally found in them, the very existence of which is
incompatible with humidity. This argument for the igneous origin of the
strata is applicable to them all, but especially to those of coal, as
abounding with pyrites more than any other.
29. The difficulty that here naturally present itself, viz. how
vegetable matter, such as coal is supposed to have been, could be
exposed to the action of intense heat, without being deprived of its
inflammable part, is obviated by the principle formerly explained
concerning the effects of compression. The weight incumbent on
the strata of coal, when they were exposed to the intense heat of
the mineral regions, may have been such as to retain the oily and
bituminous, as well as sulphureous parts, though the whole was reduced
almost to fusion; and thus, on cooling, the sulphur uniting with iron
might crystallize, and assume the form of pyrites.
30. The compression, however, has not in every instance preserved the
bituminous, in union with the carbonic part of coal; and hence a mark
of the operation of fire quite peculiar to this fossil, and found in
those infusible kinds of it which contain no bitumen, and burn without
flame. These resemble, some of them very precisely, and all them in a
great degree, the products obtained by the distillation of the common
bituminous coal that is, they consist of charcoal, united to an earthy
basis in different proportions. It is natural therefore to conclude,
that this substance was prepared in the mineral regions by the action
of heat, which, in some instances, has driven off the inflammable part
of the coal. That the heat should, in some cases, have done so, is not
inconsistent with the general effect attributed to compression. The
conditions necessary for retaining the more volatile parts, may not
have been present every where in the same degree, so that the latter,
though they could not escape, may have been forced from one part of a
stratum, or body of strata, to another.
31. In confirmation of this it must be observed, that, as the fixed
part of coal is thus found in the bowels of the earth, separate from
the volatile or bituminous, so, in the neighbourhood of coal strata,
the latter is sometimes found without any mixture of the former. The
fountains of naphtha and petroleum are well known; and Dr Hutton
has described a stratum of limestone, lying in the centre of a coal
country, which is pervaded and tinged by bituminous matter, through
its whole mass, and has, at the same time, many close cavities in
the heart of it, lined with calcareous spar, and containing fossil
pitch, sometimes in large pieces, sometimes in hemispherical drops,
scattered over the surface of the cavities. This combination could
only be effected by a part of the inflammable matter of the beds of
coal underneath, being driven off by heat, and made to penetrate the
limestone, while it was yet soft and pervious to heated vapours.[11]
[Footnote 11: NOTE IX.]
32. Hitherto we have enumerated those fossils that are either not at
all, or very sparingly soluble in water. There are, however, saline
bodies among the mineral strata, such for instance as rock-salt, which
are readily dissolved in water; and it yet remains to examine by what
cause their consolidation has been effected.
Here the theorists who consider water as the sole agent in the
mineralization of fossils, are indeed delivered from one difficulty,
but it is only that they may be harder pressed on by another. It
cannot now be said, that the menstruum which they employ is incapable
of dissolving the substances exposed to its action, as in the case of
metallic or stony bodies; but it may very well be asked, how the water
came to deposit the salts which it held in solution, and to deposit
them so copiously as it has done in many places, without any vestige
of similar deposition in the places immediately contiguous. If they
refuse to call to their assistance any other than their favourite
element, they will not find it easy to answer this question, and must
feel the embarrassment of a system, subject to two difficulties, so
nicely, but so unhappily adjusted, that one of them is always prepared
to act whenever the other is removed. If, on the other hand, they will
admit the operation of subterraneous heat, it appears possible, that
the local application of such heat may have driven the water, in
vapour, from one place to another, and by such action often repeated in
the same spot, may have produced those great accumulations of saline
matter, that are actually found in the bowels of the earth.
33. But granting that, either in the way just pointed out, or in some
other that is unknown, the salt and the water have been separated,
some further action of heat seems requisite, before a compact, and
highly indurated body, like rock-salt, could be produced. The mere
precipitation of the salt, would, as Dr Hutton has observed, form only
an assemblage of loose crystals at the bottom of the sea, without
solidity or cohesion: and to convert such a mass into a firm and solid
rock, would require the application of such heat as was able to reduce
it into fusion. The consolidation of rock-salt, therefore, however its
separation from the water is accounted for, cannot be explained but on
the hypothesis of subterraneous heat.
34. Some other phenomena that have been observed in salt mines, come
in support of the same conclusion. The salt rock of Cheshire, which
lies in thick beds, interposed between strata of an argillaceous or
marly stone, and is itself mixed with a considerable portion of the
same earth, exhibits a very great peculiarity in its structure. Though
it forms a mass extremely compact, the salt is found to be arranged
in round masses of five or six feet in diameter, not truly spherical,
but each compressed by those that surround it, so as to have the shape
of an irregular polyhedron. These are formed of concentric coats,
distinguishable from one another by their colour, that is, probably
by the greater or less quantity of earth which they contain, so that
the roof of the mine, as it exhibits a horizontal section of them,
is divided into polygonal figures, each with a multitude of polygons
within it, having altogether no inconsiderable resemblance to a
_mosaic_ pavement. In the triangular spaces without the polygons, the
salt is in coats parallel to the sides of the polygons.
The circumstances which gave rise to this singular structure we should
in vain endeavour to define; yet some general conclusions concerning
them seem to be within our reach. It is clear that the whole mass of
salt was fluid at once, and that the forces, whatever they were, which
gave solidity to it, and produced the new arrangement of its particles,
were all in action at the same time. The uniformity of the coated
structure is a proof of this, and, above all, the compression of the
polyhedra, which is always mutual, the flat side of one being turned to
the flat side of another, and never an angle to an angle, nor an angle
to a side. The coats formed as it were round so many different centres
of attraction, is also an appearance quite inconsistent with the notion
of deposition; both these, however, are compatible with the notion of
solidity acquired by the refrigeration of a fluid, where the whole
mass is acted on at the same time, and where no solvent remains to be
disposed of after the induration of the rest.
35. Another species of fossil salt exhibits appearances equally
favourable to the theory of igneous consolidation. This is the Trona of
Africa, which is no other than soda, or mineral alkali, in a particular
state. The specimen of this fossil in Dr Black's, now Dr Hope's,
collection, is of a sparry and radiated structure, and is evidently
part of the contents of a vein, having a stony crust adhering to it,
on one side, with its own sparry structure complete, on the opposite.
It contains but about one sixth of the water of crystallization
essential to this salt when obtained in the humid way; and, what is
particularly to be remarked, it does not lose this water, nor become
covered with a powder, like the common alkali, by simple exposure to
the air. It is evident, therefore, that this fossil does not originate
from mere precipitation; and when we add, that in its sparry structure
it contains evident marks of having once been fluid, we have little
reason to entertain much doubt concerning the principle of its
consolidation.
Thus, then, the testimony given to the operation of fire, or heat,
as the consolidating power of the mineral kingdom, is not confined
to a few fossils, but is general over all the strata. How far the
unstratified fossils agree in supporting the same conclusion, will be
afterwards examined.
3. _Position of the Strata._[12]
[Footnote 12: Theory of the Earth, vol. i. p. 120.]
36. We have seen of what materials the strata are composed, and by
what power they have been consolidated; we are next to inquire, from
what cause it proceeds, that they are now so far removed from the
region which they originally occupied, and wherefore, from being all
covered by the ocean, they are at present raised in many places fifteen
thousand feet above its surface. Whether this great change of relative
place can be best accounted for by the depression of the sea, or the
elevation of the strata themselves, remains to be considered.
Of these two suppositions, the former, at first sight, seems
undoubtedly the most probable, and we feel less reluctance to suppose,
that a fluid, so unstable as the ocean, has undergone the great
revolution here referred to, than that the solid foundations of the
land have moved a single fathom from their place. This, however, is a
mere illusion. Such a depression of the level of the sea as is here
supposed, could not happen without a change proportionally great in the
solid part of the globe; and, though admitted as true, will be found
very inadequate to explain the present condition of the strata.
37. Supposing the appearances which clearly indicate submersion under
water to reach no higher than ten thousand feet above the present level
of the sea, and of course the surface of the sea to have been formerly
higher by that quantity than it is now; it necessarily follows, that
a bulk of water has disappeared, equal to more than a seven hundredth
part of the whole magnitude of the globe.[13] The existence of empty
caverns of extent sufficient to contain this vast body of water, and of
such a convulsion as to lay them open, and give room to the retreat of
the sea, are suppositions which a philosopher could only be justified
in admitting, if they promised to furnish a very complete explanation
of appearances. But this justification is entirely wanting in the
present case; for the retreat of the ocean to a lower level, furnishes
a very partial and imperfect explanation of the phenomena of geology.
It will not explain the numberless remains of ancient continents that
are involved, as we have seen, in the present, unless it be supposed
that the ancient ocean, though it rose to so great a height, had
nevertheless its shores, and was the boundary of land still higher than
itself. And, as to that which is now more immediately the object of
inquiry, the position of the strata, though the above hypothesis would
account in some sort for the change of their place, relatively to the
level of the sea; yet, if it shall be proved, that the strata have
changed their place relatively to each other, and relatively to the
plane of the horizon, so as to have had an angular motion impressed on
them, it is evident that, for these facts, the retreat of the sea does
not afford even the shadow of a theory.
[Footnote 13: NOTE X.]
38. Now, it is certain, that many of the strata have been moved
angularly, because that, in their original position, they must have
been all nearly horizontal. Loose materials, such as sand and gravel
subsiding at the bottom of the sea, and having their interstices filled
with water, possess a kind of fluidity: they are disposed to yield
on the side opposite to that where the pressure is greatest, and are
therefore, in some degree, subject to the laws of hydrostatics. On
this account they will arrange themselves in horizontal layers; and
the vibrations of the incumbent fluid, by impressing slight motion
backward, and forward, on the materials of these layers, will very much
assist the accuracy of their level.
It is not, however, meant to deny, that the form of the bottom might
influence, in a certain degree, the stratification of the substances
deposited on it. The figure of the lower beds deposited on an uneven
surface, would necessarily be affected by two causes; the inclination
of that surface, on the one hand, and the tendency to horizontality,
on the other; but, as the former cause would grow less powerful as the
distance from the bottom increased, the latter cause would finally
prevail, so that the upper beds would approach to horizontally, and the
lower would neither be exactly parallel to them, nor to one another.
Whenever, therefore, we meet with rocks, disposed in layers quite
parallel to one another, we may rest assured, that the inequalities of
the bottom have had no effect, and that no cause has interrupted the
statical tendency above explained.
Now, rocks having their layers exactly parallel, are very common, and
prove their original horizontally to have been more precise than we
could venture to conclude from analogy alone. In beds of sandstone,
for instance, nothing is more frequent than to see the thin layers of
sand, separated from one another by layers still finer of coaly, or
micaceous matter, that are almost exactly parallel, and continue so to
a great extent without any sensible deviation. These planes can have
acquired their parallelism only in consequence of the property of water
just stated, by which it renders the surfaces of the layers, which it
deposits, parallel to its own surface, and therefore parallel to one
another. Though such strata, therefore, may not now be horizontal, they
must have been so originally; otherwise it is impossible to discover
any cause for their parallelism, or any rule by which it can have been
produced.
39. This argument for the original horizontality of the strata,
is applicable to those that are now farthest removed from that
position. Among such, for instance, as are highly inclined, or even
quite vertical, and among those that are bent and incurvated in the
most fantastical manner, as happens more especially in the primary
schisti, we observe, through all their sinuosities and inflections,
an equality of thickness and of distance among their component
laminæ. This equality could only be produced by those laminæ having
been originally spread out on a flat and level surface, from which
situation, therefore, they must afterwards have been lifted up by the
action of some powerful cause, and must have suffered this disturbance
while they were yet in a certain degree flexible and ductile. Though
the primary direction of the force which thus elevated them must have
been from below upwards, yet it has been so combined with the gravity
and resistance of the mass to which it was applied, as to create a
lateral and oblique thrust, and to produce those contortions of the
strata, which, when on the great scale, are among the most striking and
instructive phenomena of geology.
40. Great additional force is given to this argument, in many cases, by
the nature of the materials of which the stratified rocks are composed.
The beds of breccia and pudding-stone, for instance, are often in
planes almost vertical, and, at the same time, contain gravel-stones,
and other fragments of rock, of such a size and weight, that they
could not remain in their present position an instant, if the cement
which unites them were to become soft; and therefore they certainly
had not that position at the time when this cement was actually soft.
This remark has been made by mineralogists who were not led to it by
any system. The judicious and indefatigable observer of the Alps,
describing the pudding-stone of Valorsine, near the sources of the
Arve, tells us, that he was astonished to find it in beds almost
vertical, a situation in which it could not possibly have been formed.
"That particles," he adds, "of extreme tenuity, suspended in a fluid,
might become agglutinated, and form vertical beds, is a thing that
may be conceived; but that pieces of stone, of several pounds weight,
should have rested on the side of a perpendicular wall, till they were
enveloped in a stony cement, and united into one mass, is a supposition
impossible and absurd. It should be considered, therefore, as a thing
demonstrated, that this pudding-stone was formed in a horizontal
position, or one nearly such, and elevated after its induration. We
know not," he continues, "the force by which this elevation has been
effected; but it is an important step among the prodigious number of
vertical beds that are to be met with in the Alps, to have found some
that must certainly have been formed in a horizontal situation."[14]
[Footnote 14: Voyages aux Alpes, tom. ii § 690.]
41. Nothing can be more sound and conclusive than this reasoning; and
had the ingenious author pursued it more systematically, it must have
led him to a theory of mountains very little different from that which
we are now endeavouring to explain. If some of the vertical strata are
proved to have been formed horizontally, there can be no reason for
not extending the same conclusion to them all, even if we had not the
support of the argument from the parallelism of the layers, which has
been already stated.
42. The highly inclined position, and the manifold inflections of the
strata, are not the only proofs of the disturbance that they have
suffered, and of the violence with which they have been forced up from
their original place. Those interruptions of their continuity which
are observed, both at the surface and under it, are evidences of the
same fact. It is plain, that if they remained now in the situation
in which they were at first deposited, they would never appear to
be suddenly broken off. No stratum would terminate abruptly; but,
however its nature and properties might change, it would constitute
an entire and continued rock, at least where the effects of waste and
_detritus_ had not produced a separation. This, however, is very far
from being the actual condition of stratified bodies. Those that are
much inclined, or that make considerable angles with the horizontal
plane, must terminate abruptly where they come up to the surface. Their
doing so is a necessary consequence of their position, and furnishes no
argument, it may be said, for their having been disturbed, different
from that which has been already deduced from their inclination. There
are, however, instances of a breach of continuity in the strata, under
the surface, that afford a proof of the violence with which they have
been displaced, different from any hitherto mentioned. Of this nature
are the _slips_ or _shifts_, that so often perplex the miner in his
subterraneous journey, and which change at once all those lines and
bearings that had hitherto directed his course. When his mine reaches
a certain plane, which is sometimes perpendicular, sometimes oblique
to the horizon, he finds the beds of rock broken asunder, those on
the one side of the plane having changed their place, by sliding in
a particular direction along the face of the others. In this motion
they have sometimes preserved their parallelism, that is, the strata
on one side of the _slip_ continue parallel to those on the other; in
other cases, the strata on each side become inclined to one another,
though their identity is still to be recognized by their possessing
the same thickness, and the same internal characters. These _shifts_
are often of great extent, and must be measured by the quantity of
the rock moved, taken in conjunction with the distance to which it
has been carried. In some instances, a vein is formed at the plane of
the shift or slip, filled with materials of the kinds which will be
hereafter mentioned; in other instances, the opposite sides of the rock
remain contiguous, or have the interval between them filled with soft
and unconsolidated earth. All these are the undeniable effects of some
great convulsion, which has shaken the very foundations of the earth;
but which, far from being a disorder in nature, is part of a regular
system, essential to the constitution and economy of the globe.
The production of the appearances now described, belongs, without
doubt, to different periods of time; and, where slips intersect one
another, we can often distinguish the less from the more ancient They
are all, however, of a date posterior to that at which the waving and
undulated forms of the strata were acquired, as they do not carry with
them any marks of the softness of the rock, but many of its complete
induration.
The same phenomenon which is thus exemplified on a great scale in
the bowels of the earth, is often most beautifully exhibited in
single specimens of stone, and is accompanied with this remarkable
circumstance, that the _integrity_ of the stone is not destroyed by the
shifts, whatever wounds had been made in it being healed, and the parts
firmly reunited to one another.[15]
[Footnote 15: NOTE XI.]
43. Though such marks of violence as have been now enumerated are
common in some degree to all the strata, they abound most among the
primary, and point out these as the part of our globe which has been
exposed to the greatest vicissitudes. At their junction with the
secondary, or where they emerge, as it were, from under the latter,
phenomena occur, which mark some of those vicissitudes with astonishing
precision; phenomena of which the nature was first accurately explored,
and the consequences fully deduced, by the geologist whose system I am
endeavouring to explain. He observed, in several instances, that where
the primary schistus rises in beds almost vertical, it is covered by
horizontal layers of secondary sandstone, which last are penetrated by
the irregular tops of the schistus, and also involve fragments of that
rock, some angular, others round and smooth, as if worn by attrition.
From this he concluded, that the primary strata, after being formed at
the bottom of the sea, in planes nearly horizontal, were raised, so as
to become almost vertical, while they were yet covered by the ocean,
and before the secondary strata had begun to be deposited on them. He
also argued, that, as the fragments of the primary rock, included in
the secondary, are many of them rounded and worn, the deposition of
the latter must have been separated from the elevation of the former
by such an interval of time, as gave room for the action of waste and
decay, allowing those fragments first to be detached, and afterwards
wrought into a round figure.[16]
[Footnote 16: NOTE XII.]
44. Indeed, the interposition of a breccia between the primary and
secondary strata, in which the fragments, whether round or angular,
are always of the primary rock, is a fact so general, and the quantity
of this breccia is often so great, that it leads to a conclusion more
paradoxical than any of the preceding, but from which, nevertheless,
it seems very difficult to withhold assent. Round gravel, when in
great abundance, agreeably to a remark already made, must necessarily
be considered as a production peculiar to the beds of rivers, or
the shores of continents, and as hardly ever formed at great depths
under the surface of the sea. It should seem, then, that the primary
schistus, after attaining its erect position, had been raised up to
the surface, where this gravel was formed; and from thence had been
let down again to the depths of the ocean, where the secondary strata
were deposited on it. Such alternate elevations and depressions of the
bottom of the sea, however extraordinary they may seem, will appear to
make a part of the system of the mineral kingdom, from other phenomena
hereafter to be described.
45. On the whole, therefore, by comparing the actual position of the
strata, their erectness, their curvature, the interruptions of their
continuity, and the transverse stratification of the secondary in
respect of the primary, with the regular and level situation which
the same strata must have originally possessed, we have a complete
demonstration of their having been disturbed, torn asunder, and moved
angularly, by a force that has, in general, been directed from below
upwards. In establishing this conclusion, we have reasoned more from
the facts which relate to the _angular elevation_ of the strata,
than from those which relate to their _absolute elevation_, or their
translation to a greater distance from the centre of the earth. This
has been done, because the appearances, which respect the absolute
lifting up of the strata are more ambiguous than those, which respect
the change of their angular position. The former might be accounted
for, could they be separated from the latter, in two ways, viz. either
by the retreat of the sea, or the raising up of the land; but the
latter can be explained only in one way, and force us of necessity to
acknowledge the existence of an expanding power, which has acted on the
strata with incredible energy, and has been directed from the centre
toward the circumference.
46. When we are assured of the existence of such a power as this in the
mineral regions, we should argue with singular inconsistency, if we did
not ascribe to it all the other appearances of motion in those regions,
which it is adequate to produce. If nature in her subterraneous abodes
is provided with a force that could burst asunder the massy pavement
of the globe, and place the fragments upright upon their edges, could
she not, by the same effort, raise them from the greatest depths of the
sea, to the highest elevation of the land? The cause that is adequate
to one of these effects is adequate to them both together; for it is a
principle well known in mechanical philosophy, that the force which
produces a parallel motion, may, according to the way in which it is
applied, produce also an angular motion, without any diminution of the
former effect. It would, therefore, be extremely unphilosophical to
suppose, that any other cause has changed the relative level of the
strata, and the surface of the sea, than that which has, in so many
cases, raised the strata from a horizontal to a highly inclined, or
even vertical situation: it would be to introduce the action of more
causes than the phenomena require, and to forget, that nature, whose
operations we are endeavouring to trace, combines the possession of
infinite resources with the most economical application of them.
47. From all, therefore, that relates to the position of the strata, I
think I am justified in affirming, that their disturbance and removal
from the place of their original formation, by a force directed
from below upwards, is a fact in the natural history of the earth,
as perfectly ascertained as any thing which is not the subject of
immediate observation. As to the power by which this great effect has
been produced, we cannot expect to decide with equal evidence, but must
be contented to pass from what is certain to what is probable. We may,
then, remark, that of the forces in nature to which our experience
does in any degree extend, none seems so capable of the effect we
would ascribe to it, as the expansive power of heat; a power to which
no limits can be set, and one, which, on grounds quite independent of
the elevation of the strata, has been already concluded to act with
great energy in the subterraneous regions. We have, indeed, no other
alternative, but either to adopt this explanation, or to ascribe the
facts in question to some secret and unknown cause, though we are
ignorant of its nature, and have no evidence of its existence.
We are therefore to suppose, that the power of the same subterraneous
heat, which consolidated and mineralized the strata at the bottom of
the sea, has since raised them up to the height at which they are now
placed, and has given them the various inclinations to the horizon
which they are found actually to possess.
48. The probability of this hypothesis will appear greatly increased,
when it is considered, that, besides those now enumerated, there are
other indications of movement among the bodies of the mineral kingdom,
where effects of heat more characteristic than simple expansion are
clearly to be discovered. Thus, on examining the marks of disorder
and movement which are found among the strata, it cannot fail to be
observed, that notwithstanding the fracture and dislocation, of which
they afford so many examples, there are few empty spaces to be met
with among them, as far as our observation extends. The breaches and
separations are numerous, and distinct; but they are, for the most
part, completely filled up with minerals of a kind quite different
from the rock on each side of them, and remarkable for containing
no vestiges of stratification. We are thus led to consider the
unstratified minerals, the second of the divisions into which the whole
mineral kingdom, viewed geologically, ought to be distinguished. These
minerals are immediately connected with the disturbance of the strata,
and appear, in many instances, to have been the instruments of their
elevation.
SECTION II.
OF THE PHENOMENA PECULIAR TO UNSTRATIFIED BODIES.
1. _Metallic Veins._
49. THE unstratified minerals exist either in veins, intersecting the
stratified, or in masses surrounded by them. Veins are of various
kinds, and may in general be defined, separations in the continuity of
a rock, of a determinate width, but extending indefinitely in length
and depth, and filled with mineral substances, different from the rock
itself. The mineral veins, strictly so called, are those filled with
crystallized substances, and containing the metallic ores.
That these veins are of a formation subsequent to the hardening and
consolidation of the strata which they traverse, is too obvious to
require any proof; and it is no less clear, from the crystallized
and sparry structure of the substances contained in them, that these
substances must have concreted from a fluid state. Now, that this
fluidity was simple, like that of fusion by heat, and not compound,
like that of solution in a menstruum, is inferred from many phenomena.
It is inferred from the acknowledged insolubility of the substances
that fill the veins, in any one menstruum whatsoever; from the total
disappearance of the solvent, if there was any; from the complete
filling up of the vein by the substances which that solvent had
deposited; from the entire absence of all the appearances of horizontal
or gradual deposition; and, lastly, from the existence of close
cavities, lined with crystals, and admitting no egress to any thing but
heat.
50. To the same effect may be mentioned those groups of crystals
composed of substances the most different, that are united in the same
specimen, all intersecting and mutually impressing one another. These
admit of being explained, on the supposition that they were originally
in fusion, and became solid by the loss of heat; a cause that acted
on them all alike, and alike impelled them to crystallize: But the
appearances of simultaneous crystallization seem incompatible with the
nature of deposition from a solvent, where, with respect 16 different
substances, the effects must take place slowly, and in succession.
51. The metals contained in the veins which we are now treating of,
appear very commonly in the form of an ore, mineralized by sulphur.
Their union with this latter substance can be produced, as we know, by
heat, but hardly by the way of solution in a menstruum, and certainly
not at all, if that menstruum is nothing else than water. The metals,
therefore, when mineralized by sulphur, give no countenance to the
hypothesis of aqueous solution; and still less do they give any when
they are found native, as it is called, that is, malleable, pure
and uncombined with any other substance. The great masses of native
iron found in Siberia and South America are well known; and nothing
certainly can less resemble the products of a chemical precipitation.
Gold, however, the most perfect of the metals, is found native most
frequently; the others more rarely, in proportion nearly to the
facility of their combination with sulphur. Of all such specimens it
may be safely affirmed, that if they have ever been fluid, or even
soft, they must have been so by the action of heat; for, to suppose
that a metal has been precipitated, pure and uncombined from any
menstruum, is to trespass against all analogy, and to maintain a
physical impossibility. But it is certain, that many of the native
metals have once been in a state of softness, because they bear on them
impressions which they could not have received but when they were soft.
Thus, gold is often impressed by quartz and other stones, which still
adhere to it, or are involved in it. Specimens of quartz, containing
gold and silver shooting through them, with the most beautiful and
varied ramifications, are every where to be met with in the cabinets of
the curious; and contain, in their structure, the clearest proof, that
the metal and the quartz have been both soft, and have crystallized
together. By the compactness, also, of the body which they form, they
show, that when they acquired solidity, it was by the concretion of the
whole mass, and not by such partial concretion as takes place when a
solvent is separated from substances which it held in solution.
52. Native copper is very abundant; and some specimens of it have been
found crystallized. Here the crystallization of the metal is a proof
that it has passed from a fluid to a solid state; and its purity is a
proof that it did not make that transition by being precipitated from a
menstruum.
53. Again, pieces of native manganese have been found possessing so
exactly the characters peculiar to that metal when reduced in our
furnaces, that it is impossible to consider them as deriving their
figure and solidity from any cause but fusion. The ingenious author
who describes these specimens, La Peyrouse, was so forcibly struck
with this resemblance, that he immediately drew the same conclusion
from it which is drawn here, attributing the only difference, which
he remarked between the native and the artificial _regulus_, to the
different energy with which the same agent works when employed by
nature and by art.[17]
[Footnote 17: Theory of the Earth, vol. i. p. 68. Journal de Phys.
Janvier, 1786.]
54. All these appearances conspire to prove, that the materials which
fill the mineral veins were melted by heat, and forcibly injected, in
that state, into the clefts and fissures of the strata. These fissures
we must conceive to have arisen, not merely from the shrinking of the
strata while they acquired hardness and solidity, but from the violence
done to them, when they were heaved up and elevated in the manner which
has already been explained.[18]
[Footnote 18: NOTE XIII.]
55. When these suppositions are once admitted, the other leading facts
in the history of metallic veins will be readily accounted for. Thus,
for instance, it is evident to what we must ascribe the fragments of
the surrounding rock that are often found immersed in the veins, and
encompassed on all sides by crystallized substances. These fragments
being no doubt detached by the concussion, which at once tore asunder
and elevated the strata, were sustained by the melted matter that
flowed at the same time upward through the vein. Large masses of rock
are often found in this manner completely insulated; one of these,
which M. De Luc has described with great accuracy, is no less than a
vast segment of a mountain.[19]
[Footnote 19: Lettres Physiques, &c. tom. iii. P. 361.]
56. The immense violence which has accompanied the formation of mineral
veins, is particularly marked by the slips and shifts of the strata on
each side of them all tending to show what mighty changes have taken
place in those regions, which our imagination erroneously paints as
the abode of everlasting silence and rest. This shifting of the strata
is best observed, where the veins make a transverse section of beds of
rock, considerably inclined to the horizon. There it is common to see
the beds on one side of the vein slipped along from the corresponding
beds on the other side, and removed sometimes in a horizontal,
sometimes in an oblique direction. In this way, not only the strata
are shifted, but veins, which intersect one another, are also shifted
themselves. They are _heaved_, as it is called in the significant
language of the miners, and forced out of their direction. It is
impossible, in such a case, but to connect in the mind the formation
of the vein, and the production of the slips which accompany it, and to
regard them as parts of the same phenomenon.
57. Where these slips are horizontal, and exhibit great bodies of
strata carried from their place, while the parts of the transferred
mass remain undisturbed relatively to one another, they furnish a dear
proof, that this change of plaice has not arisen from the falling in
of the roofs of caverns, as some geologists suppose. The horizontal
direction, and the regularity of the movement, are incompatible with
the action of such a cause as this; and indeed it is highly interesting
to remark, in the midst of the signs of disturbance which prevail in
the bowels of the earth, that there reigns a certain symmetry and
order, which indicate the action of a force of incredible magnitude,
but slow and gradual in its effects. The parts of the mass moved are
undisturbed relatively to one another; what has been broken has been
cemented; the breaches of continuity have been filled up and healed;
and every where we see the operation of a cause that could unite as
well as separate. The twofold action of heat to expand and to melt,
could scarce be pointed out more clearly by any system of appearances.
58. As a long period was no doubt required for the elevation of the
strata, the rents made in them are not all of the same date, nor the
veins all of the same formation. This is clear in the case of one vein
producing a shift or slip in another; for the vein which forces the
other out of its place, and preserves its own direction, is evidently
the more recent of the two, and must have had its materials in a
state of activity, when those of the other were inert. Sometimes,
also, at the intersection of two veins, we may trace the current of
the materials of the one, across those of the other; and here, of
consequence, the relative antiquity is determined just as in the former
instance.
59. The want of any appearance of stratification in mineral veins has
already been taken notice of. There is, however, to be observed, in
many instances, a tendency to a regular arrangement of the substances
contained in them; those of the same kind forming coats parallel to the
sides of the vein, and nearly of an equal thickness. This phenomenon
is considered as one of the strongest arguments in favour of the
Neptunian system, but has nothing in it, in the least incompatible
with that theory which ascribes the formation of veins to the action
of subterraneous heat. When melted matter from the mineral regions was
thrown up into the veins, that which was nearest to the sides would
soonest lose its heat. The similar substances, also, would unite while
this process was going forward, and would crystallize, as in other
cases of congelation, from the sides toward the interior. There is the
more reason for supposing this to have been the case, that the same
sort of coating is often observed on the inside of close cavities,
which are, nevertheless, so constructed, as to afford a demonstration
that no chemical solvent was ever included in them, (§ 74.) Some
veins, it must also be considered, may have been filled by successive
injections of melted matter, and this would naturally give rise to a
variety of separate incrustations.[20]
[Footnote 20: See some farther remarks on this subject at NOTE XIII.]
60. In the view now given of metallic veins, they have been considered
as traversing only the stratified parts of the globe. They do,
however, occasionally intersect the unstratified parts, particularly
the granite, the same vein often continuing its course across rocks
of both kinds, without suffering any material change; and, if we have
hitherto paid no attention to this circumstance, it is because the
order pursued in this essay required, that the relation of the veins
to stratified bodies should be first treated of. Besides, the facts
in the natural history of veins, whether contained in stratified or
unstratified rocks, are so nearly alike, that in a general view of
geology, they do not require to be distinguished. It is material to
remark, that, though metallic veins are found indiscriminately in all
the different kinds of rock, whether stratified or otherwise, they are
most abundant in the class of primary schisti. All the countries most
remarkable for their mines, and the mountains distinguished by the
name of metalliferous, are primary, and the instance of Derbyshire is
perhaps the most considerable exception to this rule that is known.
This preference, which the metals appear to give to the primary strata,
is very consistent with Dr Hutton's theory, which represents the rocks
of that order as being most changed from their original position, and
those on which the disturbing forces of the subterraneous regions have
acted most frequently, and with greatest energy. The primary strata are
the lowest, also, and have the most direct communication with those
regions from which the mineral veins derive all their riches.
2. _Of Whinstone._
61. Beside the veins filled with spar, and containing the metallic
ores, the strata are intersected by veins of whinstone, porphyry, and
granite, the characters of which are next to be examined.
The term _whin_, or _whinstone_, with Dr. Hutton, like the word _trap_,
with the German mineralogists, denotes a class of stones, comprehending
several distinct species, or at least varieties. The common _basalt_,
the _wacken_, _mullen_, and _crag_ of Kirwan, the _grûnstein_ of
Werner, and the _amygdaloid_, are comprehended under the name of whin.
All these stones have a tendency to a spathose structure, and discover
at least the rudiments of crystallization. They are, at the same time,
without any mark of stratification in their internal texture, as they
are also, for the most part, in their outward configuration; and, as
the different species here enumerated compose, not unfrequently, parts
of the same continuous rock, the change from one to another being made
through a series of insensible gradations, they may safely be regarded
by the geologist as belonging to the same _genus_.
62. Whin, though not stratified, exists in two different ways, that is,
either in veins, (called in Scotland _dykes_,) traversing the strata
like the veins already described, or in irregular masses, incumbent
on the strata, and sometimes interposed between them. In both these
forms, whinstone has nearly the same characters, and bears, in all its
varieties, a most striking resemblance to the lavas which have actually
flowed from volcanoes on the surface of the earth. This resemblance
is so great, that the two substances have been often mistaken for one
another; and many rocks, which have been pronounced to be the remains
of extinguished volcanoes, by mineralogists of no inconsiderable
name, have been found, on closer examination, to be nothing else than
masses or veins of whinstone. This latter stone is indeed only to be
distinguished from the former, by a careful examination of the internal
characters of both; and chiefly from this circumstance, that whinstone
often contains calcareous spar and zeolite, whereas neither of these
substances is found in such lavas, as are certainly known to have been
thrown out by volcanic explosions.
Now, from these circumstances of affinity between lava and whinstone,
on the one hand, and of diversity on the other, as the formation of
the one is known, it should seem that some probable conclusion may be
drawn concerning the formation of the other. The affinity in question
is constant and essential; the difference variable and accidental;
and this naturally leads to suspect, that the two stones have the
same origin; and that, as lava is certainly a production of fire, so
probably is whinstone.
63. But, in order to see whether this hypothesis will explain the
diversity of the two substances, without which it will not be entitled
to much attention, we must remark, that the presence of carbonate of
lime in a body that has been fused, argues, agreeably to the principles
formerly explained, that the fusion was brought about under a great
compressing force, that is to say, deep in the bowels of the earth, or
in the great laboratory of the mineral regions. We are, therefore, to
suppose that the fusion of the whin was performed in those regions,
where the compression was sufficient to preserve the carbonic gas in
union with the calcareous earth, so that these two substances melted
together, and, on cooling, crystallized into spar. In the lavas, again,
thrown out by volcanic eruption, the fusion, as we know, wherever it
may begin, continues in the open air, where the pressure is only that
of the atmosphere: the calcareous earth, which, therefore, may have
been, in the form of a carbonate, among the materials of this lava,
must be converted into quicklime, and become infusible; hence the want
of calcareous spar in lavas that have flowed at the surface.
Thus, whinstone is to be accounted a subterraneous, or _unerupted_
lava; and our theory has the advantage of explaining both the affinity
and the difference between these stony bodies, without the introduction
of any new hypothesis. In the Neptunian system, the affinity of
whinstone and lava is a paradox which admits of no solution.
64. The columnar structure sometimes found in that species of whinstone
called basaltes, is a fact which has given rise to much discussion;
and it must be confessed, that though one of the most striking and
peculiar characters of this fossil, it is not that which gives the
clearest and most direct information concerning its origin. One
circumstance, however, very much in favour of the opinion that basaltic
rocks owe their formation to fire, is, that the columnar form is
sometimes assumed by the lava actually erupted from volcanoes. Now, it
is certainly of no small importance, to have the synthetic argument
on our side, and to know, that basaltic columns can be produced by
fire; though, no doubt, to give absolute certainty to our conclusion,
it would be necessary to show, that there are in nature no other
means but this by which these columns can be formed. This sort of
evidence is hardly to be looked for; but since the power of fusion,
to produce the phenomena in question, is perfectly established, and
since the production of the same phenomena in the humid way is a mere
hypothesis, if there be the least reason to suspect the action of
subterraneous heat as one of the causes of mineralization, every maxim
of sound philosophy requires that the basaltic structure, in all cases,
should be ascribed to it.
65. The Neptunists will no doubt allege, with BERGMAN, that, in the
drying of starch, clay, and a few other substances, something analogous
to basaltic columns is produced. Here, however, a most important
difference is to be remarked, corresponding very exactly to one of the
characters which we have all along observed to distinguish the products
of aqueous, from those of igneous consolidation. The columns formed
by the substances just mentioned, are distant from one another: they
are separated by fissures which widen from the bottom to the top, and
which arise from the shrinking and drying of the mass. In the basaltic
columns, no such openings, nor vacuity of any kind is found; the
pillars are in contact, and, though perfectly distinct, are so close,
that the sharp edge of a wedge can hardly be introduced between them.
This is a great peculiarity in the basaltic structure, and is strongly
expressive of this fact, that the mass was all fluid together, and
that its parts took their new arrangement, not in consequence of the
separation of a fluid from a solid part, by which great shrinking and
much empty space might be produced; but in consequence of a cause
which, like refrigeration, acted equally on all the parts of the mass,
and preserved their absolute contact after their fluidity had ceased.
66. A mark of fusion, or at least of the operation of heat, which
whinstone possesses in common with many other minerals, is its being
penetrated by pyrites, a substance, as has been already remarked, that
is of all others most exclusively the production of fire. Another
mark of fusion, more distinctive of whin, is, that both in veins and
in masses it sometimes includes pieces of sandstone, or of the other
contiguous strata, completely insulated, and having the appearance
of fragments of rock, floating in a fluid sufficiently dense and
ponderous to sustain their weight. Though these fragments have been
too refractory to be reduced into fusion themselves, they have not
remained entirely unchanged, but are, in general, extremely indurated,
in comparison of the rock from which they appear to have been detached.
67. Similar instances of extraordinary induration are observed in the
parts of the strata in contact with whinstone, whether they form the
sides of the veins, or the floors, and roofs of the masses into which
the whinstone is distributed. The strata whether sandy or argillaceous,
in such situations, are usually extremely hard and consolidated; the
former in particular lose their granulated texture, and are sometimes
converted into perfect jasper. This interesting remark was first made
by Dr Hutton, and the truth of it has been verified by a great number
of subsequent observations.
68. To the same excellent geologist we are indebted for the knowledge
of an analogous fact, attendant on the passage of whinstone veins
through coal strata. As the beds of stone where they are in contact
with veins of whin, seem to acquire additional induration, so those
of coal, in like circumstances, are frequently found to have lost
their fusibility, and to be reduced nearly to the condition of coke,
or of charcoal. The existence of coal of this kind has been already
mentioned, and considered as a proof of the operation of subterraneous
heat. In the instances here referred to, that is, where the charring
of the coal is limited to those parts of the strata which are in
contact with the whin, or in its immediate vicinity, the heat is
pointed out as residing in the vein; and this is to be accounted for
only on the supposition of the melted whin, at a period subsequent to
the consolidation of the coal, having flowed through the openings of
the strata. The heat has been powerful enough, in many cases, to drive
off the bituminous matter of the coal, and to force it into colder and
more distant parts. Few facts, in the history of fossils, are more
remarkable than this, and none more directly assimilates the operations
of the mineral regions, with those that take place at the surface of
the earth.
69. Again, the disturbance of the strata, wherever veins of whinstone
abound, if not a direct proof of the original fluidity of the
whinstone, is a clear indication of the violence with which it was
introduced into its place. This disturbance of the position of the
strata, by shifting, unusual elevation, and other irregularities, where
they are intersected by whinstone veins, is a fact so well known to
miners, that when they meet with any sudden change in the lying of the
_metals_, they are wont to foretell their approach to masses, or veins
of unstratified matter; and, in their figurative language, point them
out as the causes of the confusion with which they are so generally
accompanied.[21] The mineral veins likewise, as well as the strata, are
often heaved and shifted by the veins of whinstone.
[Footnote 21: A _Trouble_ is the name which the colliers in this
country give to a vein of whinstone.]
70. Whinstone of every species is found frequently interposed in
tabular masses, between beds of stratified rocks; and it then adds to
the indications of its igneous origin, already enumerated, some others
that are peculiar to it when in this situation. In such instances, it
is not uncommon to find the strata in some places, contiguous to the
whin, elevated, and bent with their concavity upward, so that they
appear clearly to have been acted on by a force that proceeded from
below, at the same time that they were softened, and rendered in some
degree flexible: it is needless to remark, that these effects can be
explained by nothing but the fusion of the whin; and that the great
force with which it was impelled against the strata, could be produced
by no cause but heat, acting in the manner that is here supposed.
71. Again, if it be true that the masses of whin, thus interposed
among the strata, were introduced there, after the formation of the
latter, we might expect to find, at least in many instances, that the
beds on which the whinstone rests, and those by which it is covered,
are exactly alike. If these beds were once contiguous, and have been
only heaved up and separated by the irruption of a fluid mass of
subterraneous lava, their identity should still be recognised. Now,
this is precisely what is observed; it is known to hold in a vast
number of instances, and is strikingly exemplified in the rock of
_Salisbury Crag_, near Edinburgh.
This similarity of the strata that cover the masses of whinstone,
to those that serve as the base on which they rest, and again the
dissimilitude of both to the interposed mass, are facts which I think
can hardly receive any explanation, on the principles of the Neptunian
theory. If these rocks, both stratified and unstratified, are to be
regarded as productions of the sea, the circumstances would require to
be pointed out, which have determined the whinstone, and the beds that
are all round it, to be so extremely unlike in their structure, though
formed at the same time, and in the immediate vicinity of one another;
as also those circumstances, on the other hand, which determined the
stratified deposits above and below the whinstone, to be precisely
the same, though the times of their formation must have been very
different. The homogeneous substances, thus, placed at a distance, and
the heterogeneous brought so closely together, are phenomena equally
unaccountable, in a theory that ascribes their origin to the operation
of the same element, and that necessarily dates their formation
according to the order in which they lie, one above another.
72. If, indeed, in these instances, the gradation were insensible, as
some have asserted it to be, between the strata and the interposed
mass, so that it was impossible to point out the line where the one
ended and the other began, whatever difficulties we might perceive in
the Neptunian theory, we should find it hard to substitute a better
in its room. But the truth seems to be, that, in the cases we are now
treating of, no such gradation exists; and that, though where the two
kinds of rock come into contact a change is often observed, by the
strata having acquired an additional degree of induration, yet the line
of separation is well defined, and can be precisely ascertained. This
at least is certain, that innumerable specimens, exhibiting such lines
of separation, are to be met with; and wherever care has been taken
to obtain a fresh fracture of the stone, and to remove the effects of
accidental causes, even where the two rocks are most firmly united,
and most closely assimilated, I am persuaded that no uncertainty has
ever remained as to the line of their separation. For these reasons,
it seems probable that the gradual transition of basaltes into the
adjoining strata, is in all cases imaginary, and is, in truth, a mere
illusion, proceeding from hasty and inaccurate observation.
73. Another remarkable fact in the natural history of the whinstone
rocks, remains yet to be mentioned, and with it I shall conclude the
argument, as far as these rocks are concerned.
Some of the species of whinstone are the common matrices of agates and
chalcedonies, which lie inclosed in them in the form of round nodules.
The original fluidity of these nodules is evinced by their figured, and
sometimes crystallized structure, and indeed is so generally admitted,
that the only question concerning them is, whether this fluidity
was the effect of heat or of solution. To answer this question, Dr
Hutton observes, that the formation of the concentric coats, of which
the agate is usually composed, has evidently proceeded from the
circumference toward the centre, the exterior coats always impressing
the interior, but never the reverse. The same thing also follows from
this other fact, that when there is any vacuity within the agate, it is
usually at the centre, and there too are found the regular crystals,
when any such have been formed. It therefore appears certain, that the
progress of consolidation has been from the circumference inwards, and
that the outward coats of the agate were the first to acquire solidity
and hardness.
74. Now, it must be considered that these coats are highly
consolidated; that they are of very pure siliceous matter, and are
utterly impervious to every substance which we know of, except light
and heat. It is plain, therefore, that whatever at any time, during
the progress of consolidation, was contained within the coats already
formed, must have remained there as long as the agate was entire,
without the least possibility of escape. But nothing is found within
the coats of the agate save its own substance; therefore, no extraneous
substance, that is to say no solvent, was ever included within them.
The fluidity of the agate was therefore simple, and unassisted by any
menstruum.
In this argument, nothing appears to me wanting, that is necessary to
the perfection of a physical, I had almost said of a mathematical,
demonstration. It seems, indeed, to be impossible that the igneous
origin of fossils could be recorded in plainer language, than by the
phenomenon which has just been described.
75. The examination of particular specimens of agates and chalcedonies,
affords many more arguments of the same kind, which Dr Hutton used to
deduce with an acuteness and vivacity, which his friends have often
listened to with great admiration and delight.[22] These, however,
must be passed over at present; and I have only further to remark,
that a series of the most interesting experiments, instituted by Sir
JAMES HALL, and published in the Transactions of the Royal Society of
Edinburgh,[23] has removed the only remaining objection that could
be urged against the igneous origin of whinstone. This objection is
founded on the common observation, that when a piece of whinstone or
basaltes is actually melted in a crucible, on cooling, it becomes
glass, and loses its original character entirely; and from thence it
was concluded, that this character had not been originally produced
by fusion. The experiments above mentioned, however, have shown, in
the most satisfactory manner, that melted whin, by _regulated_ or by
slow cooling, is prevented from assuming the appearance of glass, and
becomes a stony substance, hardly to be distinguished from whinstone or
lava.
[Footnote 22: NOTE XVI.]
[Footnote 23: vol. v. p. 43.]
The experiments of another ingenious chemist, Dr KENNEDY, have shown,
that whinstone contains mineral alkali, by which, of course, its fusion
must have been assisted.[24] Dr Hutton used to ascribe its fusibility,
in a great measure at least, to the quantity of iron contained in it:
both these causes have no doubt united to render it more easily melted
than the ordinary materials of the strata.
[Footnote 24: Trans. R. S. Edin. vol. v. p. 85.]
76. In a word, therefore, to conceive aright the origin of that class
of unstratified rocks, distinguished by the name of whinstone, we must
suppose, that long after the consolidation of the strata, and during
the time of their elevation, the materials of the former were melted
by the force of subterraneous heat, and injected among the rents and
fissures of the rocks already formed. In this manner were produced
the veins or dikes of whinstone; and, where circumstances allowed
the stream of melted matter to diffuse itself more widely, tabular
masses were formed, which were afterwards raised up, together with the
surrounding strata, above the level of the sea, and have been since
laid open by the operation of those causes that continually change and
waste the surface of the land.
These unstratified rocks are not, however, all the work of the same
period; they differ evidently in the date of their formation, and it is
not unusual, to find tabular masses of one species of whin, intersected
by veins of another species. Indeed, of all the fossil bodies which
compose the present land, the veins of whin appear to be the most
recently consolidated.[25]
[Footnote 25: NOTE XIV.]
Porphyry may so properly be regarded as a variety of whin,
distinguished only by involving crystallized feldspar, that, in a
geological sketch like the present, it is hardly entitled to a separate
article. Like the other kinds of whin, it exists both in veins and
in tabular masses, having, no doubt, an origin similar to that which
has just been described. Porphyry, however, has the peculiarity of
being rarely found in any but the primary strata; it seems to be the
whinstone of the old world, or at least that which is of the highest
antiquity in the present. It no where, I believe, assumes a columnar,
or basaltic appearance, of any regularity; but this is also true of
many other varieties of whin, of all, indeed, except the most compact
and homogeneous. These differences are not so considerable as to
require our entering into any particular detail concerning the natural
history of this fossil.
3. _Granite_.
77. The term Granite is used by Dr Hutton to signify an aggregate
stone, in which quartz, feldspar, and mica are found distinct from
one another, and not disposed in layers. The addition of hornblende,
schorl, or garnet, to the three ingredients just mentioned, is not
understood to alter the _genus_ of the stone, but only to constitute a
specific difference, which it is the business of lithology to mark by
some appropriate character, annexed to the generic name of granite.
The fossil now defined exists, like whinstone and porphyry, both in
masses and in veins, though most frequently in the former. It is like
them unstratified in its texture, and is regarded here, as being also
unstratified in its outward structure.[26] One ingredient which is
essential to granite, namely, quartz, is not contained in whinstone;
and this circumstance serves to distinguish these _genera_ from one
another, though, in other respects, they seem to be united by a chain
of insensible gradations, from the most homogeneous basaltes, to
granite the most highly crystallized.
[Footnote 26: Those rocks that consist of the ingredients here
enumerated, if they have at the same time a schistose texture, or a
disposition into layers, are properly distinguished from granite, and
called Gneiss, or Granitic Schistus. But it has been questioned whether
a stone does not exist composed of these ingredients, and destitute
of a schistose texture, but yet divided into large beds, visible in
its external form. Dr Hutton supposes such a stone not to exist, or at
least not to constitute any such proportion of the mineral kingdom, as
to entitle it to particular consideration, in the general speculations
of geology.
Whether this supposition is perfectly correct, may require to be
farther considered: this, however, is certain, that a rock, in all
respects conformable to it, composes a great proportion of what are
usually called the granite mountains. See NOTE XV.]
78. Granite, it has been just said, exists most commonly in masses; and
these masses are rarely, if ever, incumbent on any other rock: they are
the basis on which others rest, and seem, for the most part, to rise
up from under the ancient, or primary strata. The granite, therefore,
wherever it is found, is inferior to every other rock; and as it also
composes many of the greatest mountains, it has the peculiarity of
being elevated the highest into the atmosphere, and sunk the deepest
under the surface, of all the mineral substances with which we are
acquainted.
Notwithstanding the circumstance of not being alternated with
stratified bodies, which constitutes a remarkable difference between
granite and whinstone, the affinity of these fossils is such as to make
the similarity of their origin by no means improbable. Accordingly,
in Dr Hutton's theory, granite is regarded as a stone of more recent
formation than the strata incumbent on it; as a substance which has
been melted by heat, and which, when forced up from the mineral
regions, has elevated the strata at the same time.
79. That granite has undergone a change from a fluid to a solid
state, is evinced from the crystallized structure in which some of
its component parts are usually found. This crystallization is
particularly to be remarked of the feldspar, and also of the schorl,
where there is any admixture of that substance, whether in slender
spiculæ, or in larger masses. The quartz itself is in some cases
crystallized, and is so, perhaps, more frequently than is generally
supposed. The fluidity of granite, in some former period of its
existence, is so evident from this, as to make it appear singular that
it should ever have been considered as a fossil that had remained
always the same, and one, into the origin of which it was needless to
inquire. If the regular forms of crystallization are not to be received
as proofs of the substance to which they belong having passed from
a fluid to a solid state, neither are the figures of shells and of
other supposed petrifactions, to be taken as indications of a passage
from the animal to the mineral kingdom; so that there is an end of
all geological theories, and of all reasonings concerning the ancient
condition of the globe. To an argument which strikes equally at the
root of all theories, it belongs not to this, in particular, to make
any reply.
80. We shall, therefore, consider it as admitted, that the materials
of the granite were originally fluid; and, in addition to this, we
think it can easily be proved, that this fluidity was not that of the
elements taken separately, but of the entire mass. This last conclusion
follows, from the structure of those specimens, where one of the
substances is impressed by the forms which are peculiar to another.
Thus, in the Portsoy granite,[27] which Dr Hutton has so minutely
described, the quartz is impressed by the rhomboidal crystals of the
feldspar, and the stone thus formed is compact and highly consolidated.
Hence, this granite is not a congeries of parts, which, after being
separately formed, were somehow brought together and agglutinated; but
it is certain that the quartz, at least, was fluid when it was moulded
on the feldspar. In other granites, the impressions of the substances
on one another are observed in a different order, and the quartz gives
its form to the feldspar. This, however, is more unusual; the quartz
is commonly the substance which has received the impressions of all
the rest; and the spiculæ of schorl often shoot both across it and the
feldspar.
[Footnote 27: Theory of the Earth, vol. i. p. 104.]
The ingredients of granite were therefore fluid when mixed, or at least
when in contact with one another. Now, this fluidity was not the effect
of solution in a menstruum; for, in that case, one kind of crystal
ought not to impress another, but each of them should have its own
peculiar shape.
81. The perfect consolidation of many granites, furnishes an argument
to the same effect. For, agreeably to what was already observed, in
treating of the strata, a substance, when crystallizing, or passing
from a fluid to a solid state, cannot be free from porosity, much
less fill up completely a space of a given form, if, at the same
time, any solvent is separated from it; because the solvent so
separated would still occupy a certain space, and, when removed by
evaporation or otherwise, would leave that space empty. The perfect
adjustment, therefore, of the shape of one set of crystallizing bodies,
to the shape of another set, as in the Portsoy granite, and their
consolidation into one mass, is as strong a proof as could be desired,
that they crystallized from a state of simple fluidity, such as, of all
known causes, heat alone is able to produce.
82. This conclusion, however, does not rest on a single class of
facts. It has been observed in many instances, that where granite and
stratified rocks, such as primary schistus, are in contact, the latter
are penetrated by veins of the former, which traverse them in various
directions. These veins are of different dimensions, some being of the
breadth of several yards, others of a few inches, or even tenths of an
inch; they diminish as they recede from the main body of the granite,
to which they are always firmly united, constituting, indeed, a part of
the same continued rock.
These phenomena, which were first distinctly observed by Dr Hutton, are
of great importance in geology, and afford a clear solution of the two
chief questions concerning the relation between granite and schistus.
As every vein must be of a date posterior to the body in which it is
contained, it follows, that the schistus was not super-imposed on
the granite, after the formation of this last. If it be argued, that
these veins, though posterior to the schisti, are also posterior to
the granite, and were formed by the infiltration of water in which the
granite was dissolved or suspended; it may be replied, _1mo_, That the
power of water to dissolve granite, is a postulatum of the same kind
that we have so often, and for such good reason, refused to concede;
and, _2do_, That in many instances the veins proceed from the main
body of the granite _upwards_ into the schistus; so that they are in
planes much elevated in respect of the horizon, and have a direction
quite opposite to that which the hypothesis of infiltration requires.
It remains certain, therefore, that the whole mass of granite, and the
veins proceeding from it, are coeval, and both of later formation than
the strata.
Now, this being established, and the fluidity of the veins, when they
penetrated into the schistus, being obvious, it necessarily follows,
that the whole granite mass was also fluid at the same time. But this
can have been brought about only by subterraneous heat, which also
impelled the melted matter against the superincumbent strata, with such
force as to raise them from their place, and to give them that highly
inclined position in which they are still supported by the granite,
after its fluidity has ceased. Thus a conclusion, rendered probable by
the crystallization of granite, is established beyond all contradiction
by the phenomena of granitic veins.[28]
[Footnote 28: NOTE XV.]
83. With the granite, we shall consider the proof of the igneous origin
of all mineral substances as completed. These substances, therefore,
whether stratified or unstratified, owe their consolidation to the same
cause, though acting with different degrees of energy. The stratified
have been in general only softened or penetrated by melted matter,
whereas the unstratified have been reduced into perfect fusion.
84. In this general conclusion we may distinguish two parts, which, in
their degree of certainty, differ perhaps somewhat from one another.
The first of these, and that which stands highest in point of evidence,
consists of two propositions; namely, that the fluidity which preceded
the consolidation of mineral substances was SIMPLE, that is, it did not
arise from the combination of these substances with any solvent; and,
next, that after consolidation, these bodies have been raised up by an
expansive force acting from below, and have by that means been brought
into their present situation. These two propositions seem to me to be
supported by all the evidence that is necessary to constitute the most
perfect demonstration.
85. The other part of the general conclusion, that fire, or more
properly heat, was the cause of the fluidity of these mineral bodies,
and also of their subsequent elevation, is not perhaps to be considered
as a truth so fully demonstrated as the two preceding propositions;
it is, no doubt, a matter of THEORY; or a portion of one of those
invisible chains by which men seek to connect in the mind the state
of nature that is present, with the states of it that are past; and
participates of that uncertainty from which our reasonings concerning
such causes as are not direct objects of perception, are hardly ever
exempted. That it participates of this uncertainty in a very slight
degree, will, however, be admitted, when it is considered that the
cause assigned has been proved sufficient for the effect; that the same
is not true of any other known cause; and that this theory accounts,
with singular simplicity and precision, for a system of facts so
various and complex, as that which is presented by the natural history
of the globe.
86. Neither can it be said that the existence of subterraneous heat is
a principle assumed without any evidence, but that of the geological
facts which it is intended to explain: on the contrary, it is proved by
phenomena within the circle of ordinary experience, namely, those of
hot springs, volcanoes, and earthquakes. These leave no doubt of the
existence of heat, and of a moving and expansive power, in the bowels
of the earth; so that the only questions are, at what depth is this
power lodged? to what extent, and with what intensity, does it act?
That it is lodged at a very considerable depth, is rendered probable by
the permanency of some of the preceding phenomena: from the earliest
times many fountains have retained their heat to the present day; and
volcanoes, though they become extinguished at length, have a very
long period allotted for their duration. The cause of earthquakes is
certainly a force that resides very deep under the surface, otherwise
the extent of the concussion could not be such as has been observed in
many instances.
87. The intensity of volcanic fire, is another circumstance that
favours the opinion of its being seated deep under the surface. That
this intensity is considerable, is certain from the experiments made
by Sir James Hall on the fusibility of whinstone and lava; from which
it appears, that the lowest temperature in which either of these
stones melt, is about 80° of Wedgewood's pyrometer. Some mineralogists
have indeed affirmed, that lava is melted, not by the intensity of
the heat applied to it, but in consequence of a certain combination
formed between it and bituminous substances, in a manner which they do
not attempt to explain, and which has indeed no analogy to any thing
that is known. That a hypothesis, formed in such direct opposition to
the most obvious principles of inductive reasoning, should have been
imagined by a philosopher who had examined the phenomena of Etna and
Vesuvius with much attention, and described them with great accuracy
and truth, is more wonderful than that it should have been adopted by
mineralogists, whose views of nature may have been confined within a
cabinet or a laboratory. It is, however, a hypothesis, which, having
never had any support but from other hypotheses, hardly merited the
direct refutation that it has received from the experiments just
mentioned.
88. But, if the intensity of volcanic heat be such as is here
stated, it will be found very difficult to account for a fire of
such activity, and of such long continuance in the same spot, by any
decomposition of mineral substances near the surface. In the place
where this combustion is supposed to exist, it must be remembered,
that there is no fresh supply of materials to replace those that have
been consumed, and that, therefore, the original accumulation of these
materials in one spot, must have been very unlike any thing that has
ever been observed concerning the disposition of minerals in the bowels
of the earth.
89. If, on the other hand, we ascribe the phenomena of volcanoes to
the central heat, the account that may be given of them is simple, and
consistent with itself. According to all the appearances from which the
existence of such heat has been inferred above, it is of a nature so
far different from ordinary fire, that it may require no circulation
of air, and no supply of combustible materials to support it. It is
not accompanied with inflammation or combustion, the great pressure
preventing any separation of parts in the substances on which it acts,
and the absence of that elastic fluid without which heat seems to have
no power to decompose bodies, even the most combustible, contributing
to the unalterable nature of all the substances in the mineral regions.
There, of consequence, the only effects of heat are fusion and
expansion; and that which forms the nucleus of the globe may therefore
be a fluid mass, melted, but unchanged by the action of heat.
90. If, from the confines of this nucleus, we conceive certain fissures
and openings to traverse the solid crust, and to issue at the surface
of the earth, the vapours ascending through these may in time heat the
sides of the tubes through which they pass to a vast distance from the
lower extremities. It is, indeed, difficult to fix the limit to which
this distance may extend, on account of the great difference between
the rate at which heat moves when it has a fluid for its vehicle, and
when it is left to make its way alone through a solid body. In the
present case, the supply of heat is rapid, as being made by a vapour
ascending through a tube of solid rock; and the dissipation of it slow,
as arising from its transmission through the rock. The waste of heat is
therefore small, compared with the supply, and grows smaller at every
given point, the longer the stream of heated vapour has continued to
flow. Such a stream, therefore, though it may at first be condensed
within a small distance of its source, will in time reach higher
and higher, and may at last be able to carry its heat to an immense
distance from the place of its original derivation. Thus, it is easy
to conceive, that vapours from the mineral regions may convey their
heat to reservoirs of water near the surface of the earth, and may in
that manner produce hot springs, and even boiling fountains, like those
of Rycum and Geyser.
91. When, instead of a heated vapour, melted matter is thrown up
through the _shafts_ or _tubes_, which thus communicate with the
mineral regions, veins of whinstone and basaltes are formed in the
interior of the earth. When the melted matter reaches to the surface,
it is thrown out in the form of lava, and all the other phenomena of
volcanoes are produced.
Lastly, where melted matter of this kind, or vapours without being
condensed, have their progress obstructed, those dreadful concussions
are produced, which seem to threaten the existence even of the earth
itself. Though terrible, therefore, to the present inhabitants of the
globe, the earthquake has its place in the great system of geological
operations, and is part of a series of events, essential, as will more
clearly appear hereafter, to the general order, and to the preservation
of the whole.
Such, according to this theory, are the changes which have befallen
mineral substances in the bowels of the earth; and though different
for the stratified and unstratified parts of those substances, they
are connected together by the same _principle_, or explained by the
same _cause_. It remains to consider that part of the history of both
which describes their changes after their elevation to the surface;
and here we shall find new causes introduced, which are more directly
the subjects of observation, than those hitherto treated of; causes,
also, which act on all fossils alike, and alike prepare them for their
ultimate destination.
SECTION III.
OF THE PHENOMENA COMMON TO STRATIFIED
AND UNSTRATIFIED BODIES.
92. THE series of changes which fossil bodies are destined to undergo,
does not cease with their elevation above the level of the sea; it
assumes, however, a new direction, and from the moment that they are
raised up to the surface, is constantly exerted in reducing them again
under the dominion of the ocean. The solidity is now destroyed which
was acquired in the bowels of the earth; and as the bottom of the
sea is the great laboratory, where loose materials are mineralized
and formed into stone, the atmosphere is the region where stones are
decomposed, and again resolved into earth.
This decomposition of all mineral substances, exposed to the air,
is continual, and is brought about by a multitude of agents, both
chemical and mechanical, of which some are known to us, and many, no
doubt, remain to be discovered. Among the various aëriform fluids which
compose our atmosphere, one is already distinguished as the grand
principle of mineral decomposition; the others are not inactive, and
to them we must add moisture, heat, and perhaps light; substances
which, from their affinities to the elements of mineral bodies, have a
power of entering into combination with them, and of thus diminishing
the forces by which they are united to one another. By the action of
air and moisture, the metallic particles, particularly the iron, which
enters in great abundance into the composition of almost all fossils,
becomes oxydated in such a degree as to lose its tenacity; so that the
texture of the surface is destroyed, and a part of the body resolved
into earth.
93. Some earths, again, such as the calcareous, are immediately
dissolved by water; and though the quantity so dissolved be extremely
small, the operation, by being continually renewed, produces a slow
but perpetual corrosion, by which the greatest rocks must in time be
subdued. The action of water in destroying hard bodies into which it
has obtained entrance, is much assisted by the vicissitudes of heat
and cold, especially when the latter extends as far as the point of
congelation; for the water, when frozen, occupies a greater space than
before, and if the body is compact enough to refuse room for this
expansion, its parts are torn asunder by a repulsive force acting in
every direction.
94. Besides these causes of mineral decomposition, the action of which
we can in some measure trace, there are others known to us only by
their effects.
We see, for instance, the purest rock crystal affected by exposure
to the weather, its lustre tarnished, and the polish of its surface
impaired, but we know nothing of the power by which these operations
are performed. Thus also, in the precautions which the mineralogist
takes to preserve the fresh fracture of his specimens, we have a proof
how indiscriminately all the productions of the fossil kingdom are
exposed to the attacks of their unknown enemies, and we perceive how
difficult it is to delay the beginnings of a process which no power
whatever can finally counteract.
95. The mechanical forces employed in the disintegration of mineral
substances, are more easily marked than the chemical. Here again water
appears as the most active enemy of hard and solid bodies; and, in
every state, from transparent vapour to solid ice, from the smallest
rill to the greatest river, it attacks whatever has emerged above the
level of the sea, and labours incessantly to restore it to the deep.
The parts loosened and disengaged by the chemical agents, are carried
down by the rains, and, in their descent, rub and grind the superficies
of other bodies. Thus water, though incapable of acting on hard
substances by direct attrition, is the cause of their being so acted
on; and, when it descends in torrents, carrying with it sand, gravel,
and fragments of rock, it may be truly said to turn the forces of the
mineral kingdom against itself. Every separation which it makes is
necessarily permanent, and the parts once detached can never be united,
save at the bottom of the ocean.
96. But it would far exceed the limits of this sketch, to pursue
the causes of mineral decomposition through all their forms. It is
sufficient to remark, that the consequence of so many minute, but
indefatigable agents, all working together, and having _gravity_ in
their favour, is a system of universal decay and degradation, which
may be traced over the whole surface of the land, from the mountain
top to the sea shore. That we may perceive the full evidence of this
truth, one of the most important in the natural history of the globe,
we will begin our survey from the latter of these stations, and retire
gradually toward the former.
97. If the coast is bold and rocky, it speaks a language easy to
be interpreted. Its broken and abrupt contour, the deep gulfs and
salient promontories by which it is indented, and the proportion which
these irregularities bear to the force of the waves, combined with
the inequality of hardness in the rocks, prove, that the present
line of the shore has been determined by the action of the sea. The
naked and precipitous cliffs which overhang the deep, the rocks
hollowed, perforated, as they are farther advanced in the sea, and at
last insulated, lead to the same conclusion, and mark very clearly
so many different stages of decay. It is true, we do not see the
successive steps of this progress exemplified in the states of the same
individual rock, but we see them clearly in different individuals;
and the conviction thus produced, when the phenomena are sufficiently
multiplied and varied, is as irresistible, as if we saw the changes
actually effected in the moment of observation.
On such shores, the fragments of rock once detached, become instruments
of further destruction, and make a part of the powerful artillery with
which the ocean assails the bulwarks of the land: they are impelled
against the rocks, from which they break off other fragments, and the
whole are thus ground against one another; whatever be their hardness,
they are reduced to gravel, the smooth surface and round figure of
which, are the most certain proofs of a _detritus_ which nothing can
resist.
98. Again, where the sea coast is flat, we have abundant evidence of
the degradation of the land in the beaches of sand and small gravel;
the sand banks and shoals that are continually changing; the alluvial
land at the mouths of the rivers; the bars that seem to oppose their
discharge into the sea, and the shallowness of the sea itself. On
such coasts, the land usually seems to gain upon the sea, whereas,
on shores of a bolder aspect, it is the sea that generally appears
to gain upon the land. What the land acquires in extent, however, it
loses in elevation; and, whether its surface increase or diminish, the
depredations made on it are in both cases evinced with equal certainty.
99. If we proceed in our survey from the shores, inland, we meet
at every step with the fullest evidence of the same truths, and
particularly in the nature and economy of rivers. Every river appears
to consist of a main trunk, fed from a variety of branches, each
running in a valley proportioned to its size, and all of them together
forming a system of vallies, communicating with one another, and having
such a nice adjustment of their declivities, that none of them join the
principal valley, either on too high or too low a level; a circumstance
which would be infinitely improbable, if each of these vallies were not
the work of the stream that flows in it.
If indeed a river consisted of a single stream, without branches,
running in a straight valley, it might be supposed that some great
concussion, or some powerful torrent, had opened at once the channel
by which its waters are conducted to the ocean; but, when the usual
form of a river is considered, the trunk divided into many branches,
which rise at a great distance from one another, and these again
subdivided into an infinity of smaller ramifications, it becomes
strongly impressed upon the mind, that all these channels have been cut
by the waters themselves; that they have been slowly dug out by the
washing and erosion of the land; and that it is by the repeated touches
of the same instrument, that this curious assemblage of lines has been
engraved so deeply on the surface of the globe.
100. The changes which have taken place in the courses of rivers, are
also to be traced, in many instances, by successive platforms, of flat
alluvial land, rising one above another, and marking the different
levels on which the river has run at different periods of time. Of
these, the number to be distinguished, in some instances, is not less
than four, or even five; and this necessarily carries us back, like
all the operations we are now treating of, to an antiquity extremely
remote: for, if it be considered, that each change which the river
makes in its bed, obliterates at least a part of the monuments of
former changes, we shall be convinced, that only a small part of the
progression can leave any distinct memorial behind it, and that there
is no reason to think, that, in the part which we see, the beginning is
included.[29]
[Footnote 29: NOTE XVI.]
101. In the same manner, when a river undermines its banks, it often
discovers deposits of sand and gravel, that have been made when it ran
on a higher level than it does at present. In other instances, the same
strata are seen on both the banks, though the bed of the river is now
sunk deep between them, and perhaps holds as winding a course through
the solid rock, as if it flowed along the surface; a proof that it must
have begun to sink its bed, when it ran through such loose materials as
opposed but a very inconsiderable resistance to its stream. A river, of
which the course is both serpentine and deeply excavated in the rock,
is among the phenomena, by which the slow waste of the land, and also
the cause of that waste, are most directly pointed out.
102. It is, however, where rivers issue through narrow defiles among
mountains, that the identity of the strata on both sides is most easily
recognised, and remarked at the same time with the greatest wonder. On
observing the Potowmack, where it penetrates the ridge of the Allegany
mountains, or the Irtish, as it issues from the defiles of Altai,
there is no man, however little addicted to geological speculations,
who does not immediately acknowledge, that the mountain was once
continued quite across the space in which the river now flows; and, if
he ventures to reason concerning the cause of so wonderful a change,
he ascribes it to some great convulsion of nature, which has torn the
mountain asunder, and opened a passage for the waters. It is only the
philosopher, who has deeply meditated on the effects which action long
continued is able to produce, and on the simplicity of the means which
nature employs in all her operations, who sees in this nothing but the
gradual working of a stream, that once flowed over the top of the ridge
which it now so deeply intersects, and has cut its course through the
rock, in the same way, and almost with the same instrument, by which
the lapidary divides a block of marble or granite.
103. It is highly interesting to trace up, in this manner, the action
of causes with which we are familiar, to the production of effects,
which at first seem to require the introduction of unknown and
extraordinary powers; and it is no less interesting to observe, how
skilfully nature has balanced the action of all the minute causes of
waste, and rendered them conducive to the general good. Of this we
have a most remarkable instance, in the provision made for preserving
the soil, or the coat of vegetable mould, spread out over the surface
of the earth. This coat, as it consists of loose materials, is easily
washed away by the rains, and is continually carried down by the rivers
into the sea. This effect is visible to every one; the earth is removed
not only in the form of sand and gravel, but its finer particles
suspended in the waters, tinge those of some rivers continually, and
those of all occasionally, that is, when they are flooded or swollen
with rains. The quantity of earth thus carried down, varies according
to circumstances; it has been computed, in some instances, that the
water of a river in a flood, contains earthy matter suspended in it,
amounting to more than the two hundred and fiftieth part of its own
bulk.[30] The soil, therefore, is continually diminished, its parts
being transported from higher to lower levels, and finally delivered
into the sea. But it is a fact, that the soil, notwithstanding, remains
the same in quantity, or at least nearly the same, and must have done
so, ever since the earth was the receptacle of animal or vegetable
life. The soil, therefore, is augmented from other causes, just as
much, at an average, as it is diminished by that now mentioned; and
this augmentation evidently can proceed from nothing but the constant
and slow disintegration of the rocks. In the permanence, therefore,
of a coat of vegetable mould on the surface of the earth, we have a
demonstrative proof of the continual destruction of the rocks; and
cannot but admire the skill, with which the powers of the many chemical
and mechanical agents employed in this complicated work, are so
adjusted, as to make the supply and the waste of the soil exactly equal
to one another.
[Footnote 30: See Lehman, Traités de Phys. &c. tom. iii. p. 359. Note.]
104. Before we take leave of the rivers and the plains, we must remark
another fact, often observed in the natural history of the latter, and
clearly evincing the former existence of immense bodies of strata, in
situations from which they have now entirely disappeared. The fact here
alluded to is, the great quantity of round and hard gravel, often to be
met with in the soil, under such circumstances, as prove, that it can
only have come from the decomposition of rocks, that once occupied the
very ground over which this gravel is now spread. In the chalk country,
for instance, about London, the quantity of flints in the soil is every
where great; and, in particular situations, nothing but flinty gravel
is found to a considerable depth. Now, the source from which these
flints are derived is quite evident, for they are precisely the same
with those contained in the chalk beds, wherever these last are found
undisturbed, and from the destruction of such beds they have no doubt
originated. Hence a great thickness of chalk must have been decomposed,
to yield the quantity of flints now in the soil of these countries; for
the flints are but thinly scattered through the native chalk, compared
with their abundance in the loose earth. To afford, for example, such
a body of flinty gravel as is found about Kensington, what an enormous
quantity of chalk rock must have been destroyed?
105. This argument, which Dr Hutton has applied particularly to the
chalk countries, may be extended to many others. The great plain of
Crau, near the mouth of the Rhone, is well known, and was regarded
with wonder, even in ages when the natural history of the globe was
not an object of much attention. The immense quantity of large round
gravel-stones, with which this extensive plain is entirely covered,
has been supposed, by some mineralogists, to have been brought down
by the Durance, and other torrents, from the Alps; but, on further
examination, has been found to be of the same kind that is contained
in certain horizontal layers of pudding-stone, which are the basis of
the whole plain. It cannot be doubted, therefore, that the vast body of
gravel spread over it, has originated from the destruction of layers
of the same rock, which may perhaps have risen to a great height above
what is now the surface. Indeed, from knowing the depth of the gravel
that covers the plain, and the average quantity of the like gravel
contained in a given thickness of rock, one might estimate how much of
the latter has been actually worn away. Whether data precise enough
could be found, to give any weight to such a computation, must be left
fer future inquiry to determine.[31]
[Footnote 31: NOTE XVII.]
106. In these instances, chalk and pudding-stone, by containing in
them parts infinitely less destructible than their general mass, have,
after they are worn away, left behind them very unequivocal marks of
their existence. The same has happened in the case of mineral veins,
where the substances least subject to dissolution have remained,
and are scattered at a great distance from their native place. Thus
gold, the least liable to decomposition of all the metals, is very
generally diffused through the earth, and is found, in a greater or
less abundance, in the sand of almost all rivers. But the native place
of this mineral is the solid rock, or the veins and cavities contained
in the rock, and from thence it must have made its way into the soil.
This, therefore, is another proof of the vast extent to which the
degradation of the land, and of the rock, which is the basis of it,
has been earned; and consequently, of the great difference between
the elevation and shape of the earth's surface in the present, and in
former ages.
107. The veins of tin furnish an argument of the same kind. The
ores of this metal are very indestructible, and little subject to
decomposition, so that they remain very long in the ground without
change. Where there are tin veins, as in Cornwall, the tin-stone or tin
ore is found in great abundance in such vallies and streams as have
the same direction with the veins; and hence the _streaming_, as it is
called, or washing of the earth, to obtain the tin-stone from it. Now,
if it be considered, that none of this ore can have come into the soil
but from parts of a vein actually destroyed, it must appear evident
that a great waste of these veins has taken place, and consequently of
the schistus or granite in which they are contained.
108. These lessons, which the geologist is taught in flat and open
countries, become more striking, by the study of those Alpine tracts,
where the surface of the earth attains its greatest elevation. If we
suppose him placed for the first time in the midst of such a scene,
as soon as he has recovered from the impression made by the novelty
and magnificence of the spectacle before him, he begins to discover
the footsteps of time, and to perceive, that the works of nature,
usually deemed the most permanent, are those on which the characters
of vicissitude are most deeply imprinted. He sees himself in the midst
of a vast ruin, where the precipices which rise on all sides with
such boldness and asperity, the sharp peaks of the granite mountains,
and the huge fragments that surround their bases, do but mark so many
epochs in the progress of decay, and point out the energy of those
destructive causes, which even the magnitude and solidity of such great
bodies have been unable to resist.
109. The result of a more minute investigation, is in perfect unison
with this general impression. Whence is it, that the elevation
of mountains is so obviously connected with the hardness and
indestructibility of the rocks which compose them? Why is it, that a
lofty mountain of soft and secondary rock is no where to be found; and
that such chains, as the Pyrenees or the Alps, never consist of any
but the hardest stone, of granite for instance, or of those primary
strata, which, if we are to credit the preceding theory, have been
twice heated in the fires, and twice tempered in the waters, of the
mineral regions? Is it not plain that this arises, not from any direct
connection between the hardness of stones, and their height in the
atmosphere, but from this, that the waste and _detritus_ to which all
things are subject, will not allow soft and weak substances to remain
long in an exposed and elevated situation? Were it not for this, the
secondary rocks, being in position superincumbent on the primary, ought
to be the highest of the two, and should cover the primary, (as they
no doubt have at one time done,) in the highest as well as the lowest
situations, or among the mountains as well as in the plains.
110. Again, wherefore is it, that among all mountains, remarkable for
their ruggedness and asperity, the rock, on examination, is always
found of very unequal destructibility, some parts yielding to the
weather, and to the other causes of disintegration, much more slowly
than the rest, and having strength sufficient to support themselves,
when left alone, in slender pyramids, bold projections, and overhanging
cliffs? Where, on the other hand, the rock wastes uniformly, the
mountains are similar to one another; their swells and slopes are
gentle, and they are bounded by a waving and continuous surface. The
intermediate degrees of resistance which the rocks oppose to the causes
of destruction, produce intermediate forms. It is this which gives to
the mountains, of every different species of rock, a different habit
and expression, and which, in particular, has imparted to those of
granite that venerable and majestic character, by which they rarely
fail to be distinguished.
111. The structure of the vallies among mountains, shows clearly to
what cause their existence is to be ascribed. Here we have first a
large valley, communicating directly with the plain, and winding
between high ridges of mountains, while the river in the bottom of it
descends over a surface, remarkable, in such a scene, for its uniform
declivity. Into this, open a multitude of transverse or secondary
vallies, intersecting the ridges on either side of the former, each
bringing a contribution to the main stream, proportioned to its
magnitude; and, except where a cataract now and then intervenes,
all having that nice adjustment in their levels, (99.) which is the
more wonderful, the greater the irregularity of the surface. These
secondary vallies have others of a smaller size opening into them;
and, among mountains of the first order, where all is laid out on the
greatest scale, these ramifications are continued to a fourth, and
even a fifth, each diminishing in size as it increases in elevation,
and as its supply of water is less. Through them all, this law is in
general observed, that where a higher valley joins a lower one, of the
two angles which it makes with the latter, that which is obtuse is
always on the descending side; a law that is the same with that which
regulates the confluence of streams running on a surface nearly of
uniform inclination. This alone is a proof that the vallies are the
work of the streams; and indeed what else but the water itself, working
its way through obstacles of unequal resistance, could have opened or
kept up a communication between the inequalities of an irregular and
alpine surface?
112. Many more arguments, all leading to the same conclusion, may
be deduced from the general facts, known in the natural history of
mountains; and, if the Oreologist would trace back the progress of
waste, till he come in sight of that original structure, of which
the remains are still so vast, he perceives an immense mass of solid
rock, naked and unshapely, as it first emerged from the deep, and
incomparably greater than all that is now before him. The operation
of rains and torrents, modified by the hardness and tenacity of the
rock, has worked the whole into its present form; has hollowed out
the vallies, and gradually detached the mountains from the general
mass, cutting down their sides into steep precipices at one place,
and smoothing them into gentle declivities at another. From this has
resulted a transportation of materials, which, both for the quantity
of the whole, and the magnitude of the individual fragments, must seem
incredible to every one, who has not learned to calculate the effects
of continued action, and to reflect, that length of time can convert
accidental into steady causes. Hence fragments of rock, from the
central chain, are found to have travelled into distant vallies, even
where many inferior ridges intervene: hence the granite of Mont Blanc
is seen in the plains of Lombardy, or on the sides of Jura; and the
ruins of the Carpathian mountains lie scattered over the shores of the
Baltic.[32]
[Footnote 32: NOTE XVIII.]
113. Thus, with Dr Hutton, we shall be disposed to consider those great
chains of mountains, which traverse the surface of the globe, as cut
out of masses vastly greater, and more lofty than any thing that now
remains. The present appearances afford no data for calculating the
original magnitude of these masses, or the height to which they may
have been elevated. The nearest estimate we can form is, where a chain
or group of mountains, like those of Rosa in the Alps, is horizontally
stratified, and where, of consequence, the undisturbed position of the
mineral beds enables us to refer the whole of the present inequalities
of the surface to the operation of waste or decay. These mountains,
as they now stand, may not inaptly be compared to the pillars of earth
which workmen leave behind them, to afford a measure of the whole
quantity of earth which they have removed. As the pillars, (considering
the mountains as such,) are in this case of less height than they
originally were, so the measure furnished by them is but a limit, which
the quantity sought must necessarily exceed.
114. Such, according to Dr Hutton's theory, are the changes which the
daily operations of waste have produced on the surface of the globe.
These operations, inconsiderable if taken separately, become great, by
conspiring all to the same end, never counteracting one another, but
proceeding, through a period of indefinite extent, continually in the
same direction. Thus every thing descends, nothing returns upward; the
hard and solid bodies every where dissolve, and the loose and soft no
where consolidate. The powers which tend to preserve, and those which
tend to change the condition of the earth's surface, are never _in
equilibrio_; the latter are, in all cases, the most powerful, and, in
respect of the former, are like living in comparison of dead forces.
Hence the law of decay is one which suffers no exception: The elements
of all bodies were once loose and unconnected, and to the same state
nature has appointed that they should all return.
115. It affords no presumption against the reality of this progress,
that, in respect of man, it is too slow to be immediately perceived:
The utmost portion of it to which our experience can extend, is
evanescent, in comparison with the whole, and must be regarded as the
momentary increment of a vast progression, circumscribed by no other
limits than the duration of the world. TIME performs the office of
integrating the infinitesimal parts of which this progression is made
up; it collects them into one sum, and produces from them an amount
greater than any that can be assigned.
116. While on the surface of the earth so much is every where going to
decay, no new production of mineral substances is found in any region
accessible to man. The instances of what are called petrifactions, or
the formation of stony substances by means of water, which we sometimes
observe, whether they be ferruginous concretions, or calcareous, or,
as happens in some rare cases, siliceous stalactites, are too few
in number, and too inconsiderable in extent, to be deemed material
exceptions to this general rule. The bodies thus generated, also, are
no sooner formed, than they become subject to waste and dissolution,
like all the other hard substances in nature; so that they but retard
for a while the progress by which they are all resolved into dust, and
sooner or later committed to the bosom of the deep.
117. We are not, however, to imagine, that there is no where any means
of repairing this waste; for, on comparing the conclusion at which
we are now arrived, viz. that the present continents are all going
to decay, and their materials descending into the ocean, with the
proposition first laid down, that these same continents are composed
of materials which must have been collected from the decay of former
rocks, it is impossible not to recognise two corresponding steps of
the same progress; of a progress, by which mineral substances are
subjected to the same series of changes, and alternately wasted away
and renovated. In the same manner, as the present mineral substances
derive their origin from substances similar to themselves; so, from the
land now going to decay, the sand and gravel forming on the sea shore,
or in the beds of rivers; from the shells and corals, which in such
enormous quantities are every day accumulated in the bosom of the sea;
from the drift wood, and the multitude of vegetable and animal remains
continually deposited in the ocean: from all these we cannot doubt,
that strata are now forming in those regions, to which nature seems
to have confined the powers of mineral reproduction; from which, after
being consolidated, they are again destined to emerge, and to exhibit a
series of changes similar to the past.[33]
[Footnote 33: NOTE XIX.]
118. How often these vicissitudes of decay and renovation have been
repeated, is not for us to determine: they constitute a series, of
which, as the author of this theory has remarked, we neither see the
beginning nor the end; a circumstance that accords well with what
is known concerning other parts of the economy of the world. In the
continuation of the different species of animals and vegetables that
inhabit the earth, we discern neither a beginning nor an end; and,
in the planetary motions, where geometry has carried the eye so far
both into the future and the past, we discover no mark, either of
the commencement or the termination of the present order.[34] It is
unreasonable, indeed, to suppose, that such marks should any where
exist. The Author of nature has not given laws to the universe, which,
like the institutions of men, carry in themselves the elements of their
own destruction. He has not permitted, in his works, any symptom of
infancy or of old age, or any sign by which we may estimate either
their fixture or their past duration. He may put an end, as be no doubt
gave a beginning, to the present system, at some determinate period;
but we may safely conclude, that this great _catastrophe_ will not
be brought about by any of the laws now existing, and that it is not
indicated by any thing which we perceive.
[Footnote 34: NOTE XX.]
119. To assert, therefore, that, in the economy of the world, we see
no mark, either of a beginning or an end, is very different from
affirming, that the world had no beginning, and will have no end. The
first is a conclusion justified by common sense, as well as sound
philosophy; while the second is a presumptuous and unwarrantable
assertion, for which no reason from experience or analogy can ever be
assigned. Dr Hutton might, therefore, justly complain of the uncandid
criticism, which, by substituting the one of these assertions for the
other, endeavoured to load his theory with the reproach of atheism and
impiety. Mr KIRWAN, in bringing forward this harsh and ill-founded
censure, was neither animated by the spirit, nor guided by the maxims
of true philosophy. By the spirit of philosophy, he must have been
induced to reflect, that such poisoned weapons as he was preparing
to use, are hardly ever allowable in scientific contest, as having
a less direct tendency to overthrow the system, than to hurt the
person of an adversary, and to wound, perhaps incurably, his mind, his
reputation, or his peace. By the maxims of philosophy, he must have
been reminded, that, in no part of the history of nature, has any mark
been discovered, either of the beginning or the end of the present
_order_; and that the geologist sadly mistakes, both the object of his
science and the limits of his understanding, who thinks it his business
to explain the means employed by INFINITE WISDOM for establishing the
laws which now govern the world.
By attending to these obvious considerations, Mr Kirwan would have
avoided a very illiberal and ungenerous proceeding; and, however he
might have differed from Dr Hutton as to the _truth_ of his opinions,
he would not have censured their _tendency_ with such rash and
unjustifiable severity.
But, if this author may be blamed for wanting the temper, or neglecting
the rules, of philosophic investigation, he is hardly less culpable,
for having so slightly considered the scope and spirit of a work which
he condemned so freely. In that work, instead of finding the world
represented as the result of necessity or chance, which might be looked
for, if the accusations of atheism or impiety were well founded, we
see every where the utmost attention to discover, and the utmost
disposition to admire, the instances of wise and beneficent design
manifested in the structure, or economy of the world. The enlarged
views of these, which his geological system afforded, appeared to Dr
Hutton himself as its most valuable result. They were the parts of it
which he contemplated with greatest delight; and he would have been
less flattered, by being told of the ingenuity and originality of his
theory, than of the addition which it had made to our knowledge of
_final causes_. It was natural, therefore, that he should be hurt by
an attempt to accuse him of opinions, so different from those which he
had always taught; and if he answered Mr Kirwan's attack with warmth or
asperity, we must ascribe it to the indignation excited by unmerited
reproach.
120. But to return to the natural history of the earth: Though there be
in it no _data_, from which the commencement of the present order can
be ascertained, there are many by which the existence of that order may
be traced back to an antiquity extremely remote. The beds of primitive
schistus, for instance, contain sand, gravel, and other materials,
collected, as already shown, from the dissolution of mineral bodies;
which bodies, therefore, must have existed long before the oldest part
of the present land was formed. Again, in this gravel we sometimes find
pieces of sandstone, and of other compound rocks, by which we are of
course carried back a step farther, so as to reach a system of things,
from which the present is the third in succession; and this may be
considered as the most ancient epocha, of which any memorial exists in
the records of the fossil kingdom.
121. Next in the order of time to the consolidation of the primary
strata, we must place their elevation, when, from being horizontal, and
at the bottom of the sea, they were broken, set on edge, and raised to
the surface. It is even probable, as formerly observed, that to this
succeeded a depression of the same strata, and a second elevation,
so that they have twice visited the superior, and twice the inferior
regions. During the second immersion, were formed, first, the great
bodies of pudding-stone, that in so many instances lie immediately
above them; and next were deposited the strata that are strictly
denominated secondary.
122. The third great event, was the raising up of this compound body of
old and new strata from the bottom of the sea, and forming it into the
dry land, or the continents, as they now exist.[35] Contemporary with
this, we must suppose the injection of melted matter among the strata,
and the consequent formation of the crystallized and unstratified
rocks, namely, the granite, metallic veins, and veins of porphyry and
whinstone. This, however, is to be considered as embracing a period
of great duration; and it must always be recollected, that veins are
found of very different formation; so that when we speak generally, it
is perhaps impossible to state any thing more precise concerning their
antiquity, than that they are posterior to the strata, and that the
veins of whinstone seem to be the most recent of all, as they traverse
every other.
[Footnote 35: NOTE XXI.]
123. In the fourth place, with respect to time, we must class the
facts that regard the detritus and waste of the land, and must
carefully distinguish them from the more ancient phenomena of the
mineral kingdom. Here we are to reckon the shaping of all the present
inequalities of the surface; the formation of hills of gravel, and
of what have been called tertiary strata, consisting of loose and
unconsolidated materials; also collections of shells not mineralised,
like those in Turaine; such petrifactions as those contained in the
rock of Gibraltar, on the coast of Dalmatia, and in the caves of
Bayreuth. The bones of land animals found in the soil, such as those
of Siberia, or North America, are probably more recent than any of the
former.[36]
[Footnote 36: NOTE XXII.]
124. These phenomena, then, are all so many marks of the lapse of
time, among which the principles of geology enable us to distinguish
a certain order, so that we know some of them to be more, and others
to be less distant, but without being able to ascertain, with any
exactness, the proportion of the immense intervals which separate them.
These intervals admit of no comparison with the astronomical measures
of time; they cannot be expressed by the revolutions of the sun or of
the moon; nor is there any synchronism between the most recent epoch as
of the mineral kingdom, and the most ancient of our ordinary chronology.
125. On what is now said is grounded another objection to Dr Hutton's
theory, namely, that the high antiquity ascribed by it to the earth,
is inconsistent with that system of chronology which rests on the
authority of the Sacred Writings. This objection would no doubt be of
weight, if the high antiquity in question were not restricted merely to
the globe of the earth, but were also extended to the human race. That
the origin of mankind does not go back beyond six or seven thousand
years, is a position so involved in the narrative of the Mosaic books,
that any thing inconsistent with it, would no doubt stand in opposition
to the testimony of those ancient records. On this subject, however,
geology is silent; and the history of arts and sciences, when traced
as high as any authentic monuments extend, refers the beginnings of
civilization to a date not very different from that which has just been
mentioned, and infinitely within the limits of the most recent of the
epoch as, marked by the physical revolutions of the globe.
On the other hand, the authority of the Sacred Books seems to be but
little interested in what regards the mere antiquity of the earth
itself; nor does it appear that their language is to be understood
literally concerning the _age_ of that body, any more than concerning
its _figure_ or its _motion_. The theory of Dr Hutton stands here
precisely on the same footing with the system of COPERNICUS; for
there is no reason to suppose, that it was the purpose of revelation
to furnish a standard of geological, any more than of astronomical
science. It is admitted, on all hands, that the Scriptures are not
intended to resolve physical questions, or to explain matters in no
way related to the morality of human actions; and if, in consequence
of this principle, a considerable latitude of interpretation were not
allowed, we should continue at this moment to believe, that the earth
is flat; that the sun moves round the earth; and that the circumference
of a circle is no more than three times its diameter.
It is but reasonable, therefore, that we should extend to the
geologist the same liberty of speculation, which the astronomer and
mathematician are already in possession of; and this may be done,
by supposing that the chronology of MOSES relates only to the human
race. This liberty is not more necessary to Dr Hutton than to other
theorists. No ingenuity has been able to reconcile the natural history
of the globe with the opinion of its recent origin; and accordingly
the cosmologies of Kirwan and De Luc, though contrived with more
mineralogical skill, are not less forced and unsatisfactory than those
of Burnet and Whiston.
126. It is impossible to look back on the system which we have thus
endeavoured to illustrate, without being struck with the novelty and
beauty of the views which it sets before us. The very plan and scope
of it distinguish it from all other theories of the earth, and point
it out as a work of great and original invention. The sole object of
such theories has hitherto been, to explain the manner in which the
present laws of the mineral kingdom were first established, or began to
exist, without treating of the manner in which they now proceed, and by
which their continuance is provided for. The authors of these theories
have accordingly gone back to a state of things altogether unlike the
present, and have confined their reasonings, or their fictions, to a
crisis which never has existed but once, and which never can return.
Dr Hutton, on the other hand, has guided his investigation by the
philosophical maxim, _Causam naturalem et assiduam quærimus, non raram
et fortuitam_. His theory, accordingly, presents us with a system of
wise and provident economy, where the same instruments are continually
employed, and where the decay and renovation of fossils being carried
on at the same time in the different regions allotted to them, preserve
in the earth the conditions essential for the support of animal and
vegetable life. We have been long accustomed to admire that beautiful
contrivance in nature, by which the water of the ocean, drawn up in
vapour by the atmosphere, imparts, in its descent, fertility to the
earth, and becomes the great cause of vegetation and of life; but now
we find, that this vapour not only fertilizes, but creates the soil;
prepares it from the solid rock, and, after employing it in the great
operations of the surface, carries it back into the regions where all
its mineral characters are renewed. Thus, the circulation of moisture
through the air, is a prime mover, not only in the annual succession
of the seasons, but in the great geological cycle, by which the waste
and reproduction of entire continents is circumscribed. Perhaps a more
striking view than this, of the wisdom that presides over nature, was
never presented by any philosophical system, nor a greater addition
ever made to our knowledge of final causes. It is an addition which
gives consistency to the rest, by proving, that equal foresight is
exerted in providing for the whole and for the parts, and that no
less a care is taken to maintain the constitution of the earth, than
to preserve the tribes of animals and vegetables which dwell on its
surface. In a word, it is the peculiar excellence of this theory, that
it ascribes to the phenomena of geology an order similar to that which
exists in the provinces of nature with which we are best acquainted;
that it produces seas and continents, not by accident, but by the
operation of regular and uniform causes; that it makes the decay of one
part subservient to the restoration of another, and gives stability to
the whole, not by perpetuating individuals, but by reproducing them in
succession.
127. Again, in the detail of this theory, and the ample induction on
which it is founded, we meet with many facts and observations, either
entirely new, or hitherto very imperfectly understood. Thus, the veins
which proceed from masses of granite, and penetrate the incumbent
schistus, had either escaped the observation of former mineralogists,
or the importance of the phenomenon had been entirely overlooked. Dr
Hutton has described the appearances with great accuracy, and drawn
from them the most interesting conclusions. At the junction of the
primary and secondary strata, the facts which he has noted had been
observed by others; but no one I think had so fully understood the
language which they speak, or had so clearly perceived the consequences
that necessarily follow from them. He is the first who distinctly
pointed out the characters which distinguish whinstone from lava, and
who explained the true relation that subsists between these substances.
He also discovered the induration of the strata, in contact with veins
of whin, and the charring of the coal in their vicinity. His theory
also enabled him to determine the affinity of whinstone and granite
to one another, and their relation to the other great bodies of the
mineral kingdom.
To the observations of the same excellent geologist, we are indebted
for the knowledge of the general and important fact, that all the hard
substances of the mineral kingdom, when elevated into the atmosphere,
have a tendency to decay, and are subject to a disintegration
and waste, to which no limit can be set but that of their entire
destruction; that no provision is made on the surface for repairing
this waste, and that there, no new fossil is produced; that the
formation of all the varied scenery which the surface of the earth
exhibits, depends on the operation of causes, the momentary exertions
of which are familiar to us, though we knew not before the effects
which their accumulated action was able to produce. These are facts in
the natural history of the earth, the discovery of which is due to Dr
Hutton; and, should we lay all further speculation aside, and consider
the theory of the earth as a work too great to be attempted by man, we
must still regard the phenomena and laws just mentioned, as forming a
solid and valuable addition to our knowledge.
128. If we would compare this theory with others, as to the invisible
agents which it employs, we must consider, that fire and water are the
two powers which all of them must make use of, so that they can differ
from one another only by the way in which they combine these powers.
In Dr Hutton's system, water is first employed to deposit and arrange,
and then fire to consolidate, mineralize, and lastly, to elevate
the strata; but with respect to the unstratified or crystallized
substances, the action of fire only is recognised. The system having
least affinity to this is the Neptunian, which ascribes the formation
of all minerals to the action of water alone, and extends this
hypothesis even to the unstratified rocks. Here, therefore, the action
of fire is entirely excluded; and the Neptunists have certainly made a
great sacrifice to the love of truth, or of paradox, in rejecting the
assistance of so powerful an auxiliary.[37]
[Footnote 37: NOTE XXIII.]
129. In the systems which employ the agency of the latter element, we
are to look for a greater resemblance to that of Dr Hutton, though
many and great marks of distinction are easily perceived. In the
cosmologies, for example, of LEIBNITZ and BUFFON, fire and water are
both employed, as well as in this; but they are employed in a reverse
order. These philosophers introduce the action of fire first, and
then the action of water, which is to invert the order of nature
altogether, as the consolidation of the rocks must be posterior to
their stratification. Indeed, the theory of Buffon is singularly
defective: besides inverting the order of the two great operations
of stratification and consolidation, and of course giving no real
explanation of the latter, it gives no account of the elevation, or
highly inclined position of the strata; it makes no distinction between
stratified and unstratified bodies, nor does it offer any but the most
unsatisfactory explanation of the inequalities of the earth's surface.
This system, therefore, has but a very distant resemblance to the
Huttonian theory.[38]
[Footnote 38: NOTE XXIV.]
130. The system of LAZZARO MORO has been remarked as approaching nearer
to this theory than any other; and it is certain, that one very
important principle is common to them both. The theory of the Italian
geologist was chiefly directed to the explanation of the remains of
marine animals, which are found in mountains far from the sea; and it
appears to have been suggested to him by the phenomena of the _Campi
Phlegræi_, and by the production of the new island of _Santorini_
in the Archipelago. He accordingly supposes, that the islands and
continents have been all raised up, like the above-mentioned island,
from the bottom of the sea, by the force of volcanic fire: that
these fires began to burn under the bottom of the ocean, soon after
the creation of the world, when as yet the ocean covered the whole
earth: that they at first elevated a portion of the land; and in this
primitive land no shells are found, as the original ocean was destitute
of fish. The volcanoes continuing to burn, under the sea, after the
creation of animated nature, the strata that were then raised up by
their action were full of shells and other marine objects; and, from
the violence with which they were elevated, arose the contortions and
inclined position which they frequently possess.[39]
[Footnote 39: Dé Crostacei, et degli altri Marini Corpi, che si trovano
su' Monti: di Ant. Lazzaro Moro. Venezia. 1740.]
This system is imperfect, as it makes no peculiar provision for the
consolidation of the strata, which, according to it, as well as the
Neptunian system, must be ascribed to the action, not of fire, but of
water. No account is given of the mineralization of the shells found
in the strata, or of the difference between them and the shells found
loose at the bottom of the sea; and no distinction is made between
stratified and unstratified substances. But, with all this, Lazzaro
Moro has certainly the merit of having perceived, that some other power
than that which deposited the strata, must have been employed for their
elevation, and that they have endured the action of a disturbing force.
131. From this comparison it appears, that Dr Hutton's theory is
sufficiently distinct, even from the theories which approach to it most
nearly, to merit, in the strictest sense, the appellation of _new_ and
_original_. There are indeed few inventions or discoveries, recorded
in the history of science, to which nearer approaches were not made
before they were fully unfolded. It therefore very well deserves to be
distinguished by a particular name; and, if it behoves us to follow
the analogy observed in the names of the two great systems, which at
present divide the opinions of geologists, we may join Mr Kirwan in
calling this the PLUTONIC SYSTEM. For my own part, I would rather
have it characterized by a less splendid, but juster name, that of the
HUTTONIAN THEORY.
132. The circumstance, however, which gives to this theory its
peculiar character, and exalts it infinitely above all others, is
the introduction of the principle of pressure, to modify the effects
of heat when applied at the bottom of the sea. This is in fact the
key to the grand enigma of the mineral kingdom, where, while one
set of phenomena indicates the action of fire, another set, equally
remarkable, seems to exclude the possibility of that action, by
presenting us with mineral substances, in such a state as they could
never have been brought into by the operation of the fires we see at
the surface of the earth. These two classes of phenomena are reconciled
together, by admitting the power of compression to confine the volatile
parts of bodies when heat is applied to them, and to force them,
in many instances, to undergo fusion, instead of being calcined or
dissipated by burning or inflammation. In this hypothesis, which some
affect to consider as a principle gratuitously assumed, there appears
to me nothing but a very fair and legitimate generalization of the
properties of heat. Combustion and inflammation are chemical processes,
to which other conditions are required, besides the presence of a high
temperature. The state of the mineral regions makes it reasonable
to presume, that these conditions are wanting in the bowels of the
earth, where, of consequence, we have a right to look for nothing but
expansion and fusion, the only operations which seem essential to
heat, and inseparable from the application of it, in certain degrees,
to certain substances. Though this principle, therefore, had no
countenance from analogy, the admirable simplicity, and the unity,
which it introduces into the phenomena of geology, would sufficiently
justify the application of it to the theory of the earth.
As another excellence of this theory, I may, perhaps, be allowed to
remark, that it extends its consequences beyond those to which the
author of it has himself adverted, and that it affords, which no
geological theory has yet done, a satisfactory explanation of the
spheroidal figure of the earth.[40]
[Footnote 40: NOTE XXV.]
133. Yet, with all these circumstances of originality, grandeur,
and simplicity in its favour, with the addition of evidence as
demonstrative as the nature of the subject will admit, this theory
has probably many obstacles to overcome, before it meet the general
approbation. The greatness of the objects which it sets before us,
alarms the imagination; the powers which it supposes to be lodged
in the subterraneous regions; a heat which has subdued the most
refractory rocks, and has melted beds of marble and quartz; an
expansive force, which has folded up, or broken the strata, and raised
whole continents from the bottom of the sea; these are things with
which, however certainly they may be proved, the mind cannot soon
be familiarized. The change and movement also, which this theory
ascribes to all that the senses declare to be most unalterable, raise
up against it the same prejudices which formerly opposed the belief
in the true system of the world; and it affords a curious proof, how
little such prejudices are subject to vary, that as Aristarchus, an
ancient follower of that system, was charged with impiety for moving
the everlasting Vesta from her place, so Dr Hutton, nearly on the same
ground, has been subjected to the very same accusation. Even the length
of time which this theory regards as necessary to the revolutions of
the globe, is looked on as belonging to the marvellous; and man, who
finds himself constrained by the want of time, or of space, in almost
all his undertakings, forgets, that in these, if in any thing, the
riches of nature reject all limitation.[41]
[Footnote 41: NOTE XXVI.]
The evidence which must be opposed to all these causes of incredulity,
cannot be fully understood without much study and attention. It
requires not only a careful examination of particular instances, but
comprehensive views of the whole phenomena of geology; the comparison
of things very remote with one another; the interpretation of the
_obscure_ by the _luminous_, and of the _doubtful_ by the _decisive_
appearances. The geologist must not content himself with examining
the insulated specimens of his cabinet, or with pursuing the nice
subtleties of mineralogical arrangement; he must study the relations of
fossils, as they actually exist; he must follow nature into her wildest
and most inaccessible abodes; and must select, for the places of his
observations, those points, from which the variety and gradation of her
works can be most extensively and accurately explored. Without such an
exact and comprehensive survey, his mind will hardly be prepared to
relish the true theory of the earth. "_Naturæ enim vis atque majestas
omnibus momentis fide caret, si quis modo partes atque non totam
complectatur animo_".[42]
[Footnote 42: PLIN. Hist. Nat. lib. vii. Cap. i.]
134. If indeed this theory of the earth is as well founded as we
suppose it to be, the lapse of time must necessarily remove all
objections to it, and the progress of science will only develope its
evidence more fully. As it stands at present, though true, it must be
still imperfect; and it cannot be doubted, that the great principles
of it, though established on an immoveable basis, must yet undergo
many modifications, requiring to be limited, in one place, or to be
extended, in another. A work of such variety and extent cannot be
carried to perfection by the efforts of an individual. Ages may be
required to fill up the bold outline which Dr Hutton has traced with so
masterly a hand; to detach the parts more completely from the general
mass; to adjust the size and position of the subordinate members; and
to give to the whole piece the exact proportion and true colouring of
nature.
This, however, in length of time, may be expected from the advancement
of science, and from the mutual assistance which parts of knowledge,
seemingly the most remote, often afford to one another. Not only
may the observations of the mineralogist, in tracts yet unexplored,
complete the enumeration of geological facts; and the experiments of
the chemist, on substances not yet subjected to his analysis, afford a
more intimate acquaintance with the nature of fossils, and a measure
of the power of those chemical agents to which this theory ascribes
such vast effects: but also, from other sciences, less directly
connected with the natural history of the earth, much information may
be received. The accurate geographical maps and surveys which are now
making; the foundings; the obsevations of currents; the barometrical
measurements, may all combine to ascertain the reality, and to fix the
quantity of those changes which terrestrial bodies continually undergo.
Every new improvement in science affords the means of delineating more
accurately the face of nature as it now exists, and of transmitting, to
future ages, an account, which may be compared with the face of nature
as it shall then exist. If, therefore, the science of the present times
is destined to survive the physical revolutions of the globe, the
HUTTONIAN THEORY may be confirmed by historical record; and the author
of it will be remembered among the illustrious few, whose systems
have been verified by the observations of succeeding ages, supported
by facts unknown to themselves, and established by the decisions of a
tribunal, slow, but infallible, in distinguishing between truth and
falsehood.
====================================
NOTES AND ADDITIONS.
====================================
NOTES AND ADDITIONS.
NOTE I. § 2.
_Origin of calcareous rocks._
135. IT has been asserted, that Dr Hutton went farther than is stated
at § 2, and maintained all calcareous matter to be _originally_ of
animal formation. This position, however, is so far from being laid
down by Dr Hutton, that it belongs to an inquiry which he carefully
avoided to enter on, as being altogether beyond the limits of
philosophical investigation.
He has indeed no where treated of the _first origin_ of any of the
earths, or of any substance whatsoever, but only of the transformations
which bodies have undergone since the present laws of nature were
established. He considered this last as all that a science, built on
experiment and observation, can possibly extend to; and willingly left,
to more presumptuous inquirers, the task of carrying their reasonings
beyond the boundaries of nature, and of unfolding the properties of
the chaotic fluid, with as much minuteness of detail, as if they were
describing the circumstances of a chemical process which they had
actually witnessed.
The idea of calcareous matter which really belongs to the Huttonian
Theory, is, that in all the changes which the terraqueous globe has
undergone in past ages, this matter existed, as it does now, either
in the form of limestone and marble, or in the composition of other
stones, or in the state of corals, shells, and bones of animals. It may
be true, that there is no particle of calcareous matter, at present
existing on the surface of the earth, that has not, at some time, made
a part of an animal body; but of this we can have no certainty, nor is
it of any importance that we should. It is enough to know, that the
rocks of marble and limestone contain in general marks of having been
formed from materials collected at the bottom of the sea; and of this
a single cockle-shell, or piece of coral, found included in a rock, is
a sufficient proof with respect to the whole mass of which it makes a
part.
The principal object which Dr Hutton had in view when he spoke of the
masses of marble and limestone, as composed of the calcareous matter
of marine bodies,[43] was to prove, that they had been all formed
at the bottom of the sea, and from materials there deposited. His
general conclusion is, "That all the strata of the earth, not only
those consisting of such calcareous masses, but others superincumbent
upon these, have had their origin at the bottom of the sea, by the
collection of sand and gravel, of shells, of coralline and crustaceous
bodies, and of earths and clays variously mixed, or separated and
accumulated. This is a general conclusion, well authenticated by the
appearances of nature, and highly important in the natural history of
the earth."[44]
[Footnote 43: Theory of the Earth, vol. i. p. 23, 24.]
[Footnote 44: Theory of the Earth, vol. i. p. 26.]
136. In his Geological Essays, Mr Kirwan says, that "some geologists,
as Buffon, and of late Dr Hutton, have excluded calcareous earth from
the number of the primeval, asserting the masses of it we at present
behold to proceed from shell-fish. But, in addition to the unfounded
supposition, that shell-fish, or any animals, possess the power of
producing any simple earth, these philosophers should have considered,
that, before the existence of any fish, the stony masses that inclose
the bason of the sea, must have existed; and, among these, there is
none in which calcareous earth is not found. Dr Hutton endeavours to
_evade_ this argument, by supposing the world we now inhabit to have
arisen from the ruins and fragments of an anterior, without pointing
at any original. If we are thus to proceed _in infinitum_, I shall not
pretend to follow him; but, if he stops any where, he will find the
same argument equally to occur."[45]
[Footnote 45: Geol. Essays, p. 13.]
The argument here employed would certainly be conclusive against any
one, who, in disputing about the _first origin_ of things, should deny
that the calcareous is as ancient as any other of the simple earths.
But this has nothing to do with Dr Hutton's speculations, which, as has
been just said, never extended to the _first origin_ of substances,
but were confined entirely to their changes; so that what he asserts
concerning the calcareous rocks, is no more than that those which we
now see have been formed from loose materials, deposited at the bottom
of the sea. It was not therefore in order to _evade_ Mr Kirwan's
argument, as the preceding passage would lead us to believe, that he
supposed the world which we now inhabit to have arisen from the ruin
and waste of an anterior world; but it was because this seemed to him
a conclusion which necessarily followed from the phenomena of geology,
and it was a conclusion that he had deduced long before he heard of Mr
Kirwan's objections to his system. Instead of an _evasion_, therefore,
any one who considers the subject fairly, will see, in Dr Hutton's
reasoning, nothing but the caution of a philosopher, who wisely
confines his theory within the same limits by which nature has confined
his experience and observation.
It is nevertheless true, that Dr Hutton has sometimes expressed himself
as if he thought that the present calcareous rocks are all composed of
animal remains.[46] This conclusion, however, is more general than the
facts warrant; and, from some incorrectness or ambiguity of language,
is certainly more general than he intended. The idea of calcareous
rocks, on which he argues throughout his whole theory, is precisely
that which is stated in the preceding article.
[Footnote 46: Theory of the Earth, vol. i. p. 23.]
NOTE II. § 6.
_Origin of coal._
137. The vegetable origin of coal seems to be sufficiently proved by
the reasoning in § 5. and 6.; and that reasoning will appear still
more satisfactory, from what is said at § 28. and 29., concerning the
consolidation of this fossil. Dr Hutton has treated both of the matter
of coal and of its consolidation. Part. I. Chap. 8., of his Theory of
the Earth [47].
[Footnote 47: Vol. i. p. 558, &c.]
The notion, however, that coal is of vegetable origin, is not peculiar
to this theory, but has been for some time the prevailing opinion.
Buffon supposes this mineral to be formed from vegetable and animal
substances, the oil and fat of which have been converted into bitumen
by the action of acids.[48] A fundamental mistake, however, is
committed by this author, and by M. GENSANNE, (author of the natural
history of Languedoc,) on whose observations he greatly relies, in
considering coal as consisting of bitumen united to earth, thus
omitting the only ingredient essential to coal, namely the carbon or
charcoal. This may truly be considered as the essential part, because
coal may exist without bitumen, as in the instance of blind coal, but
not without charcoal.
[Footnote 48: Hist. Nat. des Mineraux. tom. i. p. 429, 4to edit.]
Another theory of coal, very analogous to Dr Hutton's, is that of
ARDUINO, professor of mineralogy at Venice, in which he supposes it
formed from vegetable and animal remains from the land and sea, but
chiefly from the latter.[49] This theory of coal is contained in Dr
Hutton's, in which the animal and vegetable remains must be supposed
to come both from the earth and the sea. It seems to be without any
good reason that Arduino considers the sea as the chief source of these
materials. His remarks, however, are very ingenious, and deserving of
attention.
[Footnote 49: Saggio Fisico-mineralogico del Sig. Giov. Arduino; Atti
di Siena, tom. v. p. 228, 281, &c.]
These accounts of the origin of coal are all nearly the same; it is
in what relates to the distinction between the common coal, in which
there is no ligneous structure, and those varieties of it in which that
structure is apparent, and again in explaining the consolidation of
both, that the theory laid down here is peculiar.
138. Some other mineralogists refer one of the ingredients of coal
to the vegetable kingdom, but not the other. Unable to resist the
conviction which arises from the fibrous structure of parts of strata,
and even entire strata of coal, they have supposed, that wood, which
had been somehow buried in the earth, or perhaps deposited at the
bottom of the sea, had become impregnated with bitumen, which last,
however, they consider as of mineral origin. This appears to be the
opinion of Lehman; and also of some very late writers. There seems,
however, to be hardly less reason for referring the origin of one part
of coal to the vegetable or animal kingdom than another. The two last
are certainly capable of furnishing both the carbonic and bituminous
parts; and therefore, to derive these from different sources, is at
least a very unnecessary complication of hypotheses.
139. Another explanation of coal, very different from any of the
preceding, has lately been advanced and set up in opposition to the
Huttonian Theory. Mr Kirwan,[50] the only mineralogist, I believe, who
has attempted to derive both the carbonic and bituminous matter of coal
from the mineral kingdom, distinguishes between wood coal and mineral
coal, and gives a theory entirely new of the formation of the latter.
Wood coal is that in which the ligneous structure is so apparent, as to
leave no doubt of its vegetable origin; mineral coal is that in which
no such structure can be discovered, and is the same which Dr Hutton
derives from the vegetable juices, and other remains, comminuted,
dispersed, carried into the sea, and there precipitated, so as to
unite with different proportions of earth, and to become afterwards
mineralized.
[Footnote 50: Geol. Essays, essay vii. p. 290.]
These two species of coal, which the Huttonian theory considers as
gradations of the same substance, Mr Kirwan regards as perfectly
distinct, constituting two minerals, of an origin and formation
entirely different. He therefore endeavours to ascertain the
distinguishing characters of each, considered geologically.
140. But here the leading distinction, implied in all the rest, that
the two kinds of coal are never found in the same bed, but always in
different situations, and with different laws of stratification, is
expressly contradicted by matter of fact. Coal, as is said above, with
its ligneous texture quite apparent, and coal with no such structure
visible, are often found in the same seam, are brought up from the same
mine, and united in the same specimen. I have a specimen from a bed of
coal, in the Isle of Sky, found under a basaltic rock, consisting of a
ligneous part, which graduates into one in which there is no vestige
of a fibrous texture, and in which the surface is smooth and glossy,
with a fracture almost vitreous. The upper part of the specimen is
therefore perfect wood coal, and the under part perfect mineral-coal,
in the language of Mr Kirwan; at the same time that the transition from
the one to the other is made by insensible degrees. This specimen,
were it perfectly solitary, is sufficient to prove the identity
of the two species of coal we are now speaking of, and to show,
that the difference between them is accidental, not essential. The
specimen, however, is far from being solitary; the number of similar
appearances is so great, as hardly to have escaped the observation
of any mineralogist. Mr Kirwan admits, that wood coal is often found
under basaltes;[51] but what is essential to be remarked is, that, in
this instance, we have both the wood coal and the common mineral-coal,
lying under that rock, and the one passing gradually into the other It
appears, indeed, that many of the facts which Mr Kirwan produces, in
treating of what he calls _carboniferous_ soils, are quite inconsistent
with the distinction he would make between wood-coal and mineral
coal.[52]
[Footnote 51: Geol. Essays, p. 310.]
[Footnote 52: _Ibid._ p. 311.]
141. It is, however, true, that there are instances in which the wood
coal, or fossil wood, as it is usually called, forms entire beds, quite
unconnected with the ordinary coal, and stratified in some respects
differently. Such is the Bovey coal in Devonshire, the wood-coal in the
north of Ireland, and perhaps the Surturbrandt of Iceland. With respect
to the Bovey coal, it does by no means answer to one of Mr Kirwan's
remarks, viz. that late observations have ascertained, that no such
parallelism of the beds, as in mineral coal, nor even any distinct
number of strata is found. In the Bovey coal, the number of strata is
very well defined, by beds of clay regularly interposed; but as to the
extent of these beds, the coal having been worked only at one place,
and by an open pit, without any extensive subterraneous excavation,
nothing is known with certainty.
In the Bovey coal too, I must observe, though its beds have the
ligneous structure very distinct, the clay interposed between these
beds, which is but little indurated, contains a great deal of coaly
matter, in the form of thin flakes, interspersed through it. So far as
I know, there are no mineral reins nor shifts, nor any bed of indurated
stone, that accompany this coal; so that, though one can not doubt of
its vegetable origin, some doubt may be entertained concerning the
nature of the mineralizing operations, to which it has been subjected
The consideration of these, however, does not belong to the present
argument; and the peculiarities of this semi-mineralized coal, as it
may be called, have nothing to do with the general question, whether
wood coal and mineral coal are the same substance; about which
question, if the gradations are properly considered, I think, no
reasonable doubt can remain.
142. One of Mr Kirwan's objections to the vegetable origin of coal,
is founded on this fact, that there is, in the museum at Florence, a
cellular sandstone, the cells of which are filled with genuine mineral
coal. "Could this (adds he) have been originally wood?"[53] The answer
to the interrogatory proposed here as a _reductio ad absurdum_, is,
that most undoubtedly it may have been wood. Sandstone with charred
wood, that is, with wood coal in it, is not an uncommon phenomenon in
coal countries. I have seen a specimen of this kind from the Hales
Quarry, near Edinburgh, consisting of a piece of charred wood, imbedded
in sandstone; the wood was much altered, but the remains of its fibrous
structure were distinctly visible. This affords a perfect commentary on
the specimen in the Florence cabinet.
[Footnote 53: Geol. Essays, p. 321.]
143. If then it be granted, as I think it must, that the two kinds of
coal we have been speaking of are of the same origin, it is not very
necessary to enter on a refutation of Mr Kirwan's theory with respect
to either of them. His account of the formation of mineral coal,
however, is so singular, that it cannot be passed over without remark.
Mr Kirwan supposes, 1mo, That natural carbon was originally contained
in many mountains of the granite and porphyritic order, and also in
siliceous schistus; and might, by disintegration and decomposition, be
separated from the stony particles. 2do, That both petrol and carbon
are often contained in trap, since hornblende, which has lately been
found to contain carbon, very frequently enters into its composition.
"My opinion (adds he) is, that coal mines, or strata of coal, as well
as the mountains in which they are found, owe their origin to the
disintegration of primeval mountains, either now totally destroyed,
or whose height and bulk, in consequence of such disintegration, are
considerably lessened; and that these rocks, anciently destroyed,
contained most probably a far larger proportion of carbon and
petrol than those of the same denomination now contain, since their
disintegration took place at so early a period.[54]
[Footnote 54: Geol. Essays, p. 328, &c.]
"By the decomposition of these mountains, the feldspar and hornblende
were converted into clay; the bituminous particles, thus set free,
reunited, and were absorbed, partly by the argil, but chiefly by the
carbonaceous matter, with which they have the greatest affinity. The
carbonic and bituminous particles, thus united, being difficultly
miscible with water, and specifically heavier, sunk through the moist,
pulpy, incoherent argillaceous masses, and formed the lowest stratum,"
&c.
Such is Mr Kirwan's theory of the formation of coal, and nobody I think
will dispute the originality of it.
144. To enter on a formal refutation of an opinion so loaded with
objections, would be a task as irksome as unnecessary. A few
observations will suffice.
The notion of the great degradation of mountains, involved in this
hypothesis, is the part of it to which I am least disposed to object.
But I cannot help reminding Mr Kirwan, that the effects of waste are
not supposed less in this, than in Dr Hutton's theory; and that he has
assumed the very principle, of which that theory makes so much use,
though he has reserved to himself, as it should seem, the right of
denying it, when it does not accord with his system. It is indeed worth
while to compare what is said concerning the degradation of mountains,
in the above quotations, and still more fully in the book itself, with
what is advanced concerning their indestructibility, in another passage
of the same volume:[55]
[Footnote 55: Page 436.]
"All mountains are not subject to decay; for instance, scarce any of
those that consist of red granite. The stone of which the Runic rocks
are formed, have withstood decomposition for two thousand years, as
their characters evince," &c.
"Basaltic pillars, in general, bid defiance to decay," &c. He goes on
to deny every step of the degradation of land, by which it is wasted,
carried into the sea, and spread out over its bottom, though all these
are necessary _postulata_ in his theory of the formation of coal. One
can be at no loss about estimating the value of a system, in which such
gross inconsistencies make a necessary part.
145. The quantity of hornblende and siliceous schistus, necessary to
be decomposed, in order to produce the coal strata presently existing,
is enormous, and would lead to an estimate of what is worn away from
the primeval mountains, far exceeding any thing that Dr Hutton has
supposed. It is true, that Mr Kirwan, never at all embarrassed about
preserving a similitude between nature as she is now, and as she was
heretofore, lays it down, that the part of the primeval mountains
which is worn away, contained much more carbon than the part which is
left behind. This, however, is an arbitrary supposition; and since, in
this system, such suppositions are so easily admitted, why may we not
conceive, in the primeval mountains, a more copious source of carbonic
matter than hornblende or siliceous schistus? We have but to imagine,
that the _diamond_ existed among these mountains in such abundance, as
to constitute large rocks. This stone being made up of pure, or highly
concentrated carbon, the adamantine summits of a single ridge, by
their decomposition, might afford a carbonic basis, sufficient for the
coal beds of all the surrounding plains.
146. We may also object to Mr Kirwan, that the siliceous part of
the mountains has not been chemically dissolved; it has been only
abraded and worn away. Mechanical action has reduced the quartz to
gravel and sand, but has not produced on it any chemical change.
The carbon, therefore, could not be let loose. Experiment, indeed,
might be employed, to determine whether the siliceous matter of the
secondary, and of the primary strata contains this substance in the
same proportion.
Again, a more fatal symptom can hardly be imagined in any theory, than
that, when the circumstances of the phenomena to be explained are _a
little_ changed, the theory is under the necessity of changing _a
great deal_. Now, this is what happens to Mr Kirwan's theory, in the
attempt made to explain by it the stratum of coal described in the
_Annales de Chimie_,[56] as cutting a mountain of argillaceous strata
in two, at about three-fourths of its height. This stratum, Mr Kirwan
says, must have been formed by _transudation_ from the superior part
of the mountain,[57] Besides that this is a gratuitous supposition
of a thing, without example, it involves in it an absurdity, which
becomes evident the moment the question is asked, What occupied the
place of the coal-bed before the transudation from the upper part of
the mountain? Has the _liquid coal_, as it percolated through the upper
strata, expelled any substance from the place it now occupies? or has
it been powerful enough to raise up, or to float, as it were, the upper
part of the mountain?
[Footnote 56: Tom. xi. p. 272.]
[Footnote 57: Geol. Essays, p. 338.]
The situation of this bed of coal is not singular, and its formation
is easily explained on Dr Hutton's theory. It is part of a stratum of
coal, which has been deposited, like all others, at the bottom of the
sea; from whence certain causes, of very general operation, have raised
it up, together with the attending strata: these strata have since
been all cut down, and worn away by the operations of the surface; and
the mountain, with the coal stratum in the middle of it, is a part
of them which has been left behind. There is no wonder, that a coal
stratum should be found alternating with others, in a mountain, any
more than in the bowels of the earth, and no more need of a separate
explanation.[58]
[Footnote 58: This stratum of coal, which is described by HASSENFRATZ,
is remarkable for being in a mountain which rests immediately on
primary schistus and granite.]
147. After all, it may be asked, for what purpose is it that so
many incongruous and ill supported hypotheses are thus piled on one
another? is it only to avoid ascribing the carbonic and bituminous
matter of coal to a substance in which we know with certainty that such
matter resides in great abundance, in order to derive it from other
substances, in which a subtle analysis has shown, that it exists in a
very small proportion? Such reasoning is so great a trespass on every
principle of common sense, not to say of sound philosophy, that, to
bestow any time on the refutation of it, is, in some degree, to fall
under the same censure.
NOTE III. § 7.
_Primitive mountains._
148. The enumeration of the different kinds of primary schistus, at
§ 7, is not proposed as at all complete. It will be less defective,
however, if we add to it _talcose schistus_, and _lapis ollaris_ or
_potstone_.[59]
[Footnote 59: Kirwan's Mineralogy, vol. i. p. 155.]
149. The rocks called here by the name of primary, were first
distinguished, as forming the basis of all the great chains of
mountains, and as constituting a separate division of the mineral
kingdom, by J. G. LEHMAN, director of the Prussian mines. See his
work, intituled, _Essai d'une Histoire Naturelle des Couches de la
Terre_.[60] These rocks were regarded by Lehman as parts of the
original nucleus of the globe, which had undergone no alteration, but
remained now such as they were at first created; and, agreeably to
this supposition, he bestowed on them, and on the mountains composed
of them, the name of primitive. He remarks, nevertheless, their
distribution into beds, either perpendicular to the horizon, or highly
inclined, and the super-position of the secondary and horizontal
strata. However mineralogists may now differ in their theories from
Lehman, they must consider this distinction as a great step in the
science of geology, and very material to the right arrangement of the
natural history of the earth.
[Footnote 60: Tom. iii. p. 239, &c. The French translation is in 1759,
but the original preface is dated at Berlin, 1756.]
150. Several mineralogists have agreed with him in the supposition,
that these rocks are a part of the original structure of the globe, and
prior to all organized matter. Of this number is PALLAS;[61] and also
DE LUC, who applies the term _primordial_ to the rocks in question,
and considers them as neither stratified nor formed by water.[62]
In his subsequent writings, however, he admits their formation from
aqueous deposition, as the Neptunists do in general, but holds them to
be more ancient than organized bodies.
[Footnote 61: Observations sur la Formation des Montagnes.]
[Footnote 62: Lettres Phys. Sur l'Histoire de la Terre, tom. ii. p.
206.]
151. PINI, professor of natural history at Milan, has denied the
stratification of primitive mountains, in a memoir on the mineralogy
of St Gothard, and in another on the revolutions of the globe.[63]
His reasonings are opposed by SAUSSURE,[64] and are certainly, in
many respects, very open to attack. They proceed on a comparison
between the division of rocks, by what is called the planes of their
stratification, and their division by transverse fissures: two
things, which he thinks so much alike, that they ought not to be
referred to different causes; and, as the one cannot be regarded as
the effect of aqueous deposition, so neither should the other. This
is a very fallacious argument, because it confounds two things that
are essentially different; and, instead of inquiring about a matter
of fact, inquires about its cause. The truth is, that the dispute has
arisen from not distinguishing the granite from the schistus mountains,
and from involving both under the name of primitive. M. Pini seems
to be in the right, when he holds the granite of St Gothard to be
unstratified; but it is without any good reason, that he would extend
the same conclusion to the schistus of that mountain. CHARPENTIER, and
Saussure, in his last two volumes, contend even for the stratification
of granite.[65]
[Footnote 63: Memoria sulle Rivoluzioni del Globo Terrestre; Memorie
della Societa Italiana, tom. v. p. 222, &c.]
[Footnote 64: Voyages aux Alpes, tom. iv. § 1881.]
[Footnote 65: See NOTE XV. on Granite.]
As the consent, if not universal, is very general for the
stratification of the primary schistus, and the fact itself abundantly
obvious, in almost all the instances I have ever met with, I have not
considered it as necessary to enter here into any argument on this
subject.
NOTE IV. § 8.
_Primary strata not primitive._
152. An account of the facts referred to § 8, may be found in Hutton's
Theory, vol. i. p. 332, &c. To what is there said, of the shells
contained in the primary limestone of Cumberland, I must add, that
I have since had an opportunity of verifying the conjecture, that
the limestone rock, in which the shells were found, near the head of
_Coniston_ Lake, is part of the same body of strata, where shells were
found, in a quarry between Ambleside and Low-wood. The limestone of
that quarry contains several marine objects; it is in strata declining
about 10° from the perpendicular, toward the S. E., and forms a belt,
stretching across the country from N. E. to S. W.
In a quarry where the argillaceous schistus, on the south side of this
limestone belt, is worked for pavement, are impressions of what I think
may safely be accounted marine objects; they have the form of shells,
are much indurated, and full of pyrites. They seem to be of the same
kind with the impressions said to be found in a slate quarry, near the
village of Mat in Switzerland.[66]
[Footnote 66: Hutton's Theory, vol. i. p. 327.]
Another spot, affording instances of shells in primary limestone, is
in Devonshire. On the sea shore on the east side of Plymouth Dock,
opposite to Stonehouse, I found a specimen of schistose micaceous
limestone, containing a shell of the bivalve kind: it was struck
off from the solid rock, and cannot possibly be considered as an
adventitious fossil.
Now, no rocks can be more decided primary than those about Plymouth.
They consist of calcareous strata, in the form either of marble or
micaceous limestone, alternating with varieties of the same schistus,
which prevails through Cornwall to the west, and extends eastward into
Dartmoor, and on the sea-coast, as far as the Berry-head. These all
intersect the horizontal plane, in a line from east to west nearly;
they are very erect, those at Plymouth being elevated to the north.
Though, therefore, the remains of marine animals are not frequent among
the primary rocks, they are not excluded from them; and hence the
existence of shell-fish and zoophytes, is clearly proved to be anterior
to the formation even of those parts of the present land which are
justly accounted the most ancient.
153. The rocks which contain sand or gravel, which are of a granulated
texture, must also be considered as carrying in themselves a testimony
of the most unequivocal kind, of their being derived from the
_detritus_ and waste of former rocks. Now, the fact stated in the text,
concerning sand found in schistus, most justly accounted primary,
might be exemplified by actual reference to many spots on the earth's
surface. A few such will be sufficient in this place.
St Gothard is a central point, in one of the greatest tracts of primary
mountains on the face of the earth, yet arenaceous strata are found in
its vicinity. Between Ayrolo and the Hospice of St Gothard, Saussure
found a rock, composed of an arenaceous or granular paste, including in
it hornblende and garnets. He is somewhat unwilling to give the name
_gres_ to this stone, which M. Besson had done; but he nevertheless
describes it as having a granulated structure.[67]
[Footnote 67: Voyages aux Alpes, tom. iv. § 1822.]
Among the most indurated rocks that compose the mountains of this
island, many are arenaceous. Thus, on the western coast of Scotland,
the great body of high and rugged mountains on the shores of Arafaig,
&c. from Ardnamurchan to Glenelg, consists, in a great measure, of a
granitic sandstone, in vertical beds. This stone sometimes occupies
great tracts; at other times it is alternated with the micaceous, or
other varieties of primary schistus; it occurs, likewise, in several of
the islands, and is a fossil which we hardly find described or named
by the writers on mineralogy. Much, also, of a highly indurated,
but granulated quartz, is found in several places in Scotland, in
beds or strata, alternated with the common schistus of the mountains.
Remarkable instances of this may be seen on the north side of the ferry
of Balachulish, and again on the sea-shore at Cullen. At the latter,
the strata are remarkably regular, alternating with different species
of schistus. At the former, the quartz is so pure, that the stone has
been mistaken for marble.
These examples are perhaps sufficient; but I must add, that in the
micaceous and talcose schisti themselves, thin layers of sand are
often found, interposed between the layers of mica or talc. I have
a specimen, from the summit of one of the highest of the Grampian
mountains, where the thin plates, of a talcky or asbestine substance,
are separated by layers of a very fine quartzy sand, not much
consolidated.
The mountain from which it was brought, consists of vertical strata,
much intersected by quartz veins. It is impossible to doubt, in this
instance, that the thin plates of the one substance, and the small
grains of the other, were deposited together at the bottom of the sea,
and that they were alike produced from the degradation of rocks, more
ancient than any which now exist.
154. In the Neptunian system, as improved by WERNER, an attempt is made
to take off the force of such instances as are produced in § 8, 9, and
152, &c. by distinguishing rocks, as to their formation, into three
different orders, the primitive, the intermediate, and the secondary,
or, to speak more properly, into primary, secondary, and tertiary. The
same mineralogist distinguishes, among the materials of these rocks,
between what he terms chemical and mechanical deposits. By mechanical
deposits, are understood sand, gravel, and whatever bears the mark of
fracture and attrition; by chemical deposits, those which are regularly
crystallized, or which have a tendency to crystallization, and in which
the action of mechanical causes cannot be traced. This distinction
is founded in nature, and proceeds on real and palpable differences;
but the application made of it to the three kinds of strata just
enumerated, seems by no means entitled to the same praise.
The primitive rocks contain, it is said, none but chemical deposits,
and are entirely composed of them: the intermediate contain a mixture
of both, and also some vestiges of organized bodies: the secondary
consist almost entirely of the mechanical, or of the remains of such
bodies, with little of the chemical. The first of these, then, are held
to contain no mark or vestige whatsoever of any thing more ancient
than themselves, and are, in the strictest sense, primeval, or formed
of the first materials, deposited by the immense ocean which originally
encompassed the globe.
After them were formed the intermediate, mostly consisting of chemical
deposits, but containing also some animal remains, and some spoils from
the land, subjected to the various kinds of destruction, which even
then made a part of the order of nature. These rocks, it is alleged,
are chiefly argillaceous, are less indurated than the primary, and not
intersected by veins of quartz.
The secondary were formed from the remains of the other two, and
contain more mechanical deposits than any other.
This sketch of what I understand to be Werner's opinion concerning the
different formation of the strata, is chiefly taken from a view of his
system, in the _Journal de Physique_ for 1800.
155. The main objection to the distinction here made between the
primary and the intermediate strata, is founded on the facts that
have been just stated. The sandstone of St Gothard is from a country
having every character of a primary one in the highest perfection. The
instances I have mentioned from the Highlands of Scotland, are from
mountains, less elevated indeed than the Alps, but where the rock is
micaceous, talcose, or siliceous, in planes erect to the horizon, and
intersected by veins' of quartz. The shells from Plymouth are from a
rock, that Werner would, I think, admit to be truly primitive. Those
from the lakes, also, are from the centre of a country, occupied by
porphyry, schorl, hornstone-schistus, and many others, about the order
of which there can be no dispute. It is true, that in this tract
there are argillaceous strata, of the kind that might be accounted
intermediate, were they not interposed among those that are certainly
primary; and this very intermixture shows, how little foundation there
is for the distinction attempted to be made between the formation of
the one and of the other. If there is any principle in mineralogy,
which may be considered as perfectly ascertained, it is, that rocks
similarly stratified, and alternated with one another, are of the same
formation.
Hence we conclude, that there is _no order of strata yet known_, that
does not contain proofs of the existence of more ancient strata. We see
nothing, in the strict sense, primitive. It must be understood, that
what is here said has no reference to granite, which I do not consider
as a stratified rock, and in which neither the remains of organized
bodies, nor sand, have I believe been ever found; though some
instances will be hereafter mentioned, where granite contains fragments
of other stone, viz. of different kinds of primary schistus.
To the instances of sand involved in primary schistus, I might have
added many from the rocks of that order on the coast of Berwickshire,
of which mention is so often made in these Illustrations; but I wished
to draw the evidence from those rocks that are most unequivocally
primary, and to which the Wernerian distinction of _intermediate_ could
not possibly be applied.
If any one assert, as M. De Luc has done, that sand is a chemical
deposit, a certain mode of crystallization which quartz sometimes
assumes, let him draw the line which separates sand from gravel; and
let him explain why quartz, in the form of sand, is not found in
mineral veins, in granite, nor in basaltes, that is, in none of the
situations where the appearances of crystallization are most general
and best ascertained.
NOTE V. § 10.
_Transportation of the
materials of the strata._
156. The great transportation or _travelling_ of the materials of the
strata, supposed by Dr Hutton, has been treated as absurd by some
of his opponents, particularly De Luc and Kirwan. These philosophers
seem not to have observed, that their own system, and indeed every
system which derives the secondary strata from the primary, involves
a transportation of materials, hardly less than is supposed in the
Huttonian theory, and a degradation of the primeval mountains, in
many instances much greater. To form some notion of this degradation,
it must be recollected, that the primeval mountains, which furnished
the materials of the secondary strata in the plains, cannot have
stood in the place now occupied by these plains. This is obvious; and
therefore we must necessarily regard the secondary strata as derived
from the primitive mountains which are the nearest to them, and of
which a part still remains. This part is sufficient to define the
base of the original mountains; and the quantity of the secondary
strata which surround them may help us to make some estimate of their
height. Let us take, for instance, the extensive tract of secondary
country about Newcastle, where coal mines have been sunk through a
succession of secondary strata, to the depth of more than a thousand
feet. This secondary country may be considered as comprehending almost
the whole of the counties of Northumberland and Durham, and probably
as extending very far under the part of the German Ocean which washes
their coasts; and the whole strata composing it must be derived, on
the hypothesis we are now considering, from the Cheviot Hills, on one
side, and from those in the high parts of Westmoreland and Cumberland
on the other, comprehending the Alston-Moor Hills, and the large group
of primary mountains, so well known from the sublime and romantic
scenery of the _Lakes_. Now, the mountains which stood on this base,
had not only to supply the materials for the tract already mentioned,
on the east, but had also their contingent to furnish to the plains on
the west and north; the Cheviots to Roxburghshire and Berwickshire;
the Northumberland mountains to the coal strata about Whitehaven, and
along the sea coast to Lancashire. On the whole, we shall not exceed
the truth, if we suppose, that the secondary strata, at the feet of the
above mountains, are six or seven times more extensive than the base
of the mountainous tract. If then we take the medium depth of these
secondary strata to be one thousand feet, it is evident, that the mass
of stone which composes them, if it were placed on the same base with
the primitive mountains, would reach to the height of six thousand
feet. This is supposing the mass to preserve the breadth of its base
uniformly to the summit; but if it be supposed to taper, as mountains
usually do, we must multiply this six thousand by three, in order to
have the height of these primeval mountains, which, therefore, were
originally elevated not less than eighteen thousand feet; in height,
therefore, they once rivalled the Cordilleras, and are now but poorly
represented by the hills of Skidaw and Helvellyn. It were easy to show,
that this estimate is still below the result that strictly follows from
the Neptunian hypothesis; but it is unnecessary to proceed further,
than to prove, that the principle of the degradation of mountains, is
involved in that hypothesis to an excessive and improbable degree; and
that the supporters of it, have either been guilty of the inconsistency
of refusing to Dr Hutton the moderate use of a principle, which they
themselves employ in its utmost extent, or of not having sufficiently
adverted to the consequences of their own system.
157. The formation of secondary strata from the degradation of the
contiguous mountains, on close examination, is subject to many
other difficulties of the same kind. Mountains of secondary strata,
and nearly horizontal, are found in this island of the height of
three thousand feet. Such are Ingleborough, Wharnside, and perhaps
some others on the west of Yorkshire. The whole chain, indeed,
for secondary mountains, is of great elevation. The strata are of
limestone, and of a very coarse-grained sandstone, alternating with it.
No mountains can more clearly point out, that the strata of which they
consist were once continued quite across the vallies which now separate
them; and hence, if the materials of those strata were indeed furnished
from any contiguous primitive mountains, the latter must have been, out
of all proportion, higher than any mountains now in Britain.
158. Thus, a great degradation of the primitive mountains, and of
course a great travelling of their materials, is proved to make a
necessary part of the Neptunian theory. The extent of this travelling
or transportation may be rendered more evident, if we apply a similar
mode of reasoning to larger portions of the globe. The north-west of
Europe furnishes us an instance of a very extensive tract of secondary
country, comprehending the greater part of Britain, the whole of
Flanders and Holland, part of Germany, the northern provinces of
France, and probably the bed of the German Ocean, at least for a great
extent. Within this circle almost all is secondary, and on the sides
of it all round are placed ridges or groups of primitive mountains,
namely the mountains of Auvergne, at least in part, and going round by
the east, the Alps, the Vosges, the Hartz, the Highlands and Western
Islands of Scotland, the hilly countries of Cumberland, Wales, and
Cornwall. This zone of primitive mountains, on the supposition of the
Neptunists, must have risen up in the form of islands in the great
ocean, that originally covered the earth, forming a kind of circular
Archipelago, including in its bosom a sea, which was from seven to
five hundred miles in diameter. Over the whole of this extent, the
_detritus_ of the above mountains must have been carried, in order to
form the flat interjacent countries which are now exposed to our view.
Such then, even on their own supposition, is the extent to which the
Neptunists must admit that the materials of the primeval mountains were
transported by the ocean.
159. This transportation of materials, may not be so great as that
which is involved in Dr Hutton's theory, but is such as should make the
enemies of his system consider, how nearly the principles they _must_
introduce, agree with those that they _would_ reject. This is one
fact, out of many, which shows, that there is at present a much nearer
agreement between the systems of geology, than between their authors.
160. To these facts, demonstrating the great transportation of fossils
in some former conditions of the globe, we may add another, recognised
by all mineralogists. The animal exuviæ contained in limestone and
marble, are often known to belong to seas, extremely remote from the
countries where they are now found. In the chalk-beds of England, in
the limestones of France, a great proportion of the petrifactions
belong to the tropical seas, and appear to have been brought from the
vicinity of the equator. Buffon observes, that of the fossil shells
found in France, it has been disputed, whether the foreign are not
more numerous than the native; and, though he is himself of opinion
that they are not, it is evident that they must bear a considerable
proportion to the whole.[68] In the petrifactions of Monte Bolca, near
Verona, where the impressions of fish are preserved between the laminæ
of a calcareous schistus, one hundred and five different species have
been enumerated, of which thirty-nine are from the Asiatic seas, three
from the African, eighteen from those of South, and eleven from those
of North America.[69] Similar observations have been made on the marine
plants, and the impressions of vegetables, found in rocks, in different
parts of Europe. At St Chaumont, near Lyons, is found an argillaceous
schistus, covering a bed of coal, every lamina of which is marked with
the impressions of the stem, leaf, or other part of some plant; and it
happens, says M. FONTENELLE, by an unaccountable destination of nature,
that not one of these plants is a native of France. They are all ferns
of different species, peculiar to the East Indies, or the warmer
climates of America. Here also was found the fruit of a tree, which
grows only on the coasts of Malabar and Coromandel.[70]
[Footnote 68: Buffon, Théorie de la Terre, art. 8.]
[Footnote 69: Saussure, Voyages aux Alpes, tom. iii. § 1535.]
[Footnote 70: Mém. De l'Acad. Des Sciences, 1718, p. 3 and 287; and
1721, p. 89, &c.]
The same holds of the bodies of amphibious animals which now make a
part of the fossil kingdom. The head and the bones of crocodiles have
been found in the island of Shepey, at the mouth of the Thames; and the
remains of an animal of the same species, but of a variety now peculiar
to the Ganges, have been discovered in the alum rocks on the coast of
Yorkshire.[71] These proofs of the transportation of materials by the
sea, have the advantage of involving nothing hypothetical, and of being
equally addressed to the geologists of every persuasion.
[Footnote 71: Phil. Trans. vol. l. p. 688. CAMPER denies that the
remains here mentioned belong to the crocodile, or any amphibious
animal, and refers them to the balænaæ. He passes the same judgment on
those fossil bones from St Peter's Mount, near Maestricht, which have
been supposed to belong to the crocodile; he looks on them as belonging
to whales, though of an unknown species. In this Mount, so famous for
its petrifactions, he finds many specimens of bones, which he thinks
belong to the turtle. Phil. Trans. vol lxxvi. p. 443. The opinion of an
author, so well skilled in comparative anatomy, must be regarded as of
great weight: if it takes from our argument in one part, it adds to it
in another, and the acquisition of the turtle makes up abundantly for
the loss of the crocodile.]
On this subject I cannot help observing, that the accurate comparison
of the animal exuviæ of the mineral kingdom, with their living
archetypes, is not merely a curious inquiry, but is one that may lead
to important consequences, concerning the nature and direction of the
forces which have changed, and are continually changing, the surface of
the earth.
161. These remarks I have thought it proper to add to the proofs of the
composition of the present from former strata, in order to show, that
the great transportation of materials involved in that supposition, is
not only conformable to the hypothesis of the Neptunists concerning
the secondary strata, but is also proved by the most direct evidence,
independently of all hypothesis. All this reasoning regards the ancient
state of the globe. Whether such a travelling of stony bodies makes
a part of the system now actually carrying on, will be considered in
another place.[72]
[Footnote 72: See NOTE XIX.]
NOTE VI. § 13.
_Mr Kirwan's notion of precipitation._
162. The Neptunist who has provided the means of dissolving the
materials of the strata, has only performed half his work, and must
find it a task of equal difficulty to force this powerful menstruum
to part with its solution. Mr Kirwan, aware in some degree of this
difficulty, has attempted to obviate it in a very singular way. First,
he ascribes the solution of all substances in water, or in what he
calls the chaotic fluid, to their being finely pulverised, or created
in a state of the most minute division. Next, as to the deposition, the
solvent being, as he acknowledges, very insufficient in quantity, the
precipitation took place, (he says,) on that account the more rapidly.
If he means by this to say, that a precipitation without solution
would take place the sooner the more inadequate the menstruum was to
dissolve the whole, the proposition may be true; but will be of no
use to explain the crystallization of minerals, (the very object he
has in view,) because to crystallization, it is not a bare subsidence
of particles suspended in a fluid, but it is a passage from chemical
solution to non-solution, or insolubility, that is required.
If, on the other hand, he means to say, that the solution actually took
place more quickly, and was more immediately followed by precipitation,
because the quantity of the menstruum was insufficient, this is to
assert, that the weaker the cause, the more instantaneous will be its
effect.
Of two propositions the one of which is nugatory, and the other absurd,
it is not material to inquire which the author had in view.
NOTE VII. § 16.
_Compression in the mineral regions._
163. It is worthy of remark, that the effects ascribed to compression
in the Huttonian Theory, very much resemble those which Sir ISAAC
NEWTON supposes to be produced in the sun and the fixed stars by
that same cause. "Are not," says he, "the sun and fixed stars great
earths, vehemently hot, whose heat is conserved by the greatness of
the bodies, and the mutual action and reaction between them, and the
light which they emit; and _whose parts are kept from fuming away,
not only by their fixity, but also by the vast weight and density of
the atmospheres incumbent upon them, and very strongly compressing
them_."[73]
[Footnote 73: Newton's Optics, Query 11.]
164. The fact of water boiling at a lower temperature under a less
compression, is sufficient to justify the supposition, that bodies may
be made by pressure to endure extreme heat, without the dissipation
of their parts, that is, without evaporation or combustion. A further
_postulatum_ is introduced in Dr Hutton's theory, namely, that compound
bodies, such as carbonate of lime, when the compression prevents their
separation, may admit of fusion, notwithstanding that the fixed part
may be infusible when separated from the volatile. This assumption is
supported by the analogical fact of the fusion of the carbonate of
barytes, as mentioned in the text.
165. In a region where the action of heat was accompanied with such
compression as is here supposed, there could be no fire, properly so
called, and no combustion; this is admitted by Dr Hutton, and it is
therefore a fallacious argument which is brought against his theory,
from the impossibility of fire being maintained in the bowels of the
earth. This impossibility is precisely what he supposes; and yet Mr
Kirwan's arguments are directed, not against the existence of heat in
the interior of the earth, but against the existence of burning and
inflammation.
After taking notice,[74] that Saussure had succeeded, though with
extreme difficulty, in melting a particle of limestone, so small as
to be visible only with a microscope, "what (adds he) must have been
the heat necessary to melt whole mountains of this matter? Judging
by all that we at present know of heat, such a high degree could
only be produced by the purest air, acting on an enormous quantity
of combustible matter. Now, EHRMAN observed, that the combustion of
two hundred and eighty cubic inches of air, acting on charcoal, was
not able to effect the fusion of one grain of Carrara marble; from
whence it is apparent, that all the air in the atmosphere, nor in ten
atmospheres, would not melt a single mountain of this substance, of
any extent, even if there were a sufficient quantity of inflammable
matter for it to act upon. Judging also of subterraneous heat by what
we know of that of volcanoes, no such heat exists: the highest they
in general produce, is that requisite for the fusion of the volcanic
glass called obsidian, which Saussure found not to exceed 115° of
Wedgewood; but basaltine, which requires 140° of Wedgewood, is never
melted in the lavas of Ætna. How little capable, then, would volcanic
heat be to effect the fusion of Carrara marble, which, according to
the same excellent author, would require a heat of upwards of 6300°
of Wedgewood, if this pyrometer could extend so far? And in what
circumstances does Dr Hutton suppose this astonishing heat to have
existed, and even still to exist, under the ocean, in the bowels of
the earth, where neither a sufficient quantity of pure air, nor of
combustible matter, capable of such mighty effects, can, with any
appearance of probability, be supposed to exist: and, without these,
such degrees of heat cannot even be imagined, without flying into the
region of chimeras."
[Footnote 74: Geol. Essays, p. 453.]
166. Now, this reasoning is not applicable to Dr Hutton's hypothesis
of subterraneous heat, because it is grounded on experiments, where
that very separation of the volatile and fixed parts takes place, which
is excluded in that hypothesis. When limestone or marble is exposed
to such heat as is here mentioned, or even to heat of a degree vastly
inferior, the carbonic gas is expelled, and the body is reduced to pure
lime; from the refractory nature of which, as we learn from the fact
relative to barytes, mentioned above, no conclusion can be drawn as to
the infusibility of the same substance, when combined with the carbonic
gas. The Carrara marble may require a heat of 6300° of Wedgewood, to
melt it in the open air, where the carbonic gas escapes from it; but
under such a pressure as would retain this gas, it cannot be inferred,
that it might not melt with the heat of a glass-house furnace. In like
manner, it may be true, that two hundred and eighty cubic inches of
air, acting on charcoal, cannot effect the fusion of one grain of this
marble, after its fixed air is driven off from it; but we cannot from
thence draw any inference, applicable to a case where the carbonic gas
is retained, and where the action of heat is independent of atmospheric
air.
Nothing, therefore, can be more inconclusive than this reasoning,
as it proceeds on the supposition, that Dr Hutton's system admits
propositions, which in fact it expressly denies.
167. Of the production and maintenance of heat, in circumstances
so different from those of ordinary experience, we can hardly be
expected to give any explanation; but we are not entitled, merely
on that account, to doubt of the existence of such heat. Mr Kirwan
thinks otherwise: "Judging," he says, "from all we at present know of
heat, such a high degree of it, (as will melt limestone,) could only
be produced by the purest air, acting on an enormous quantity of
combustible matter. Without these, such degrees of heat cannot even be
imagined, without flying into the region of chimeras."[75]
[Footnote 75: Geol. Essays, p. 454.]
Now, in the first place, the high degree of temperature which is
here understood, is probably not necessary to the purposes of
mineralization, as has just been shown; and, in the second place, it is
not FIRE, in the usual sense of the word, but HEAT, which is required
for that purpose; and there is nothing chimerical in supposing, that
nature has the means of producing heat, even in a very great degree,
without the assistance of fuel or of vital air. Friction is a source
of heat, unlimited, for what we know, in its extent, and so perhaps
are other operations, both chemical and mechanical; nor are either
combustible substances, or vital air, concerned in the heat thus
produced. So also the heat of the sun's rays in the focus of a burning
glass, the most intense that is known, is independent of the substances
just mentioned; and, though that heat certainly could not calcine a
metal, nor even burn a piece of wood, without oxygenous gas, it would
doubtless produce as high a temperature in the absence as in the
presence of that gas.
It is true, that it is not by the solar rays that subterraneous heat
is produced; but still, from this instance, we see, that there is no
incongruity in supposing the production of heat to be independent of
combustible bodies, and of vital air. We are indeed, in all cases,
strangers to the origin of heat; philosophers dispute, at this moment,
concerning the source of that which is produced by burning; and much
more are they at a loss to determine, what upholds the light and heat
of the great luminary, which animates all nature by its influence. If
we would form any opinion on this object, we shall do well to attend
to the suggestions of that great philosopher, who was hardly less from
others by his doubts and conjectures, than by his most rigorous and
profound investigations. "May not great, dense, and fixed bodies, when
heated beyond a certain degree, emit light so copiously, as, by the
emission and reaction of its light, and the reflections and refractions
of its rays within its pores, to grow still hotter, till it comes to
a certain period of heat, such as is that of the sun? And, are not
the sun and fixed stars great earths, vehemently hot, whose heat is
conserved by the greatness of the bodies, and the mutual action and
reaction between them and the light which they emit?"[76]
[Footnote 76: Newton's Optics, _ubi supra_.]
168. Some recent experiments, seem to make the suggestions in this
query applicable to an opaque body like the earth, as well as to
luminous bodies, such as the sun and fixed stars. The radiation of
heat, where there is no light, was first rendered probable by the
experiments of M. PICTET of Geneva;[77] and the only objections to
which the conclusions from those experiments seemed liable, are removed
by the late very important discoveries of Dr HERSCHEL.[78] From these
it appears, that heat is capable of refraction and reflection, as well
as light, so that it is not absurd to suppose, that _the heat of great,
dense, and faced bodies, may be conserved by the greatness of the
bodies, and the mutual action and reaction between them and the heat
which they emit_.
[Footnote 77: Essai sur le Feu.]
[Footnote 78: Phil. Trans. 1800, p. 84.]
The existence of subterraneous heat is still further rendered probable
from the researches of MAIRAN, which tend to show, that there is
another source of terrestrial heat besides the influence of the solar
rays.[79]
[Footnote 79: Mém. de l'Acad. des Sciences, 1765, p. 143.]
Whatever be the truth with regard to these conjectures, it is certain,
that the first and original source of heat is independent of burning.
Burning is an _effect_ of the concentration of heat; and though, by a
certain reaction, it has the power of continuing and augmenting that
heat, it never can be regarded as its primary and material cause. When,
therefore, we suppose a source of heat, independent of fire and of
burning, we suppose what certainly exists in nature, though we are not
informed of the manner of its existence, nor of its place, otherwise
than from considering the phenomena of the mineral kingdom.
169. Lastly, we are not entitled, according to any rules of
philosophical investigation, to reject a principle, to which we are
fairly led by an induction from facts, merely because we cannot give
a satisfactory explanation of it. It would be a very unsound view of
physical science, which would induce one to deny the principle of
gravitation, though he cannot explain it, or even though the admission
of it reduces him to great metaphysical difficulties. If indeed a
downright absurdity, or inconsistency with known and established
facts, be involved in any principle, it ought not to be admitted,
however it may seem calculated to explain other appearances. If, for
instance, Dr Hutton held, that combustion was carried on in a region
where there was no vital air, we should have said, that he admitted
an absurdity, and that a theory founded on such _postulata_ was worse
than chimerical. But, if the only thing imputable to him is, that,
being led by induction to admit the fusion of mineral substances in the
bowels of the earth, he has assumed the existence of such heat as was
sufficient for this fusion, though he is unable to assign the cause
of it, I believe it will be found, that his system only shares in an
imperfection, which is common to all physical theories, and which the
utmost improvement of science will never completely remove.
170. Thus, then, we are led, it must be allowed, into the _region of
hypothesis_ and conjecture, but by no means into that of chimeras.
Indeed, the reproach of flying into the latter region, may be said to
come but ill from one, who has trode so often the crude consistence of
the chaos, and who delights to dwell beyond the boundaries of nature.
By sojourning there long, it is not impossible that the eye may become
so accustomed to fantastic forms, that the figures and proportions of
nature shall appear to it deformed and monstrous.
NOTE VIII. § 24.
_Sparry structure of
calcareous petrifactions._
171. When the shells and corals in limestone are quoted by
mineralogists, it is not always considered in what state they are
found. In general, they have a sparry structure, very different
from that of the original shell or coral, of which, however, they
retain the figure with wonderful exactness, though probably sometimes
altered in size. Though sparry, they are often foliated, and preserve
their animal, in conjunction with their mineral, texture. Now, this
crystallization is a mark of some operation, quite different from any
that can be ascribed to the water in which these bodies had their
origin, and by which they were brought into their place. They were
impervious to water; and it cannot be said that their sparry structure
has been derived from the percolation of that fluid, carrying new
calcareous matter into their pores. We can account for the change
produced in them, I think, only by supposing them to have been softened
by heat, so as to permit their parts to arrange selves anew, and to
assume the characteristic organization of mineral substances.
All shells have not the change effected on them that is here referred
to; those in chalk, for instance, retain very much their original form
in all respects. This is what we might expect from the very different
degree of intensity, with which the mineralizing cause has acted on
chalk, and on limestone or marble. In general, it is in the hardest
and most consolidated limestone, that the marine objects are most
completely changed into spar.
It would be exceedingly interesting to examine, whether any of the
phosphoric acid remains united to shells of either of these kinds. We
might most readily expect it to be united, in a certain degree, to the
shells that are least mineralized.
This experiment would enable us also to appreciate the force of Mr
Kirwan's argument against the finer marbles, such as the Carrara,
containing shells.[80] This argument proceeds on an experiment,
mentioned in the _Turin Memoirs_ for 1789, from which it appears,
that no phosphoric acid is found in pure limestone; and its absence,
Mr Kirwan says, cannot be attributed to fusion, as phosphoric acid is
indestructible by heat.
[Footnote 80: Geol. Essays, p. 458.]
He calls this a demonstration; but, in order to entitle it to that
name, it will be necessary, first, to prove, that phosphoric acid
exists in those limestones which evidently consist of shells in a
mineralized state. If these are found without phosphoric acid, it is
evident that the preceding argument fails entirely. If they are found
to contain that acid, it will then no doubt afford a probability,
though not a demonstration, that Carrara marble does not directly
originate from shells.
That nature has some process, by which the above acid is separated
from the earth of bones, and probably also from the earth of shells,
is evident from the state in which the bones are found in the caves of
Bayreuth. Those that are the most recent, and least petrified, contain
most of the phosphoric acid. Where the petrifaction has proceeded far,
that acid is not found.
172. Among many of the strata, such a fluidity has prevailed, as
to enable some of the substances included in them to crystallize.
Calcareous spar and siliceous crystals are often found in stratified
rocks, forming veins of secretion, or lining close cavities, included
on all sides by the uncrystallized rock. In the instances of gneiss,
and many species of marble, almost the whole matter of the stratum is
crystallized. This union of a stratified and crystallized structure
in the same substance, has a great affinity to that union of the
crystallized with the organic structure of shells and corals which has
just been mentioned; and both are doubtless to be referred to the same
cause.
NOTE IX. § 81.
_Petroleum, &c._
173. According to the theory of coal laid down above, its two chief
materials, charcoal and bitumen, being furnished by the vegetable and
animal kingdoms, both of the land and of the sea, have formed with one
another a new combination, by the action of subterraneous heat; but
have also, in some cases, been separated by that same action, where
the degree of compression necessary for their union, happened to be
wanting. The carbonic part, when thus separated from the bituminous,
forms an infusible coal, which burns without flame: the bituminous
part, when separated from the carbonic, is found in the various states
of naphtha, petroleum, asphaltes, and jet.
The great resemblance of infusible or blind coal, to the residuum
obtained by the distillation of bituminous coal; and again, the
coincidence of the bitumens just named, with the volatile part, or
the matter brought over by such distillation, are strong arguments
in favour of this theory. The other facts in the natural history of
coal, serve to confirm the same conclusion; but it must be confessed,
that what we know of the pure bitumens, except the circumstance
just mentioned, is of a more ambiguous nature, and may be reconciled
with different theories. The drops of petroleum contained within the
cavities of the limestone, mentioned at § 31, are however strong facts
in confirmation of Dr Hutton's opinions, and they are furnished by the
substances purely bituminous. A careful examination would probably
make us acquainted with others of the same kind, for limestone is very
often the matrix in which petroleum and asphaltes are contained. The
greatest mine of asphaltes in Europe, that in the _Val de Travers_ in
the territory of Neufchâtel, is in limestone, from which, though it in
some places exudes, it is in general extracted by the application of
heat. The strata for several leagues are impregnated with bitumen; and,
if examined with attention, would probably afford specimens similar to
those which have just been mentioned.
174. It is a general remark, that, where petroleum is found, on
digging deeper, they come to asphaltes; and, at a depth still greater,
they discover coal. This probably does not hold invariably; but
it is certain, that most of the fountains of petroleum are in the
neighbourhood of coal strata. Petroleum and asphaltes are found in
great abundance in Alsace, in a bed of sand, between two beds of clay
or argillaceous schistus, and the same country also affords coal.[81]
This is true likewise of the fossil pitch of Coal-Brookdale; and of the
petroleum found in St Catharine's Well, near Edinburgh. Auvergne[82]
contains abundance of fossil pitch, which exudes, in the warm season,
from a rock impregnated with it through its whole mass. There are also
coal strata in the same country, not far distant.
[Footnote 81: Encyclopédia, mot, _Asphalte_.]
[Footnote 82: Voyage en Auvergne, par Lagrand, tom. i. p. 351.]
A very satisfactory observation relating to this subject, has lately
been communicated from a country, with whose natural history we were
till of late entirely unacquainted. In the Burmha empire, petroleum is
dug up in an argillaceous earth, from the depth of seventy cubits. This
argillaceous earth, or schistus, lies under a bed of freestone; and
under all, about one hundred and thirty cubits from the surface, is a
bed of coal.[83]
[Footnote 83: Asiatic Researches, vol. vi, art. 6. p. 130.]
175. In the petroleum lake of the Island of Trinidad, described _Phil
Trans._ 1789, the petroleum evidently exudes from the rock, and is
collected in a variety of springs in the bottom, after which it
hardens, and acquires the consistency of pitch. The manner, therefore,
in which petroleum exists in the strata, is very consistent with the
idea of its having been introduced in the form of a hot vapour.
Even amber appears to have some relation to coal. It is found in the
unconsolidated earth in Prussia and Pomerania; but I am not sure
whether this earth is _travelled_ or not. In the same earth where the
amber is found, there is often a mixture of coaly matter, which burns
in the fire; it is apparently fibrous, and has been considered as a
kind of fossil-wood.[84]
[Footnote 84: Buffon, Hist. Nat. des Mineraux, tom. ii. p. 5.]
These circumstances make out a connection between the purer bitumens
and ordinary coal; but do not, it must be acknowledged, establish any
thing with respect to the more immediate relation, supposed in this
theory to exist between them and blind coal. It is probable, indeed,
that, to discover any facts of that kind, the natural history of both
substances must be more carefully examined; the natural history of
blind coal, in particular, has hitherto been but little attended to.
176. A fact is mentioned by Mr Kirwan, which must not be regarded as
less valuable for being adverse to this theory. It is, that neither
petroleum, nor any fossil bitumen, is found in the vicinity of the
Kilkenny coal, as might be expected, if that coal was deprived of
its bituminous part by subterraneous distillation.[85] This, however,
admits of explanation. Though a general connection, on the above
hypothesis, might be expected between bitumens and infusible coal, we
cannot look for it in every instance. The heat which drove off the
bitumen from one part of a stratum of coal, may only have forced it to
a colder part of the same stratum; and thus, in separating it from one
portion of carbonic matter, may have united it to another. Blind coal
may therefore be found where no bitumen has been actually extricated.
In like manner, bitumen may have been separated, where the coal was
not reduced to the state of coke, as a part of the bitumen only may
have been driven off, and enough left to prevent the coal from becoming
absolutely infusible.
[Footnote 85: Geol. Essays, p. 473.]
It should be considered too, if the bitumen was really separated, and
forced, in the state of vapour, into some argillaceous or limestone
stratum, that this stratum may have been wasted and worn away long ago,
so that the bitumen it contained may have entirely disappeared. It does
not therefore necessarily follow, that, wherever we find blind coal,
there also we should discover some of the purer bitumens.
NOTE X. § 37.
_The height above the level of the sea
at which the marks of aqueous
deposition are now found._
177. We have two methods of determining the _minimum_ of the change
which has happened to the relative level of the sea and land; or
for fixing a limit, which the true quantity of that change must
necessarily exceed. The one is, by observing to what height the regular
stratification of mountains reaches above the present level of the sea;
the other is, by determining the greatest height above that level,
at which the remains of marine animals are now found. Of these two
criteria, the first seems preferable, as the fact on which it proceeds
is most general, and least subject to be affected by accidental causes,
or such as have operated since the formation of the rocks. The results
of both, however, if we are careful to select the extreme cases, agree
more nearly than could have been expected.
178. The mountain Rosa, in the Alps, is entirely of stratified rocks,
very regularly disposed, and nearly horizontal.[86] The highest summit
of this mountain is, by Saussure's measurement, 2430 toises, or 14739
English feet, above the level of the sea, or lower than the top of
Mont Blanc only by 20 toises, or 128 feet.[87] This is, I believe, the
highest point on the earth's surface, at which the marks of regular
stratification are certainly known to exist; for though, by the account
of the same excellent mineralogist, Mont Blanc itself is stratified,
yet, as the rock is granite, the stratification vertical, and somewhat
ambiguous, it is much less proper than Monte Rosa for ascertaining the
limit in question.
[Footnote 86: Voyages aux Alpes, tom. iv. § 2138.]
[Footnote 87: _Ibid._ § 2135.]
179. Again, in the new continent, we have an instance of shells
contained in a rock, not much lower than the summit of Monte Rosa.
This is one described by Don ULLOA, near the quicksilver mine of
Guanca-Velica, in Peru. The height at which a specimen of these shells,
given by Ulloa to M. Le GENTIL, was found, was 2222 1/3 toises, or
14190 feet English, above the level of the seas.[88] This height agrees
with the preceding, within 549 feet, a quantity comparatively small.
[Footnote 88: See Hist. Acad. des Sciences, 1770. Phys. Générale, No.
7.]
180. The last of the facts just mentioned is curiously commented on by
Mr Kirwan. As he has proved, he says, that the mountains higher than
8500 feet were all formed before the creation of fish, it follows,
that the shells found at Guanca-Velica, must have been carried there
by the deluge.[89] Now, without objecting to the proof here referred
to, (though it seems very open to objection,) it is sufficient to
remark, that, if the shells at Guanca-Velica were carried there by
the deluge, or any other cause that operated after the formation of
the rock of which the mountain consists, they can make no part of
that rock, but must lie, like other adventitious fossils, loose and
detached on the surface, or at most externally agglutinated to the
stone. This, however, is certainly not the fact; for, in the account
just quoted, we read, that Don Ulloa told M. le Gentil, "qu'il avoit
détaché ces coquilles d'un banc fort épais." This seems plainly to
indicate, that the shells were included in a bed of rock; But, granting
that the expression is a little ambiguous, on turning to the _Mémoires
Philosophiques_ of the same author, the difficulty is completely
removed, and it is made evident, that these shells are in fact
integrant parts of the rock. "On voit dans ces montagnes-là, (about
Guanca-Velica, and particularly at that in which is the quicksilver
mine,) des coquilles entières, petrifiées et enfermées au milieu de la
roche, que les eaux de pluie mettent à decouvert. Ces coquilles font
corps avec la pierre; mais malgré cela, on remarque que la partie qui
fut coquille, se distingue par la couleur, la structure, la qualité de
la matière de tout autre corps pierreux qui l'enferme, et du massif qui
s'est fixé entre les deux ecailles,"[90] &c. He goes on to say, that
one can distinguish marks of these shells having been worn, before they
were included in the stone.
[Footnote 89: Geol. Essays, p. 54.]
[Footnote 90: Mém. Philosophiques de Don Ulloa, Discours xvi. vol. i.
p. 364.]
181. Thus it appears, that whatever proof any fossil shell affords,
that the rock in which it is found was formed under the sea, the
same is afforded by the fossil shells of Guanca-Velica; and we are,
therefore, perfectly entitled to conclude, that the relative level of
the sea and land has changed, since the formation of the latter, by
more than 14000 feet. The height assumed in § 37 is therefore much
under the truth; and the water, for which the Neptunists must provide
room in subterraneous caverns, might very well have been stated at
more than a five-hundredth part of the whole mass of the earth.
Thus also the argument by which the Neptunists would connect the
creation of fish with the beginning of the secondary mountains, falls
entirely to the ground. Indeed, it is strange that Mr Kirwan should
have supposed it possible, that the shells in question were loose and
unconnected with the rock, and had continued so, ever since the deluge,
in such elevated ground, where the torrents wear and cut down the
mountains with unexampled violence, and have hollowed out _Quebradas_
so much deeper and more abrupt than the glens or vallies among other
mountains. He had not, I believe, seen the passage I have quoted from
Ulloa; but the circumstances did not warrant the shells in question to
be regarded as extraneous and adventitious fossils. A geologist should
have known better than to suppose this possible. When we see VOLTAIRE
ascribing to accidental causes the transportation of those shells
which he had been told were often found among the Alps, we can excuse
in a Poet and a Wit, that ignorance of the facts in mineralogy, which
concealed from him the extreme absurdity of his assertion; but when a
Chemist or Mineralogist talks and reasons in the same manner, we cannot
consider him as entitled to the same indulgence.
NOTE XI. § 42.
_Fracture and dislocation
of the strata._
182. The greatest part of the facts relative to the fracture and
dislocation of the strata, belongs to the history of veins. The
instances of slips, where no new mineral substance is introduced
between the separated rocks, are what properly belong to this place.
The frequency of these, and their great extent, are well known wherever
mines have been wrought. In some of them no opening is left, but the
slipped strata remain contiguous; in other cases, there is introduced
an unconsolidated earth, often a clay, which may be supposed to have
come from above, arid very probably to have been carried down by the
water. In some such cases, however, there are not wanting appearances,
which show the matter in the slip to have been forced up from below,
as we find it to contain substances which could not have come from the
surface.[91]
[Footnote 91: Unconsolidated earth contained between the sides of
a rock that has slipped, is frequent in Cornwall, and is called a
_Fleukan_. ]
183. A very remarkable fact of this kind occurred not long ago, in
digging the Huddersfield canal in Yorkshire; and a very distinct
account of it is given in the _Philosophical Transactions_, by the
engineer who directed the work. In carrying a tunnel into the heart of
a hill, the miners came to what is called in the description a _fault_,
_throw_, or _break_, or what we have here called a shift, which was
filled with _shale_ set on edge, mixed with softer earth, and in some
places with small lumps of coal. The fault or space filled with these
materials, was in general about four yards broad, and lay nearly in
the direction of the tunnel, so that a considerable extent of it was
visible. Beside the shale, it contained a _rib_ of limestone, about
four feet thick, which run parallel to the sides of the _fault_, and
about four feet from the southern margin of it. On each side of this
rib were found balls of limestone, promiscuously scattered, and of
various sizes, from an ounce to one hundred pounds weight. The balls,
when broken, were found to contain some pyrites near their edges; they
were not perfectly globular, but flattened on the opposite sides, and
similar to one another.[92] At the time when the account was written,
about seventy yards of the rib had been discovered.
[Footnote 92: Phil. Trans. 1796, p. 350.]
184. Now, it is certain, that neither this rib of limestone, nor the
balls that accompanied it, can have come from above, as there is no
limestone within twenty miles of the place where they were found. They
must, therefore, have been forced up from below, and no doubt belong
to some limestone strata, which lie there at a great depth under the
surface. The length of this fragment of rock, which, from the account,
one must suppose to have been entire, conveys no mean idea, either of
the intensity or regularity of the force by which it was brought into
its present situation. In veins, it is not uncommon to meet with stones
that appear to have come from a greater depth: but this is probably the
most remarkable instance of the same phenomenon, which has appeared in
a mere slip, and none, I think, can speak a language less liable to be
misunderstood.
185. I shall here mention another mark of violent fracture, that has
been observed in rocks of breccia or pudding-stone, which, though not
of the same kind with the preceding, and of a nature quite peculiar,
belongs rather to this place than any other. In rocks of the kind,
just mentioned, it sometimes happens, that considerable portions are
separated from one another, as if by a mathematical plane, which had
cut right across all the quartzy pebbles in its way. None of the
pebbles are drawn out of their sockets, that is, out of the cement that
surrounds them, but are divided in two with a very smooth and even
fracture. The pebbles, in the instances which I have seen, were of
quartz, and other species of primary and much indurated rock.
Lord WEBB SEYMOUR and I observed pudding-stone rocks, exhibiting,
instances of this singular kind of fracture, near Oban, in Argyleshire,
about three years ago. The phenomenon was then entirely new to us both;
but I have since met with an instance of the same kind in Saussure's
last work. As the fact is of so particular a kind, I shall state it in
his own words: The place was on the sea shore, near the little town of
Alassio, between Nice and Genoa.
"En passant entre ces blocs de breche, j'admirai quelques-uns
d'entr'eux, d'une grandeur considérable, et taillés en cubes, avec
la plus parfaite régularité. Il y avoit ceci de remarquable, c'est
que l'action de la pesanteur, qui avoit taillé ces cubes en rompant
leurs couches, avoit coupé tous les cailloux des breches à fleur de
la surface de la pierre, aussi nettement que si c'eût été une masse
molle qu'on eût tranchée verticalement avec un rasoir. Cependant parmi
ces cailloux, la plupart calcaires, il s'en trouvoit de très durs, de
petrosilex, par exemple, même de jade, qui étoient tranchées tout
aussi nettement que les autres."[93]
[Footnote 93: Voyages aux Alpes, tom. iii. § 1731.]
186. This description is no doubt accurate, though it involves in it
something of theory, viz. that the fracture was made by the weight of
the stone. This may indeed be true: the operation probably belongs
altogether to the surface, and is one with which the powers of the
mineral regions are not directly concerned. The phenomenon, however,
appears to me, on every supposition, very difficult to explain. In the
specimen which I brought from Oban, the smallest pieces of stone are
cut in two, as well as the largest. The consolidation and hardness of
the mass are very great, and the connection of the different fragments
so perfect, that it is no wonder the whole should break as one stone.
But still, that the fracture should be so exactly in one plane, and
without any shattering, is not a little enigmatical; if it is indeed
a fracture, it must be the consequence of an immense impulse, very
suddenly communicated.
NOTE XII. § 43.
_Elevation and inflection
of the strata._
187. The evidence of the different formation of the primary and
secondary strata, and of the changes which the former have undergone,
is best seen at the points where those strata come into contact with
one another. Dr Hutton was not the first who observed these junctions,
though the first who rightly interpreted the appearances which they
exhibit. He has mentioned observations of this sort by De Luc on the
confines of the Hartz; by the author of the _Tableau de la Suisse_,
at the pass of Yetz; by Voight, in Thuringia; and Schreiber, at the
mountain of Gardette.[94]
[Footnote 94: Theory of the Earth, vol. i. p. 410 to 453.]
The leading facts to be remarked, are,
I. The vertical or very upright position of the primary or lower strata.
II. The superstratification of the secondary, in a position nearly
horizontal, so as to be at right angles to those on which they rest.
III. The interposition of a breccia between them or, as happens in
many cases, the transition of the lowest of the secondary beds into a
breccia, containing fragments sometimes worn, sometimes angular, of the
primary rock.
This last is a phenomenon extremely general, and all our subsequent
information confirms Dr Hutton's anticipations concerning it. "It will
be very remarkable," he says, "if similar appearances, (such as those
of the breccia described by Voight,) are always found upon the junction
of the Alpine with the level countries."[95] Saussure, in a part of his
work, not published when Dr Hutton wrote this passage, has attested the
generality of the fact with respect to the whole Alps, from the Tyrol
to the Mediterranean: "Un sait que l'on observe sans aucune exception,
ce sont les amas de débris, sous la forme de blocs, de breches, de
poudingues, de grès, de sable, ou amoncelés, et formant des montagnes,
ou des collines, dispersés sur le bord exterieur, ou même dans les
plaines qui bordent la chaine des Alpes."[96]
[Footnote 95: Theory of the Earth, vol. i. p. 448.]
[Footnote 96: Voyages aux Alpes, tom. iv. § 2330.]
This passage is perfectly decisive as to the generality of the fact,
that the Alps, from the Tyrol to the Mediterranean, are bordered all
round by pudding-stones or breccias. At the same time, it is necessary
to remark, that M. Saussure, by enumerating loose blocks and sand,
along with pudding-stones, breccias and grit, confounds together things
which are extremely different, and which have had their origin at
periods extremely remote from one another. The consolidated rocks of
breccia, pudding-stone and grit, though they are indications of waste,
have received their present character at the bottom of the sea: the
loose blocks of stone, the sand and gravel, on the other hand, are the
effects of the waste now going forward on the surface of the land, and
are the materials out of which rocks of the three kinds just mentioned
may hereafter be composed. If so skilful a mineralogist as Saussure
is guilty of such inaccuracy, it must be ascribed to the confusion
necessarily arising from the system which he followed, and not to his
own want of discrimination.
188. The same phenomenon, of a breccia circumscribing the primary
mountains, is met with in Scotland; and the Grampians, wherever they
are bounded by secondary strata, whether on the south or north, afford
examples of it. The breccia generally consists of the fragments of
the primary rock, most commonly rounded, but sometimes also angular,
united by a cement of secondary formation, and the whole disposed
in horizontal beds. It was on the constancy of this accompaniment
of the primary strata, and on the great quantity of highly polished
gravel often included in these breccias, that Dr Hutton grounded the
hypothesis of the double raising up and letting down of the ancient
strata. See § 43.
189. As the spots where the primary and secondary rocks may be seen
in contact with one another are of great importance in geology, and
present to the senses the most striking monuments of the high antiquity
and great revolutions of the globe, it may be useful to point out such
of them as have been observed in this island. To those which Dr Hutton
has described, I have a few more to add, the result of some geological
excursions, which I made in company with the Right Honourable Lord
WEBB SEYMOUR, to whose assistance I have been much indebted in the
prosecution of these inquiries.
190. The most southern junction which we observed is at Torbay, where
the ancient schistus which prevails along the coast, from the Land's
End to that point, receives a covering of red horizontal sandstone,
the same which composes the greater part of Devonshire. The spot where
the immediate contact is visible, is on the shore, a little to the
south of Paynton; and one circumstance, which among many others serves
to distinguish the different formation of the two kinds of rock, is,
that the schistus, which is elevated here at an angle of about 45°, is
full of quartz veins, which veins are entirely confined to it, and do
not, in as far as we could observe, penetrate into the sandstone, in a
single instance. It is probable, that on the north shore of the bay,
the same line of junction is visible: we saw it at Babicomb Bay, still
more to the northward.
191. From this place, the secondary strata of different kinds prevail
without interruption, along the coast of the British Channel, and
of the German Ocean, as far as Berwick upon Tweed, and for some
miles beyond it. The sea coast then intersects a primary ridge, the
Lammermuir Hills, which traverses Scotland from east to west, uniting,
near the centre of the country, with the metalliferous range of
Leadhills, and afterwards with the mountains of Galloway. The section
which the sea coast makes of the eastern extremity of this ridge, is
highly instructive, from the great disturbance of the primary strata,
and the variety of their inflections. The junction of these strata
with the secondary, on the south side, is near the little sea-port of
Eyemouth, but the immediate contact is not visible.
On the north side of the ridge, the junction is at a point called the
_Siccar_, not far from Dunglass, the seat of Sir James Hall, Baronet.
By being well laid open, and dissected by the working of the sea,
the rock here displays the relation between the two orders of strata
to great advantage. Dr Hutton himself has described this junction;
_Theory of the Earth_, vol. i. p. 464.
192. From the point just mentioned, the secondary strata continue as
far as Stonehaven, where the southern chain of the Grampian mountains
is intersected by the sea-coast. Here a great mass of pudding-stone
appears to lie on the primary strata, but their immediate contact has
not been observed.
193. Going along the coast toward the north, the next junctions which
we saw were on the shore, one near Gardenston, and another near Cullen,
in Banffshire. The latter is very distinct; it is about a mile to the
westward of the rocks called _The Three Kings_, where a red sandstone,
the lower beds of which involve much quartzy gravel, lies horizontally
upon very regular, upright, and highly indurated strata. Some of these
strata are micaceous, and others of the granulated quartz, mentioned in
§ 153.
194. This last is, I believe, the most northern junction which has
been observed in our island. The western coast furnishes several more,
which however are not all visible. The line of separation, between
the primary schistus of the Grampians and the sandstone which covers
it, is intersected at its western extremity by the Frith of Clyde,
not far from Ardencaple in Dunbartonshire. The two kinds of stone can
be traced within a few yards of each other, but not to the actual
contact: the beds of sandstone nearest the schistus form as usual a
breccia, loaded with fragments of the primary rock. The secondary rock,
which begins here, continues for about fifty miles south, to Girvan in
Ayrshire, where the primary schistus again rises up, but is not seen in
contact with the secondary. It extends to the Mull of Galloway and the
shores of the Solway Frith.
The Isle of Arran, however, not far distant from this part of the
coast, contains a junction at its northern extremity, where secondary
strata of limestone lie immediately on a primary micaceous schistus.
This is described by Dr Hutton, and was the first phenomenon of the
kind which he had an opportunity of examining.[97] The junction is
visible but at one spot, and is not seen so distinctly as in some of
the instances just mentioned; but the great quantity of pudding-stone
near it, renders it more interesting than it would be otherwise. As the
greater part of this little island is surrounded by secondary strata,
other junctions might be expected to be visible.
[Footnote 97: Theory of the Earth, vol. i. p. 429.]
195. On the coast of England and Wales, from the Solway Frith to the
Land's End, though there are several alternations from secondary to
primary strata, I know not that any of them have been observed. At St
Bride's Bay, in Pembrokeshire, the primary and secondary strata are
seen very near their junction; but the precise line I believe is not
visible. The coal-pits in the secondary strata, approach here within a
few hundred yards of the primary. The secondary strata which commence
at this place, occupy both sides of the Bristol Channel, and meet the
Cornish schistus, which extends across the north of Devonshire to the
Quantock Hills, in a line that may be looked for on the sea coast, some
where between Watchett and Minehead.
196. Besides the sea coast, the beds of rivers may be expected to
afford information on this subject. To the instances I have mentioned,
I have accordingly two others from the inland country to be added. One
of them is from the river Jed, a little way above Jedburgh, where the
secondary strata are seen lying horizontally on the primary, a section
of both being made by the bed of the river. The phenomena here are very
distinct, and strongly marked; Dr Hutton has described and represented
them in a plate.[98] He has mentioned another junction, not far from
this, which he saw in the Tiviot. Both these belong to the same primary
ridge with the Siccar point.
[Footnote 98: Theory of the Earth, vol. i. p. 430; also plate 3.]
197. I shall mention only one other, which was discovered by Lord Webb
Seymour and myself, at the foot of the high mountain of Ingleborough,
in Yorkshire. As we went along the Askrig road from Ingleton, about a
mile and a half from the latter, an opening appeared in the side of
the hill, on the right, about one hundred yards from the road, formed
by a large stone, which lay horizontally, and was supported by two
others, standing upright. On going up to the spot, we found it was the
mouth of a small cave, the stone lying horizontally, being part of a
limestone bed, and the two upright stones, vertical plates of a primary
argillaceous schistus. The limestone bed, which formed the roof of the
cave, was nearly horizontal, declining to the south-east; the schistus
nearly vertical, stretching from north-west by west, to south-east by
east. The schistus, though close in contact with the limestone, seemed
to contain nothing calcareous, and did not effervesce with acids in the
slightest degree.
As this cave is at the foot of Ingleborough, a cold wind, 24° below the
temperature of the external air, which issued from the mouth of it,
might very well be supposed to come from the inmost recesses of that
mountain. Ingleborough, which consists entirely of strata of limestone
and grit, nearly horizontal, and alternating with one another, rises
to the height of 1800 or 2000 feet above the spot where we now stood.
This, I believe, is the greatest thickness of secondary strata that
has ever been observed incumbent on the primary, and it is therefore
a geological fact highly deserving of attention. The country all
round, to a very great extent, is composed of limestone, with a few
beds of grit interposed, and forming, beside Ingleborough, some other
high mountains, such as Wharnside and Pennigant, all resting, it is
probable, on the same foundation.
At the spot just described, no breccia appeared to be interposed
between the primitive and secondary rock; but we found a breccia at
another point of the same junction, not far distant. This was at a
cascade, in the river Greta, called Thornton Force, about two miles
and a half from the place just mentioned. The Greta here precipitates
itself from a horizontal rock of limestone; and, after a fall of
about eighteen or twenty feet, is received into a bason which it has
worked out in the primary schistus. This schistus is in beds almost
perpendicular; it exactly resembles that which has Just been described,
and stretches nearly in the same direction. On the south side of the
river a breccia was seen, lying upon the schistus, or rather, it might
be said, that the lowest beds of limestone contained in them many
rounded fragments of stone, which, on comparison, resembled exactly
the schistus underneath. The primary rock itself is here seven or eight
hundred feet above the level of the sea.
The same schistus, somewhat lower down the valley, and nearer to
Ingleton, appears in large quantities, and is quarried for slate. Here,
however, the immediate junction of the limestone and schistus does not
appear.
I have dwelt longer on the description of these appearances than on
any others of the same kind, because, from the great mass of secondary
strata which here covers the primary, the circumstances are such as we
cannot expect to see very often exemplified.
198. The Lakes of Cumberland are much visited by travellers; and it may
be worth remarking, on that account, that, as the site of these lakes
is a patch of primary country, bounded on all sides by secondary, so,
in the rivers that run from the lakes, such junctions as we are now
treating of may be expected to be found. Under Dun-Mallet, on the side
toward Ulles Water, we observed a breccia, which was in horizontal
layers, and seemed to lie on the primary schistus, so that the whole
hill is perhaps a piece of more indurated breccia, or secondary rock,
which has resisted the wearing and washing down of the rivers better
than the rest.
199. After ascertaining the fact of the disturbance of the strata, and
their removal from their original position, it is of consequence to
inquire into the direction of the force by which these changes have
been produced. Now, if the disturbed or elevated strata, were every
where in planes, without bending or sinuosity, it might perhaps be
hard to determine, whether that force had acted in the direction of
gravity, or in the opposite. Either supposition would account for the
appearances; and, as gravity is a known force, providing we can find
some place fit to receive the matter impelled downward by it, its
action would furnish the most probable solution of the difficulty.
It is on this principle that the Neptunian system proceeds, imagining,
that certain great caverns or vacuities having been opened in the
interior of the globe, a great part of the waters which formerly
covered its surface, retired into them, and much of the solid rock also
sunk down at the same time. In this way, one extremity of a stratum
has been elevated, while the other has been depressed, and a certain
inclination to the horizon has been given to the whole of it. Thus one
cause serves two purposes; the vacuities in the interior of the earth
account, both for the depression of the sea, and the elevation of the
land; and the Neptunists, if the phenomena were all such as have been
now stated, might boast of a felicity of explanation, not very usual in
their system.
But this appearance of success vanishes, when the elevation and
disturbance of the strata are more minutely examined, and are found
to include waving and inflection, in a great variety of forms. It
then becomes evident, that the beds of rock, at the time when they
were disturbed from their horizontal position, had not their present
hardness and rigidity, but were, in a certain degree at least, soft
and flexible. Without these qualities, they could not have received,
as they have often done, the curvature of a circle, not many feet,
nay, not many inches, in diameter; nor could they have been bent into
superficies, with their curvature in opposite directions, so that
the same surface is in one part convex, and in another concave, on
the same side, with a line of contrary flexure interposed. These are
appearances, not reconcilable with the mere falling in, and breaking
down of indurated rocks.
200. The inflections and wavings that we are here speaking of, though
not peculiar to the primary strata, are found most frequently among
them, and are perfectly familiar to every one who his travelled among
mountains with any view to the study of geology. The following are a
few instances of this phenomenon out of a great number which might be
produced.
Saussure, in describing the route from Geneva to Chamouni, mentions
many remarkable instances of the bending of the strata, and
particularly where the small stream of Nant d'Arpenaz forms a cascade,
by falling over the face of a perpendicular limestone rock. The strata
of this rock are bent into circular arches, extremely regular, and with
their concavity turned to the left. What deserves particularly to be
remarked, is, that a mountain behind the cascade has its strata bent
in a direction opposite to the former, or with their concavity to the
right. There is no doubt that the strata of both rocks are the same, so
that a vertical section of them would give a curve, in the figure of an
S.[99] These circumstances are mentioned by Saussure, and from them we
may infer this other property of these strata, that their section by a
horizontal plane, must exhibit a system of straight lines, probably all
parallel to one another.
[Footnote 99: Voyages aux Alpes, vol i. § 472; also, Theory of the
Earth, vol. ii. p. 30.]
The same mineralogist describes the calcareous strata which compose
the mountain Axenberg, on the side of the Lake of Lucerne, as having
from top to bottom of the mountain the form of the letter S compressed,
(_ecrasée_) with their curvature in some places very great. These
inflections are repeated several times, and often in contrary
directions; the layers are sometimes broken, where their curvature is
greatest.[100]
[Footnote 100: Voyages aux Alpes, tom. iv. § 1935.]
On the side of the same lake, is another instance of bent strata, in a
mountain, of which the beds are horizontal in the lower part, but are
bent at one end upwards, in the form of the letter C. The horizontal
part is of great extent, and the rock is also calcareous,[101]
[Footnote 101: _Ibid._ § 337.]
The Montagne de la Tuile, near Montmelian, receives its name from the
beds of rock being incurvated in form of a tyle.[102] Among secondary
mountains, the same kind of phenomena are observed, though less
frequently, and with less variety of inflection. The chain of Jura is
secondary, and the beds which compose it are of limestone, or of grit:
they are bent in such a manner, that in a transverse section of the
mountain, each layer would have the figure of a parabola.[103]
[Footnote 102: _Ibid._ vol. iii. § 1182, and plate i.]
[Footnote 103: _Ibid._ tom. i. § 334.]
201. The Pyrenees furnish abundance of phenomena of the same kind,
as we learn from the _Essai sur la Mineralogie des Pyrenées_. The
calcareous strata of the valley of Aspe, represented plate v. of that
work, deserve particularly to be remarked.
202. Our own island abounds with examples of the bending and inflection
of the strata, especially the primary, and many of them very much
resembling those in the Alps and Pyrenees. On the top of the mountain
of _Ben-Lawers_, in Perthshire, there is a rock, the face of which
exhibits a section of a great number of thin equidistant layers, bent
backwards and forwards like those described by Saussure; and this
unequivocal proof of the rock having once existed in the state of a
flexible and tenacious paste, is rendered more striking, by the great
elevation of the spot, and the ruggedness and induration, both of
the stone itself, and of every thing that surrounds it. Many other
mountains in this tract consist of a schistus, which is talcose rather
than micaceous, and subject, in a remarkable degree, to the sort of
sinuosity and inflection here treated of.
The appearances of the primary strata on the coast of Berwickshire,
have been already mentioned, as affording much valuable instruction in
geology. They also exemplify the waving and inflection of the strata
on a large scale, and with great variety. A section of some of them
is given by Dr Hutton, in his _Theory of the Earth_, vol. i. from a
drawing made by Sir James Hall. The nature of the curve superficies
into which the schistus is bent, is the better understood from this,
that, besides transverse sections from north to south, the deep
indentures which the sea has made, and the projecting points of rock,
exhibit many longitudinal sections, in a direction from east to west.
203. The dock-yards at Plymouth are in several places cut out of a
solid rock of primary schistus, singularly incurvated. The inflections
are seen there to great advantage, being exhibited in three sections,
at right angles to one another, transverse, longitudinal and horizontal.
204. From these instances, to which it were easy to add many more, two
conclusions may be drawn. The first of these is very obvious, viz.
that the strata must have been pliant and soft when they acquired
their present form. The bending of an indurated bed of stone into an
arch of great curvature, and without fracture, as in the preceding
examples, is a physical impossibility. Saussure has indeed observed a
fracture to accompany the bending, in one or two cases; but it is an
uncommon phenomenon, and, where it happens, must no doubt be understood
to indicate an imperfect flexibility. Now, if it be granted that the
strata were at any time soft and flexible, since their complete
formation, it will be found impossible to deny their having been
softened by the application of heat.
205. The second conclusion, alluded to above, results from a property,
which belongs very generally, if not universally, to the inflections of
the strata. This consists in their curvature being simple, or in one
dimension only, like a cylindric superficies, not double, or in two
dimensions, like the superficies of a sphere or spheroid. This may be
otherwise expressed by saying, that the sections of the bent strata,
by a horizontal plane, are straight lines, parallel to one another. On
this account, every such stratum seems as if it were bent over all axis
and the axes of all these different bendings, for a great extent of
country, are nearly parallel.
The truth of this is evident, where the strata are seen both
transversely and longitudinally. It holds remarkably of the primary
schistus on the coast of Berwickshire; where the beds of rock, if
cut transversely, by a vertical plane, exhibit the figures of very
complicated curves, with various maxima and minima, and points of
contrary flexure; but, if they are cut by a horizontal plane, the
section will produce nothing but straight lines, nearly parallel.
206. The constancy of the direction of the primary strata, when
estimated by their intersection with the horizontal plane, is often
very remarkable. Their elevation and flexure are subject to great and
sudden changes, so as to pass not only from greater to less, but from
one side to the opposite, within a small distance; but the horizontal
line in which they _stretch_, usually preserves the same bearing
to a great extent. The general direction of the primary strata, in
the south part of Scotland, is from E.N.E. to W.S.W.; and the same
is nearly true of those which compose the ridge of the Grampians on
the north, and the hills of Cumberland and Westmoreland toward the
south, though between the schistus of these three tracts, there is no
communication at the surface, each being entirely separated from the
one next it, by the interposition of secondary strata. I have already
mentioned the observations of Lord Webb Seymour and myself, at the
foot of Ingleborough; and it appears from them, that the vertical
schistus on which that mountain rests, though it still preserves an
eastern and western direction, varies several points from that of the
more northern strata. The strata of Wales return more to the first
mentioned direction, and those of Devonshire and Cornwall agree with
it very nearly. In all this, it will be easily conceived, that I do
not mean to speak with absolute precision, or to deny the existence of
great local irregularities. The result given is only a kind of average,
deduced from observations hardly susceptible of great exactness, and
not yet sufficiently multiplied to give to the conclusion all the
accuracy it may attain.
207. This tendency of the primary strata to take a uniform direction,
has also been observed in other countries. Saussure remarked in the
Alps, that the beds of schistus are generally parallel to the chains of
mountains composed of them;[104] and this remark is probably applicable
to all mountains consisting of primary strata. The general direction,
therefore, of the schistus of the Alps, must be confined between W.
10° S. and W. 40° S. In the Pyrenees, the direction of the strata is
about W.N.W.[105] If Saussure's rule may be depended on, the schistus
of the Altaic, and most of the other great chains in the old continent,
are in directions that run considerably to the south of west. The
Urals, and perhaps some other of the northern chains, are however
entirely different. In the Urals, as we learn not only from the general
direction of the chain, but from a section of it in the 10th volume of
the Nova Acta of Petersburgh (Tab. 12,) the direction of the strata is
nearly from N. to S. This last is probably the direction in the great
chains of South America; so that the uniformity of direction in the
primary strata, which some mineralogists would extend to those of the
whole earth, is certainly imaginary, though there can be no doubt that
it extends over very large portions of the earth's surface.[106]
[Footnote 104: Voyage aux Alpes, tom. i. § 577.]
[Footnote 105: Essai sur la Mineralogie des Pyrenées.]
[Footnote 106: It is perhaps unnecessary to observe, that the two
propositions, that the intersections of the strata with the horizon
are parallel lines; and that they are lines which reserve the same
bearing with respect to the points of the compass; are nearly the same
thing for tracts of moderate extent, but for large portions of the
earth's surface are extremely different. If, for instance, the belt
of primary vertical schistus, which traverses the south of Scotland,
were to be produced eastward in the same plane, from its northern
extremity, where its direction is E.N.E. and its latitude 55° 57', it
would cut the meridian always less obliquely as it advanced, till,
having increased it longitude about 26° 28', it would be at right
angles to the meridian, and its direction of consequence due east and
west. This would happen in the parallel of 58° 51', (on the shore of
the Gulf of Finland, near Revel,) the strata being now extended about
880 G. miles from the Siccar Point. Conversely, vertical strata, having
the same bearing with respect to the meridian, may be in planes very
much inclined to one another. A stratum which bears east and west in
Cornwall, and one that does the same at the east end of the Altaic,
will be in planes, which, if produced, would cut one another at right
angles. All this is sufficiently plain from the doctrine of the
sphere, and is mentioned here merely as a caution to prevent too hasty
conclusions from being drawn from any correspondence of bearing among
the strata of remote countries.
For the sake of those who would deduce the medium bearing of any body
of strata from a number of observations, it may be proper to take
notice, that the true average is not to be found by simply taking
an arithimetical mean among all the observations. A more exact way
is to work by the traverse table, as in keeping a ship's reckoning,
(supposing the distance run to be always unity,) and to compute from
the observed bearings the amount of all the southing or northing, and
also all the easting and westing. The sum of all the latter, divided
by the sum of all the former, is the tangent of the angle which the
general direction of the strata makes with the meridian.]
208. The tendency of the primary strata to remain straight in the
horizontal direction, and to be bent in the vertical, is a phenomenon
which points very directly to the causes from whence it has arisen. A
surface of simple curvature, or a surface straight in one direction,
is what the application of forces to different points of a plane,
which is flexible, though with a certain degree of rigidity, will
naturally produce. The supposition, therefore, that these strata were
once flat and horizontal, and were impelled upward from that situation
before they had become rigid or hard, will explain their having the
kind of curvature which removes them as little as possible from their
original condition. But no other hypothesis affords any reason why
they should have that curvature more than any other. From the falling
in of roofs of caverns, we might expect fracture and dislocation,
without any order or regularity; but certainly no bending or sinuosity,
nor any symmetrical arrangement. If, as some mineralogists allege,
the curvature, as well as inclination of the strata, arose from the
irregularities of the bottom on which they were deposited, why is the
former in one dimension only, and why is it not in every direction,
like that of hills and valleys, or the actual surface of the earth?
Or, lastly, if the whole structure of the primitive mountains is an
effect of crystallization, and if these mountains are now such as
they have ever been from the time of their consolidation, whence is
it, that, in their bendings the law just mentioned is so constantly
observed? Indeed, the idea of ascribing the inflections of the strata
to crystallization, though suggested by Saussure,[107] and since
become a favourite system with several mineralogists, appears to me
in the highest degree unsatisfactory and illusive. The purpose for
which crystallization is here introduced, is not to give a specific
figure to a particular substance, but to arrange the substances which
it has formed and figured, according to certain rules; a work which
we know not how it is to perform, and in which we have no experience
of its power. Accordingly, this principle does not account, in any
way whatever, for the circumstances which attend the inflection of
the strata, for the simple curvature which they affect, nor for that
parallelism of their layers, which, in all their bendings, is so
accurately preserved. It does, indeed, so little serve to explain
these facts, that, were the appearances completely reversed; did the
strata assume the most complex, instead of the most simple curvature;
instead of equidistant, were they converging, or alternately receding
and approaching to one another; the theory of crystallization might
be equally applied to them. The state of the phenomena is a matter of
perfect indifference to such a theory as this; all things are explained
by it with the same facility; the straight and the crooked, the square
and the round, the moveable and the immoveable. Is it not evident that
such an explanation is a mere word; or, if any thing more than a word,
an expression of our ignorance, so awkward and indirect, as to deprive
us of whatever credit might have been gained by a plain and candid
avowal of it?
[Footnote 107: Voyages aux Alpes, tom. i. § 475.]
It should never be forgotten, that a theory which accounts for _any
thing_, and a theory which accounts for _nothing_, stand precisely
on the same footing, and ought to be banished from all parts of
philosophy, as they have been from those sciences which are justly
honoured with the name of accurate. The animated orbs of Aristotle,
and the vortices of Des Cartes, have long ceased to be mentioned in
physical astronomy; the first, because, they accounted for every thing
alike; the second, because, when they accounted for one thing, they
never could be made to account for another. Both theories, therefore,
have very properly been rejected; and, when geology shall undergo a
similar purification, the principle we have been considering will not
be the only sacrifice required of the Neptunian system.
209. An appearance observed in some kinds of primary schistus, which
clearly indicates their deposition by water, and in planes very
different from those in which we now see them, though it might have
been introduced before, is also much connected with the present
argument. This appearance consists of small wavings or undulæ on the
surface of the plates of schistus, precisely similar to these marks
which are left by the sea on a gently inclining beach of sand, at the
ebbing of the tide. All the species of schistus do not seem to afford
instances of these wavings. The rocks which do so, are, I think,
chiefly of the argillaceous kind, but often highly indurated; so that
the laminæ containing the impressions are not to be torn asunder but
with great difficulty. Instances of it abound in the schistus of
Berwickshire, and are also not unfrequent in that of Galloway. All must
agree about the agent which produced these marks; it could be no other
than the sea; but it must have been the sea acting on loose, small and
round particles, lying on a surface which was nearly horizontal.
210. Dr Hutton's theory is no where stronger, than in what relates
to the elevation and inflection of the strata; points in which all
others are so egregiously defective. The phenomena to be connected
are here extremely various, and even in appearance contradictory:
the horizontally of one part of the strata; the inclined or vertical
position of another; the perfect planes in which one set are extended;
the breaking and dislocation found in a second; the inflection and
sinuosity of a third; and almost every where the utmost rigidity and
induration, combined with appearances of the greatest softness and
flexibility; the preservation of a parallelism of superficies in the
midst of so much irregularity, and the assumption of a determinate
species of curvature, under circumstances the most dissimilar; all
these appearances were to be connected with one another, and with
the consolidation of the strata, and this is done by the twofold
hypothesis, of aqueous deposition, and the action of subterraneous
heat. When these circumstances are fairly considered, and when the
shifts which other systems are put to on this occasion are remembered,
I think it will be granted, that few attempts at generalization have
been more successful, than that which has been made by the Huttonian
Theory.
211. To the fact of the elevation of the strata, the study of geology
is much indebted. The stratified form of a great proportion of the
earth's surface, gives to minerals that organization and regularity,
which makes their disposition an object of science, and their inclined
position serves to bring that organization into view, from far greater
depths than we can ever reach by artificial excavations. If, for
instance, the termination of strata, that make with the horizon an
angle of 30°, lying one over another, is seen for a horizontal distance
of two miles; then it is certain, that if these strata have that extent
under ground, which may be reasonably supposed, the thickness of the
whole mass, measured by a line perpendicular to its stratification, is
half the horizontal distance, or amounts to one mile. It would also
require a pit to be sunk from the uppermost of these strata, to the
depth of (2 miles × tan 30°, =) 6093 feet before it could intersect the
undermost; and therefore, if we suppose the same stratum to preserve
the same character for the extent of some miles, we obtain the same
information from inspecting the edge-seams, and see in reality as far
into the bowels of the earth, as if we had sunk a perpendicular shaft
to the depth of 6000 feet.
In general, the length of the horizontal line drawn across the strata,
from the lowest in position to the highest, multiplied into the sine of
the inclination of the strata to the horizon, gives the thickness of
the whole, measured perpendicularly to the plane of the stratification:
and the same horizontal distance, multiplied into the tangent of the
inclination, gives the actual depth at which the lowest stratum would
meet a perpendicular to the horizon, drawn from the highest extremity
of the upper stratum.
In many cases, the extent of stratified materials admitting of such
an examination as this, is much greater than has now been supposed.
M. Pallas describes a range of hills on the south-east side of the
peninsula of the Tauride, which is cut down perpendicularly toward the
sea, and offers a complete section of the parallel beds of a primary,
or, as he calls it, an ancient limestone, inclined at an angle of
45° to the horizon; and this section continues for the length of 130
_versts_, or about 86 English miles. The beds are so regular, that M.
Pallas compares them to the leaves of a book.[108] The height of these
hills does not exceed 1200 feet, but the real height of the uppermost
stratum above the undermost, is 86 × √1/2 = 86 × 5/7 = 61 miles nearly.
[Footnote 108: See Nova Acta Acad. Petropol. tom. x. (1792,) p. 257.]
If therefore we conceive that there is no shift in all this great
system of strata, we in reality are enabled, by means of it, to see no
less than 61 miles into the interior of the earth, nearly a 65th part
of the radius of the globe. It is true, that we can hardly suppose so
great a body of strata to have been raised without shifting, so that
we must diminish this depth considerably; but were it reduced even to
one-half it will appear, that men see much farther into the interior
of the globe than they are aware of, and that geologists are reproached
without reason for forming theories of the earth, when all that they
can do is but to make a few scratches on its surface. Art indeed can do
little more; but nature supplies the deficiency, and makes discoveries
to the attentive observer, on the same great scale with her other
operations.
The simplest account that can be given of the vast body of parallel and
highly inclined strata just mentioned, is, that it consists of the ends
of horizontal strata, or of strata not greatly inclined, that have been
forced up when they were all soft and flexible. This is a much more
conceivable supposition than Pallas's, viz. that the greater part of
this mass has sunk down into some vast cavern in the interior of the
earth.
NOTE XIII. § 53.
_Metallic Veins._
212. The large specimens of native iron found in Siberia and Peru,
mentioned above, § 51, are among the most curious facts in the natural
history of metals. It has been doubted, however, by some, whether they
really belong to natural history, or are not rather to be accounted
artificial productions. If they had been found in the heart of rocks,
or in the midst of metallic veins, no doubt of this sort could possibly
have been entertained; but, as they lie quite on the surface, in the
middle of flat countries, and at a distance from any known vein of
metal, the conjecture that they may be artificial, and the remains of
the iron founderies of ancient and unknown nations, is at first sight
not entirely destitute of probability. This probability, however, will
appear to be the less, the more carefully the specimens are examined.
The metal is too perfect, and the masses too large, to have been melted
in the furnaces, or to have been transported by the machinery, of a
rude people. The specimen in South America weighs 300 quintals, or
about 15 tons, and is soft and malleable.[109] The Siberian specimen,
described by Pallas, is also very large; it is soft and malleable, and
full of round cavities, containing a substance, which, on examination,
has been found to be chrysolite.[110] Now, it is certainly quite
impossible, that, in an artificial fusion, so much chrysolite could
have come by any means to be involved in the iron; but, if the
fusion was natural, and happened in a mineral vein, the iron and the
chrysolite were both in their native place, and their meeting together
has nothing in it that is inexplicable.
[Footnote 109: Phil. Trans. 1788, p. 37. also p. 183, &c.]
[Footnote 110: Kirwan's Mineralogy, vol. ii. art. Native Iron.]
213. Some circumstances in the description of the specimen in South
America, such as the impressions of the feet of men and of birds on its
surface, are not to be accounted for on any hypothesis, and certainly
require more careful investigation. It is said, that this iron is
very little subject to rust, and the analysis of a piece of it by
PROUST makes it probable, that it owes this quality to its union with
nickel.[111] It appears, also, that the country of Chaco, where this
specimen was found, affords many others of the same kind, one of which
is mentioned in the description above referred to. That country lies
on the east side of the Plata, and is a plain extremely level, and of
vast extent, without any appearance of mineral veins; but such veins
may nevertheless exist undiscovered, in a tract subject to periodical
inundations, and where the native rock is covered with alluvial earth
and gravel to a great depth. The veins maybe washed away, and the more
durable substances, such as those pieces of native iron, may be left
behind; and, though they must be of a formation extremely ancient,
according to this hypothesis, they may not have been very long on the
surface.
[Footnote 111: Annales de Chimie, tom. xxxv. Messidor, p. 47.]
214. Specimens of native iron have been found, less remarkable than
the preceding for their size, but in circumstances that excluded
all idea of artificial fusion. Of this sort was MARGRAAF'S specimen
of native iron, the first of the kind that was known; it consisted
of small bits of soft and malleable iron, found in the heart of a
brown iron-stone.[112] This makes it certain, that native iron is a
natural production, and the mere circumstance of great magnitude, in
the specimens before mentioned, does not entitle us to doubt of their
having that same origin. It is a circumstance, besides, not in the
least material to this argument; the smallest piece of native iron
being as much a proof of fusion as the greatest; and the specimen
of Margraaf being just as conclusive in favour of the Huttonian
Theory, as those of Pallas or De Celis, supposing their reality in
mineral productions to be completely established. À metal malleable
and ductile, in ever so small a quantity, cannot be the result of
precipitation from a menstruum, without a very particular combination
of circumstances. Such a metal, can the other hand, can be readily
produced by igneous fusion; so that here the negative and affirmative
parts of the inductive argument may both be regarded as complete.
[Footnote 112: Kirwan's Mineralogy, vol. ii. p. 156.]
215. Mr Kirwan, in order to account for the magnitude of the two large
specimens mentioned above, supposes, that small pieces of native iron
(about the formation of which he appears to have no difficulty) have
been originally agglutinated by petroleum, and left bare, when the
surrounding stony or earthy masses either withered or were washed
off.[113] This is no doubt the most singular of all the opinions which
have been advanced on the subject; and, as it borrows nothing from
analogy, it admits of no proof, and requires no refutation. None but a
chemist of eminence could have ventured with impunity on an assertion
so inconsistent with all the phenomena and principles of his science.
[Footnote 113: Geol. Essays, p. 405.]
216. A remark of the same author, on the subject of the native gold
found in the county of Wicklow in Ireland, is entitled to more
attention. "That these lumps of native gold," he says, "were never in
fusion, is evident from their low specific gravity, and the grains of
sand found in the midst of them. I found the specific gravity of a lump
of the size of a nutmeg to be only 12800, whereas, after fusion, it
became 18700."[114]
[Footnote 114: _Ibid._ p. 402.]
This argument is plausible; but, I think, nevertheless inconclusive.
The sand found in the gold, accounts, at least in part, for its
lightness. It is only by repeated fusions that any of the metals is
brought to its utmost purity and highest specific gravity; and on no
supposition can the melting of gold in the mineral regions, be very
likely to separate it from heterogeneous substances. That quartzy
sand should be found in it, after such a process, is naturally to be
expected. The impressions which the quartz crystals have left on the
Wicklow gold, would be received as a full proof of the fusion of that
metal, if geologists always regulated their theories by the principles
which determine the belief of ordinary men.
217. Don Rubin de Celis, in the paper referred to above, mentions some
masses of silver found at Quantajaia, and also some dust of platina, in
terms that excite a strong desire to have more information concerning
them. They are considered by him as effects of volcanic fire; so we
may conclude, that they contain evident marks of fusion, and would in
this system be ascribed to that heat, from which volcanic fire is but a
partial and accidental derivation.
218. The state also in which gold and silver are often found pervading
masses of quartz, and shooting across them in every direction,
furnishes a strong argument for the igneous origin, both of the metal
and the stone. From such specimens, it is evident, that the quartz
and the metal crystallized, or passed from a fluid to a solid state,
at the same time; and it is hardly less clear, that this fluidity did
not proceed from solution in any menstruum: For the menstruum, whether
water or the _chaotic fluid_, to enable it to dissolve the quartz,
must have had an alkaline impregnation; and, to enable it to dissolve
the metal, it must have had, at the same time, an acid impregnation.
But these two opposite qualities could not reside in the same subject;
the add and alkali would unite together, and, if equally powerful,
form a neutral salt, (like sea-salt,) incapable of acting either on
the metallic or the siliceous body. If the acid was most powerful, the
compound salt might act on the metal, but not at all upon the quartz;
and if the alkali was most powerful, the compound might act on the
quartz, but not at all on the metal. In no case, therefore, could it
act on both at the same time. Fire or heat, if sufficiently intense,
is not subject to this difficulty, as it could exercise its force with
equal effect on both bodies.
219. The simultaneous consolidation of the quartz and the metal is
indeed so highly improbable, that the Neptunists rather suppose, that
the ramifications in such specimens as are here alluded to, have been
produced by the metal defusing itself through _rifts_ already formed
in the stone.[115] But it may be answered, that between the channels
in which the metal pervades the quartz, and the ordinary cracks or
fissures in stones, there is no resemblance whatever: That a system of
hollow tubes, winding through a stone, (as the tubes in question, must
have been, according to this hypothesis, before they were filled by
the metal,) is itself far more inconceivable than the thing which it
is intended to explain; and lastly, that if the stone was perforated
by such tubes, it would still be infinite to one that they did not all
exactly join, or inosculate with one another.
[Footnote 115: Geol. Essays, p. 401.]
220. The compenetration, as it may be called, of two heterogeneous
substances, has here furnished a proof of their having been melted by
fire. The inclusion of one heterogeneous substance within another,
as happens among the spars and drusens, found so commonly in mineral
veins, often leads to a similar conclusion. Thus, from a specimen of
chalcedony, including in it a piece of calcareous spar, Dr Hutton
has derived a very ingenious and satisfactory proof, that these two
substances were perfectly soft at the same time, and mutually affected
each other at the moment of their concretion.[116]
[Footnote 116: Theory of the Earth, vol. i. p. 93.]
Each of these substances has its peculiar form, which, when left to
itself, it naturally assumes; the spar taking the form of rhombic
crystals, and the chalcedony affecting a mammalated structure, or a
superficies composed of spherical segments, contiguous to one another.
Now, in the specimen under consideration, the spar is included in the
chalcedony, and the peculiar figure of each is impressed on the other;
the angles and planes of the spar are indented into the chalcedony,
and the spherical segments of the chalcedony are imprinted on the
planes of the spar. These appearances are consistent with no notion of
consolidation that does not involve in it the simultaneous concretion
of the whole mass; and such concretion cannot arise from precipitation
from a solvent, but only from the congelation of a melted body. This
argument, it must be remarked, is not grounded on a solitary specimen,
(though if it were it might still be perfectly conclusive,) but on a
phenomenon of which there are innumerable instances.
221. According to this theory, veins were filled by the injection of
fluid matter from below; and this account of them, which agrees so
well with the phenomena already described, is confirmed by this, that
nothing of the substances which fill the veins is to be found any where
at the surface. It is not with the veins as with the strata, where, in
the loose sand on the shore, and in the shells and corals accumulated
at the bottom of the sea, we perceive the same materials of which these
strata are composed. The same does not equally hold of metallic veins:
"Look," says Dr Huston, "into the sources of our mineral treasures? Ask
the miner from whence has come the metal in his veins? Not from the
earth or air above, not from the strata which the vein traverses: these
do not contain an atom of the minerals now considered. There is but
one place from whence these minerals may have come; this is the bowels
of the earth; the place of power and expansion; the place from whence
has proceeded that intense heat, by which loose materials have been
consolidated into rocks, as well as that enormous force, by which the
regular strata have been broken and displaced."[117]
[Footnote 117: Theory of the Earth, vol. i. p. 130.]
222. The above is a very just and natural reflection; but if, instead
of interrogating the miner; we consult the Neptunist, we will receive
a very different reply. As this philosopher never embarrasses himself
about preserving a uniformity in the course of nature, he will tell
us, that though it may be true, that neither the air, the upper part
of the earth's surface, nor even the sea, contain at present any thing
like the materials of the veins, yet the time was when these materials
were all mingled together in the chaotic mass, and constituted one
vast fluid, encompassing the earth; from which fluid it was, that the
minerals were precipitated and deposited in the clefts and fissures of
the strata.
223. It is alleged, in proof of this hypothesis, that mineral veins are
found to be less rich as they go farther down, whereas they ought to
be richer if they were filled by the projection of melted matter from
below. But the fact, that mines are less rich as they descend farther,
though it may hold in some instances, is not general, and may therefore
be supposed to arise from local causes, such as are, in respect of us,
accidental, and beyond the limits to which our theories can be expected
to reach. Thus the mines of Mexico and Peru are said to be subject
to the preceding rule; but in the mines of Derbyshire and Cornwall,
the very contrary is understood to take place. Besides, what we are
pleased to call the riches of a mine, are riches relatively to us, and
relatively to a distinction which nature does not recognise. The spars
and veinstones which are thrown out in the rubbish of our mines, may be
as precious in the eyes of nature, as conducive to the great objects
of her economy, and are certainly as characteristic of mineral veins,
as the ores of silver or gold, to which we attach so great a value.
Unless the former are in smaller quantity, or less highly crystallized
at great than at small depths, which I believe is not alleged, no
conclusion can be drawn from substances, which occupy in general but a
small proportion of any vein, and, in their dissemination through it,
do not seem to be always guided by the same law.
224. Again, if the veins were filled by deposition from above, we ought
to discover in them such horizontal stratification as is the effect of
deposition from water, and we should perceive no marks of the materials
having been introduced with violence into their place. The Neptunists
cannot object to the trial of their theory by these two frets.
As to the first, it is acknowledged, that there is a certain regular
disposition of the substances in mineral veins, as stated § 59, but it
is one which has hardly any thing in common with the real phenomena
of stratification. It consists in the distribution of the principal
substances in coats parallel to the sides of the vein, each substance
forming a separate coat. In a vein, for instance, containing quartz,
fluor, calcareous spar, lead, &c. we might expect to find a lining of
quartz crystals, applied immediately to the walls of the mine, and
following exactly the irregularities of their surface; next, perhaps,
a coat of fluor, then of calcareous spar, and last of lead ore in the
centre of the vein, the same order being observed on the opposite
side. These successive coats, it is material to remark, are not in
planes, but in uneven surfaces, of which the inequalities are evidently
determined by those of the walls, that is, of the rock which forms
the sides of the vein; neither are they horizontal, but are parallel
to the walls, whether these be perpendicular or inclined. Here,
therefore, there is no appearance of the action of that statical law
which has directed the arrangement of the other strata, and which tends
to make the plane of every stratum deposited by water perpendicular
to the direction of gravity. The coating of the veins has therefore
been performed under the conduct of some other power than that which
presides over aqueous deposition. If, as the Neptunists maintain, the
materials in the veins were deposited by water, in the most perfect
tranquillity, it is wonderful that we do not find those materials
disposed in horizontal layers, across the vein, instead of being
parallel to its sides; and it seems very unaccountable, that the common
strata, deposited as we are told while the water was in a state of
great agitation, have so rigorously obeyed the laws of hydrostatics, (§
38.) and acquired a parallelism in the planes of their stratification,
which approaches so often to geometrical precision; while the materials
of the veins, in circumstances so much more favourable for doing the
same, have done nearly the reverse, and taken a position, often at
right angles to that which hydrostatical principles require. This is a
paradox which the Neptunian system has created, and which therefore it
is not very likely to resolve.
225. Mere words should have little power to mislead, in a science which
treats of sensible objects, such as are always easily subjected to the
examination of sight or of touch; yet there is some appearance as if
the Neptunists were misled in this, and other instances, by the term
_stratification_. Though an incrustation on the perpendicular face of a
rock has very little affinity to a stratum, such as we are accustomed
to see deposited by water, yet the same name being once imposed on
both, mineralogists have proceeded to reason concerning them, as if
they were precisely the same thing, and were both to be ascribed to
the same cause. Indeed every perpendicular or highly inclined bed of
stone, is inexplicable as an effect of aqueous deposition, in a system,
unprovided, as the Neptunian is,[118] with the means of raising up
such beds from a horizontal into a vertical position. This observation
may also be extended to all cases of vertical stratification. Water
cannot directly arrange its deposits in planes highly inclined, and
therefore I have often wondered to see the Neptunists contending so
eagerly for the stratification of certain rocks, such as granite,
which, being vertical, or highly inclined, was much less friendly to
their system than the entire absence of all stratification would have
been. I was disposed to admire their candour, when the use which
they made of the fact convinced me, that I ought only to wonder at
their inconsequential reasoning. The Huttonian Theory is, indeed, the
only one which possesses the means of reconciling the elevation of
the strata with their horizontal deposition, and which is entitled to
consider stratification, in whatever plane it may be, as originally the
work of the ocean. The geologists who attach themselves exclusively to
the action of water, will never be able to extend the dominion of that
element so far as Dr Hutton has done, by combining it with fire.
[Footnote 118: See preceding note.]
226. But, though the Neptunian system were provided with engines,
powerful enough to raise up strata from a level to a vertical plane,
this would avail nothing in the present instance; since, on no
supposition, can the incrustations on the perpendicular sides of a
vein have ever been horizontal. On no supposition, therefore, can
these incrustations be received as a proof of aqueous deposition: it
may indeed be certainly inferred from them, that the matter which they
consist of was fluid at the time of their formation; but the absence of
all appearance of a horizontal disposition, in any part of the vein,
amounts nearly to a demonstration, that this fluidity did not proceed
from solution in a menstruum. We must therefore conceive the coats
to have been formed during the refrigeration of the melted matter
injected from the mineral regions into the clefts and fissures of the
strata. (§ 59.)
227. Mineral veins, particularly at their intersections with one
another, contain abundant marks of the most violent and repeated
disturbance (§ 56.). Not to mention that they owe their first formation
to the fracture and displacing of rocks already consolidated, it
appears, that they have originated at very different periods, and that
the birth of each has been accompanied with convulsions, which shook
the foundations of the earth. In Cornwall, for instance, the principal
veins, and those which they distinguish particularly by the name of
_Lodes_, have nearly the same direction with the strata or vertical
schistus, extending from about E. N. E. to W. S. W. These, however,
are often intersected nearly at right angles by other mineral veins,
called _Cross Courses_, and this hardly ever happens without the latter
moving, or, as it is called, heaving the former out of their direction.
This plainly indicates, that the cross courses are of later origin
than the others, and that their formation was accompanied with such a
force, as must, in many substances, have moved the whole body of rock
which constitutes the promontory of Cornwall, and probably much more,
for several yards, in a horizontal direction. Sometimes, also, both
the longitudinal and the cross vein are forced out of their place by
a third. These disturbances arise not only from mineral veins, but
from veins of porphyry and granite, the production of which has been
attended with no less violence than of the others.
228. What is here said of Cornwall, is the history, in some degree,
of all mineral countries whatever. The great horizontal _translation_
which has thus accompanied the formation of veins; the movement
impressed on such vast bodies of rock, and the frequent renewal of
these immense convulsions; are not to be explained by the mild and
tranquil dominion of the watery element. They require the utmost power
that is known any where to exist, and were it not for the admonitions
of the volcano and the earthquake, we might doubt if even subterraneous
heat itself possessed an energy adequate to these astonishing effects.
229. From the _heaving_ of one vein by another, it is evident, that
there was a force of protrusion in the direction of one of them, that
acted at the time of its formation. This force cannot be accounted
for on the supposition that veins were produced by the mere shrinking
of the strata; for the rocks could not, in that case, have been rent
asunder, and impelled forward at the same time. It appears most likely,
that fissures in the strata were made, at least in many instances,
and the matter poured into them, nearly at the same time, both being
effects of the same cause, the expansive force of subterraneous heat.
230. It is remarked, at § 56, that the shifting of the strata is best
observed where the veins make a transverse section of beds of rock,
considerably inclined to the horizon. It is also true, that in some
cases the near approach of the strata to the level, may make the shifts
produced by the veins very easy to be discovered. Thus in Derbyshire,
where the mineral veins are in secondary strata, nearly horizontal,
there is almost no instance in which the corresponding strata are not
observed to be on different levels, on the opposite sides of the same
vein.
231. The fact described by De Luc, and referred to at § 55, may, for
what we know of it, admit of being explained in two ways. The great
wedge of rock which appears to be insulated between two branches of the
same vein, may either be a mass that has been broken off, and sustained
by the melted matter that flowed all around it; or, it may be a mass
of rock contained between two veins that are in reality distinct, and
of different formation. Whether this last supposition is the truth,
would probably be evident from a careful examination of both parts
of the vein; as some difference of character cannot fail to be the
consequence of different formation. If no such difference is observed,
the two branches must be supposed to belong to the same vein, and the
only probable explanation of the insulation of so large a mass of rock
will be by the first mentioned supposition. This fact, therefore,
notwithstanding the great attention M. De Luc has bestowed on it,
still requires further examination, before it can be decided whether
it inclines to the Huttonian Theory, as on the first supposition, or
is, as on the latter hypothesis, equally balanced between it and the
_Wernerian_.
232. Whatever be the case with this fact, the general one of pieces of
rock being found insulated in veins, is certainly favourable to the
notion of an injected and ponderous fluid having originally sustained
them. Where, as happens in some instances, the stones contained in
the veins have no affinity to any of the rocks above, they cannot
be supposed to have come any how but from below, and to have been
carried up by the matter of the vein. The instance from the slip at the
Huddersfield Canal has been already mentioned.
233. The preceding observations have been principally directed against
that theory of veins which supposes them to have been filled by
deposition from water. There is another theory maintained by some
of the Neptunists, that the metals in veins were introduced there by
infiltration.[119] This opinion is sufficiently refuted by the fact,
that rarely any metallic ore is found out of the vein, or in the rock
on either side of it, and least of all where the vein is richest.
This is inconsistent with the notion of the ore being carried into
the vein by water percolating through the adjacent rocks, unless some
satisfactory reason is assigned, which determined the water to leave
the ore in the vein and no where else. Besides, this hypothesis does
not account for the formation of the spars and veinstones which fill
the vein, and which appear clearly to have been brought there at the
same time with the ore, and no doubt by the same cause.
[Footnote 119: Geol. Essays, p. 401.]
234. The veins, properly so called, are indefinitely extended; but
there are also thin plates of spar, and of crystals of different kinds,
often found included in rocks, and shut in on all sides, to which the
name of veins is commonly applied. These last ought certainly to be
distinguished from the former, and may not improperly be called _Plate
Veins_ or _Lenticular Veins_, the plate or cake of spar of which they
consist having very often the form of a lens, though, as may be
supposed, considerably irregular. Either of these terms being derived
entirely from external characters, has the advantage of involving
nothing theoretical.
The lenticular veins are certainly not formed like the usual mineral
veins, by injection, since they are shut in, on all sides, by the solid
rock. When they are found, therefore, in stratified rocks, such as
have not themselves been melted, we must conceive them to be composed
of materials more fusible than the surrounding rock, so that they
have been brought into fusion by a degree of heat which the rest of
the rock was able to resist, and, on cooling, have assumed a sparry
structure. When they are found in rocks, of which the whole has been
fluid, they must be considered as component parts of that mass, which,
by an elective attraction, have united with one another, and separated
themselves from the substances to which they had less affinity.
The veins of this kind seem to be connected with those called in
Derbyshire _Pipe Veins_, in which the ores of metals are sometimes
found. The pipe veins, indeed, are not in all cases completely
insulated, but sometimes communicate with the veins properly called
mineral. I am too little acquainted, however, with their natural
history, to Be able to say with certainty to which of the two species
they ought to be referred.
NOTE XIV. § 75.
_On Whinstone._
235. To the facts and reasonings given above, I shall, in this note,
add a few remarks, tending to show, that whinstone is not of volcanic,
nor of aqueous, but certainly of igneous origin.
It is asserted, (§ 62,) that carbonate of lime and zeolite are often
contained in whinstone, but never in lava, and that this circumstance
may sometimes serve to distinguish these stones from one another. With
respect to carbonate of lime, in particular, it seems evident, that
this substance cannot enter into the original composition of any lava,
because the same heat which melted the lava, would, where there was no
greater pressure than the weight of the atmosphere, expel the carbonic
acid and produce quicklime. Notwithstanding this, rocks containing
carbonate of lime, have often been considered as lavas, into the pores
and cavities of which, calcareous matter having been carried by the
infiltration of water, had crystallized into spar. Thus SPALLANZANI,
in his account of the Euganean Hills, in Lombardy, describes some of
the rocks as abounding at their surface, and even in their interior,
with air-bubbles of various sizes, from such as are hardly perceptible,
to some that are half an inch in diameter; and which, he says, are all
of an oval figure, with their longest diameters in the same direction.
This he considers as a proof that the rock is a genuine lava; for the
air-bubbles prove the stone to have had its fluidity from fire; and
by their elongation in the same direction they prove, that the mass
when fluid was also in motion. Spallanzani adds, that _many of these
cavities are filled with crystals of the carbonate of lime, an effect
of the infiltration of water_.[120]
[Footnote 120: Voyages dans les deux Siciles, tom. iii. p. 157. Edit.
de Faujas de St Fond.]
236. Though the argument here advanced for the igneous origin of the
rock may be admitted as conclusive, the introduction of calcareous
spar into it by infiltration must still be questioned. Lava, except
in a state of decay or decomposition, is not readily penetrated by
water; and, if it were, the filling of cavities with spar, by means
of the water percolating through them, would still be subject to many
difficulties, (§12.). Besides, whinstone rocks are frequently found so
full of calcareous spar, or of zeolite, that they would become porous
to such a degree, if the cavities filled with these latter substances
were all empty, that they could hardly sustain their own weight, and
much less that of the great masses of rock incumbent on them. In such
cases, it is certain, that the crystallized substances were part of the
original composition of the rock. The truth is, that the infiltration
of the water is a mere gratuitous assumption, introduced for the
purpose of explaining the existence of carbonated lime in a stone which
had endured the action of intense heat: and this assumption ought of
course to be rejected, if the phenomenon can be explained by a theory,
that is in other respects conformable to nature. The spar, then, may be
considered as a proof, that the rocks in question are to be numbered
with those unerupted lavas which have flowed deep in the bowels of the
earth, and under a great compressing force. This is the more probable,
that the Euganean Hills, like some whinstone hills in our own country,
have, in certain places, a covering of slaty and calcareous strata
incumbent on them, even at their summits,[121] so that the torrent of
melted stone, of which they are admitted to consist, cannot have flowed
from the mouth of a volcano. I do not mean to say, that there are
among these hills no vestiges of volcanic explosion. I am very far from
having _data_ sufficient for drawing this conclusion; but I believe it
may be safely affirmed, that the bulk of them is no more composed of
volcanic lava, than the basaltes of Staffa, or of the Giant's Causeway.
[Footnote 121: Phil. Trans. 1775, p. 34.]
237. But, besides the evidence deduced from calcareous spar and
zeolite, against the rocks containing them being real lava, there are
other marks, even less equivocal perhaps, that distinguish the lavas
which we suppose to have flowed in the mineral regions, from those
which have actually flowed on the surface. These are what we collect
from the disposition, the organization, or, as we may say, the physical
geography of whinstone countries, unlike, in so many respects, to that
of volcanic countries. The shape of whinstone hills; their large flat
terraces, rising one above another; their perpendicular faces, and the
correspondence of their heights even at considerable distances; have
nothing similar to them in the irregular torrents of volcanic lavas.
The phenomena of the former are also on a scale of magnitude very far
exceeding the latter, and clearly indicate, that though both have been
produced by fire, it has been by fire in very different circumstances,
and regulated by very different laws. The structure of the two kinds
of rock agrees, in many respects, and so does their chemical analysis;
but their disposition and arrangement are so dissimilar, that they
cannot be supposed to be of the same formation.
238. This argument, I believe, was first stated by Mr Strange, in
a letter to Sir JOHN PRINGLE, published in the 65th volume of the
_Philosophical Transactions_.[122] That intelligent observer, after
visiting the countries in Europe most remarkable either for burning,
or for what are accounted, extinguished volcanoes, and examining them
with a very discriminating eye, remained convinced, that there are two
distinct species of rock, which both owe their origin to fire; but
to fire acting in circumstances and situations extremely different.
The first is the common volcanic lava; the other, to which he gives
the name of a basaltine rock, comprehends such rocks as the Giant's
Causeway, the basaltes of the Vivarais, of the Euganean Hills, &c.
and differs in nothing from that which is called here by the name
of whinstone. Mr Strange conceived, that the one of these kinds of
stone could, no more than the other, be accounted the work of aqueous
deposition, but was led to the distinction just mentioned, by observing
the organization and arrangement in the rocks of the latter kind, and
comparing them with the disorder and ruin that every where mark the
footsteps of volcanic fire He does not pretend to determine the nature
of the fire to which the basaltine rocks owe their formation, nor the
circumstances in which it has acted: he is satisfied with the negative
conclusion, that it is not volcanic; and his paper affords a specimen
of what is perhaps rare in any of the sciences, and certainly most rare
of all in geology, viz. a philosophic induction carried just as far as
the facts will bear it out, and not a single step beyond that point.
[Footnote 122: Account of Two Giants' Causeways in the Venetian State,
&c. by John Strange, Esq. Phil. Trans. vol. lxv. (1775,) p. 5, &c.]
239. Several other hints contained in this paper are highly deserving
of notice; for we not only find in it the notion of a formation of
basaltic rocks, igneous though not volcanic, but also that of their
simultaneous crystallization,[123] together with the suggestion, that
granite and basalt are of the same origin,[124] These opinions had not,
I believe, occurred at that time to any mineralogist except Dr Hutton,
nor had they been communicated by him to any but a few of his most
intimate friends; so that Mr Strange has without doubt all the merit
of a first discoverer. Indeed, without the knowledge of the principle
of compression, such as it is laid down by Dr Hutton, it was hardly
possible for him to proceed further than he has done. He remarked
the unburnt limestone that lies on the tops of some of the Euganean
basaltes, and seems to have been aware of the great difficulty, which
it was reserved for the Huttonian Theory to overcome. His letter
contains also some excellent general remarks on the rocks of the
Vivarais and Velay, which he had visited, before FAUJAS DE ST FOND had
published his curious and elaborate description of these countries.
[Footnote 123: Phil. Trans, _ubi supra_, p. 17.]
[Footnote 124: _Ibid._ p. 36 and 37.]
240. The cause of the peculiar structure which has just been observed
to distinguish whinstone from volcanic countries, is easily assigned
in the Huttonian Theory. According to that theory, the whinstone rocks
were formed, in the bowels of the earth, of melted matter poured into
the rents and openings of the strata. They were cast, therefore,
in those openings, as in a mould; and received the impression and
character of the rocks by which they were surrounded. Hence the tabular
masses of whinstone, which when soft have been interposed between
strata, and compressed by their weight, so as almost to have themselves
acquired the appearance of stratification. Hence the perpendicular
faces of the same rocks, produced by their being abutted when yet
soft, against the abrupt sides of the strata. The rocks which formed
those moulds have, in many cases, entirely disappeared; in others, a
part still remains, surrounding, or even covering, the basaltes, as in
the Euganean Hills, in those of the Val di Noto in Sicily, the rocks
near Lisbon,[125] and in different parts of Great Britain.
[Footnote 125: Recherches sur les Volcans Eteints du Vivarais; Lettre
du Dolomieu, p. 443.]
Above all, the veins of whinstone which intersect the strata, are the
completest proofs of the theory here given of these rocks, and the most
inconsistent, in all respects, with the hypothesis of their volcanic
origin.
241. If these _criteria_ are applied to what are called extinguished
volcanoes, I have no doubt that many which have been reckoned of that
number, will be found to derive their origin more directly from the
fire of the mineral regions. The basaltic rocks of the Vivarais, I am
well persuaded, belong to this class; and I conclude that they do so,
not only from the account of them given by Mr Strange, but from the
description of Faujas himself, who, though under the influence of the
opposite theory, seems very fair and accurate in his description of
phenomena. The most unequivocal mark of real whinstone rock, and of
a formation in the strictest sense mineral, is where veins of that
kind of rock intersect the strata. Now, in a letter to Buffon, on the
streams of lava found in the interior of certain calcareous rocks in
the lower Vivarais, Faujas describes what can be accounted nothing
else but a vein or dike of whinstone, accompanied with several of
its most remarkable and characteristic appearances: "Figurez-vous un
courant de lave, de la nature du basalte noir, dur et compacte, qui a
percé à travers les masses calcaires, et s'est fait jour dans quelques
parties, paroissant et disparoissant alternativement: Cette coulée
de matière volcanique s'enfonce sous une partie de la ville, bâtie
sur le rocher; elle reparoit dans la cave d'un maréchal, se cache et
se montre encore de temps en temps en descendant dans le vallon, &c.
Ce qu'il y a d'admirable, c'est que la lave forme deux branches bien
extraordinaires, dont l'une s'éleve sur la crête du rocher, tandis que
l'autre coupe horizontalement de grands bancs calcaires escarpés, qui
sont à découvert, et bordent le chemin.
"Quels efforts n'-a-t-il pas fallu pour forcer cette lave se prendre
une telle direction, et se percer cette suite de rochers calcaires?
Si cette longue coulée de lave avoit eu 200 ou 300 toises de largeur,
je ne serois pas surpris qu'un torrent de matière en fusion de ce
volume eut pu produire, des effets extraordinaires et violens; _mais
figurez-vous, Monsieur, que dans les endroits les plus larges, elle n'a
tout-au-plus qu'environ 12 ou 15 pieds; elle n'en a que 3 ou 4 dans
certaines parties_".[126]
[Footnote 126: Volcains Eteints du Vivarais, p. 328, &c.]
This narrow stream is to be traced across the strata for more than a
league and a half; and the whole appeared to Faujas so marvellous,
that he says he almost doubted the testimony of his senses. He would
have done much better, however, to have doubted the conclusions of his
theory; for it was by them that the phenomena before him were rendered
so mysterious and incredible. While he continued to regard what is
described above as a stream of melted lava, which had descended from
the top of one mountain, and climbed up the sides of the opposite, like
water in a conduit pipe, piercing occasionally through vast bodies of
solid rock, it is no wonder that he considered as marvellous what is
indeed physically impossible. Had his belief in the volcanic theory
permitted him to see in all this, not a superficial current, but one of
indefinite depth, he would have beheld the object divested, not of what
was curious and interesting, but of what was incredible or absurd, and
reduced to the same class of things with mineral veins. That it belongs
really to this class, and is no more than a vein or dike of whinstone,
intersecting the strata to an unknown depth, and most probably, like
other veins, communicating with the mineral regions, cannot be doubted
by any one who has studied the subject of basaltine rocks, through
any other medium than the volcanic theory. The ramifications which
run from it into the calcareous rock, contrived, Faujas says, just as
if on purpose to perplex mineralogists, is one of the well known and
characteristic appearances of basaltic veins.
242. It can hardly be doubted, that the lava described by the same
author as heaving up a mass of granite,[127] and including pieces of
it, is a rock of real whinstone. The same may be said of many others;
and, though I pretend not to affirm that there is nothing volcanic in
the Vivarais, I must say, that nothing decidedly volcanic appears in
the description of that country, but many things that are certainly of
a very different origin.
[Footnote 127: Volcains Eteints du Vivarais, fol. p. 365, &c.]
In the present state of geological science, a skilful mineralogist
could hardly employ himself better, than in traversing those ambiguous
countries, where so much has been ascribed to the ancient operation
of volcanic fire, and marking out what belongs either clearly to the
erupted or unerupted lavas, and what parts are of doubtful formation,
containing no mark by which they may be referred to the one of these
any more than the other. Such a work would contribute very materially
to illustrate the natural history of the earth.
243. One of the most ingenious attempts to support the volcanic theory,
is the system of _submarine volcanoes_, imagined by the celebrated
mineralogist DOLOMIEU. The phenomenon that led to this hypothesis,
was what he had observed in the hills near Lisbon, and still more
remarkably in those of the Val di Noto in Sicily, where the basaltine
rocks had regular strata incumbent on them, and in some cases
interposed or alternated with them.[128] It seemed from this evident,
that the strata were of later formation than the stone on which they
rested; and as they must, on every supposition, be held to be deposited
by water, it was concluded, that the lava which they covered had been
thrown out by volcanoes at the bottom of the sea; that the strata had
afterwards been deposited on this lava; and that, in some cases, there
had been frequent alternations of these eruptions and depositions.[129]
[Footnote 128: Mémoire de Deodate de Dolomieu, sur les Volcains
Eteints du Val di Noto, en Sicile. Journal de Phys. tom. xxv. (1784.
Septembre.) p. 191.]
[Footnote 129: Near Vizini, in the Val di Noto, Dolomieu tells us,
that he counted eleven beds, alternately calcareous and volcanic, in
the perpendicular face of a hill, which at a distance appeared like a
piece of cloth, striped black and white; _ubi supra_. He has since made
similar observations in the Vicentine and in Tyrol. Journal de Phys.
tom. xxxvii. (1790), partie 2, p. 200.]
244. Though this hypothesis does certainly deliver the system of
the Volcanists from one great difficulty, it is itself liable to
insurmountable objections. I shall just mention some of the principal.
1. The regular and equidistant strata that we often see covering the
tops of whinstone or basaltic rocks, could not have been deposited in
the oblique and very much inclined position which they now occupy.
This is remarkable in the strata which cover the basaltic rock of
Salisbury _Craig_, near Edinburgh, at its northern extremity. The
strata are very regular, and must have been deposited in a plane nearly
horizontal; yet the surface of the basaltes on which they now rest is
very much inclined, dipping rapidly to the north-east. The necessity of
a horizontal deposition in strata, which, though not now horizontal,
have their planes nearly parallel to one another, has been proved at §
38.
2. If there is any truth in the principles established above, even the
strata themselves have not been consolidated without the action of
fire. By Dolomieu's system, therefore, the consolidation of the strata
which cover the basaltes is not accounted for.
3. There are no means furnished by the hypothesis of submarine
volcanoes for bringing the basalt, and the strata which cover it,
above the level of the sea. If it is said that the waters of the sea
have been drained off, the objections are all incurred that have been
stated at § 37.[130] If it is said, that the rocks themselves have been
elevated by a force, impelling them upwards, we say, that the existence
of such a force, when admitted, furnishes another means of explaining
the whole phenomenon, namely, that of the injection of melted matter
among the strata, the same that is used in the Huttonian Theory.
[Footnote 130: Dolomieu adopts this supposition; he thinks, that the
surface of the sea must have been formerly 500 or 600 toises above its
present level. _Ibid._ p. 196.]
4. The phenomena of basaltic veins are not in the least explained by
the hypothesis of submarine volcanoes. That hypothesis, then, even
if the foregoing objections were removed, does not serve to explain
all the facts respecting the rocks of this genus, and wants, of
consequence, one of the most important characters of a true theory.
It must be allowed, however, that it makes a considerable approach to
such a theory, and that the submarine volcanoes of Dolomieu, have an
affinity to the unerupted lavas of Dr Hutton.
245. Though in these remarks I have endeavoured to expose the errors
of the volcanic system, I cannot but consider that system as coming
infinitely nearer to the truth than the Neptunian. It has the merit
of distinguishing an order of rocks, which bears no marks of aqueous
formation, and in which the crystallized, sparry, or lava-like
structure, bespeaks their primeval fluidity, and refers their origin
to fire. The Neptunian system, on the other hand, strives to confound
the most marked distinction in the mineral kingdom, and to explain
the formation, both of the stratified and unstratified rocks, by
the operation of the same element. Though chargeable with this
inconsistency, it has become the prevailing system of geology; and the
arguments which support it are therefore entitled to attention.
246. It will no doubt be thought singular, that the same mineralogist,
whom we have just seen exerting his ingenuity in defence of the
volcanic system, should now appear equally strenuous in defence of the
Neptunian. Though Dolomieu contends for the volcanic origin of some
basaltic rocks, he does not admit that all basaltes is volcanic, nor
even all of igneous formation. Thus he states, that he had examined at
Rome some of the most ancient monuments of art, executed in basaltes,
brought from Upper Egypt, and that he could discover no mark of the
action of fire in any of them.[131]
[Footnote 131: Journal de Physique, tome xxxvii. (1790,) partie 2, p.
193.]
On the contrary, he found that some of them consisted of green
basaltes, which changes its colour to a bronze, when exposed even to a
moderate heat, and which therefore, he argues, can never have endured
any strong action of fire.
The answer to this argument is very plain, if we admit the effects
ascribed by Dr Hutton to the compression which necessarily takes place
in the mineral regions. If indeed the heat in those regions resembled
exactly that of our fires at the surface, it would not be easy to deny
the above conclusion, which therefore certainly holds good against
the volcanic origin of the Egyptian basaltes. But there is no reason
why, under strong compression, the colouring matter of these stones
might not be fixed, and indestructible by heat, though it can be easily
volatilized or consumed when such compression is removed. This argument
then is against the volcanic; but not against what has been called the
_Plutonic_ formation of basaltes.
247. As to the other marks of fire which Dolomieu sought for and did
not find in the above mentioned stones, we are not exactly informed
in what they consisted. If the crystallized or spathose texture that
belongs to this description of stones was wanting, the specimens were
not to be considered as of the real basaltic or whinstone genus,
whatever their name or history may seem to indicate. If they did
possess that texture, they had the only mark of an igneous origin that
could be expected, supposing that origin to have been in the bowels of
the earth. No part, therefore, of the observations of this ingenious
mineralogist, can be considered as inconsistent with the theory of
basaltic rocks which has been laid down above.
248. Bergman had before reasoned on this subject precisely in the same
manner, but from better data, as the stones from which he derived
his argument were in their native place: "Trap," says that ingenious
author, (that is whinstone,) "is found in the stratified mountains
of West Gothland, in a way that deserves to be described. The lower
stratum, which is several Swedish miles in circuit, (10-1/2 of these
miles make a degree,) is an arenaceous stone, horizontal, resting on
granite, and having its particles agglutinated by clay. The stratum
above this is calcareous, full of the petrifactions of marine animals,
and above this is the trap. These three kinds of rock compose the
greater part of the mountains just mentioned, though there are some
other beds, particularly very thin beds of marl and of clay, which
separate the middle stratum, both from that which is under it and
over it, and are frequently so penetrated with bitumen that they
burn in the fire. This schistus is black; when burnt it becomes red,
and afterwards, when washed with water, affords alum. How can it be
supposed," he adds, "that the trap has ever been violently heated,
while the shistus on which it is incumbent retains its blackness, which
however it loses by the action even of a very weak fire?"[132]
[Footnote 132: Bergman de Productis Volcaniis, Opuscula, tom. iii. p.
214, &c.]
The answer to this argument is already given. The reasoning, as
in the former instance, is conclusive only against the action of
volcanic fire, or fire at the surface; but not against the action of
heat deep in the bowels of the earth, and under the pressure of the
superincumbent ocean. In such a situation, the bituminous schistus
might be in contact with the melted basalt, and yet there might be no
evaporation of the volatile, nor combustion of the inflammable parts.
It does not, however, always happen, that the bituminous substances, or
substances alterable by fire, which are found in contact with basaltes,
are without any mark of having endured the operation of fire. Instances
in which such operation is apparent are given above, § 30; and more
will be added in the conclusion of this note.
249. The same mineralogist founds another argument for the aqueous
formation of whin or trap on the existence of that stone in the form
of veins, included in primeval rocks: "Invenitur hoc saxum (trap) in
Suecia pluribus locis, sæpeque in montibus primævis, angustas implens
venas, adeo subtilis structuræ, ut particulæ sint impalpabiles, et, dum
niger est, genuinum efficit lapidem Lydium. In hisce montibus, nulla
adsunt ignis subterranei vestigia."[133]
[Footnote 133: Opuscula, _ubi supra_.]
The phenomena here described, namely, a vein of compact whinstone
traversing a primary rock, is, without doubt, as incapable of being
explained by the operation of a volcano, as it is by that of aqueous
deposition. It is, however, a most complete proof of the original
softness of the substance of which the veins consist, and affords one
of the strongest possible arguments for such an operation of fire as is
supposed in the present theory. The main arguments, therefore, which
have been proposed as subversive of the igneous origin of basaltes, are
only subversive of their formation by one modification of fire, viz.
of fire acting near the surface; and thus the weapons which directly
pierce the armour of the Volcanist, and inflict a mortal wound, are
easily turned aside by the superior temper of the _Plutonic_ mail.
250. An argument founded on facts very similar to some of the
preceding, and leading to the same conclusion, is employed by the
mineralogist to whom the Neptunian system owes its chief support.
Werner, in his observations on volcanic rocks and on basaltes, has
rested his proof of the aqueous formation of the latter, on their
interposition between beds of stone in mountains regularly stratified,
and obviously formed by water. He describes an instance of this in the
basaltic hill of _Scheibenberg_; and the facts, though most of them are
not uncommon, are highly deserving of attention. Near the top of this
hill, and above the basaltic rock which composes the body of it, he
tells us, that there was a sand-pit; a circumstance which he appears
to consider as not a little singular. It was, however, at the bottom of
the hill, that he met with the appearances which chiefly attracted his
notice: "First," says he; "or lowest, was a thick bank of quartzy sand,
above that a bed of clay, then a bed of the argillaceous stone called
wacke; and upon this last rested the basaltes." "When I saw," adds he,
"the three first beds running almost horizontally under the basaltes,
and forming its base; the sand becoming finer above, then argillaceous,
and at last changing into real clay, as the argil was converted into
wacke in the superior part; and, lastly, the wacke into basaltes; in a
word, when I found a perfect transition from pure sand to argillaceous
sand, from the latter to a sandy clay, and from this sandy clay,
through many gradations, to a fat clay, to wacke, and at last basaltes,
I was irresistibly led to conclude, that the basaltes, the wacke, the
clay, and the sand, are all of one and the same formation; and that
they are all the effect of a chemical precipitation during one and the
same submersion of this country."[134]
[Footnote 134: "Combien je fus surpris de voir en arrivant au fond,
un epais _banc de sable quartzeux_, puis au-dessus une _couche
d'argile_, enfin une couche de la pierre argileuse nommée _Wacke_, et
sur celle-ci reposer le _basalte_. Quand je vis les trois premiéres
couches s'enfoncer _presqu' horizontalement sous le basalte_, et former
ainsi sa _base_; le sable devenir plus fin au-dessus, puis argileux,
et se changer enfin en vraie argile, comme l'argile se convertissoit
en wacke dans sa partie supérieure; et finalement la wacke en basalte;
en un mot, de trouver ici une _transition parfaite_ du _sable pur_ au
_sable argileux_, de celui-ci à _l'argile sablonneuse_, et de _l'argile
sablonneuse_, par plusieurs gradations, à l'argile _grasse_, à la
_wacke_ et enfin au _basalte_.
"A cette vue, je fus sur le champ et irrésistiblement entrainé à
penser, (comme l'auroit été sans doute tout connoisseur impartial
frappé des conséquences de ce phénomène;) je fus, dis je,
irrésistiblement entrainé aux id es suivantes: Ce _basalte_, cette
_wacke_, cette _argile_, et ce _sable, sont d'une seule et même
formation_; ils sont tous l'effet d'une _precipitation par voie humide_
dans une seule et même submersion de cette contrée; les eaux qui la
couvroient alors transportoient d'abord le _sable_, puis deposoient
l'_argile_, et changoient peu-à-peu leur précipitation en _wacke_, et
enfin en vraie _basalte_.--Journal de Physique, tom. xxxviii. (1791,)
Partie i. p. 415.]
First, as to the sand on the top of this basaltic hill, it is most
probably the remains of certain sandstone strata that originally
covered the basaltic part, but are now worn away. We are therefore to
consider this as an instance of a basaltic rock, interposed between
strata that are undoubtedly of marine origin. In this, however, there
is nothing inconsistent with Dr Hutton's theory of basaltes; on the
contrary, it is one of the principal facts on which that theory is
founded. It has indeed been argued by some mineralogists, that bodies
thus contiguous must owe their origin to the same element, and that
a mineral substance cannot be of more recent formation than that
which lies above it. But the maxim, that a fossil must have the same
origin with those that surround it, does not hold, unless they have a
certain similarity of structure. It is, for instance, the want of this
similarity, that authorizes us to assign different periods of formation
to mineral veins, and to the rocks in which they are included.
In a succession of strata, no one can doubt, that the lowest were the
first formed, and the others in the order in which they lie; but, when
between two strata of sandstone or of limestone we find an intermediate
rock, so different as to resemble lava, and to have nothing schistose
or stratified in its composition, the same instrument cannot be
supposed to have been employed in the formation of both; nor is there
any reason why we may not suppose, that the intermediate body was
interposed between the other two, by some action subsequent to their
formation. It was thus that Dolomieu concluded, when he saw a lava-like
stone interposed between calcareous strata in the Val di Noto, that,
though contiguous, these two rocks could not possibly be of the same
formation; and thus far it is certain, that every unprejudiced observer
must agree with him.
251. But the circumstance on which Werner seems to lay the greatest
stress, is the gradual transition from the sand to the basalt, through
the intermediate steps of clay and wacke; this gradual transition he
considers as a direct proof, that they are all of the same formation.
A gradual transition of one body into another, can only be said to
take place, when it is impossible to define their common boundary,
or to determine the line where the one begins and the other ends.
Now, if this be the proper notion of gradual transition, I must say,
that after much careful examination, I have never seen an instance,
in which such a transition takes place between whinstone and the
contiguous strata. The _line_ of separation, though in some places
less evident than in others, has, on the whole, been marked out with
great precision; and, though the stones have been firmly united, or,
as one may say, welded one upon another, yet, when a fresh fracture
was obtained, the stratified and unstratified parts have rarely failed
to be distinguished. The fresh fracture is indeed often necessary, for
many species of whinstone get by decomposition a granulated texture at
the surface, so as hardly to be distinguished from real sandstone.
Some of the kinds of primary schistus also, particularly the
argillaceous, when much indurated, have in their structure a
considerable resemblance to whinstone; they are slightly granular, or
laminated, and have a tendency to a sparry texture. Where it happens
that this sort of schistus and whinstone are contiguous, it is natural
to expect, that their common boundary will be traced with difficulty,
and in many parts will be quite uncertain. Still, however, if a careful
examination is made; if the effects of accidental causes are removed;
and, above all, if the more ambiguous instances are compared with the
more decisive, and interpreted by them, though single specimens may be
doubtful, we will hardly ever find that any uncertainty remains with
respect to entire rocks.
252. This general fact, which I state on much better authority than
that of my own observations, viz. on those of Dr Hutton, is not given
as absolutely without exception. The theory of whinstone which has
been laid down here, leads us indeed to look for some such exceptions.
It is certain, that the basis of whinstone, or the material out of
which it is prepared by the action of subterraneous heat, is clay in
some state or other, and probably in that of argillaceous schistus.
It follows, of consequence, that argillaceous schistus may by heat be
converted into whinstone, or the material out of which is prepared by
the action of subterraneous heat, is clay in some state or other, and
probably in that of argillaceous schistus. It follows, of consequence,
that argillaceous schistus may by heat be converted into whinstone.
When, therefore, melted whinstone has been poured over a rock of such
shistus, it may, by its heat, have converted a part of that rock into
a stone similar to itself; and thus may now seem to be united, by
an insensible gradation, with the stratum on which it is incumbent;
and phenomena of this kind may be expected to have really happened,
though but rarely, as a particular combination of circumstances seems
necessary to produce them. Hence it is evident, that stones may
graduate into one another, without being of the same formation; and
that it is fallacious to conclude, from the insensible transition
of one kind of rock into another, without any other circumstance of
affinity, that they have both the same origin.
I am disposed, therefore, to make some limitation to what is said
in § 72, where I have expressed an absolute incredulity as to such
transitions as are here referred to. The great skill and experience
of the mineralogist who has described the strata at Scheibenberg, do
not allow us to doubt of his exactness, though some of the appearances
are such as decomposition and wearing might well enough be supposed to
produce. The fairest way is to take Mr Werner's observations just as
they are given us, and to try whether they cannot be explained without
the assistance of his theory. In effect, the wacken which he describes,
rests, it would seem, on an unconsolidated bed of clay; and it may be
supposed, that a part of this bed has been converted into wacken by
the heat of the incumbent mass, and has thus produced the apparent
gradation from the one substance to the other. As the appearances of
the rocks of Scheibenberg seem to be considered by Werner as furnishing
a very strong, and even an unexpected confirmation of his system, I
cannot help thinking, that an explanation of them, on the principles
of Dr Hutton, without any straining or forcing of those principles,
contributes not a little toward extending the empire of the latter over
all the phenomena of geology.
253. Another fact, which has been much infilled on of late, in proof
of the aqueous formation of basaltic rocks, is that shells are found
in them. Of the reality of this fact, however, or at least of the
instances hitherto produced, great doubts I think may be reasonably
entertained. The specimens of the supposed basaltes, with shells
included in them, that are chiefly relied on, are found at Portrush in
Ireland, a rocky promontory to the westward of the Giant's Causeway,
and separated from it by a considerable body of calcareous strata.
Some of these specimens were brought to Edinburgh about a year ago,
and were supposed, I believe, to contain an irrefragable proof of the
Neptunian origin of the basaltic promontory where they were found. I
went to see these specimens in company with Lord Webb Seymour and Sir
James Hall; and, on examining them carefully, we were all of opinion,
that the stones which contained the shells, or the impressions of the
shells, were no part of the real basaltes. They were all very compact,
and had all more or less of a siliceous appearance, such as that of
chert; they had nothing of a sparry or crystallized structure; their
fracture was conchoidal, and but slightly uneven. In two of them,
one of which bore the impression of a _cornu ammonis_, the schistose
texture might be distinctly perceived. A specimen which accompanied
them, but in which there was no shell, served very exactly to explain
the relation between these stones and the true basaltes. Part of this
specimen was a true basalt, and the rest a sort of hornstone, exactly
the same with that in which the shells were, and not unlike the jasper
that is under the whinstone of Salisbury Crag, and in contact with
it; so that on the whole it was evident, that the rock containing the
shells is the schistus or stratified stone, which serves as the base of
the basaltes, and which has acquired a high degree of induration, by
the vicinity of the great ignited mass of whinstone.
This solution of the difficulty has since been confirmed by
observations made on the spot by Dr Hope, who discovered two or three
alternations of the basaltic rock, with the beds of the schistus in
which the shells are contained.
254. This also explains some observations of Spallanzani, made in
the island of Cerigo, on the coast of Greece, the Cythæra of the
ancients.[135] The base of that island is limestone; but it abounds
also in unstratified rocks, which the Italian naturalist supposes to
be of volcanic origin; but which, if I mistake not, we would regard
as whinstone, or perhaps porphyry; and they are said to contain
oyster-shells and pectinites of a large size, perfectly mineralized.
These petrifactions, however, Spallanzani says, are not contained
in the lava that has actually flowed, but in stones which have only
endured a slighter action of fire. Without the commentary afforded by
the Portrush specimens, it would be difficult to make out any thing
very precise from this description. By help of the information derived
from those specimens, we may conclude, that the condition of the
shells in them, and in the rocks of Cerigo, is perfectly alike; and
that, in both cases, the shells are involved in parts of the rock which
are truly stratified, but which have been, in some degree, assimilated
to the basaltes by the heat which they have endured. Spallanzani would
probably have used exactly the same terms which he employs in speaking
of Cerigo, if he had been required to describe the petrified shells at
Portrush.
[Footnote 135: Journal de physique, tom. xlviii. (1798,) p. 278.]
255. In the instances just mentioned, the petrified marine objects are
not found in the real whinstone; but if they were found in it, when it
borders on stratified rocks containing such objects, the thing would
not be at all surprising, nor furnish any argument against the igneous
consolidation of the stone. If a torrent of melted matter was poured
in among the strata, by a force which at the same time broke up and
disordered those strata, nothing could be more natural, than that this
matter should contain fragments of them, and of the objects peculiar to
them.
In one instance, mentioned by Mr Strange, this seems actually to have
taken place. In the Veronese, a country remarkable for a mixture of
limestone strata, containing marine objects, with volcanic or basaltine
hills, he assures us, that he had seen a mass of stone, which had
evidently concreted from fusion, in which the marine fossil bodies,
originally, as he supposes, contained in the strata, were perfectly
distinguishable, though variously disfigured.[136] It may be, that
in this, as in the foregoing examples, it was not real basaltes, or
real lava, which contained the shells, but the conterminal rock; but,
supposing it to be as Mr Strange represents it, there appears to be
no inconsistency between the phenomenon, and the igneous origin of
the rock in which the shells were included. Here, however, it should
be remarked, that the presence of great pressure, to prevent the
conversion of the shells into quicklime, seems absolutely necessary;
and that the phenomenon of these basaltic petrifactions, requires the
application of heat to have been deep under the surface of the earth.
[Footnote 136: Phil. Trans. 1775, p. 25.]
256. The phenomena we have been considering, have been selected as the
most unfavourable to the igneous origin of basaltic rocks; and we have
seen, that when duly examined, they are not at all inconsistent with
it. We are now to take a view of some appearances, that seem quite
irreconcilable with the aqueous formation of these rocks.
Where whinstone rocks are found in masses, bounded by the strata,
and insulated among them, they subject the Neptunian system to great
difficulties. For, supposing it true that this stone may be produced by
the precipitation and crystallization of mineral substances dissolved
in water, yet it seems unaccountable, that this effect has been so
local and limited in extent, as often to be confined to an irregular
figure of a few acres, while, all round, the substances deposited have
had no tendency to crystallization, and have been formed into the
common secondary strata. The rock of Salisbury _Craig_, for instance,
is a mass of whinstone, having a perpendicular face eighty or ninety
feet high toward the west, and extending from north to south with
a circular sweep about 900 yards. The whole of this rock rests on
regular beds of secondary sandstone, not horizontal, but considerably
depressed toward the north-east: the rock is loftiest in the middle,
and decreases in thickness toward each end, terminating at its northern
extremity in a kind of wedge. It is covered at top, toward that
extremity, with regular beds of sandstone, perfectly similar to those
on which it is incumbent; and it is not improbable, that this covering
formerly extended over the whole.
Now, what cause can have determined the column of water, which rested
on the base at present occupied by this rock, to deposit nothing but
the materials of whinstone, while the water on the south, west, and
north, was depositing the materials of arenaceous and marly strata?
Wherefore, within this small space, was the precipitate every where
_chemical_, to use the language of Werner, while close to it, on either
side, it was entirely _mechanical?_ Why is there, in this case, no
gradation? and why is a mere mathematical line the boundary between
regions where such different laws have prevailed? Whence also, we may
ask, has the basaltic deposit been abruptly terminated toward the west,
so as to produce the steep face which has just been mentioned? The
operation of currents, or of any motion that can take place in a fluid,
will furnish no explanation whatever of these phenomena; yet they are
phenomena far from being peculiar to a single hill; they are among the
most general and characteristic appearances in the natural history of
whinstone mountains; and a geological theory which does not account for
them, is hardly entitled to any consideration.
257. The basaltic rock, just described, is also covered, at least
partly, with strata perfectly similar to those that lie under it.
Now, it appears altogether unaccountable, that after the water had
done depositing the materials of the whin on the spot in question,
the former order was so quickly resumed, and a deposition of sand,
and of the other materials of the strata, took place just as before.
All this is quite unintelligible; and the principles of the Neptunian
system seem here to stand as much in need of explanation, as any of the
appearances which they are intended to account for.
258. The unequal thickness, and great irregularity in the surface
of the whinstone mass, here treated of, and of many rocks of the
same kind, is also a great objection to the notion of their aqueous
formation. This seems to have been perceived by Werner, in the instance
of the rocks formerly mentioned; and he endeavours to explain it, by
supposing, that much of these rocks has been destroyed by waste and
decomposition, so that an irregularity of their surface, and want of
correspondence has been given to them, which they did not originally
possess. In the instance of Salisbury _Craig_, however, we have a
proof, that the great irregularity of surface, and the inequality of
thickness, do not always arise from these causes. The thinnest part
of that rock, toward its northern extremity, is still covered by the
strata in their natural place, and has been perfectly defended by them
from every sort of wearing and decay. The cuneiform shape, therefore,
which this rock takes at its extremities, and the great difference of
its thickness at them and in the middle, is a part of its original
constitution, and can be attributed to nothing casual, or subsequent to
its consolidation.
The same may be said of many other basaltic rocks, where an inequality
of thickness, most unlike to what belongs to aqueous deposits, is known
to exist in beds of whinstone that are still deep under the surface.
Thus the toadstone of Derbyshire, even where it has a thick covering of
strata over it, has been found, by the sinking of perpendicular shafts,
to vary from the thickness of eighteen yards to more than sixty, within
the horizontal distance of less than a furlong. Nothing of this kind
is ever found to take place in those beds of rock which are certainly
known to originate from aqueous deposition, and no character can more
strongly mark an essential difference of formation.
259. We have had frequent occasion to consider the characters of
those masses of whinstone which are so often found interposed between
stratified rocks. These have been found in general very adverse to the
Neptunian system; and two of them which yet remain to be mentioned,
are even more so than any of the rest.
Where a bed or tabular mass of whinstone is interposed between strata,
and wherever an opportunity offers of seeing its termination, if the
strata under it are not broken, it may be remarked, that they do not
abut themselves bluff and abrupt against the whin. On the contrary,
if we mark the course of the stratum which covers the whinstone, and
of that which is the base of it, we shall find they converge toward
one another, the interposed mass growing thinner and thinner, like a
wedge. When the latter terminates, the two former come in contact, and
have no stratum interposed between them. Thus the roof and base of the
whinstone rock are contiguous beds, that appear as if they had been
lifted up and bent, and separated by an interposed mass. Had the whole
been an effect of simultaneous deposition, the regular strata must have
been abruptly terminated by the whin, like two courses of different
forts of masonry where they meet with one another.
260. From this wedge-form of the whinstone masses, and in general from
the irregularity of their surfaces, another conclusion follows, similar
to the preceding, and one which has been already mentioned. Where the
surface of the interposed mass is greatly inclined to the horizon, the
strata which rest on this inclined plane, are nevertheless as exactly
parallel to that plane, and to one another, as if they were really
horizontal. It is certain, therefore, that they were not deposited on
the same inclined plane on which they now rest; for, if so, they would
have been still nearly horizontal, and by no means parallel to the
inclined side of the whinstone. This follows from the nature of aqueous
deposition, as already explained.
We have a remarkable instance of the phenomenon here referred to, in
the rock of Salisbury _Craig_, of which mention has been so often made,
and in which almost every circumstance is united, that can serve to
elucidate the natural history of basaltic rocks. The north end of that
rock is in the figure of a wedge, with its inclined side considerably
steep, and covered by strata of grit, perfectly regular, and parallel
to the surface on which they lie. The inspection of them will convince
any one, that they were not deposited by the water, on a bottom so
highly inclined as that on which they now rest. They are of a structure
very schistose; their layers very thin; so that any inaccuracy of
their parallelism would be readily observed. The appearances of the
horizontal deposition of these strata, are indeed so clear, and so
impossible to be misunderstood, that the followers of the Huttonian
system would not risk much, if they were to leave the whole theory of
whinstone to the decision of this single fact, and should agree to
abandon that theory altogether, if the Neptunists can shew any physical
or statical principle, on which the deposition now described can
possibly have been made; or will point out the rule, by which nature
has given a structure so nicely stratified to arenaceous beds deposited
on a surface so highly inclined. If no such principle can be pointed
out, though we cannot conclude that the Huttonian Theory is true, we
certainly may conclude that the Neptunian is false.
261. Proofs of the igneous formation of whinstone, still more direct,
are derived from the induration of the contiguous strata; from their
disturbance when interfered by veins of whinstone; and from the
charring of the coal which happens to be in contact with these veins.
These are considered above at § 66, 67, &c.; and it is particularly
taken notice of at § 66, that pieces of sandstone are sometimes found
as if floating in the whinstone, and, at the same time, greatly altered
in their texture. One of the best and most unequivocal instances of
this sort which I have seen, is to be found on the south side of
_Arthur's Seat_, near Edinburgh. The rock which composes the upper
part of the hill, on that side, is a whinstone breccia, such as we have
many examples of, and, I believe, very much resembling what is called
a _lava brecciata_ by the volcanic geologists. The stony fragments
included in this compound mass, are for the greater part rounded;
and some of them are of whinstone, others of porphyry, strongly
characterized by rectangular maculæ of feldspar, and many seem to be
of sandstone, but so considerably altered, as to leave it at least
disputable whether they really are so or not. In one part, however,
where the face of the rock is nearly perpendicular, a narrow ridge is
seen standing out from the rest, and of a different colour, being more
entirely covered with moss than the rock round about it, and, as may
be presumed from that circumstance, less liable to decomposition. On
examination I found, that this ridge does not consist of whinstone, but
of a very hard and highly consolidated sandstone. It appears to be the
edge of a stratum, of the thickness of about nine or ten inches, and of
the height of fifteen or sixteen feet. It is not perfectly straight,
but slightly waved, its general direction being nearly vertical; and
it is on both sides firmly embraced by the whinstone. When broken, it
appears that this sandstone resembles in colour, and in every thing
but its greater consolidation, and more vitreous structure, the common
grit found at the bottom of the hill, and over all the adjacent plain.
262. If all these circumstances are put together, there appears but
one conclusion that can be drawn from them. We have here the manifest
marks of some power which could lift up this fragment of rock from its
native place, distant at least several hundred yards from its present
situation, place it upright on its edge, encompass it with a solid
rock, of a nature quite heterogeneous to itself, and bestow on it, at
the same time, a great addition of solidity and induration. If the mass
in which this stone is now imbedded, be supposed to have been once in
fusion, and forcibly thrown up from below, invading the strata, and
carrying the fragments along with it, the whole phenomena now described
admit of an explanation, and all the circumstances accord perfectly
with one another; but, without this supposition, they are so many
separate prodigies, which have no connection with one another, nor with
any thing that is known. It is indeed impossible, that the effects of
motion and heat can be more clearly expressed than they are here, or
the subject in which these powers resided more distinctly pointed out.
263. The preceding facts being susceptible but of one interpretation,
are on that account extremely valuable. The phenomena of Salisbury
_Craig_, near the same place, are almost equally free from ambiguity.
The basaltic rock which forms that precipice, rests on arenaceous or
marly strata; and these, in their immediate contact with the former,
afford an instance of what is mentioned § 67, namely, the conversion
of the strata in such situations into a kind of petrosilex, or even
jasper. The line which separates the one rock from the other, is,
at the same time, so well defined, as, in the eyes even of the most
determined Neptunist, to exclude all idea of insensible gradation.
264. The same rock affords some remarkable instances of the disturbance
of the strata contiguous to the whinstone. The beds of the former are
bent upwards in several places; and, at one in particular, form an
arch, with its convexity downward, so as to make it evident, that the
force which produced this bending was directed from below upwards.
265. It is, however, where whinstone takes the form of veins,
intersecting the strata, that the induration of the latter is most
conspicuous. The coast of Ayrshire, and the opposite coast of Arran,
exhibit these veins in astonishing variety and abundance. The strata
are, in many instances, so _reticulated_ by the veins, and intersected
at such small distances, that it seems necessary to suppose, that the
fissures in them were hardly sooner made than filled up. This at least
is true, if the veins are to be accounted all of the same formation;
and, in the greatest number of instances by far, there is no mark of
the one being posterior to the other.
266. The induration of the sides of these veins, in some cases, has
been such, that the sides have become more durable than the vein
itself; so that the whinstone has been worn away by the washing of the
waves, and has left the sides standing up, with an empty space, like
a _ditch_, between them. One of these I remarked on the south side of
Brodick Bay, in Arran, which, where it met the face of an abrupt cliff
was not less than forty or fifty feet in depth.
267. I shall pass over whatever argument might be drawn in favour
of our system, from the slender ramifications of the veins, and
the varieties of their sizes, from a few inches to many fathoms in
diameter, and also from the connection which they often appear to have
with the great tabular masses of basaltes; and shall only add a few
remarks on the charring of coal in the vicinity of veins or masses
of whinstone. The connection between the charring of coal and the
presence of whinstone, was first observed by Dr Hutton; and, as far as
opportunities of verifying the observation have yet occurred, appears
to be a fact no less general than it is curious and interesting. In the
coal mines of Scotland, it certainly holds remarkably, particularly in
those about Saltcoats in Ayrshire, where a whinstone dike is known to
stretch across the whole of the coal country, and to be every where
accompanied with blind or uninflammable coal. At Newcastle, dikes of
the same kind are met with, and one, in particular, in what is called
the _Walker_ Colliery, has proved the action of subterraneous fire, to
the satisfaction of mineralogists nowise prejudiced in favour of the
Huttonian system.
The coal found under basaltes, in the Island of Sky, has been already
mentioned, § 140. To what was said concerning the fibrous structure of
the parts of that fossil in immediate contact with the whin, it may be
added, that it is also charred in those parts, so as to have hardly
any flame when it is burnt, though further down it is of the nature of
ordinary coal. Indeed, if there be any truth in Mr Kirwan's general
remark, that it is common to find wood coal under basaltes, it must
be understood to arise from this, that the coal in contact with the
basaltes is frequently charred, and its fibrous structure, by that
means, rendered more visible.
268. It has been objected to the supposition of coal having its
bituminous part driven off by the heat of the whinstone, that this
ought not, on Dr Hutton's principles, to happen in the mineral
regions. But it may be replied, as has been done above, that the local
application of heat might certainly produce this effect, and might
drive off the volatile ports from a hotter to a colder part of the
same stratum. The bitumen has not been so volatilized and expanded as
entirely to escape from the mineral regions; but it has been expelled
from some parts of a mass, only to be condensed and concentrated in
others. This supposition coincides exactly with the appearances.
269. The native or fossil coke which accompanies whinstone, has been
distinguished into two varieties. The first is the most common, in
which, though the coal is perfectly charred, it is solid, and breaks
with a smooth and shining surface. The second is also perfect charcoal,
but is very porous and spongy. This substance is much rarer than the
other. Dr Hutton mentions an instance of it at the mouth of the river
Ayr, where there is a whinstone dike.[137] I had the satisfaction of
visiting it along with him. It was in the bed of the river, below the
high water mark; the specimens had the exact appearance of a _cinder_.
[Footnote 137: Theory of the Earth, vol. i. p. 611.]
In the banks of the same river, some miles higher up, he found a piece
of coal, belonging to a regular stratum, involved in whinstone, and
extremely incombustible. It consumed very slowly in the fire, and
deflagrated with nitre like plumbago. This be considered as the same
fossil which has been described under the name of _plombagine_. Near
it, and connected with the same vein of whinstone, was a real and
undoubted plumbago.
From these circumstances he also concluded, that plumbago is the
extreme of a gradation, of which fossil-coal is the beginning, and is
nothing else than this last reduced to perfect charcoal This agrees
with the chemical analysis, which shows plumbago to be composed of
carbon, combined with iron.
In confirmation of this theory, he mentions a specimen, in his
possession, of steatical whinstone, from Cumberland, containing nodules
of a very perfect and beautiful plumbago; and he also takes notice of a
mine of this last, in Ayrshire, which, on the authority of Dr Kennedy,
who has examined it with great care, I can state as being contained,
or enveloped in whinstone; and I hope the public will soon be favoured
with a particular description of this very interesting spot, by the
same ingenious and accurate observer.
270. Thus the mineralogical and chemical discoveries agree in
representing coal, blind coal, plombagine, plumbago, as all
modifications of the same substance, and as exhibiting the same
principle, carbon, in a state of greater or less combination. As the
last and highest term of this series should be placed the _diamond_;
but we are yet unacquainted with the matrix of this curious fossil,
and its geological relation to other minerals. When known, they will
probably give to this substance the same place in the geological, as in
the chemical arrangement: in the mean time, it is hardly necessary to
remark, how well all the preceding facts agree with the hypothesis of
the igneous formation of whinstone, and how anomalous and unconnected
they appear, according to every other theory.
271. Notwithstanding all this accumulated and unanswerable evidence
for the igneous formation of basaltes, a great objection would still
remain to our theory, were it not for the very accurate and conclusive
experiments concerning the fusion of this fossil, referred to above, §
75. A strong prejudice against the production of any thing like a real
stone by means of fusion, had arisen, even among those mineralogists,
who were every day witnesses of the stony appearance assumed by
volcanic lava. They still maintained, on the authority of their own
imperfect experiments, that nothing but glass can ever be obtained
by the melting of earths or of stones, in whatever manner they are
combined.
An ingenious naturalist, after describing a block of basaltes, in which
he discovered such appearances, as inclined him to admit its igneous
consolidation, rejects that hypothesis, merely from the imaginary
inability of fire to give to any substance a stony character: "Quelque
mélange,"says he, "de terres que l'on suppose, quelque soit le degré
de feu que l'on imagine, quelque soit le tems que l'on emploie, il est
très certain que l'on n'obtiendra pas, par le seul fluide igné, ni
basalte, ni rien qui lui ressemble."[138]
[Footnote 138: Journal de Phys. tom. xlix. (1799,) p. 36.]
Sir James Hall's experiments have completely demonstrated the contrary
of what is here asserted: they have added much to the evidence of the
Huttonian system; and, independently of all theory, have narrowed the
circle of prejudice and error.
NOTE XV. § 83.
_On Granite._
1. _Granite Veins._
272. It is said above, § 77, that granite is found in unstratified
masses, and in veins. In the former of these conditions, it constitutes
entire mountains, and forms the central ridge of many of the greatest
chains that traverse the surface of the earth. It is the granite of
this kind that has been most generally described by travellers and
mineralogists. The veins have not been so much attended to, though
they are of peculiar importance for ascertaining the relation between
granite and other fossils.
273. Though Dr Hutton was the first geologist who explained the nature
of granite veins, and who observed with attention the phenomena
which accompany them, he is not the first who has mentioned them. M.
Besson found veins of this kind in the Limoges, in an argillaceous
schistus, and unconnected, as far as appeared, with any large mass of
granite.[139]
[Footnote 139: Journal de Phys. tom. xxix. p. 89.]
Saussure met with granite veins in the Valorsine, but did not see them
distinctly. He ascribed them to infiltration.[140] The date of this
observation is in 1776: He afterwards discovered similar appearances at
Lyons.[141]
[Footnote 140: Voyage au Alpes, tom. i. § 598, 599.]
[Footnote 141: _Ibid._ § 601.]
Werner also, in enumerating the substances of which veins are formed,
reckons granite as one of them.
274. Veins of granite may be considered as of two kinds, according as
they are connected, or not connected apparently with any large mass of
granite, it is probable, that these two kinds of veins only differ in
appearance, and that both are connected with masses of the same rock,
though that connection is visible in some instances, and invisible in
others. The distinction, however, whatever it be with respect to the
thing observed, is real with respect to the observer; and, as it is
right, in a description of facts, to avoid every thing hypothetical, I
shall speak of these veins separately.
275. Veins of granite, having no communication, so far as can be
discovered, with any mass of the same rock, are found in the Western
Islands of Scotland, peculiarly in that of Coll, where they traverse
the beds of gneiss and hornblende schistus, which compose the main
body of the island. They are sometimes several fathoms in thickness,
obliquely intersecting the planes of the strata just mentioned, which
are nearly vertical. In these veins the feldspar is predominant; it is
very highly crystallized, and of a beautiful flesh colour. Many smaller
veins are also to be met with in the same place; but no large mass of
granite is found, either in this or the adjacent island of Tiree.
276. The Portsoy granite, of which mention has been already made, §
80, also constitutes a vein or dike, traversing a highly indurated
micaceous schistus, about a mile to the eastward of the little town
of Portsoy, and not visibly connected with any large mass of the same
kind. More dikes than one of this granite have been observed near the
same spot.
A similar granite is likewise found inland, in the neighbourhood of
Huntly, about eighteen miles south of Portsoy; but whether in the shape
of a vein or a mass, I have not been able to learn.
277. Veins of granite are also frequent in Cornwall, where they are
known by the name of _lodes_, the same name which is applied in that
country to metallic veins. The granite veins frequently intersect
the metallic, and are remarkable for producing shifts in them, or
for throwing them out of their natural direction. The mineral veins,
particularly those that yield copper and tin, run nearly from east to
west, having the same direction with the beds of the rock itself, which
is a very hard schistus. The granite lodes, as also those of porphyry,
called _elvan_ in Cornwall, are at right angles nearly to the former;
and it is remarked, that they generally heave the mineral veins, but
that the mineral veins seldom or never heave the cross-veins. In this
country, therefore, the veins of granite and porphyry are posterior in
formation to the metallic veins. These veins of granite may perhaps be
connected with the great granitic mass that runs longitudinally through
Cornwall, from Dartmoor to the Land's End. This much is certain, that
their directions in general are such, that, if produced, they would
intersect that mass, nearly at right angles.
278. The granite veins in Glentilt, where Dr Hutton made his first
observations on this subject, are not, I believe, visibly connected
with any large mass of the same rock.[142] The bed of the river Tilt,
in the distance of little more than a mile, is intersected by no
less than six very powerful veins of granite, all of them accompanied
with such marks of disorder and confusion in the strata, as indicate
very strongly the violence with which the granite was here introduced
into its place. These veins very probably belong to the great mass
of granite which is known to form the central ridge of the Grampians
further to the north; but they are several miles distant from it, and
the connection is perhaps invisible in the present state of the earth's
surface.
[Footnote 142: Trans. Royal Society Edin. Vol iii. p. 77, &c.]
279. The second kind of granite vein, is one which proceeds visibly
from a mass of that rock, and penetrates into the contiguous strata.
The importance of this class of veins, for ascertaining the relation
between granite and other mineral bodies, has been pointed out, §
82; and by means of them it has been shown, that the granite, though
inferior in position, is of more recent formation than the schistus
incumbent on it; and that the latter, instead of having been quietly
deposited on the former, has been, long after its deposition and
consolidation, heaved up from its horizontal position, by the liquid
body of granite forcibly impelled against it from below.
It has been alleged, in order to take off the force of the argument
derived from granite veins, that these veins are formed by
infiltration, though, to give any probability to this supposition,
it would be necessary to show, that water is able to dissolve the
ingredients of granite; and even if this could be done, the direction
which the veins have, in many instances, rising up from the granite,
is a proof, as remarked § 82, that they cannot be the effect of
infiltration.
Another objection has been thrown out, namely, that the veins here
referred to are not of true granite, according to the definition
which mineralogists have given of that substance. The force of a
fact, however, is not to be lessened by a change of names, or the
use of arbitrary definitions. The general fact is, that the granitic
mass, and the vein proceeding from it, constitute one continuous,
and uninterrupted body, without any line of separation between them.
The geological argument turns on this circumstance alone; and it is
no matter whether the rock be a syenite, a granitelle, or a real
granite. The phenomenon speaks the same language, and leads to the same
conclusion, whatever be the technical terms the mineralogist employs in
describing it.
280. It must, however, be admitted, that a difference of character is
often to be observed between the granite mass and the veins proceeding
from it; sometimes the substances in the latter are more highly
crystallized than in the former; sometimes, but more rarely, they are
less crystallized, and, in some instances, an ingredient that enters
into the mass seems entirely wanting in the vein. These varieties,
for what we yet know, are not subject to any general rule; but they
have been held out as a proof, that the masses and the veins are not
of the same formation. It may be answered, that a perfect similarity
between substances that, on every hypothesis, must have crystallized
in very different circumstances, is not always to be looked for; but
the most direct answer is, that this perfect similarity does sometimes
occur, insomuch that, in certain instances, no difference whatsoever
can be discovered between the mass and the vein, but they consist of
the same ingredients, and have the same degree of crystallization. Some
instances of this are just about to be remarked.
281. A strong objection to the supposed origin of granitic veins
from infiltration, and indeed to their formation in any way but by
igneous fusion, arises from the number of fragments of schistus, often
contained, and completely insulated in those veins. How these fragments
were introduced into the fissures of the schistus, and sustained till
they were surrounded by the matter deposited by water, is very hard
to be conceived; but if they were carried in by the melted granite,
nothing is more easily understood.
The following are some of the places where the phenomena of granite
veins may be distinctly seen.
282. The island of Arran, remarkable for collecting into a very small
compass a great number of the most interesting facts of geology,
exhibits many instances of the penetration of schistus by veins of
granite. A group of granite mountains occupies the northern extremity
of the island, the highest of which, Goatfield, rises nearly to the
height of 3000 feet, and on the south side is covered with schistus
to the height of 1100. From thence, the line of junction, or that
at which the granite emerges from under the schistus, winds, so far
as I was able to observe, round the whole group of mountains, with
many wavings and irregularities, rising sometimes to a greater, and
descending sometimes to a much lower level, than that just mentioned.
Along this line, particularly on the south, wherever the rock is laid
bare, and cut into by the torrents, innumerable veins of granite are to
be seen entering into the schistus, growing narrower as they advance
into it; and being directed, in very many cases, from below upwards,
they are precisely of the kind which the infiltration of water could
not produce, even were that fluid capable of dissolving the substances
which the vein consists of. From this south face of the mountain, and
from the bed of a torrent that intersects it very deeply, Dr Hutton
brought a block of schistus, of several hundred weight, curiously
penetrated by granite veins, including in them many insulated fragments
of the schistus.
From this point, the common section of the granite and schistus
descends towards the west side of the mountain, and is visible at the
bottom of a deep glen, (Glen-Rosa,) which detaches Goatfield from the
hills farther to the west. The junction is laid bare at several places
in the bed of the river which runs in the bottom of this glen; and in
all of them exhibits, in a greater or less degree, the appearances of
disturbance and violence which have accompanied the injection of the
granite veins. Many circumstances render this spot interesting to a
geologist, and, among others, an intersection of the granite, a little
above its junction with the schistus, by a dike or vein of very compact
whinstone.
The same line of junction is found on the opposite, or north-east, side
of the mountain, where it is intersected by another little river, the
Sannax, which on this side determines the base of the mountain. This
junction is no less remarkable than the other two.
The island of Arran contains, I have no doubt, many other spots where
these phenomena are to be seen; but I have had no opportunity of
observing them, nor do I find that Dr Hutton met with any others in his
visit to this island.
283. Another series of granite veins is found in Galloway, which was
first discovered by Dr Hutton and his friend Mr Clerk, and afterwards
more fully explored by Sir James Hall and Mr Douglas, the present Earl
of Selkirk. The two last traced the line of separation between a mass
of granite and the schistus incumbent upon it, all round a tract of
country, about eleven miles by seven, extending from the banks of Loch
Ken westward; and in all this tract they found, "that wherever the
junction of the granite with the schistus was visible, veins of the
former, from fifty yards, to the tenth of an inch in width, were to be
seen running into the latter, and pervading it in all directions, so
as to put it beyond all doubt, that the granite of these veins, and
consequently of the great body itself, which was observed to form with
the veins one uninterrupted mass, must have flowed in a soft or liquid
state into its present position."[143] I have only farther to add, that
some of these veins are remarkable for containing granite, not sensibly
different, in any respect, from the mass from which they proceed.
[Footnote 143: Trans. Royal Society Edin. vol. iii. p. 8.]
284. In Invernessshire, between Bernera and Fort Augustus, the same
phenomena occur on the north side of Loch Chloney, where some granite
mountains rise from under the schistus. In travelling near this place,
Lord Webb Seymour and myself were advertised of our approach to a
junction of granite and schistus, by finding among the loose stones on
the road many pieces of schistus, intersected with veins of feldspar
and granite. We walked along this junction for more than a mile; and
toward the east end, where the road leaves it, we saw, in the bed of
a stream that runs into Loch Chloney, many beautiful specimens of
granitic veins pervading the schistus, and branching out into very
minute ramifications.
285. The last instance I have to mention from my own observation, is
at St Michael's Mount in Cornwall. That mount is entirely of granite,
thrust up from under a very hard micaceous schistus, which surrounds
it on all sides. At the base of it, on the west side, a great number
of veins run off from the granite, and spread themselves like so many
roots fixed in the schistus: they are seen at low water. In the smaller
veins, the granite is of very minute, though distinct parts; in the
larger, it is more highly crystallized, and is undistinguishable from
the mass of the hill.
Besides the above, Cornwall probably affords many other instances of
the same kind, which I have not had an opportunity to examine. Such
instances may in particular be looked for at the Land's End, where a
promontory, consisting of a central part of granite, and covered by
a micaceous schistus on both sides of it, is cut transversely by the
sea coast, and the contact of the granite and schistus of course twice
exposed to view.
286. Scotland also affords other examples of granite veins, and some
of them have been actually described. Mr Jameson has taken notice of
some which he saw in the bottom of the river Spey, at Glen Drummond, in
Badenach, and has represented them in an engraving.[144] They traverse
the strata in various directions, and inclose pieces of the micaceous
schistus; and, from the great number of loose blocks which he found,
exhibiting portions of such veins, it is probable, that they are very
numerous in this quarter. The same mineralogist mentions some instances
of similar veins in the Shetland Isles.[145]
[Footnote 144: Mineralogy of the Scottish Isles, vol. ii. p. 3.]
[Footnote 145: _Ibid._ p. 216.]
In Ross-shire, Sir George Mackenzie has observed a great variety of
granite veins, some of them of large size. One of them, in particular,
not far from Coul, when first discovered, was supposed to be a
single mass, rising from under the schistus; but, on a more careful
examination, has been found to be a part of a great system of veins
which intersects the micaceous schistus of this tract in various
directions.
287. The granite veins are not the only proof that this stone is more
recent than some other productions of the mineral kingdom. Specimens
of granite are often found, containing round nodules of other stones,
as, for example, of gneiss or micaceous schistus. Such is the specimen
of granite containing gneiss, which Werner himself is said to be in
possession of, and to consider as a proof, that the schistus is of
greater antiquity than the granite. Such also seemed to me some pieces
of granite, which I met with in Cornwall, near the Land's End; and
others which I saw in Ayrshire, in loose blocks, on the sea coast
between Ayr and Girvan. It is impossible to deny that the containing
stone is more modern than the contained. The Neptunists indeed admit
this to be true, but allege, that all granite is not of the same
formation; and that, though some granite is recent, the greater part
boasts of the highest antiquity which belongs to any thing in the
fossil kingdom. This distinction, however, is purely hypothetical; it
is a fiction contrived on purpose to reconcile the fact here mentioned
with the general system of aqueous deposition, and has no support from
any other phenomenon.
2. _Granite of Portsoy._
288. The granite of Portsoy is one of the most singular varieties of
this stone, and is remarkable for this circumstance, that the feldspar
is the substance which has assumed the figure of its proper crystal,
and has given its form to the quartz, so that the latter is impressed
both with the acute and obtuse angles belonging to the rhombic figure
of the former. The angular pieces of quartz thus moulded on the
feltspar, and ranged by means of it in rows, give to this stone the
appearance of rude alphabetical writing.
Now, Dr Hutton argued, that substances precipitated from a solution,
and crystallizing at liberty, cannot be supposed to impress one another
in the manner here exemplified; and that they could do so only when
the whole mass acquired solidity at the same time, or at the same time
nearly.[146] Such simultaneous consolidation can be produced in no way
that we know of, but by the cooling of a mass that has been in fusion.
[Footnote 146: Theory of the Earth, vol. i. P. 104.]
289. A granite, brought from Daouria by M. PATRIN, and described by him
in the Journal de Physique for 1791, p. 295, under the name of _pierre
graphique_, seemed to Dr Hutton to have so great a resemblance to the
granite of Portsoy, that he ventured to consider them both as the same
stone, and as both containing quartz moulded on feltspar.[147] It
should seem, however, from further explanations, which M. Patrin has
since given, but Dr Hutton was mistaken in his conjecture, and that, in
the _pierre graphique_ of the former mineralogist, the quartz gives its
form to the feltspar, preserving in its crystals their natural angle of
120 degrees[148] It is impossible, I think, to doubt of the accuracy
of this statement; and the graphical stone of Portsoy must therefore
be admitted to differ materially from that of Daouria. They are not,
however, without some considerable affinity, besides that of their
outward appearance; for, though the quartz in the former is generally
moulded on the feltspar, the feldspar is also occasionally impressed by
the quartz, and sometimes even included in it. They may be considered
as varieties of the same species of granite; and the _pierre graphique_
of Corsica is probably a third variety, different from them both.
[Footnote 147: Trans. Royal Society Edin. vol. iii. p. 83.]
[Footnote 148: Journal Britainnique, (of Geneva,) 1798, vol. viii.
Sciences et Arts, p. 78.]
290. It would seem, however, that all these stones lead exactly to
the same conclusion. M. Patrin describes his specimen as containing
quartz crystals, that are for the most part only _cases_, having their
interior filled with feltspar "Le feltspath en masse contient des
crysteaux quartzeux, qui n'ont le plus souvent que la carcasse, et dont
l'interieur est rempli de feltspath; souvent il manque à ces carcasses
quelques unes de leurs faces, et souvent la section de cette pierre
dans un sens transversal aux crysteaux, presente une suite de figures
qui sont des portions d'hexagones, et qui ne resemblent pas mal à des
caractères Hebraiques."[149]
[Footnote 149: Journal Britannique, _Ibid._]
These imperfect hexagonal cases of quartz, filled with feldspar,
certainly indicate the crystallization of substances, which all
assumed their solidity at the same time, and, in doing so, constrained
the figures of one another. To use the words of Dr Hutton, "whether
crystallizing quartz inclose a body of feltspar, or concreting feltspar
determine the shape of fluid quartz, particularly if we have, as is
here the case, two solid bodies including and included, it amounts to
a demonstration, that those bodies have concreted from a fluid state
of fusion, have not crystallized, in the manner of salts, from a
solution."[150]
[Footnote 150: Trans. Royal Society Edin. _ubi supra_, p. 84.]
291. The quartz in granite so generally receives the impressions of
all the other substances, particularly of the feldspar and schorl,
and appears to be so passive a body, that it has been doubted by some
mineralogists, whether in this stone it ever assumes its own figure,
except where cavities afford room for its crystallization. But it
is certain that, beside the Daourian granite just mentioned, there
are others, in which the quartz is completely crystallized. Of this
sort are some specimens, found in a granite vein on the west side of
the hill of St Agnes, in Cornwall. The vein traverses the primitive
schistus, of which that hill consists, from south to north nearly:
the stone is much decomposed, and the feldspar in general is almost
reduced to the state of clay. In this decomposed mass, quartz crystals
are found, having the shape of double hexagonal pyramids, perfectly
regular and complete. The side of the hexagon, which is the base of the
two opposite pyramids, varies from half a tenth to a tenth of an inch
in length, and is the same with the altitude of each of the pyramids.
In some few specimens, the two pyramids do not rest on the same base,
but are separated by a very short, though regular, hexagonal prism.
The surfaces of these crystals are rough, and somewhat opaque, with
slender spiculæ of schorl frequently traversing them. This roughness
is occasioned by slight furrows on the surface of the crystal, very
regularly disposed, and parallel to one another, being without doubt
impressions from the thin plates of the feldspar, which surrounded
the crystal, and slightly indented it. They very much resemble some
impressions, remarked by Dr Hutton in the granite of Portsoy, and
ascribed by him also to a similar cause. He has represented these in
his Theory of the Earth, vol. i. plate ii. fig. 4. The action and
reaction of two crystallizing bodies, hardly admits of a stronger and
more unequivocal expression, than in these two instances.
Where the granite was little decomposed, the quartz was not easily
disengaged from the mass it was imbedded in, and often broke in pieces
before it could be extricated. The crystallization of the quartz,
therefore, would not have been discovered, but for the decomposition of
the feltspar; and it is probable, that similar crystallizations exist
in many granites where they are not perceived.
292. Some mineralogists are inclined to think, that the regular
crystallization of quartz is to be found only in what they call
secondary granites, or in those that are of a formation subsequent
to the great masses which constitute the granite mountains. It is
indeed true, that in the instances given here, both from Cornwall and
Daouria, the granites containing quartz crystals are from veins that
intersect the primary schistus, and are therefore, on every hypothesis,
of a formation subsequent to that schistus. But it does not follow
from thence, that they are less ancient than the great masses of
unstratified granite; with these last they are most probably coëval,
nor can there be any reason for thinking the crystallization of quartz
a mark of more recent formation than that of feltspar.
3. _Stratification of Granite._
293. What are the various modes in which granite exists, is a question
not absolutely decided among mineralogists. 1. That it exists as a
schistose stone of a fissile texture, in gneiss and _veined granite_,
is on all hands admitted, though in this state the name of granite
is generally withheld from it. 2. That it exists often without any
indication of a fissile texture, and altogether unstratified, is
likewise acknowledged. 3. That it is found in veins, intersecting
the strata, has been shown above. The only mode of its existence
subject to dispute, is that in which it is said to be stratified in
its outward configuration, but not schistose in its texture. On this
point mineralogists do not perfectly agree: Dr Hutton did not think
that this was a state in which granite ever appears. When not schistose
in its structure, he supposed it to be unstratified altogether; and
he considered it as a body which, like whinstone, was originally in
a state of igneous fusion, and, in that condition, injected among
the strata. The school of Werner, on the other hand, maintain, that
granite, if not always, is generally stratified, and disposed in beds,
sometimes horizontal, though more frequently vertical, or highly
inclined.
In forming an opinion where there are great authorities on opposite
sides, a man must trust chiefly to his own observations, and ought
to esteem himself fortunate if these lead to any certain conclusion.
Mine incline me to differ from Dr Hutton, on the one hand, and from
the Neptunists, on the other, as they convince me, that granite does
form strata where it has no character of gneiss; and, at the same
time, induce me to suspect, that the stratification ascribed by the
Neptunists to the granite mountains, is, in many instances, either an
illusion, or at least something very different from what, in other
stones, is accounted stratification.
294. The first example I ever saw of granite that was stratified,
and yet had no character of gneiss, was at Chorley Forest, in
Leicestershire. The greater part of that forest has for its base a
hornstone schistus, primary and vertical; and, on its eastern border,
particularly near Mount Sorrel, are beds of granite, holding the same
direction with those of the schistus. The stone is a real granite;
it has nothing in its internal structure of a schistose or fissile
appearance; and its beds, which it is material to remark, are no
thicker than those of the hornstone strata in the neighbourhood. This
granite is remarkable, too, for being close to the secondary sandstone
strata; I did not see their contact, but traced them within a small
distance of one another; so that I think it is not likely that any body
of rock intervenes. At the same time that I state my belief of this
rock of granite being in regular strata, I must acknowledge, that a
very intelligent mineralogist, who viewed these rocks at the same time,
and whose eye was well practised in geological observation, remained in
doubt concerning them.
295. Another instance of a real granite, disposed in regular beds, but
without any character of gneiss, is one which I saw in Berwickshire,
in Lammermuir, near the village of Priestlaw. The little river of
Fassnet cuts the beds across, and renders it easy to observe their
structure. The beds are not very thick; they run from about S.S.W. to
N.N.E. like the schistus on either side of them. I was in company with
Sir James Hall when I saw these rocks; we examined them with a good
deal of attention, and traced them for more than a mile in the bed of
the river; and, if I mistake not, our opinions concerning them were
precisely the same.
296. What exists in two instances may exist in many, and, after these
observations, I should be guilty of great inconsistency, in refusing
to assent to the accounts of Pallas, De Luc, Saussure, and many other
mineralogists, who so often represent granite as formed into strata.
In some cases, however, it is certain, that the stratification they
describe is extremely unlike that in the two instances just mentioned,
and indeed very unlike any thing that is elsewhere known by the name
of stratification. For example, the stratification must be very
ambiguous, and very obscurely marked, that was not discovered till
after a series of observations, continued for more than twenty years,
by a very skilful and distinguishing mineralogist. Yet such undoubtedly
is the stratification of Mont Blanc, and of the granite mountains in
its neighbourhood, as it escaped the eyes of Saussure, in the repeated
visits which he made to them, during a period of no less extent than
has just been mentioned. It was not till near the conclusion of those
labours, to which the geologists of every age will consider themselves
as highly indebted, that, having reached the summit of Mont Blanc, he
perceived, or thought that he perceived, the stratification of the
granite mountains. The _Aiguilles_ or Needles which border the valley
of Chamouni, and even Mont Blanc itself, appeared to be formed of vast
tabular masses of granite, in position nearly vertical, and so exactly
parallel, that he did not hesitate to call them by the name of strata.
Till this moment, these same mountains, viewed from a lower point,
had been regarded by him as composed of great plates of rock, nearly
vertical indeed, but applied, as it were, round an axis, and resembling
the leaves of an artichoke;[151] and the fissures by which they are
separated from one another, had been considered as effects of waste and
degradation. "But now," (says he, speaking of the view from the top of
Mont Blanc,) "I was fully convinced, that these mountains are entirely
composed of vast plates of granite, perpendicular to the horizon, and
directed from N.E. to S.W. Three of these plates, separated from each
other, formed the top of the _Aiguille du Midi_, and other similar
plates, decreasing gradually in height, compose its declivity to the
south."[152]
[Footnote 151: Voyages aux Alpes, tom. ii. § 910, &c.]
[Footnote 152: Voyages aux Alpes, tom. iv. § 1996.]
297. Saussure was so strongly impressed with the appearances of what
he accounted regular stratification, such as water only can produce,
and such as must have been in the beginning horizontal, that, placed
as he now was, on one of the highest points of the earth's surface, he
formed the bold conception, that the summit on which he was standing
had been once buried under the surface, to the depth at least of half
the diameter of the mountain, and horizontally distant from its present
place by a line not less than the whole height of the mountain; the
granite beds which compose that mountain, having been raised by some
enormous power from their horizontal position, and turned as on an
axis, till they were brought into the vertical plane. In this notion,
which suits so well with the nature of mountains really composed of
vertical strata, and which does credit to the extent of Saussure's
views, it is wonderful that he did not see the overthrow of the
geological system be had adopted, which is provided with no means
whatsoever of explaining these great effects.
Such, then, were the ideas suggested to Saussure, by viewing the
mountains of the Alps from the highest of their summits. His great
experience, his accurate knowledge of the objects before him, and the
power he had acquired of dissipating those illusions, to which, in
viewing mountainous tracts, the eye is peculiarly subject, all conspire
to give great weight to his opinion. Yet, as this opinion is opposed
by that which he himself had so long entertained, before it can be
received with perfect confidence, it will require to be verified by new
observations. It seems certain, that the beds of rock here described,
differ from all ordinary strata, both horizontal and vertical, in the
circumstance of their vast thickness, three of them being so large as
to form the main body of a mountain. Their parallelism cannot easily
be ascertained; and they have at best but a very slight resemblance to
such beds as water is known to produce.
298. Their parallelism is difficult to be ascertained; for, on account
of the magnitude and inaccessibility of the objects, it is impossible
to place the eye in any situation, where it shall not be much nearer to
one part of the planes whereof the parallelism is to be estimated, than
to another. Indeed, one can perceive a cause which may have rendered
the parallelism of the plates of granite which compose the _aiguilles_,
more accurate in appearance than in reality, when viewed from a point
so elevated as the summit of Mont Blanc. For, even on the supposition
that the comparison of those plates to leaves of artichokes was just,
and that the planes of their separation converged toward one another,
in ascending to the top, when they were viewed from a point more
elevated than that top, this convergency would be diminished, and, by
the force of the perspective, might even be converted into parallelism.
We cannot at present ascertain what effect this cause of deception may
have actually produced.
299. The observations of Saussure concerning the stratification
of granite, are not, however, in all instances, liable to these
objections; and it seems to be on much less exceptionable grounds
that he pronounces the granite of St Gothard to be stratified. The
gneiss and micaceous schistus which constitute the lower part of that
mountain, are succeeded by a granite without any schistose appearance,
but divided into large plates, exactly parallel to the beds of the
former gneiss. These he regards as real strata. On studying them in
detail, he says, considerable irregularities were to be observed, but
not greater than in the case of limestone or micaceous schistus.[153]
It may be inferred from this, that these plates of granite are not so
thick but that they admit of comparison with beds that are known with
certainty to be of aqueous formation, and I am therefore disposed to
believe, that the granite of St Gothard, in this part at least, is
stratified. The transition from gneiss to granite en masse, is not
uncommon, as Saussure has observed in other instances, and as we are
just about to consider more particularly.
[Footnote 153: Voyages aux Alpes, tom. iv. § 1830.]
300. In the mountains of our own country, some difficulties concerning
the stratification of granite have also occurred. In Arran, for
instance, the mountain of Goatfield, which I have mentioned above
as affording an instance of granite sending out many veins into
the schistus, and rivetted, as it were, by means of them to the
superincumbent rock, when I visited it, with, a view of verifying
on the spot the interesting observations which Dr Hutton had there
made, appeared to me to be without any vestige of stratification in
its granitic part, as did also the whole group of mountains to which
it belongs. It was, therefore, not without a good deal of surprise,
that I lately read, in an account of that island, by a very accurate
and ingenious mineralogist, that Goatfield consists of stratified
granite.[154] The impression which the appearance of that mountain
made on my mind, is just the reverse; and though I saw large tabular
masses, sometimes nearly vertical, separated by fissures, they
appeared to be much too irregular, too little extended in length and
height, and vastly too much in thickness, to be reckoned the effects
of stratification. For all this, I would by no means be understood
to set my observations in opposition to those of Mr Jameson. In my
visit to Arran, I did not direct my inquiries much toward this point;
the general appearance of the rocks did not suggest the necessity of
doing so, and I was not perfectly aware how much the stratification of
granite had been insisted on by some mineralogists; so that I applied
myself entirely to study some other of the interesting phenomena which
this little island offers in so great abundance. I therefore carry
my confidence in the appearances which seemed to indicate a want of
stratification in the granite of Arran no further than to remain
sceptical both as to Mr Jameson's conclusions and my own, till an
opportunity shall occur of verifying the one or the other by actual
observation.
[Footnote 154: Mineralogy of the Scottish Isles, vol. i. p. 35, 36.]
301. The stratification of granite, though it made no part of Dr
Hutton's system, does by no means embarrass his theory with any new
difficulty. Rocks, of which the parts are highly crystallized, are
already admitted as belonging to the strata, and are exemplified in
marble, gneiss, and veined granite. In the two last, we have not only
stratification, but a schistose, united with a crystallized structure,
and the effects of deposition by water, and of fluidity by fire, are
certainly no where more singularly combined. The stratification of
these substances is therefore more extraordinary than even that of the
most highly crystallized granite. Neither the one nor the other can be
explained but by supposing, that while such a degree of fluidity was
produced by heat, as enabled the body when it cooled to crystallize,
the whole mass was kept in its place by great pressure acting on all
sides, so that the shape was preserved as originally given to it by
the sea. As we cannot, however, suppose, that the intensity of the
heat, or the fusibility of the substance through all the parts of a
stratum, were precisely the same, we may expect to find in the same
stratum, or in the same body of strata, that in some parts the marks
of stratification are completely obliterated while in others they
remain entire. It is thus that _veined granite_, of what I think should
be called granitic schistus, often graduates into granite _in mass_,
that is, granite without any schistose or fissile texture. Saussure
says, that to be veined or not veined, is an affection of granite,
that seems, in many cases, accidental;[155] as, in the midst of rocks
of that substance, most clearly fissile, large portions appear without
any vestige of stratification. Of this phenomenon, which is frequent
in the Alps, instances are also to be met with in the granite rocks
of Scotland, and the adjacent isles; and I know that Dr Hope, in a
mineralogical excursion which he lately made among the Hebrides,
observed many interesting and curious examples of it. Indeed, when
rocks were so much fused as to crystallize, and so compressed, at the
same time, as to remain stratified, they were evidently on the verge of
change; two opposite forces were very nearly balanced, and each carried
as far as it could go without entirely overcoming the other; so that
a small alteration in the conditions may have made a great alteration
in the effects. Hence a sudden transition from a stratified to an
unstratified texture, which is only found in rocks highly crystallized,
and such as have endured the most violent action of the mineralizing
powers.
[Footnote 155: Voyages aux Alpes, tom. iv. § 2143.]
302. Now, though the stratification of granite, or the mixture of the
stratified with the unstratified rocks of that genus, is not only
reconcilable with the principles of the Huttonian geology, but might
even have been deduced as a corollary from those principles, before
it was actually observed, it may be considered as inconsistent with
the theory of granitic veins that has just been given. A stratum,
though soft or fluid, could not invade the surrounding strata with
violence, nor send out veins to penetrate into them. It might, if,
strongly compressed by another stratum less fluid than itself, fill up
any fissures or cracks that were in that other, but this would hardly
produce, such large veins, and of such considerable length, as often:
penetrate from the granite into the schistus, nor could it give rise
to any appearance of disturbance. If, therefore, veins were found
proceeding from such stratified granite as that of Chorley Forest or
Lammermuir, I should think, that the explanation of them was still a
_desideratum_ in geology. The Neptunian theory of infiltration would
indeed be as applicable to them as to any other veins; for it is but
little affected by the condition of the phenomena to be explained.
Indeed, it is very difficult to set any limits to the explanations
which this theory affords; and it would certainly puzzle a Neptunist,
to assign any good reason why infiltration has not produced veins of
one schistus running into another, or veins of schistus running into
granite, as well as of granite running into, schistus. He will find it
a hard task to restrain the activity of his theory, and to confine its
explanations to those things that really exist.
303. As the Huttonian system cannot boast of theories of equal
versatility, it would be not a little embarrassed to account for veins
of great magnitude proceeding from a rock distinctly stratified, and
accompanied with marks of having disturbed the rocks through which they
pass. I am, however, inclined to believe, that this embarrassment will
never occur; and that the granite veins do not proceed from the rocks
that are really stratified, but from such as have never been deposited
by water, and where the appearances of stratification, if there are
any, are altogether illusory. This anticipation, however, requires to
be verified by future observation; and it remains to be seen, whether
granitic veins ever accompany real granitic strata, or are peculiar to
those in which the appearances of regular beds are either ambiguous,
or are entirely wanting. The decision of this question is an object
highly worthy of the attention of geologists.
304. An argument, directed at once against the igneous origin and
unstratified nature of all granite, is given in a work already
mentioned. "If granite had flowed from below, how does it happen, that,
after it had burst through the strata of micaceous schistus, &c. it did
not overflow the neighbouring country? If this hypothesis were true,
Mont Blanc could never have existed."[156]
[Footnote 156: Mineralogy of the Scottish Isles, vol. ii. p. 166.]
A theory is never more unfairly dealt with, than when those parts
are separated which were meant to support one another, and each left
to stand or fall by itself. This, however, is precisely what is done
in the present instance; for Dr Hutton's theory of granite would
not deserve a moment's consideration, if it were so inartificially
constructed, as to suppose that granite was originally fluid, and yet
to point out no means of hindering this fluid from diffusing itself
over the strata, and settling in a horizontal plane. The truth is, that
his theory, at the same time that it conceives this stone to have been
in fusion, supposes it to have been, in that state, injected among the
strata already consolidated; to have heaved them up, and to have been
formed in the concavity so produced, as in a mould. Thus Mont Blanc,
supposing that it is unstratified, is understood to consist of a mass
that was melted by subterraneous heat under the strata, and being
impelled upwards by a force, that may stand in some comparison with
that which projected the planets in their orbits, heaved up the strata
by which it was covered, and in which it remained included on all sides.
305. The covering of strata, thus raised up, may have been burst
asunder at the summit, where the curvature and elevation were the
greatest; but the melted mass underneath may have already acquired
solidity, or may have been sustained by the beds of schistus incumbent
on its sides. This schistus, forming the exterior crust, was
immediately acted on by the causes of waste and decomposition, which
have long since stripped the granite of a great part of its covering,
and are now exercising their power on the central mass. That even Mont
Blanc itself, as well as other unstratified mountains, was once covered
with schistus, will appear to have in it nothing incongruous, when we
consider the height to which the schistus still rises on its sides,
or in the adjacent mountains; and when we reflect, that, from the
appearances of waste and degradation which these mountains exhibit, it
is certain, that the schistus must have reached much higher than it
does at present.
It is obvious, therefore, that when the corresponding parts are brought
together, and placed in their natural order, no room is left for the
reproach, that this system is inconsistent with the _existence_ of
granite mountains. I have no pleasure in controversial writing; and,
notwithstanding the advantages which a weak attack always gives to a
defender, I cannot but regret, that Dr Hutton's adversaries have been
so much more eager to refute than to understand his theory.
[Illustration]
306. A remark which Dr Hutton has made on the quantity of granite that
appears at the surface, compared with that of other mineral bodies,
has been warmly contested. Having affirmed, that the greater part of
rocks bear marks of being formed from the waste and decomposition of
other rocks, he alleges that granite, (a stone which does not contain
such marks) does not, for as much as appears from actual observation,
make up a tenth, nor perhaps even a hundredth part of the mineral
kingdom.[157] Mr Kirwan contends, that this is a very erroneous
estimate, and that the quantity of granite visible on the surface, far
exceeds what is here supposed.[158] The question is certainly of no
material importance to the establishment of Dr Hutton's theory: it is
evident, too, that an estimation, which varies so much as from a tenth
to a hundredth part, cannot have been meant as any thing precise; yet
it may not be quite superfluous to show, that the truth probably lies
nearer to the least than the greatest of the limits just mentioned.
[Footnote 157: Theory of the Earth, vol. i. p. 211.]
[Footnote 158: Geol. Essays, p. 480.]
307. Though granite forms a part, generally the central part, of all
the great chains of mountains, it usually occupies a much less extent
of surface than the primary schistus. Thus in the Alps, if a line be
drawn from Geneva to Ivrea, it will be about eighty-five geographical
miles in length, and will measure the breadth of this formidable
chain of mountains, at the place of its greatest elevation. Now, from
the observations of Saussure, who crossed the Alps exactly in this
direction, it may be collected, that less than nine miles of this line,
or not above a tenth part of it, in the immediate vicinity of Mont
Blanc, is occupied by granite.
308. In some sections of the Alps, no granite at all appears. Thus, in
the route from Chambery to Turin, across Mont Cenis, which measures by
the road not less than ninety miles, no granite is found, at least of
that kind which is distinctly in mass, and different from gneiss or
veined granite.[159]
[Footnote 159: Voyages aux Alpes, tom. iii. § 1190, &c.]
309. In some other places of the same mountains, the granite is more
abundant. A line from the lake of Thun, along the course of the Aar,
and over the mountains to the upper end of Lago Maggiore, crosses a
very elevated tract, and passes by the sources of the Rhone, the Rhine,
and the Tessino, which last runs into the Po. A good deal of granite is
discovered here, in the mountains of Grimsel and St Gothard; but by far
the greater part of it is the veined granite, the granite in mass being
confined chiefly to the north side of the Grimsel. Both together do not
occupy more than one-third of the line, and therefore the latter less
than one-sixth.
310. The essay on the mineralogy of the Pyrenees, by the Abbé PALASSO,
contains a mineralogical chart of those mountains. From this chart I
have found, by computation, that the granite does not occupy one-fifth
of the horizontal surface on the north side of the ridge, reckoning
from one end of it to the other. Indeed, many great tracts, even of the
central parts of the Pyrenees, contain no granite whatsoever; and not a
few of the highest mountains consist entirely of calcareous schistus.
A large deduction should be made from the fraction 1/5 on account of
the substances unknown, which, from the construction of the chart, are
often confounded with the granitic tract.
311. I might add other estimations of the same kind, all confessedly
rude and imperfect, but still conveying, by means of numbers, a better
idea of the limit to which our knowledge approximates, than could be
done simply by words; and, on the whole, it would appear, that if we
state the proportion of granite to schistus to be that of one to four,
we shall certainly do no injustice to the extent of the former.
It remains to form a rough estimate from maps, and from the accounts
of travellers, of what proportion of the earth's surface consists
of primary, and what of secondary rocks. After supplying the want
of accurate measurement by what appeared to me the most probable
suppositions, I have found, that about 1/18 of the surface of the old
continent may be conceived to be occupied by primitive mountains; of
which, if we take one-fifth, we have 1/90 for the part of the surface
occupied by granite rocks, which differs not greatly from the least of
the two limits assigned by Dr Hutton.
312. In estimating the granite of Scotland, Dr Hutton has certainly
erred considerably in defect,[160] and Mr Kirwan, who always differs
from him, is here nearest the truth; though he is right purely by
accident, as the information on which he proceeds is vague and
erroneous.
[Footnote 160: Dr Hutton in this case no doubt made a very loose
estimate. He says, the granite does not perhaps occupy more than a
500dth part of the whole surface. The whole surface of Scotland is not
much more than 23,000 geographical miles, the 500dth part of which is
exactly 46; and this is exceeded by the granite in Kirkcudbrightshire
done, as may be gathered from what is said § 283.]
The places in Scotland where granite is found, are very well known; but
the extent of some of the most considerable of them is not accurately
ascertained. In the southern parts, except the granite of Galloway,
which is found in two pretty large insulated tracts, there is no
other of any magnitude. The granite of the north extends over a large
district. If we suppose a line to be drawn, from a few miles south
of Aberdeen to a few miles south of Fort William, it will mark out
the central chain of the Grampians in its full extent, passing over
the most elevated ground, and by the heads of the largest rivers,
in Scotland. Along this line there are many granite mountains, and
large tracts in which granite is the prevailing rock. There are,
however, large spaces also in which no granite appears, though, if we
were permitted to speak theoretically, and if the question did not
entirely relate to a matter of observation, we might suppose, that,
in no part of this central ridge is the granite far from the surface,
notwithstanding that in some places it may be covered by the schistus.
313. A great part of the Grampian mountains is on the south side of the
line just mentioned, but hardly any granite is found in this division
of them, except such veins as those of Glentilt. On the north side of
the line, the granite extends in various directions; and, if from Fort
William a line is drawn to Inverness, the quadrilateral figure, bounded
on two sides by these lines, and on the other two by the sea, will be
found to contain much granite, and many districts consisting entirely
of that stone. This is in fact the great granite country of Scotland:
it is a large tract, containing about 3170 square geographical
miles, or about a seventh part of the whole: but the proportion of
it occupied by granite cannot at present be ascertained with any
exactness, nor will, till some mineralogist shall find leisure to
examine the courses of the great rivers, the Dee, the Spey, &c. which
traverse this country. If we call it one-fourth of the whole surface,
its extent is certainly not underrated, and will amount to 790 square
miles nearly; to which adding 150, as a very full allowance for all the
other granite contained in Scotland, exclusive of the isles, we shall
have 940 square miles, between a twenty-fourth and twenty-fifth part of
the surface of the whole.
This computation, it must be observed, aims at nothing precise, but I
think it is such, that a more accurate survey would rather diminish
than increase the proportion assigned in it to the granite rock.
314. This result may perhaps fall as much short of Mr Kirwan's notion,
as it exceeds the estimate made by Dr Hutton. If it shall not, and if
the former has, in this instance, come nearest the truth, it cannot
be ascribed to the accuracy of his information, or the soundness of
the principles which directed his research. Mr WILLIAMS, whom he
quotes, was a miner, of great skill and experience in some branches
of his profession, to which, if he had confined himself, he might
have written a book full of useful information. What he says on the
subject of granite, is, in the main I believe just; but it is far too
general to authorize the conclusion which Mr Kirwan derives from it.
Dr Ash, for whose judgment I have great respect, cannot, I think, have
meant, when he used the expression granitic rocks, to describe granite
strictly so called. He says, in the passage quoted by Mr Kirwan, that
"from Galloway, Dumfries, and Berwick, there is a chain of mountains,
commonly schistose, but often also granitic." Now, the fact is, that
the great belt of primary rock, here alluded to, which traverses the
south of Scotland, consists of vertical schistus of various kinds;
but except in Galloway, and again in Lammermuir, near Priestlaw, it
appears, as already mentioned, to contain no granite whatsoever. If the
German mineralogist quoted by Mr Kirwan, when he says that the Grampian
mountains consist of micaceous limestone, gneiss, porphyry, argillite,
and granite, alternating with one another, means only to affirm that
all these stones are found in the Grampians, he is certainly in the
right, and the catalogue might easily be enlarged; but, if he either
means to say, that these are nearly in equal abundance, or that the
granite is commonly found in strata alternating with other strata, I
must say, that these are propositions quite contrary to any thing
I have ever seen or heard of those mountains. But it is probable
that this is not meant, and that the fault lies in understanding
the expressions much too literally. Mr Kirwan accuses Dr Hutton of
not knowing where to look for the granite; not aware of how much,
notwithstanding any error committed in the present estimate, he was
skilled in the art of mineralogical observation; an art, which those
who have not practised do not always know how to appreciate. But,
however imperfect Mr Kirwan's knowledge of this subject has been,
he has here had the good fortune to correct a mineralogist of very
superior information. The mere disposition to oppose is not always
without its use: no man is in every thing free from error, and, to
controvert indiscriminately all the opinions of any individual, is an
infallible secret for being sometimes in the right.
NOTE XVI. § 100.
_Rivers and Lakes._
315. Rivers are the causes of waste most visible to us, and most
obviously capable of producing great effects. It is not, however, in
the greatest rivers, that the power to change and wear the surface of
the land is most clearly seen. It is at the heads of rivers, and in the
feeders of the larger streams, where they descend over the most rapid
slope, and are most subject to irregular or temporary increase and
diminution, that the causes which tend to preserve, and those that tend
to change the form of the earth's surface, are farthest from balancing
one another, and where, after every season, almost after every flood,
we perceive some change produced, for which no compensation can be
made, and something removed which is never to be replaced. When we
trace up rivers and their branches toward their source, we come at
last to rivulets, that run only in time of rain, and that are dry, at
other seasons. It is there, says Dr Hutton, that I would wish to carry
my reader, that he may be convinced, by his own observation, of this
great; fact, _that the rivers have, in general, hollowed out their
valleys_. The changes of the valley of the main river are but slow;
the plain indeed is wasted in one place, but is repaired in another,
and we do not perceive the place from whence the repairing matter has
proceeded. That which the spectator sees here, does not therefore
immediately suggest to him what has been the state of things before
the valley was hollowed out. But it is otherwise in the valley of the
rivulet; no person can examine it without seeing, that the rivulet
carries away matter which cannot be repaired, except by wearing away
some part of the surface of the place upon which the rain that forms
the stream is gathered. The remains of a former state are here visible;
and we can, without any long chain of reasoning, compare what has been
with what is at the present moment. It requires but little study to
replace the parts removed, and to see nature at work, resolving the
most hard and solid masses, by the continued influences of the sun and
atmosphere.[161] We see the beginning of that long journey, by which
heavy bodies travel from the summit of the land to the bottom of the
ocean, and we remain convinced, that, _on our continents, there is no
spot on which a river may not formerly have run_.[162]
[Footnote 161: Theory of the Earth, vol. ii. 294.]
[Footnote 162: _Ibid._ p. 296.]
316. The view thus afforded of the operations, in their nascent state,
which have shaped out and fashioned the present surface of the land,
is necessary to prepare us for following them to the utmost extent
of their effects. From these effects, the truth of the proposition,
that rivers have cut and formed, not the beds only, but the whole of
the valleys, or rather system of valleys, through which they flow, is
demonstrated on a principle which has a close affinity to that on which
chances are usually calculated, § 99. In order to conceive rightly the
course of a great river, and the communication subsisting between the
main trunk and its remotest branches, let us take the instance of the
Danube and cast our eyes on one of the maps constructed by MARSIGLI,
for illustrating the natural history of that great river.[163] When
it is considered, that over all the vast and uneven surface, which
reaches from the Alps to the Euxine, and from the mountains of Crapack
to those of Hæmus, a regular communication is kept up between every
point and the line of greatest depression, in which the river flows, no
one can hesitate to acknowledge, that it is the agency of the waters
alone which has opened them a free passage through all the intricacies
of this amazing labyrinth. In effect, suppose this communication to be
interrupted, and that some sudden operation of nature were to erect a
barrier of mountains to oppose the Theise or the Drave, as they rolled
their waters to the Danube. From this what could possibly result, but
the damming up of those rivers till their waters were deep, or high
enough to find a vent, either under the bases or over the tops of the
opposing ridge. Thus there would be formed immense lakes and immense
cataracts, which, by filling up what was too low, and cutting down what
was too high, would in time restore such a uniform declivity of surface
as had before prevailed. Just so in the times that are past, whatever
may have been the irregularities of the surface at its first emerging
from the sea, or whatever irregularities may have been produced in
it by subsequent convulsions, the slow action of the streams would
not fail in time to create or renew a system of valleys communicating
with one another, like that which we at present behold. Water, in all
circumstances, would find its way to the lowest point; though, where
the surface was quite irregular, it would not do so till after being
dammed up in a thousand lakes, or dashed in cataracts over a thousand
precipices. Where neither of these is the case; and where the lake and
the cataract are comparatively rare phenomena; there we perceive that
constitution of a surface, which water alone, of all physical agents,
has a tendency to produce; and we must conclude, that the probability
of such a constitution having arisen from another cause, is, to the
probability of its having arisen from the running of water, in such a
proportion as unity bears to a number infinitely great.
[Footnote 163: Histoire du Danube, tom. i. tab. 34.]
317. The courses of many rivers retain marks that they once consisted
of a series of lakes, which have been converted into dry ground, by the
twofold operation of filling up the bottoms, and deepening the outlets.
This happens, especially, when successive terraces of gravelly and
flat land are found on the banks of a river, § 100. Such platforms, or
_haughs_ as they are called in this country, are always proofs of the
waste and _detritus_ produced by the river, and of the different levels
on which it has run; but they sometimes lead us farther, and make it
certain, that the great mass of gravel which forms the successive
terraces on each side of the river, was deposited in the basin of a
lake. If, from the level of the highest terrace, down to the present
bed of the river, all is alluvial, and formed of sand and gravel, it
is then evident, that the space as low as the river now runs must have
been once occupied by water; at the same time, it is dear, that water
must have stood, or flowed as high at least, as the uppermost surface
of the meadow. It is impossible to reconcile these two facts, which are
both undeniable, but by supposing a lake, or body of stagnant water,
to have here occupied a great hollow, (which by us must be held as
one of the original inequalities of the globe, because we can trace
it no farther back,) and that this hollow, in the course of ages, has
been filled up by the gravel and alluvial earth brought down by the
river, which is now cutting its channel through materials of its own
depositing. There is no great river that does not afford instances
of this, both in the hilly part of its course, and where it descends
first from thence into the plain. Were there room here for the minuter
details of topographical description, this might be illustrated by
innumerable examples.
318. It is said above, that the water must have run or stood, in
former times, as low as the present bottom of the river; but there is
often clear evidence, that it has run or stood much lower, because the
alluvial land reaches far below the present level of the river. This is
known to hold in very many instances, where it has happened that pits
have been sunk to considerable depths on the banks of large rivers. By
that means, the depth of the alluvial ground, under the present bed of
the river, has been discovered to be great; and from this arises the
difficulty, so generally experienced, of finding good foundations for
bridges that are built over rivers in large valleys, or open plains,
the ground being composed of travelled materials to an unknown depth,
without any thing like the native or solid strata. In such cases, it
is evident, that formerly the water must have been much lower, as well
as much higher, than its present level, and this is only consistent
with the notion, that the place was once occupied by a deep lake.
319. If, following the light derived from these indications, we go
back to the time when the river ran above the highest of those levels
at which it has left any traces of its operations, we shall see it
composed of a series of lakes and cataracts, from which, by the
filling up of the one, and the wearing down of the other, the waters
have at length worked out to themselves a quiet and uninterrupted
passage to the ocean. We may, indeed, on good evidence, go back still
farther than the succession of such meadows or terraces, as are above
mentioned, will carry us, and may consider the whole valley, or
trough of the river, as produced by its own operations. The original
inequalities of the surface, and the disposition of the strata, must
no doubt have determined the water courses at first; but this does not
hinder us from considering the rivers as having modified and changed
those inequalities, and as the proximate causes of the shape and
configuration which the surface has now assumed.
320. From this gradual change of lakes into rivers, it follows, that
a lake is but a temporary and accidental condition of a river, which
is every day approaching to its termination; and the truth of this is
attested, not only by the lakes that have existed, but also by those
that continue to exist. Where any considerable stream enters a lake,
a flat meadow is usually observed increasing from year to year. The
soil of this meadow is disposed in horizontal strata: the meadow is
terminated by a marsh; which marsh is acquiring solidity, and is soon
to be converted into a meadow, as the meadow will be into an arable
field. All this while the sediment of the river makes its way slowly
into the lake, forming a mound or bank under the surface of the water,
with a pretty rapid slope toward the lake. This mound increases by the
addition of new earth, sand, and gravel, poured in over the slope; and
thus the progress of filling up continually advances.
321. In small lakes, this progress may easily be traced; and will be
found singularly conspicuous in that beautiful assemblage of lakes,
which so highly adorns the mountain scenery of Westmoreland and
Cumberland. Among these a great number of instances appear, in which
lakes are either partially filled up, or have entirely disappeared. In
the Lake of Keswick, we not only discover the marks of filling up at
the upper end, which extend far into Borrowdale, from which valley a
small river flows into the lake; but we have the clearest proof, that
this lake was once united to that of Bassenthwaite, and occupied the
whole valley from Borrowdale to Ouse-Bridge. These two lakes are at
present joined only by a stream, which runs from the former into the
latter, and their continuity is interrupted by a considerable piece of
alluvial land, composed of beds of earth and gravel, without rock, or
any appearance of the native strata. This separation, therefore, seems
no other than a _bar_, formed by the influx of two rivers, that enter
the valley here from opposite sides, the Greata from the east, and
Newland's water from the west. The surface of this meadow is at present
twelve or fifteen feet at least above the level of either lake and a
quantity of water of that depth must therefore have been drawn off by
the deepening of the issue at Ouse-Bridge, through which the water of
both lakes passes, in its way to the ocean.
Many more examples, similar to this, may be collected from the same
lakes; there are indeed few places from which, in this branch of
geology, more information may be collected.
322. The larger lakes exemplify the same progress. Where the Rhone
enters the Lake of Geneva, the beach has been observed to receive an
annual increase; and the Portus Valesiæ, now Port Valais, which is at
present half a league from the lake, was formerly close upon its bank.
Indeed, the sediments of the Rhone appear clearly to have formed the
valley through which it runs, to a distance of about three leagues at
least from the place where the river now discharges itself into the
lake. The ground there is perfectly horizontal, composed of sand and
mud, little raised above the level of the river, and full of marshes.
The deposition made by the Rhone after it enters the lake, is visible
to the eye; and may be seen falling down in clouds to the bottom.
The great lakes of North America are undergoing the same changes, and,
it would seem, even with more rapidity. As the rivers, however, which
supply these vast reservoirs, are none of them very great, the filling
up is much less remarkable than the draining off of the water, by the
deepening of the outlet. An intelligent traveller has remarked, that
in Lake Superior itself the diminution of the waters is apparent,
and that marks can be discovered on the rocks, of the surface having
been six feet higher than it is at present. In the smaller lakes this
diminution is still more evident.[164] In some of those far inland, the
ground all round appeared to the same traveller to be the deposit from
the rivers, of which the lakes themselves may be considered as a mere
expansion.[165]
[Footnote 164: Mackenzie's Voyages through the Continent of North
America to the Frozen and Pacific Oceans, p. xlii. and xxxvi.]
[Footnote 165: _Ibid._ p. 122.]
323. In order to give uniform declivities to the rivers, the lakes
must not only be filled up or drained, but the cataract, wherever
there is one, must be worn away. The latter is an operation in all
cases visible. The stream, as it precipitates itself over the rocks,
hurries along with it, not only sand and gravel, but occasionally large
stones, which grind and wear down the rock with a force proportioned
to their magnitude and acceleration. The smooth surface of the rocks
in all waterfalls, their rounded surface, and curious excavations,
are the most satisfactory proofs of the constant attrition which they
endure; and, where the rocks are deeply intersected, these marks
often reach to a great height above the level on which the water now
flows. The phenomena, in such instances, are among the arguments best
calculated to remove all incredulity respecting the waste which rivers
have produced, and are continuing to produce. They suffer no doubt to
remain, that the height and asperity of every waterfall are continually
diminishing; that innumerable cataracts are entirely obliterated; that
those which remain are verging toward the same end, and that the Falls
of Montmorenci and Niagara must ultimately disappear.
324. Though there can be no doubt of the justness of the preceding
conclusions, when applied to lakes in general, some apparent
exceptions occur, in which the progress of draining and filling up
seems to have been suspended, or even to have gone in a contrary
direction. These exceptions consist of the lakes which appear to
have received a greater quantity of materials than was sufficient to
have filled them up. Such, for example, is the Lake of Geneva, which
receives the Rhone descending from the Valais, one of the deepest and
longest valleys on the surface of the earth. Now, if this valley, or
even a large proportion of it, had been excavated by the Rhone itself,
as our theory leads us to suppose, the lake ought to have been entirely
filled up, because the materials brought down by the river seem to be
much greater than the lake, on any reasonable supposition concerning
its original magnitude, can possibly have received. What, then, it
may be said, has become of all that the Rhone has brought down and
deposited in it? The lake, at this moment, retains, in some places, the
depth of more than 1000 feet; and yet, of all that the Rhone carries
into it, nothing but the pure water issues. If it has been continuing
to diminish, both in superficial extent and in depth, from the time
when the Rhone began to run into it, what must have been its original
dimensions?
I cannot pretend to remove entirely the difficulty which is here
stated; yet I think the following remarks may go some length in doing
so.
325. It is certain, that from the present state of the Lake of Geneva,
and of the ground round it, we can hardly draw any inference as to its
original dimensions. Saussure has traced, with his usual skill, the
marks of the course of the Rhone, on a level greatly above the present;
and, by observations on the side of Mont Saleve, has found proofs of
the running of water, at least 200 toises above the present superficies
of the lake. But, if ever the superficies of the lake stood at this
height, or at this height nearly, though we can conjecture but little
concerning the state of the adjacent country, which no doubt was also
on a higher level, the lake may very well be supposed to have been of
far greater dimensions than it is now. It may have occupied the whole
space from Jura to Saleve, and included the Lake of Neufchâtel; so
that it may have been of magnitude sufficient to receive the spoils of
the Valais, which, as the surface of its waters lowered, may have been
washed away and carried down to the sea. Thus it may have afforded a
temporary receptacle for the _debris_ of the Alps, and may have served
for an _entrepot_, as it were, where those _debris_ were deposited,
before they were carried to the place of their ultimate destination.
326. But the great depth which the lake has at present, still remains
to be explained, because no mud or gravel could be carried beyond the
gulf, of a thousand feet deep, which was here ready to receive it. The
reality of this difficulty must be acknowledged; and some cause seems
to act, if not in the generation, yet certainly in the preservation of
lakes, with which we are but little acquainted We can indeed imagine
some causes of that kind to occur in the course of the degradation
of the land, which may produce new lakes, or increase the dimensions
of the old. The wearing away of a stratum, or body of strata, may
lay bare, and render accessible to the water, some beds of mineral
substances soluble in that fluid. The district, for instance, in
Cheshire, which contains rock-salt, extends over a tract of fourteen or
fifteen miles, and is covered by a thick stratum of clay, more or less
indurated, which defends the salt from the water at the surface, and
preserves the whole mass in a state of dryness. Should this covering be
broke open by any natural convulsion, or should it be worn away, as it
must be in the progress of the general detritus, the water would gain
admission to the saline strata, would gradually dissolve them, and
form of course a very deep and extensive lake, where all was before dry
land. This event is not only possible, but it should seem, that in the
course of things it must necessarily happen.
327. Something of this kind may have taken place in the track of the
Rhone, and may have produced the Leman Lake. It is not impossible,
that, at a very remote period, the Rhone descended from the Alps
without forming any lake, or at least any lake of which the remains are
now existing; and this supposition, which is more probable than that of
§ 325, we shall soon find to be conformable to appearances of another
kind. The river may have wore away the secondary limestone strata over
which it took its course after it left the schistus of the mountains;
and, in doing so, may have reached some stratum of a saline nature, and
this being washed out, may have left behind it a lake, which is but
modern compared with many of the revolutions that have happened on the
surface of the earth.[166]
[Footnote 166: There are salt springs at Bex, near Aigle, about ten
miles from the head of the lake: saline strata, therefore, are probably
at no great distance.]
This explanation is no doubt hypothetical; but it is proposed in one
of those cases, in which hypothetical reasonings are warranted by
the strictest rules of philosophical investigation. It is proposed
in a case, where the causes visible to man seem inadequate to the
effect, and where we must therefore have recourse to an agent that is
invisible. If the operations ascribed to this agent are conformable to
the analogy of nature, it is all that can in reason be required.
328. Another circumstance may also influence the generation and
preservation of lakes; but it is also one with which we are but little
acquainted. The strata, and indeed the whole body of mineral substances
which forms the basis of our land, have been raised up from the bottom
of the sea, by a progress that should seem in general to have been
gradual and slow. Appearances, however, are not wanting, which show,
that this progress is not uniform; and that both rising and sinking in
the surface of the land, or in the rocks which are the base of it, have
happened within a period of time, which is by no means of great extent.
In this progress, the elevations and depressions may not be the same
for every spot. They may be partial, and one part of a stratum, or body
of strata, may rise to a greater height, or be more depressed, than
another. It is not impossible, that this process may affect the depth
of lakes, and change the relative level of their sides and bottom.
329. All lakes, however, do not involve the difficulty which the
preceding conjectures are intended to remove. The great lakes of North
America do not, for instance, receive their supply from very large
rivers. Of course, it is not from a tract great in comparison of
themselves, that the waste and detritus is brought down into them; and
it seems not at all wonderful, that, without being filled up, they have
been able to receive it. The same, in a degree at least, is true of
many other lakes.
It should also be considered, that we may err greatly in the estimate
we make of the materials actually carried down and deposited in any
lake. To judge of their entire amount, we should know the original
form of the inequalities on the earth's surface; of the quantity of
depression which existed, independently of the rivers; and though,
in general, these original inequalities may be overlooked, and the
present considered as made by the running of water, yet, in particular
instances, this may be far from true. The Valais, for example, which
we consider as the work of the Rhone, may, when the Alps rose out of
the sea, have included many depressions of the surface, which the river
joined together, and, from being a series of lakes, formed into one
great valley.
* * * * *
330. The mouths by which rivers on bold rocky coasts discharge their
waters into the sea, afford a very striking confirmation of the
conclusions concerning the general system of waste and degradation
which have been drawn above. At these mouths we usually see, not only
the bed of the river, but frequently a considerable valley, cut out
of the solid rock, while that rock preserves its elevation, and its
precipitous aspect, wherever it is not intersected by a run of water.
No convulsion that can have torn asunder the rocks; no breach that can
have been made in them, antecedent to the running of the waters, will
account for the circumstance of every river finding a corresponding
opening, by which it makes its way to the sea; for that opening being
so nearly proportional to the magnitude of the river, and for such
breaches never occurring but where streams of water are found.
331. The actual survey of any bold and rocky coast, will make this
clearer than any general statement can possibly do. Let us take, for an
example, the coast of the British Channel, from Torbay to the Land's
End, which is faced by a continued rampart of high cliffs, formed
of much indurated and primeval rock. If we consider the breaches in
this rampart, at the mouths of the Dart, of the Plym and Tamer, of
the river at Fowey, of the Fal, the Hel, &c. it will appear perfectly
clear, that they have been produced by their respective streams. Where
there is no stream, there is no breach in the rock, no softening in
the bold and stern aspect which this shore every where presents to the
ocean. If we look at the smaller streams, we find them working their
way through the cliffs at the present moment; and we see the steps by
which the larger valleys of the Dart and the Tamer have been cut down
to the level of the sea. If we would have still clearer evidence, that
no breaches made antecedently to the running of the rivers have opened
a way for them, we need only look to the opposite side, or northern
shore, of the same promontory, where we also find a series of outlets,
all originating in the ridge of the country, and becoming deeper as
they approach the sea, but altogether unconnected with the openings
on the south side; and this could hardly have been the case, had they
been the effects of previous concussions, or of any peculiarity in the
original structure of the rocks.
332. In contemplating such coasts as these, when we go back to the
time when the rivers ran upon a level as high as the highest of the
cliffs on the sea shore, we must suppose, that the land then extended
many miles farther into what is now occupied by the sea. When at
Plymouth, for instance, the Tamer and the Plym flowed on the level of
Mount Edgecombe or of Staten Heights, if the rivers ran with a moderate
declivity into the sea, the coast must have advanced many miles beyond
its present line. Thus the land, when higher, was also more extended,
and the limits of our island in that ancient state, were doubtless very
different from these by which it is at present circumscribed.
If with the same views we consider any other of the bold coasts which
the map of the world presents us with, we shall quickly remark, that
wherever a deep intersection of the sea is made into the land, as on
the western shores of our own island, or on those of Norway, a river
runs in at the head of it, and points out by what means such inlets
are formed, viz. by the united powers of the sea and of the land, the
waters of the latter having opened the way by which those of the former
have penetrated so far into the country.
333. It is not meant assuredly to deny the irregularities of the sea
coast, as it may have originally existed; these irregularities no doubt
determined the initial operations of that waste and decay, by which, in
process of time, they were themselves entirely effaced. The line of
our coasts may be compared to one of those curves, which are sometimes
treated of in the higher geometry, where the ordinates are functions,
not only of, their abscissæ, but also of the time elapsed since a
certain epocha. The form of the curve at that epocha, or when the time
began to flow, corresponds to the original form of the sea coast, on
its emerging from the ocean, and before the powers of wasting and decay
had begun to act upon it. To speak strictly, the original figure, in
both cases, influences all the subsequent; but the farther removed from
it in point of time, the less is that influence; so that, in physical
questions, and for the purpose of such approximations as suit the
imperfection of our knowledge, the consideration of the original figure
may be wholly left out.
NOTE XVII. § 105.
_Remains of Decomposed Rocks._
334. THE plain of Crau was the _Campus Lapideus_ of the ancients; and,
as mythology always seeks to connect itself with the extraordinary
facts in natural history, it was said to be the spot where Hercules,
fighting with the sons of Neptune, and being in want of weapons, was
supplied from heaven by a shower of stones: hence it was called _Campus
Herculeus_.
This plain is on the east side of the Rhone, between Salon and Aries:
it is of a triangular form, about twenty square leagues in extent, and
is covered almost entirely with quartzy gravel. This immense collection
of gravel has been supposed by some to have been brought down by the
Durance from the Alps of Dauphine; by others it has been ascribed to
the Rhone; and by many to the sea, as being a work too great for any
river. The explanation mentioned above, § 105, namely, that the loose
gravel on the plain arises from the decomposition of a great stratum
of pudding-stone, which is the basis of the whole, is the opinion of
Saussure, and is founded on his own observations.[167]
[Footnote 167: See Voyages aux Alpes, tom. iii. § 1592 et 1597. See
also on this subject a Memoir by Lamanon, Journal de Physique, tom.
xxii. p. 477; and another by M. De Servieres, _ibid._ p. 270.]
335. The theories that have been contrived for explaining the
phenomena of the plain of Crau, afford an instance of the necessity
of generalizing our observations before we can explain a particular
appearance: in other words, they prove the truth of Lord Bacon's
maxim, That the explanation of a phenomenon should not be sought for
from the study of that phenomenon alone, but from the comparison of it
with others. One of the theories of this plain is, that the breccia,
which is the base of it, is formed from the consolidation of the loose
gravel of the plain, by water percolating through it, and carrying some
cementing substance along with it, or some _lapidific juice_, as it
is called. And indeed, whether the gravel is formed from the breccia,
or the breccia from the gravel, is a question which probably could
never be resolved by the mere examination of the plain itself. But the
question is very soon decided, when we compare what is observed here
with other appearances in the natural history of the earth's surface,
and consider how much more frequent the decomposition of solids is,
than their reconsolidation, in any place above the level of the sea.
336. The argument for the decomposition of stony substances which is
afforded by the state of this singular plain, may be confirmed by
the appearances observed in many extensive tracts of land all over
the world, and especially in some parts of Great Britain. The road
to Exeter from Taunton Dean, between the latter and Honiton, passes
over a large heath or down, considerably elevated above the plain of
Taunton. The rock which is the base of this heath, as far as can be
discovered, is limestone, and over the surface of it large flints, in
the form of gravel, are very thickly spread. There is no higher ground
in the neighbourhood from which this gravel can be supposed to have
come, nor any stream that can have carried it, so that no explanation
of it remains, but that it is formed of the flints contained in beds
of limestone, which are now worn away. The flints on the heath are
precisely of the kind found in limestone; many of them are not much
worn, and cannot have travelled far from the rock in which they were
originally contained. It seems certain, therefore, that they are the
_debris_ of limestone strata, now entirely decomposed, that once lay
above the strata which at present form the base of this elevated plain,
and probably covered them to a considerable height. This explanation
carries the greater probability with it, that any other way of
accounting for the fact in question, as the travelling of the gravel
from higher grounds, or the immersion of the surface under the sea,
will imply changes in the face of the country, incomparably greater
than are here supposed. Our hypothesis seems to give the _minimum_ of
all the kinds of change that can possibly account for the phenomenon.
337. The same remarks may be made on the high plain of Blackdown, which
the road passes over in going from Exeter to the westward. The flints
there are disseminated over the surface as thickly as in the other
instance, and can be explained only on the same supposition.
Again, in the interior of England, beginning from about Worcester
and Birmingham, and proceeding north-east through Warwickshire,
Leicestershire, Nottinghamshire, as far as the south of Yorkshire, a
particular species of highly indurated gravel, formed of granulated
quartz, is found every where in great abundance. This same gravel
extends to the west and north-west, as far as Ashburn in Derbyshire,
and perhaps still farther to the north. The quantity of it about
Birmingham is very remarkable, as well as in many other places; and the
phenomenon is the more surprising, that no rock of the same sort is
seen in its native place. It is such gravel as might be expected in a
mountainous country, in Scotland, for instance, or in Switzerland, but
not at all in the fertile and secondary plains of England.
This enigma is explained, however, when it is observed, that the basis
of the whole tract just described is a red sandstone, often containing
in it a hard quartzy gravel, perfectly similar to that which has just
been mentioned. From the dissolution of beds of this sandstone, which
formerly covered the present, there can be no doubt that this gravel
is derived. But, as the gravel is in general thinly dispersed through
the sandstone, and abounds only in some of its layers, it should
therefore seem, that a vast body of strata must have been worn away and
decomposed, before such quantities of gravel as now exist in the soil
could have been let loose.
338. I have said, that a rock capable of affording such gravel as
this, is not to be found in the tract of country just mentioned.
This however, is not strictly true; for in Worcestershire, between
Bromesgrove and Birmingham, about seven miles from the latter, a rock
is found consisting of indurated strata, greatly elevated, and without
doubt primitive, from the detritus of which such gravel as we are
now speaking of might be produced. These strata seem to rise up from
under the secondary, where they are intersected by the road; and, for
as much as appears, are not of great thickness, so that they cannot
have afforded the materials of this gravel directly, though they may
have done so indirectly, or through the medium of the red sandstone;
that is to say, a primary rock of which they are the remains, may have
afforded materials for the gravel in the sandstone; and this sandstone
may in its turn have afforded the materials of the present soil, and
particularly the gravel contained in it.
339. Pudding-stones being very liable to decomposition, have probably,
in most countries, afforded large proportion of the loose gravel now
found in the soil The mountains, or at least hills, of this rock, which
are found in many places, prove the great extent of such decomposition.
Mount Rigi, for instance, on the side of the Lake of Lucerne, is
entirely of pudding-stone, and is 742 toises in height, measured from
the level of the lake. By the descriptions given of it, as well as
of other hills of the same kind in Switzerland, we may, without due
attention, be led to suppose that they are entirely formed of loose
gravel. Even M. Saussure's description is chargeable with this fault,
though, when attended to, it will be found to contain a sufficient
proof, that this hill is composed of real pudding-stone.[168] The
nature of the thing also, would be sufficient to convince us, that a
hill, more than 4000 feet in height, could not consist of loose and
unconsolidated materials.
If, then, we regard Mount Rigi as the remains of a body of
pudding-stone strata, we must conclude, that these strata were
originally more extensive, and the adjacent valleys and plains will
serve, in some degree, to measure the quantity of them which time has
destroyed.
[Footnote 168: Voyages aux Alpes, tom. iv. § 1941.]
340. If the theory of unstratified mountains, namely those of
whinstone, porphyry, and granite, be admitted as laid down above, it
will furnish a measure of the destruction which has taken place in the
stratified rocks, and of the vast depredations which have been made
upon them since they were raised up from the bottom of the sea. Like
every other measure, however, of wasting, by a thing that is itself
subject to waste, it can only give a _minimum_, or a limit which the
quantity wasted must necessarily exceed.
The abrupt face of a whinstone rock must be understood as an evidence,
that some body of strata which supported it when fluid, remained
in contact with it, when it was become solid; and if this part of
the mould in which the whinstone was cast, has disappeared, it must
generally be ascribed to the operation of waste and decomposition.
Such a face, for instance, as that which Salisbury _Craig_ presents to
the west, viz. a perpendicular wall of whinstone, about ninety feet
high, raised on a body of sandstone strata of the height of about
300 feet, can have been produced only by having been abutted against
some stratified rock, equally abrupt, and of the same elevation with
itself. Of this rock no part remains.
The basaltic rock of Edinburgh Castle is nearly in the same state. Its
perpendicular sides on the south, west, and north, are now disengaged
from the strata by which they were once encompassed.
341. The granite mountains also, where they are quite unstratified,
give rise to the same conclusion. Those central chains which we find
in so many instances towering above the schistus which cover their
sides, have probably been once completely enveloped by the latter;
and, on this supposition, an estimate may sometimes be formed of the
original height of such mountains. In these estimations, however, some
uncertainty must arise, from our being unable to distinguish between
the effects which are to be ascribed to the fracture and dislocation
that took place when the compound body of stratified and unstratified
rocks was raised up from the bottom of the sea, and the effects
produced by the subsequent waste and decomposition at the surface. In
this, as in many other instances, we are not always able to separate
between the original inequalities of the surface, and those which
wearing has produced.
342. It would be important to ascertain the rate at which the elevation
of mountains decreases, and this is what we may perhaps expect to
be accomplished, by the progress of geological science, and the
multiplying of accurate observations. It has been supposed, that the
Pyrenees diminish about ten inches in a century; but what confidence is
to be put in this estimate, I am unable to determine.[169]
[Footnote 169: Essai sur la Mineralogie des Pyrenées, p. 87.]
A very unequivocal mark of the degradation of mountains is often to
be met with in the heaps of loose stones found on their tops. These
stones, it is obvious, cannot have come from any other place by natural
means, and they are accordingly always sharp and angular, and have none
of the characters of transported rocks. They are said sometimes to
have been brought by men's hands; but this is highly improbable, their
quantity is often so considerable, and the difficulty of transportation
so great. Where any purpose was to be served by heaping them together,
men have availed themselves of the stones that they found ready
prepared on the summit, and have constructed from them cairns, which
have served as signals, useful in their pastoral, and sometimes in
their military occupations.
NOTE XVIII. § 112.
_Transportation of Stones, &c._
343. NATURE supplies the means of tracing with considerable certainty
the migration of fossil bodies on the surface of the earth, as only
the more indurated stones, and those most strongly characterized, can
endure the accidents that must befal them in travelling to a distance
from their native place.
It is a fact very generally observed, that where the valleys among
primitive mountains open into huge plains, the gravel of those plains
consists, of stones, evidently derived from the mountains. The nearer
that any spot is to the mountains, the larger are the gravel stones,
and the less rounded is their figure; and, as the distance increases,
this gravel, which often forms a stratum nearly level, is covered
with a thicker bed of earth or vegetable soil. This progression has
particularly been observed in the valleys of Piémont and the plains of
Lombardy, where a bed of gravel forms the basis of the soil, from the
foot of the Alps to the shores of the Hadriatic.[170] We may collect
from GUETTARD, that a similar gradation is found in the gravel and
earth which cover the great plain of Poland, from Mount Krapack to the
Baltic.[171] The reason of this gradation is evident; the farther the
stones have travelled, and the more rubbing they have endured, the
smaller they grow, the more regular is the figure they assume, and the
greater the quantity of that finer detritus which constitutes the soil.
The washing of the rains and rivers is here obvious; and each of the
three quantities just mentioned, if not directly proportional to the
distance which the stones have migrated from their native place, may
be said, in the language of geometry, to be at least proportional to a
certain function of that distance.
[Footnote 170: Voyages aux Alpes, tom. iii. § 1315.]
[Footnote 171: Mém. Acad. des Sciences, 1762, p. 234, 293, &c.]
344. The immense quantity of _cailloux roulés_, or rounded gravel,
collected in the immediate vicinity of mountainous tracts, has led
some geologists to suppose the existence of ancient currents, which
descended from the mountains, in a quantity, and with a _momentum_,
of which there is no example in the present state of the world. Thus
Saussure imagines, that the hill of Supergue, near Turin, which is
formed of gravel, can only be explained by supposing such currents as
are just mentioned, or what he terms a _debacle_, to have taken place
at some former period.[172] If, however, we ascribe to the mountains
a magnitude and elevation vastly greater than that which they now
possess; if we regard the valleys between them as cut out by the rivers
and torrents from an immense rampart of solid rock, neither materials
sufficiently great, nor agents sufficiently powerful, will appear to
be wanting, for collecting bodies of gravel and other loose materials,
equal to any that are found on the surface of the earth. The necessity
of introducing a _debacle_, or any other unknown agent, to account for
the transportation of fossils, seems to arise from underrating the
effects of action long continued, and not limited by such short periods
as circumscribe the works, and even the observations, of men.
[Footnote 172: Voyages aux Alpes, tom. iii. § 1303.]
345. The supply of gravel and _cailloux roulés_, for the plains
extended at the feet of primitive mountains, is doubtless in many cases
much increased by the pudding-stone, interposed between the secondary
and the primary strata. The beds of pudding-stone contain gravel
already formed on the shores of continents, that ceased to exist before
the present were produced; and the cement of this gravel, yielding
easily to the weather, allows the stones included in it to be washed
down by the torrents, and scattered over the plains. I know not if
the hill of Supergue above mentioned, is not in reality a mass of the
pudding-stone which forms the border of the Alps, and of which the
materials have suffered no transportation since the time of their last
consolidation. This at least is certain, that Saussure, notwithstanding
his accuracy, has sometimes confounded the loose gravel on the surface
with that which is consolidated into rock; an inaccuracy which is to be
charged, as I have elsewhere observed, rather against his system than
himself.
346. The loose stones found on the sides of hills, and the bottoms
of valleys, when traced back to their original place, point out with
demonstrative evidence the great changes which have happened since the
commencement of their journey; and in particular serve to show, that
many valleys which now deeply intersect the surface, had not begun to
be cut out when these stones were first detached from their native
rocks. We know, for instance, that stones under the influence of such
forces as we are now considering, cannot have first descended from one
ridge, and then ascended on the side of an opposite ridge. But the
granite of Mont Blanc has been found, as mentioned above, on the sides
of Jura, and even on the side of it farthest from the Alps. Now, in
the present state of the earth's surface, between the central chain of
the Alps, from which these pieces of granite must have come, and the
ridge of Mont Jura, besides many smaller valleys, there is the great
valley of the Rhone, from the bottom of which, to the place where they
now lie, is a height of not less than 3000 feet. Stones could not, by
any force that we know of, be made to ascend over this height. We must
therefore suppose, that when they travelled from Mont Blanc to Jura,
this deep valley did not exist, but that such an uniform declivity, as
water can run on with rapidity, extended from the one summit to the
other. This supposition accords well with what has been already said
concerning the recent formation of the Leman Lake, and of the present
valley of the Rhone.
347. We can derive, in a matter of this sort, but little and from
calculation; yet we may discover by it, whether our hypothesis
transgresses materially against the laws of probability, and is
inconsistent with physical principles already established. The
horizontal distance from Mont Jura to the granite mountains, at the
head of the Arve, may be accounted fifty geographic miles. Though
we suppose Mont Blanc, and the rest of those mountains, to have been
originally much higher than they are at present, the ridge of Jura must
have been so likewise; and though probably not by an equal quantity,
yet it is the fairest way to suppose the difference of their height to
have been nearly the same in former ages that it is at present, and
it may therefore be taken at 10,000 feet. The declivity of a plane
from the top of Mont Jura to the top of Mont Blanc, would therefore be
about one mile and three quarters in fifty, or one foot in thirty; an
inclination much greater than is necessary for water to run on, even
with extreme rapidity, and more than sufficient to enable a river or
a torrent to carry with it stones or fragments of rock, almost to any
distance.
Saussure, in relating the fact that pieces of granite are found among
the high passes near the summits of Mont Jura, alleges, that they are
only found in spots from which the central chain of the Alps may be
seen. But it should seem that this coincidence is accidental, because,
from whatever cause the transportation of these blocks has proceeded,
the form of the mountains, especially of Mont Jura, must be too much
changed to admit of the supposition, that the places of it from which
Mont Blanc is now visible, are the same from which that mountain was
visible when these stones were transported hither. It may be, however,
that the passes which now exist in Mont Jura are the remains of valleys
or beds of torrents, which once flowed westward from the Alps; and it
is natural, that the fragments from the latter mountains should be
found in the neighbourhood of those ancient water-tracks.
348. Saussure observed in another part of the Alps, that where the
Drance descends from the sides of Mont Velan and the Great St Bernard,
to join the Rhone in the Valais, the valley it runs in lies between
mountains of primary schistus, in which no granite appears, and yet
that the bottom of this valley, toward its lower extremity, is for
a considerable way covered with loose blocks of granite.[173] His
familiar acquaintance with all the rocks of those mountains, led him
immediately to suspect, that these stones came from the granite chain
of Mont Blanc, which is westward of the Drance, and considerably higher
than the intervening mountains. This conjecture was verified by the
observations of one of his friends, who found the stones in question to
agree exactly with a rock at the point of Ornes, the nearest part of
the granite chain.
[Footnote 173: Voyages aux Alpes, tom. ii. § 1022.]
In the present state of the surface, however, the valley of Orsiere
lies between the rocks of Ornex and the valley of the Drance, and would
certainly have intercepted the granite blocks in their way from the one
of these points to the other, if it had existed at the time when they
were passing over that tract. The valley of Orsiere, therefore, was not
formed, when the torrents, or the glaciers transported these fragments
from their native place.
Mountainous countries, when carefully examined, afford so many facts
similar to the preceding, that we should never have done were we
to enumerate all the instances in which they occur. They lead to
conclusions of great use, if we would compare the machinery which
nature actually employs in the transportation of rocks, with the
largest fragments of rock which appear to have been removed, at some
former period, from their native place.
349. For the moving of large masses of rock, the most powerful engines
without doubt which nature employs are the glaciers, those lakes or
rivers of ice which are formed in the highest valleys of the Alps,
and other mountains of the first order. These great masses are in
perpetual motion, undermined by the influx of heat from the earth, and
impelled down the declivities on which they rest by their own enormous
weight, together with that of the innumerable fragments of rock with
which they are loaded. These fragments they gradually transport to
their utmost boundaries, where a formidable wall ascertains the
magnitude, and attests the force, of the great engine by which it was
erected. The immense quantity and size of the rocks thus transported,
have been remarked with astonishment by every observer,[174] and
explain sufficiently how fragments of rock may be put in motion, even
where there is but little declivity, and where the actual surface of
the ground is considerably uneven. In this manner, before the valleys
were cut out in the form they now are, and when the mountains were
still more elevated, huge fragments of rock may have been carried to a
great distance; and it is not wonderful, if these same masses, greatly
diminished in size, and reduced to gravel or sand, have reached the
shores, or even the bottom, of the ocean.
[Footnote 174: The stones collected on the _Glacier de Miage_, when
Saussure visited it, were in such quantity as to conceal the ice
entirely. Voyages aux Alpes, tom. ii. § 854.]
350. Next in force to the glaciers, the torrents are the most powerful
instruments employed in the transportation of stones. These, when
they descend from the sides of mountains, and even where the declivity
of their course is not very great, produce effects which nothing but
direct experience could render credible. The fragments of rock which
oppose the torrent, are rendered specifically lighter by the fluid
in which they are immersed, and lose by that means at least a third
part of their weight: they are, at the same time, impelled by a force
proportional to the square of the velocity with which the water rushes
against them, and proportional also to the quantity of gravel and
stones which it has already put in motion. Perhaps, after taking all
these circumstances into computation, in the midst of a scene perfectly
quiet and undisturbed, a philosopher might remain in doubt as to the
power of torrents to move the enormous bodies of rock which are seen
in the bottom of the narrow valleys or deep glens of a mountainous
country; but his incredulity, says an experienced traveller, will
cease altogether, if he has been surprised by a storm in the midst of
some Alpine region; if he has seen the number and impetuosity of the
cataracts which rushed down the sides of the mountains, and beheld the
ruin which accompanied them; and if, when the tempest was passed, he
has viewed those meadows, which a few hours before were covered with
verdure, now buried under heaps of stones, or overwhelmed by masses of
liquid mud, and the sides of the mountains cut by deep ravines, where
the track of the smallest rivulet was not before to be discovered.[175]
[Footnote 175: See an account of a thunder storm near Bareges, in the
Essai sur la Mineralogie des Pyrenées, p. 134.]
It is but rarely, however, even on occasions like these, that such
vast masses of rock can be seen actually in motion, as are often found
on the surface, apparently removed to a great distance from their
native place. The magnitude of these is so great, in many instances,
that their transportation cannot be explained without supposing,
that the surface was very different when these transportations took
place from what it is at present; that the elevation of the mountains
was greater, and the ground smoother and more uniform, at least in
some directions. If these suppositions are admitted, and they are
countenanced, as we have already seen, by almost every phenomenon in
geology, the difficulties which present themselves here will not appear
insurmountable.
351. One of the largest blocks of granite that we know of, is on
the east side of the lake of Geneva, called _Pierre de Gouté_,
about ten feet in height, with a horizontal section of fifteen by
twenty.[176] Another block not far from it, and nearly of the same
size, has some remains of schistus attached to it. These stones very
much resemble those which have fallen from the _Aiguilles_, in the
valley of Chamouni. The distance from their present situation to those
_Aiguilles_ is about thirty English miles, with many mountains and
valleys at present interposed. By whatever means, therefore, these
blocks were transported, their motion must have been over a surface
of much more uniform declivity than the present. If the surface was
without great inequalities, and its general declivity about one foot
in thirty, as already computed, the glaciers, in the first place, and
the torrents afterwards, may have served for the transportation even of
these rocks.
[Footnote 176: Voyages aux Alpes, tom. i. § 308.]
352. Again, in the narrow vale or glen which separates the Great from
the Little Saleve, the strata are all calcareous, but a great number
of loose blocks of granite and primary schistus are scattered over the
surface. A block of the former, near the lower end of the valley, is
about the size of 1200 cubic feet. Two other large blocks of the same
kind of stone rest on a base of horizontal limestone, elevated two
or three feet above the rest of the surface. This elevation arises
no doubt from the protection which the stones have afforded to the
calcareous beds on which they lie, so that these beds do not wear away
so fast as those which are fully exposed to the weather. But it is
surely to take a very limited view of the operations on the surface,
to suppose, with Saussure, that the parts of the calcareous rock under
these stones has suffered no waste whatsoever, so that the stones
remain now in the identical spot where they were placed by the great
_debacle_ which brought them down from the high Alps.[177] For my part,
I have no doubt that the Arve, which is still at no great distance,
when it ran on a higher level, and in a line different from the
present, aided by the glaciers and superior elevation of the mountains,
was an engine sufficiently powerful for effecting the transportation of
these stones.
[Footnote 177: _Ibid._ § 227.]
353. These phenomena are not peculiar to the Alps, but prevail, in
a greater or less degree, in the vicinity of all primary or granite
mountains. In the island of Arran, a fragment of the same kind with
that which constitutes the upper part of Goatfield, is found on the sea
shore, at least three miles from the nearest granite rock, and with a
bay of the sea intervening. Its dimensions are not far from those of
the _pierre de gouté_. In some former state of the granitic mountains
in that island, the declivity from the top of Goatfield may have been
very uniform, and more rapid than it is at present.
354. Besides glaciers and torrents, which have no doubt been the
principal instruments in producing these changes, other causes
may have occasionally operated. Large stones, when once detached,
and resting on an inclined plane, from the effects of waste and
decomposition, may advance horizontally, at the same time that they
descend perpendicularly, and this will happen though they be not urged
by any torrent, or any thing but their own weight; for the surface
of the ground, as it wastes, remains higher under the stone, and for
a little way round it, than at a greater distance, on account of the
protection which it receives from the stone, as in the instances at
Saleve, just mentioned. The stone itself also becomes rounded at the
bottom; and thus the surface in contact with the ground is diminished
in extent, and the two surfaces rendered convex towards one another. It
must therefore happen, that the support, continually weakening, will at
length give way, and the stone incline or roll toward the lower side,
and may even roll considerably, if its centre of gravity has been high
above its point of support, and if its surface has had much convexity:
Thus the horizontal may very far exceed the perpendicular motion;
and, in the course of ages, the stone may travel to a great distance.
A stone, however, which travels in this manner, must diminish as it
proceeds, and must have been much greater in the beginning than it is
at present.
355. This kind of motion may be aided by particular circumstances.
When a stone rests on an inclined plane, so as to be in a state
not very remote from equilibrium, if a part be taken away from the
upper side, the equilibrium will be lost, and the stone will thereby
be put in motion. That stones which lie on other stones, may, by
wearing, be brought very near an equilibrium, is proved by what are
called _rocking-stones_, or in Cornwall _Logan stones_, which have
sometimes been mistaken for works of art; but are certainly nothing
else than stones, which have been subjected to the universal law
of wasting and decay, in such peculiar circumstances, as nearly to
bring about an equilibrium of that stable kind, which, when slightly
disturbed, re-establishes itself.[178] The Logan stone at the Land's
End, is a mass of granite, weighing more than sixty tons, resting
on a rock of granite, of considerable height, and close on the sea
shore. The two stones touch but in a small spot, their surfaces being
considerably convex towards one another. The uppermost is so nearly
in an equilibrium, that it can be made to vibrate by the strength
of a man, though to overset it entirely would require a vast force.
This arises from the centre of gravity of the stone being somewhat
lower than the centre of curvature of that part of it on which it
has a tendency to roll; the consequence of which is, that any motion
impressed on the stone, forces its centre of gravity to rise, (though
not very considerably,) by which means it returns whenever the force
is removed, and vibrates backward and forward, till it is reduced to
rest. Were it required to remove the stone from its place, it might be
most easily done, by cutting off a part from one side, or blowing it
away by gunpowder; the stone would then lose its balance, would tumble
from its pedestal, and might roll to a considerable distance. Now, what
art is here supposed to perform, nature herself in time will probably
effect. If the waste on one side of this great mass shall exceed that
on the opposite in more than a certain proportion, and it is not likely
that that proportion will be always maintained, the equilibrium of the
Logan stone will be subverted, never to return. Thus we perceive how
motion may be produced by the combined action of the decomposition and
gravitation of large masses of rock.
[Footnote 178: I do not presume so far as to say, that all
rocking-stones are produced by natural means: I have not sufficient
information to justify that assertion; but the great size of that
at the Land's End, its elevated position, and the approaches toward
something of the same kind which are to be seen in other parts of that
shore, prove that it is no work of art. They who ascribe it to the
Druids, do not consider the rapidity with which the Cornish granite
wastes, nor think how improbable it is, that the conditions necessary
to a rocking-stone, whether produced by nature or art, should have
remained the same for sixteen or seventeen hundred years.]
356. Besides the gradual waste to which stones exposed to the
atmosphere are necessarily subject, those of a great size appear to be
liable to splitting, and dividing into large portions, no doubt from
their weight. This may be observed in almost all stones that happen to
be in such circumstances as we are now considering; and from this cause
the subversion of their balance may be more sudden, and of greater
amount, than could be expected from their gradual decay.
Thus, if to the wasting of a stone at the bottom, we add the accidents
that may befal it in the wasting of its sides, we see at least the
physical possibility of detached stones being put in motion, merely
by their own weight. It is indeed remarkable, that some of the largest
of these stones rest on very narrow bases. Those at the foot of
Saleve touch the ground only in a few points: The Boulder stone of
Borrowdale is supported on a narrow ridge like the keel of a ship, and
is prevented from tumbling by a stone or two, that serve as a kind of
shores to prop it up. Very unexpected accidents sometimes happen to
disturb the rest of such fragments of rock as have once migrated from
their own place. Saussure mentions a great mass of _lapis ollaris_[179]
that lies detached on the side of a declivity in the valley of Urseren,
in the canton of Uri. The people use this stone as a quarry, and are
working it away on the upper side, in consequence of which it will
probably be soon overset, and will roll to the bottom of the valley.
[Footnote 179: Voyages aux Alpes, tom. iv. § 1851.]
357. In many instances it cannot be doubted, that stones of the kind
here referred to are the remains of masses or veins of whinstone or
granite, now worn away, and that they have travelled but a very short
way, or perhaps not at all, from their original place. Many of the
large blocks of whinstone which we find in this country, sometimes
single, and sometimes scattered in considerable abundance over a
particular spot, are certainly to be referred to this cause. But the
most remarkable examples of this sort are the stones found at the Cape
of Good Hope, on the hill called _Paarlberg_, which takes its name from
a chain of large round stones, like the pearls of a necklace, that
passes over the summit. Two of these, placed near the highest point,
are called the Pearl and the Diamond, and were mentioned several years
ago in the Philosophical Transactions.[180] From a more recent account,
these stones appear to be a species of granite, though the hill on
which they lie is composed of sandstone strata.[181] The Pearl is a
naked rock, that rises to the height of 400 feet above the summit of
the hill; the Diamond is higher, but its base is less, and it is more
inaccessible.
[Footnote 180: Vol. lxviii. p. 102.]
[Footnote 181: Barrow's Travels into Southern Africa, p. 60.]
From the above stones forming a regular chain, as well as from the
immense size of the two largest, it is impossible to suppose that they
have been moved; and it is infinitely more probable, that they are
parts of a granite vein, which runs across the sandstone strata, and
of which some parts have resisted the action of the weather, while the
rest have yielded to it. The whole geological history of this part of
Africa seems highly interesting, since, as far as can be collected from
the accounts of the ingenious traveller just mentioned, it consists
of horizontal beds of sandstone or limestone, resting immediately on
granite, or on primary schistus. Loose blocks of granite are seen in
great abundance at the foot of the Table Mountain, and along the sea
shore.
* * * * *
358. The system which accounts for such phenomena as have been
considered in this and some of the preceding notes, by the operation
of a great deluge, or _debacle_, as it is called, has been already
mentioned. In Dr Hutton's theory, nothing whatever is ascribed to such
accidental and unknown causes; and, though their existence is not
absolutely denied, their effects, whatever they may have been, are
alleged to be entirely obliterated, so that they can be referred to
no other class but that of mere possibilities. A minute discussion,
however, of the question, Whether there are, on the surface of the
earth, any effects that require the interposition of an extraordinary
cause, would lead into a longer digression than is suited to this
place. I shall briefly state what appear to be the principal
objections to all such explanations of the phenomena of geology.
359. The general structure of valleys among mountains, is highly
unfavourable to the notion that they were produced by any single great
torrent, which swept over the surface of the earth. In some instances,
valleys diverge, as it were from a centre, in all directions. In
others, they originate from a ridge, and proceed with equal depth and
extent on both sides of it, plainly indicating, that the force which
produced them was _nothing_, or evanescent at the summit of that ridge,
and increased on both sides, as the distance from the ridge increased.
The working of water collected from the rains and the snows, and
seeking its way from a higher to a lower level, is the only cause we
know of, which is subject to this law.
360. Again, if we consider a valley as a space, which perhaps with many
windings and irregularities, has been hollowed out of the solid rock,
it is plain, that no force of water, suddenly applied, could loosen
and remove the great mass of stone which has actually disappeared.
The greatest column of water that could be brought to act against
such a mass, whatever be the velocity we ascribe to it, could not
break asunder and displace beds of rock many leagues in length, and
in continuity with the rock on either side of them. The slow working
of water, on the other hand, or the powers that we see every day in
action, are quite sufficient for this effect, if time only is allowed
them.
361. Some valleys are so particularly constructed, as to carry with
them a still stronger refutation of the existence of a _debacle_. These
are the longitudinal valleys, which have the openings by which the
water is discharged, not at one extremity, but at the broadside Such is
that on the east side of Mont Blanc, deeply excavated on the confines
of the granite and schistus rock, and extending parallel to the beds of
the latter, from the Col de la Segne to the Col de Ferret; its opening
is nearly in the middle, from which the Dora issues, and takes its
course through a great valley, nearly at right angles to the chain of
the Alps, and to the valley just mentioned. From the structure of these
valleys, Saussure has argued very justly against Buffon's hypothesis,
concerning the formation of valleys by currents at the bottom of the
sea.[182] It affords indeed a complete refutation of that hypothesis:
and it affords one no less complete of the system which Saussure
himself seems on some occasions so much inclined to support. For if it
be said, that this valley was cut out by the current of a _debacle_,
that current must either have run in the direction of the valley of
Ferret, or in that of the Dora, which issues from it. If it had the
direction of the first, it could not cut out the second; and if it had
the direction of the second, it could not cut out the first. Besides,
the force which excavated this valley must have been _nothing_ at the
two extreme points, viz. at the Col de la Segne and the Col de Ferret,
and must have increased with the distance from each. It can have been
produced, therefore, only by the running of two streams in opposite
directions, on a surface that was but slightly uneven, these streams at
meeting taking a new direction, nearly at right angles to the former. A
clearer proof could hardly be required than is afforded in this case,
that what is now a deep valley was formerly solid rock, which the
running of the waters has gradually worn away; and that the waters,
when they began to run, were on a level as high, at least, as the tops
of those mountains by which the valley is bounded toward the lower side.
[Footnote 182: Voyage aux Alpes, tom. ii. § 920.]
362. Longitudinal valleys, with the water bursting out transversely
from their sides, like the preceding, are by no means confined to
mountains of the first order. We have a very good example, though
on a small scale, of a valley of this sort, within a few miles of
Edinburgh. The Pentland Hills form a double ridge, separated by a
small longitudinal valley, that runs from N. E. to S. W., the water of
which issues from an opening almost in the middle, and directed towards
the south. This, therefore, is not the work of any great torrent, which
overwhelmed the country; for no one direction, which it is possible
to assign to such a torrent, will afford an explanation, both of the
valley and its outlet.[183]
[Footnote 183: In Scotland there is one valley, of a kind that I
believe is extremely rare in any part of the world, in accounting for
which, the hypothesis of a torrent or _debacle_ might, if any where,
be employed to advantage. This is the valley which extends across the
island, from Inverness to Fort William, or from sea to sea, being open
at both ends, and very little elevated in the middle. It is nearly
straight, and of a very uniform breadth, except that towards each end
it widens considerably. The bottom, reckoning transversely, is flat,
without any gradual slope from the sides towards the middle. From the
sides the mountains rise immediately, and form two continued ridges
of great height, like ramparts or embankments on each side of a large
fossé. A great part of the bottom of this singular valley is occupied
by lakes, namely, Loch Ness, Loch Oich, and Loch Lochy. Its length is
about sixty-two miles, and the point of partition from which the waters
run different ways, viz. north-east to the German Ocean, and south-west
to the Atlantic, is between Loch Oich and Loch Lochy; and, by the
estimation of the eye, I should hardly think that it is elevated more
than ten or fifteen feet above the surface of either lake. The country
on both sides is rugged and mountainous, and the streams which descend
from thence into the valley, either fall directly into the lakes, or
turn off almost at right angles when they enter the valley. Though
the bottom of this valley, therefore, is every where alluvial, with
the exception, perhaps, of a few rocks which appear at the surface,
it is certainly not excavated by the rivers which now flow in it. The
direction of the valley, it is to be observed, is the same with that of
the vertical strata which compose the mountain on either side.
Here, then, we have a valley, not cut out by the working of any streams
which now appear; and we may therefore make trial of the hypothesis
of a _debacle_. This, however, will afford us no assistance; because,
if we suppose what is now hollow to have been once occupied by the
same kind of rock which is on either side, no force of torrents can
have suddenly loosened and removed from its place a body of such vast
magnitude. A greater column of water, than one having for its base a
transverse section of the valley, could not act against it, and this
would have to overcome the cohesion and inertia of a column of rock
of the same section, and of the length of sixty-two miles. It is not
hazarding much to affirm, that no velocity which could be communicated
to water, not even that which it could acquire by falling from an
infinite height, could give to it a force in any degree adequate to
this great effect.
The explanation of this valley, which appears to me the most probable,
is the following. It will be shown hereafter, that there is good reason
to suppose, that, in most parts of our island, the relative level of
the sea and land has been in past ages considerably higher than it is
at present. In such circumstances, this valley may have been under
the surface of the sea, the highest part of it being scarcely 100
feet above that level at present. It may have been a kind of sound,
therefore, or strait, which connected the German Sea with the Atlantic;
and the strong currents, which, on account of the different times
of high water in these two seas, must have run alternately up and
down this strait, may have produced that flatness of the bottom, and
straightness of the sides, and that widening at the extremities, which
are mentioned above. In this way, too, some difficulties are removed
relative to Loch Ness, which is so deep as hardly to be consistent with
the indefinite length of the period of waste that must be ascribed to
the mountains on each side of it. Its depth is said, where greatest,
not to be less than 180 fathoms. According to this hypothesis, it may,
at no very distant period, have been a part of the bottom of the sea.]
363. They who maintain the existence of the _debacle_, will no doubt
allege, that though these valleys were not cut out by means of it, yet
others may. But it must be recollected, that if some of the greatest
and deepest valleys on the face of the earth, such as that just
mentioned, on the east side of Mont Blanc, are thus shown to be the
work of the daily wasting of the surface, what other inequalities can
be great enough to require the interposition of a more powerful cause?
If a _dignus vindice nodus_ does not exist here, in what part of the
natural history of the earth is it likely to be found?
364. The large masses of rock so often met with at a distance from
their original place, are one of the arguments used for the _debacle_.
It has, however, been shown, that, supposing a form of the earth's
surface considerably different from the present, especially, supposing
the absence of the valleys which the rivers have gradually cut out, the
transportation of such stones is not impossible, even by such powers
as nature employs at present. Now, without the supposition that the
surface was more continuous, and that its present inequalities did not
exist, no force of torrents, whatever their velocity and magnitude may
have been, could have produced this transportation. No force of water
could raise a stone like the _pierre de goutté_ from the bottom of
a valley, to the top of a steep hail. Indeed, if we suppose a great
fragment of rock to be hurried along on a horizontal or an inclined
plane, by the force of water, the moment it comes to a deep valley, and
has to rise up over an ascent of a certain steepness, it will remain
at rest; the water itself will lose its velocity, and the heavy bodies
which it carried with it will proceed no farther. Thus, therefore, we
have the following dilemma. If the surface is not supposed to have
had a certain degree of uniformity in past times, a _debacle_ is
insufficient for the transportation of stones: If it is supposed to
have had that uniformity, a _debacle_ is unnecessary.
365. Another fact, which has been supposed favourable to the opinion
of the action of great torrents at some former period, is, that in
countries like that round Edinburgh, where whinstone hills rise up from
among secondary strata, a remarkable uniformity is observed in the
direction of their abrupt faces. Thus, in the country just mentioned,
the steep faces generally front the west, whiles in the opposite
direction, the slope is gentle, and the hills decline gradually into
the plain. Hence it is supposed, that a torrent, sweeping from west to
east, has carried off the strata from the west side of these hills,
but, being obstructed by the whinstone rock, has left the strata on
the east side in their natural place.
But, besides that no force which can ever be ascribed to a torrent
could have removed at once bodies of strata 300 or 400 feet, nay even
800 or 1000 in thickness, which must have been the case if this were
the true explanation of the fact, there is a circumstance which may
perhaps enable us to explain these phenomena without the assistance of
any extraordinary cause. The secondary strata in which the whinstone
hills are found in this part of Scotland, are not horizontal, but
rise or _head_ towards the west, dipping towards the east. The side,
therefore, of the whinstone hills which is precipitous, is the same
with that towards which the strata rise. Now, from the manner in which
these hills are supposed to have been elevated, the strata are likely
to have been most broken and shattered towards that side, while, on the
opposite, they had the support of the whinstone rock. They would become
a prey, therefore, more easily to the common causes of erosion and
waste on the upper side than on the lower. The streams that flowed from
the higher grounds would wear them on the former most readily; and the
action of these streams would be resisted by the superior hardness of
the whinstone, just as the great torrent of the _debacle_ is supposed
to have been.
It should also be observed, that this fact of the uniform direction of
the abrupt faces of mountains, is often too hastily generalized. In
primitive countries, it is no farther observed than by the steep faces
of the mountains being most frequently turned toward the central chain.
In Scotland, as soon as you leave the flat country, and enter the
Highlands, the scarps of the hills face indiscriminately all the points
of the compass, and are directed as often to the east as to the west.
366. Where the strata are nearly horizontal, they afford the most
distinct information concerning the direction and progress of the
wasting of the land. The inclined position of the strata, which in all
other cases must enter for so much into our estimate of the causes
which have produced the present inequality of the earth's surface,
disappears there entirely; and the whole of that inequality is to be
ascribed to the operations at the surface, whether they have been
sudden or gradual. A very important fact from a country of this
sort, is related by BARROW, in his Travels into Southern Africa. The
mountains about the Cape of Good Hope, and as far to the north as that
ingenious traveller prosecuted his journey, are chiefly of horizontal
strata of sandstone and limestone, exhibiting the appearance, on their
abrupt sides, of regular layers of masonry, of towers, fortifications,
&c. Now, among all these mountains, he observed, that the high or
steep sides look constantly down the rivers, while the sloping or
inclined sides have just the opposite direction. When, in travelling
northward, he passed the line of partition, where the waters from
running south take their direction to the north, he found, that the
gradual slope, which had hitherto been turned to the north, was now
turned to the south: The abrupt aspect of the mountains, in like
manner, from facing the south, was directed to the north; so that,
in both cases, the hills turned their backs on the line of greatest
elevation.[184]
[Footnote 184: Barrow's Travels into Southern Africa, p. 245.]
It is evident, therefore, that the form of this land has been
determined by the slow working of the streams. The causes which
produced the effects here described, began their action from the line
of greatest elevation, and extended it from thence on both sides, in
opposite directions. This is the most precise character that can mark
the alluvial operations, and distinguish them from the overwhelming
power of a great _debacle_.
367. Lastly, if there were any where a hill, or any large mass composed
of broken and shapeless stones, thrown together like rubbish, and
neither worked into gravel nor disposed with any regularity, we must
ascribe it to some other cause than the ordinary _detritus_ and
wasting of the land. This, however, has never yet occurred; and it
seems best to wait till the phenomenon is observed, before we seek for
the explanation of it.
368. These arguments appear to me conclusive against the necessity of
supposing the action of sudden and irregular causes on the surface of
the earth. In this, however, I am perhaps deceived: neither Pallas,
nor Saussure, nor Dolomieu, nor any other author who has espoused
the hypothesis of such causes, has explained his notions with any
precision; on the contrary, they have all spoken with such reserve and
mystery, as seemed to betray the weakness, but may have concealed the
strength of their cause. I have therefore been combating an enemy, that
was in some respects unknown; and I may have supposed him dislodged,
only because I could not penetrate to his strongholds. The question,
however, is likely soon to assume a more determinate form. A zealous
friend of Dr Hutton's theory, has lately[185] declared his approbation
of the hypothesis which has here been represented as so adverse to that
theory; and, from his ability and vigour of research, it is likely to
receive every improvement of which it is susceptible.
[Footnote 185: Trans. Royal Society Edin. vol. v. p 68.]
NOTE XIX. § 117.
_Transportation of Materials by the Sea._
369. THE existence of the great and extensive operations, by which the
spoils of the land are carried all over the ocean, and spread out on
the bottom of it, may be supposed to require some further elucidation.
We must attend, therefore, to the following circumstances.
When the detritus of the land is delivered by the rivers into the sea,
the heaviest parts are deposited first, and the lighter are carried
to a greater distance from the shore. The accumulation of matter
which would be made in this manner on the coast, is prevented by the
farther operation of the tides and currents, in consequence of which
the substances deposited continue to be worn away, and are gradually
removed farther from the land. The reality of this operation is
certain; for otherwise we should have on the sea shore a constant and
unlimited accumulation of sand and gravel, which, being perpetually
brought down from the land, would continually increase on the shore, if
nature did not employ some machinery for removing the advanced part
into the sea, in proportion to the supply from behind.
The constant agitation of the waters, and the declivity of the
bottom, are no doubt the causes of this gradual and widely extended
deposition. A soft mass of alluvial deposit, having its pores filled
with water, and being subject to the vibrations of a superincumbent
fluid, will yield to the pressure of that fluid on the side of the
least resistance, that is, on the side toward the sea, and thus will
be gradually extended more and more over the bottom. This will happen
not only to the finer parts of the detritus, but even to the grosser,
such as sand and gravel. For suppose that a body of gravel rests on
a plane somewhat inclined, at the same time that it is covered with
water to a considerable depth, that water being subject not only to
moderate reciprocations, but also to such violent agitation as we see
occasionally communicated to the waters of the ocean; the gravel,
being rendered lighter by its immersion in the water, and on that
account more moveable, will, when the undulations are considerable,
be alternately heaved up and let down again. Now, at each time that
it is heaved up, however small the space may be, it must be somewhat
accelerated in its descent, and will hardly settle on the same point
where it rested before. Thus it will gain a little ground at each
undulation, and will slowly make its way towards the depths of the
ocean, or to the lowest situation it can reach. This, as far as we may
presume to follow a progress which is not the subject of immediate
observation, is one of the great means by which loose materials of
every kind are transported to a great distance, and spread out in beds
at the bottom of the ocean.
370. The lighter parts are more easily carried to great distances,
being actually suspended in the water, by which they are very gradually
and slowly deposited. A remarkable proof of this is furnished from an
observation made by Lord Mulgrave, in his voyage to the North Pole.
In the latitude of 65° nearly, and about 250 miles distant from the
nearest land, which was the coast of Norway, he sounded with a line of
688 fathoms, or 4098 feet; and the lead, when it struck the ground,
sunk in a soft blue clay to the depth of ten feet.[186] The tenuity
and fineness of the mud, which allowed the lead to sink so deep into
it, must have resulted from a deposition of the lighter kinds of
earth, which being suspended in the water, had been carried to a great
distance, and were now without doubt forming a regular stratum at the
bottom of the sea.
[Footnote 186: Phipps's Voyage, p. 74, 141.]
371. The quantity of detritus brought down by the rivers, and
distributed in this manner over the bottom of the sea, is so great,
that several narrow seas have been thereby rendered sensibly shallower.
The Baltic has been computed to decrease in depth at the rate of
forty inches in a hundred years. The Yellow Sea, which is a large
gulf contained between the coast of China and the peninsula of Corea,
receives so much mud from the great rivers that run into it, that it
takes its colour, as well as its name, from that circumstance; and the
European mariners who have lately navigated it, observed, that the
mud was drawn up by the ships, so as to be visible in their wake to a
considerable distance.[187] Computations have been made of the time
that it will require to fill up this gulf, and to withdraw it entirely
from the dominion of the ocean: but the data are not sufficiently exact
to afford any precise result, and are no doubt particularly defective
from this cause, that much of the earth carried into the gulf by the
rivers, must be carried out of it by the currents and tides, and
the finer parts wafted probably to great distances in the Pacific
Ocean.[188] The mere attempt, however, towards such a computation,
shows how evident the progress of filling up is to every attentive
observer; and, though it may not ascertain the measure, it sufficiently
declares the reality of the operations, by which the waste of the
present continents is made subservient to the formation of new land.
[Footnote 187: Staunton's Account of the Embassy to China, vol. i. p.
448.]
[Footnote 188: Perouse, in sailing along the coast of China, from
Formosa to the strait between Corea and Japan, though generally
fifty or sixty leagues from the land, had soundings at the depth of
forty-five fathoms, and sometimes at that of twenty-two. Atlas du
Voyage de la Perouse, No. 43.]
372. Sandbanks, such as abound in the German Ocean, to whatever they
owe their origin, are certainly modified, and their form determined,
by the tides and currents. Without the operation of these last, banks
of loose sand and mud could hardly preserve their form, and remain
intersected by many narrow channels. The formation of the banks on the
coast of Holland, and even of the Dogger Bank itself, has been ascribed
to the meeting of tides, by which a state of tranquillity is produced
in the waters, and of consequence a more copious deposition of their
mud. Even the great bank of Newfoundland seems to be determined in its
extent by the action of the Gulf stream. In the North Sea, the current
which sets out of the Baltic, has evidently determined the shape of
the sandbanks opposite to the coast of Norway, and produced a circular
sweep in them, of which it is impossible to mistake the cause.
In proof of the action here ascribed to the waters of the sea, in
transporting materials to an unlimited extent, we may add the well
known observation, that the stones brought up by the lead from the
bottom of the sea, are generally round and polished, hardly ever sharp
and angular. This could never happen to stones that were not subject to
perpetual attrition.
373. Currents are no doubt the great agents in diffusing the detritus
of the land over the bottom of the sea. These have been long known to
exist; but it is only since the later improvements in navigation, that
they have been understood to constitute a system of great permanence,
regularity, and extent, connected with the trade winds, and other
circumstances in the natural history of the globe. The Gulf stream was
many years since observed to transport the water, and the temperature
of the tropical regions into the climates of the north; and we are
indebted to the researches of Major RENNELL, for the knowledge of a
great system of currents, of which it is only a part. That geographer,
who is so eminent for enriching the details of his science with the
most interesting facts in history or in physics, has shown, that
along the eastern coast of Africa, from about the mouth of the Red
Sea, a current fifty leagues in breadth sets continually towards the
south-west.[189] It doubles the Cape of Good Hope, runs from thence
north-west, preserving on the whole the direction of the coast, but
reaching so far into the ocean, that, about the parallel of St Helena,
its breadth exceeds 1000 miles. From thence, as it approaches the line,
its direction is more nearly east; and meeting in the parallel of 3°
north, with a current which has come along the western coast of Africa
from the north, the two united stretch across the Atlantic, in a line
somewhat south of west, and in a very wide and rapid stream. This
stream meets the American land at Cape St Roque, where it is joined
by another coming up along the eastern shore of that continent, and
directed towards the north. They proceed northward together till they
enter the Gulf of Florida, from which being as it were reflected, they
form the Gulf stream, passing along the coast of North America, and
stretching across the Atlantic to the British Isles. From thence the
current turns to the south, and, proceeding down the coast of Spain
and Africa, meets the stream ascending from the south, as already
described, and thus continues in perpetual circulation. The velocity of
these currents is not less remarkable than their extent. At the Cape
of Good Hope, the rate is thirty nautical miles in twenty four hours;
in some places forty five; and under the line seventy seven. When the
Gulf stream issues from the Straits of Bahama, it runs at the rate of
four miles an hour, and proceeds to the distance of 1800 miles, before
its velocity is reduced to half that quantity. In the parallel of 38°,
near 1000 miles from the above strait, the water of the stream has been
found ten degrees warmer than the air.
[Footnote 189: Geography of Herodotus, p. 672.]
374. The course of the Gulf stream is so fixed and regular, that
nuts and plants from the West Indies are annually thrown ashore on
the Western Islands of Scotland. The mast of a man of war, burnt at
Jamaica, was driven several months afterwards on the Hebrides,[190]
after performing a voyage of more than 4000 miles, under the direction
of a current, which, in the midst of the ocean, maintains its course as
steadily as a river does upon the land.
[Footnote 190: Pennant's Arctic Zoology, Introd. p. 70.]
The great system of currents thus traced through the Atlantic, has no
doubt phenomena corresponding to it in the Indian and Pacific Oceans,
which the industry of future navigators may discover. The whole appears
to be connected with the trade winds, the figure of our continents, the
temperature of the seas themselves, and perhaps with some inequalities
in the structure of the globe. The disturbance produced by these causes
in the equilibrium of the sea, probably reaches to the very bottom of
it, and gives rise to those counter currents, which have sometimes been
discovered at great depths under the surface.[191]
[Footnote 191: Histoire Naturelle de Buffon, Supplément, tom. ix. p.
479. 8vo.]
The great transportation of materials that must result from the
action of these combined currents is obvious, and serves not a little
to diminish our wonder, at finding the productions of one climate
so frequently included among the fossils of another. Amid all the
revolutions of the globe, the economy of nature has been uniform, in
this respect, as well as in so many others, and her laws are the only
thing that have resisted the general movement. The rivers and the
rocks, the seas and the continents, have been changed in all their
parts; but the laws which direct those changes, and the rules to which
they are subject, have remained invariably the same.
375. Objections have been made to that translation of materials by the
waters of the ocean which is supposed in this theory, particularly
by Mr Kirwan, in his Geological Essays; and, though I might perhaps
content myself with the remark already made, that the Neptunian system
involves suppositions concerning the transportation of solid bodies by
the sea, in the early ages of the world, as wonderful as those which,
according to our theory, are common to all ages, I am unwilling to
remain satisfied with a mere _argumentum ad hominem_, where the fallacy
of the reasoning is so easily detected.
376. One of Mr Kirwan's objections to the deposition of materials at
the bottom of the sea, is thus stated: "FRISI has remarked, in his
mathematical discourses, that if any considerable mass of matter were
accumulated in the interior of the ocean, the diurnal motion of the
globe would be disturbed, and consequently it would be perceptible;
a phenomenon, however, of which no history or tradition gives any
account."[192]
[Footnote 192: Geol. Essays, p. 441.]
The appeal made here to Frisi is singularly unfortunate, as that
philosopher has demonstrated the very contrary of Mr Kirwan's
position, and has proved, that the disturbance given to the diurnal
motion by the causes here referred to may be real, but cannot be
perceptible. Having investigated a formula expressing the law which
all such disturbances must necessarily observe, he concludes, "Hàc
autem formulâ manifestum fiet, ex iis omnibus variationibus quæ in
terrestri superficie observari solent, montium et collium abrasione,
dilapsu corporum ponderosiorum in inferiores telluris sinus, nullam
oriri posse variationem _sensibilem_ diurni motûs. Nam si statuamus
data aliqua annorum periodo terrestrem superficiem ad duos usque pedes
abradi undique, eam vero materiæ quantitatem ad profunditatem pedum
1000 dilabi; erit omne quod inde orietur incrementum velocitatis diurni
motûs 30000/(19638051)^2 = 1/12855068184."[193]
[Footnote 193: Frisii Opera, tom. iii. p. 269.]
Here, it is evident, that Frisi admits those very changes on the
surface which we are contending for, and shows, that their tendency is
to accelerate the earth's diurnal motion, but, by a quantity so small,
that, in a space of time amounting at least to 200 years, the increase
of the diurnal motion would only be such a part of the whole as the
preceding fraction is of unity.[194]
[Footnote 194: The time requisite for taking away by waste and erosion
two feet from the surface of all our continents, and depositing it at
the bottom of the sea, cannot be reckoned less than 200 years. The
fraction 1/12855068184, reduced to parts of a day, is 1/148554 of a
second; so that it would require 200 years to shorten the length of
the day, by the above fraction of a second; and therefore it would
require 148554 times 200 years, or 29710800 years, to diminish it an
entire second. The accumulated effect, however, of all the diminutions
during that period, would amount to much more: and if we had any
perfectly uniform standard to compare the motion of the earth with,
its difference from that standard would increase as the squares of the
time, and the total acceleration would amount to one second in 77080
years. Whatever relation this bears to the age of the globe itself, it
exceeds more than ten times the age of any historical record.
Though Frisius concludes, as is stated here, that the acceleration
produced in the diurnal motion of the earth, is far too inconsiderable
to become the object of astronomical observation, he makes a
supposition difficult to be reconciled with this conclusion, namely,
that the acceleration has had a sensible effect on the figure of the
earth, or rather of the sea, having increased the centrifugal force,
and thereby accumulated the waters under the equator, in the present,
more than in former ages. Such an accumulation, he thinks agreeable
to certain appearances that have been observed respecting the ancient
level of the sea. These appearances will be afterwards considered: it
is sufficient to remark here, that though the fraction, expressing the
increment of the centrifugal force, must be double that which expresses
the acceleration, it must be too small to have any perceptible effect
in elevating the sea, except after an immense interval of time; and the
compensations which arise from other causes, probably must prevent it
from becoming sensible in any length of time whatsoever.]
377. The instance just given may serve as one of many, to shew what
confidence is to be placed in that indigested mass of facts and
quotations which Mr Kirwan, without discrimination, and without
discussion, has brought together from all quarters. He has no
intention, I believe, to deceive his readers; but we may judge, from
this specimen, of the precautions he has taken against being deceived
himself.
In some respects, the result of Frisi's investigation must be
considered as imperfect. If there were no relative motion in the parts
of our globe, but that by which things descend from a higher to a lower
level, a continual acceleration of its rotation, though extremely slow,
would take place, as above computed. But as, in the interior of the
earth, there are undoubtedly motions of a tendency opposite to those on
the surface, and directed from the centre towards the circumference,
they must produce a retardation in the diurnal revolution; and from
this must arise an inequality, not uniformly progressive in the same
direction, but periodical, and confined within certain limits, as the
causes are by which it is produced.[195]
[Footnote 195: Even in the descent of bodies from a higher to a lower
level at the surface of the earth, the whole tendency is not to
increase the velocity of the earth's rotation, and many compensations
take place, which, when the matter is considered only in general,
are necessarily overlooked. This will appear evident, if we reflect,
that it is not simply the approach of a body towards the centre of
the earth, or its removal from that centre, which tends to disturb
the rotation of the earth; but its approach to the axis of the earth,
or its removal from that axis. The velocity with which a particle of
matter revolves, whether on the surface, or in the interior of the
globe, is proportional to its distance from the axis of rotation;
and therefore, when a body comes nearer to the axis, it loses a part
of the motion which it had before; which part, of consequence, is
communicated to the whole mass of the earth, and therefore tends to
increase the velocity with which it revolves. The contrary happens when
a body recedes from the axis; for it then receives an addition to its
velocity, which, of course, is taken away from the rotatory motion of
the earth.
Hence, bodies moving in a horizontal plane, may increase or diminish
the swiftness of the diurnal motion, according as they move towards the
poles or towards the equator; and those which descend from a higher to
a lower level, disturb the earth's rotation, much more in consequence
of their horizontal, than of their perpendicular motion. The Ganges,
for instance, though its source is probably elevated no less than
7000 feet above the level of the sea, tends to retard the earth's
rotation, by bringing its waters, and the mud contained in them, from
the parallel of 31° to that of 22°, and so increasing their distance
from the earth's axis by more than 1/12 th part. Had the Ganges flowed
towards the north, as the Nile does, its effect would have been just
the contrary.
In the same manner, a stone descending from the top of a mountain, may
accelerate or retard the earth's rotation, according to the direction
in which it descends. If it descend on the side of the elevated pole,
it will then produce acceleration, because its distance from the axis
will be diminished; but if it descend on the side of the depressed
pole, and if the direction in which it is moved, be over a line less
inclined, than a line drawn from the same point to the depressed pole,
it will then produce a retardation, because its distance from the axis
will be increased.
Let us suppose, for example, that the top of Mount Blanc is in latitude
45° 49′, and that its height is 2450 toises above the level of the sea.
The point at which a line drawn from the top of this mountain, parallel
to the earth's axis, will meet the superficies of the sea, (supposing
that superficies continued inland from the Mediterranean), must be
about 2382 toises in horizontal distance, or about 2-1/2 minutes south
of the summit, that is, in the parallel of 45° 46-1/2′; and if this
parallel be continued all round the globe, the points of the earth's
surface between it and the equator, are all more distant from the
earth's axis than the top of Mount Blanc is; whereas all the points
to the north of it are nearer to that axis. A stone, therefore, from
the top of Mount Blanc, if carried any where to the south of the above
parallel, will retard the earth's diurnal motion; but if carried any
where to the north of the same line, will accelerate that motion.
The same quantity of matter, however, carried an equal distance
toward the pole, and toward the equator, from any point, will lose
more velocity in the former case than it will gain in the latter, as
easily follows from the nature of circle. Therefore, supposing an equal
dispersion of the detritus of a mountain in all directions, the parts
that go toward the pole will most disturb the diurnal motion; and hence
a balance on their side, or in favour of acceleration, as already
observed.]
378. Mr Kirwan's second objection is founded on the misapprehension
of a well-known fact in the natural history of the earth. "Rivers,"
says this author, "do not carry into the sea the spoils which they
bring from the land, but employ them in the formation of deltas of low
alluvial land at their mouths, according to what Major Rennell has
proved." The fact of the formation of _deltas_ from the spoils which
the rivers carry from the higher grounds, is perfectly ascertained;
and the detail into which Major Rennell has entered in the passage
referred to by Mr Kirwan, does credit to the acuteness and accuracy
of that excellent geographer. But it is not there asserted, that
rivers employ _all_ the materials which they carry with them, in the
formation of those deltas, and deliver none of them into the sea. On
the contrary, they carry from the _delta_ itself mud and earth, which
they can deposit no where but in the sea; and it is this circumstance
chiefly that limits the increase of those alluvial lands, and makes
them either cease to increase, or makes them increase very slowly after
a certain period, though the supply of earth from the higher grounds
remains nearly the same. To make Mr Kirwan's argument conclusive, it
would be necessary to prove, that _all_ the mud carried down by the
Nile or the Ganges, was deposited on the low lands before these rivers
enter the sea; a thing so obviously absurd, that nothing but his haste
to obtain a conclusion unfavourable to the Plutonic system, could have
prevented him from perceiving it[196].
[Footnote 196: The instance mentioned in the Geological Essays, from
the travels of the Abbé Fortis, concerning urns thrown into the
Adriatic, upwards of 1400 years ago, and not yet covered with mud,
must be explained from peculiar circumstances, or local causes, with
which we are unacquainted, as it makes against the deposition of earth
near the shore, and in narrow seas; a general fact which, I think,
every body admits.]
379. A remark which Major Rennell has made concerning the mouths of
rivers, in his Geography of Herodotus, deserves Mr Kirwan's attention,
though perhaps he may not be able to put on it an interpretation quite
so favourable to his system. The remark is, that the mouths of great
rivers are often formed on principles quite opposite to one another, so
that some of them have a real delta or triangle of flat land at their
mouths, while others have an estuary, or what may not improperly be
called a _negative_ delta. Of the latter kind are some of the greatest
rivers in the world, the Plata, the Oroonoko and the Maranon, and by
far the greatest number of our European rivers. Nobody can doubt, that
the three rivers just named carry with them as much earth as the Nile,
or the Euphrates, or any other river in the world. All this they have
deposited in the sea, and committed to the currents, which sweep along
the shore of the American continent, and by these they have been spread
out over the unlimited tracts of the ocean.
Indeed, nothing can be more just than Dr Hutton's observation, that
where low land is formed at the mouths of rivers, there the rivers
bring down more than the sea is able to carry away; but that where
such land is not formed, it is because the sea is able to carry off
immediately all the deposit which it receives.
380. Mr Kirwan has denied on another principle the power of the sea to
carry to a distance the materials delivered into it: "Notwithstanding,"
says he, "many particles of earth are by rivers conduced to the sea,
yet _none are conveyed in any distance_, but are either deposited at
their mouths, or rejected by currents or by tides; and the reason
is, because the tide of flood is always more impetuous and forcible
than the tide of ebb, the advancing waves being pressed forward by
the countless number behind them, whereas the retreating are pressed
backward by a far smaller number, as must be evident to an attentive
spectator; and hence it is that all floating things cast into the sea,
are at last thrown on shore, and not conveyed into the mid regions of
the sea, as they should be if the reciprocal undulations of the tides
were equally powerful "[197]
[Footnote 197: Kirwan's Geol. Essays, p. 439.]
381. But if the _attentive spectator_, instead of trusting to a vague
impression, or listening to some crude theory of undulations, reflects
on one of the most simple facts respecting the ebbing and flowing of
the tides, he will be very little disposed to acquiesce in the above
conclusion. He has only to consider, that the flowing of the tide
requires just six hours, and the ebbing of it likewise six hours; so
that the same body of water flows in upon the shore, and retreats from
it, in the same time. The quantity of matter moved, therefore, and the
velocity with which it is moved, are in both cases the same; and it
remains for Mr Kirwan to show in what the difference of their force can
possibly consist.
The force with which the waves usually break upon our shores, does not
arise from the velocity of the tide being greater in one direction
than in another. In the main ocean, the waves have no progressive
motion, and the columns of water alternately rise and fall, without
any other than a reciprocating motion: a kind of equilibrium takes
place among the undulations, and each wave being equally acted upon by
those on opposite sides, remains fixed in its place. Near the shore
this cannot happen; the water on the land side from its shallowness
being incapable of rising to the height necessary to balance the
great undulations which are without. The water runs, therefore, as it
were, from a higher to a lower level, spreading itself towards the
land side. This produces the breakers on our shores, and the surf
of the tropical seas. A rock or a sandbank coming within a certain
distance of the surface, is sufficient, in any part of the ocean, to
obstruct the natural succession of undulations; and, by destroying the
mutual reaction of the waves, to give them a progressive instead of a
reciprocating motion.
382. It is, however, but from a small distance, that the waves are
impelled against the shore with a progressive motion. The border of
breakers that surrounds any coast is narrow, compared with the distance
to which the _detritus_ from the land is confessedly carried; the
water, while it advances at the surface, flows back at the bottom;
and these contrary motions are so nearly equal, that it is but a very
momentary accumulation of the water that is ever produced on any shore.
If it were otherwise, and if it were true that the sea throws out every
thing, and carries away nothing, we should have a constant accumulation
of earth and sand along all shores whatsoever, at least wherever a
stream ran into the sea. This, as is abundantly evident, is quite
contrary to the fact.
So, also, the bars formed at the mouths of rivers, after having
attained a certain magnitude, increase no farther, not because they
cease to receive augmentations from the land, but because their
diminution from the sea, increasing with their magnitude, becomes at
length so great, as completely to balance those augmentations. When
properly examined, therefore, the phenomena, which have been proposed
as most inconsistent with the indefinite transportation of stony
bodies, afford very satisfactory proofs of that operation.
383. It is true, that bodies which float in the water, when carried
along on the tops of the waves towards a shelving beach, having
acquired a certain velocity, are thrown farther in upon the land than
the distance they would have floated to, if they had been simply
sustained by the water. The depth of water, therefore, at the place
where they take the ground, is not likely to be such as to float them
again, and to carry them out towards the sea. They are, therefore, left
behind; and this produces an appearance of a force impelling floating
bodies towards the land, much greater and more general than really
takes place.
These observations may serve to show, how unsound the principles are
from which Mr Kirwan's conclusions are deduced: they are perhaps more
than is necessary for that purpose: it might have been sufficient to
observe, that the increase of land on the sea shore is limited, though
the augmentation from the land is certainly indefinite, a proof that
the diminution from the sea is constant and equal to the increase.
384. "Mariners," says Mr Kirwan, "were accustomed, for some centuries
back, to discover their situation, by the kind of earth or sand brought
up by their sounding plummets; a method which would prove fallacious,
if the surface of the bottom did not continue invariably the same."[198]
[Footnote 198: Geol. Essays, p. 440.]
The fact here stated, that mariners, when navigation was more imperfect
than it is now, had very frequent recourse to this method, and that
they still use it occasionally, is very true. But from this, the only
inference that can be fairly deduced is, that the changes at the bottom
of the sea are very slow, and the variation but little; not merely
from one year to another, but even from one century to another. The
rules by which the mariner judged of his position from the quality of
the earth which the lead brought up, and which were deduced no doubt
from observations made at no very great distance of time, might be
sufficient for his purpose, though a slow change had been all the while
going forward. Such observations could at best have little accuracy,
and could not be affected by small variations. It is the slowness of
the change, that makes the experience of one age applicable, in this,
as in innumerable other instances, to the observations of the next. If
a long interval is taken, we will look in vain for the same uniformity
of results. A pilot, who would at present judge of his position in the
German Ocean, by comparing his soundings with those taken by PYTHEAS,
(supposing them known) in his navigation of that sea, more than 2000
years ago, could hardly be expected to determine his latitude and
longitude with great exactness; and I know not if the most zealous
advocate for the immutability of the earth's surface, would be willing
to trust his safety in a ship that was guided by such antiquated rules.
NOTE XX. § 118.
_Inequalities in the Planetary Motions._
385. The assertion that, in the planetary motions, we discover no
mark, either of the commencement or termination of the present order,
refers to the late discoveries of LA GRANGE and LA PLACE, which have
contributed so much to the perfection of physical astronomy. From
the principle of universal gravitation, these mathematicians have
demonstrated, that all the variations in our system are periodical;
that they are confined within certain limits; and consist of alternate
diminution and increase. The orbits of the planets change not only
their position, but even their magnitude and their form: the longer
axis of each has a slow angular motion; and, though its length remains
fixed, the shorter axis increases and diminishes, so that the form of
the orbit approaches to that of a circle, and recedes from it by turns.
In the same manner, the obliquity of the ecliptic, and the inclination
of the planetary orbits, are subject to change; but the changes are
small, and, being first in one direction, and then in the opposite,
they can never accumulate so as to produce a permanent or a progressive
alteration. Thus, in the celestial motions, no room is left for the
introduction of disorder; no irregularity or disturbance, arising from
the mutual action of the planets, is permitted to increase beyond
certain limits, but each of them, in time, affords a correction for
itself. The general order is constant, in the midst of the variation of
the parts; and, in the language of La Place, there is a certain mean
condition, about which our system perpetually _oscillates_ performing
small vibrations on each side of it, and never receding from it
far.[199] The system is thus endowed with a stability, which can refill
the lapse of unlimited duration; it can only perish by an external
cause, and by the introduction of laws, of which at present no vestige
is to be traced.
[Footnote 199: Exposition du Systéme du Monde, par La Place, Livre iv.
chap. 6. p. 199. 2d edit.]
386. The same _calculus_ to which we are indebted for these sublime
conclusions, informs us of two circumstances, which mark the law
here treated of as an effect of wise design, to the entire exclusion
both of necessity and chance. One of these circumstances consists in
the planetary motions being all in the same direction, or all _in
consequentia_ as it is called by the astronomers. This is essential
to the compensation and stability above mentioned:[200] had one planet
circulated round the sun in a direction from east to west, and another
in a direction from west to east, the disturbances they would have
produced on one another's motion would not necessarily have been
periodical; their irregularities might have continually increased, and
they might have deviated in the course of ages from their original
condition, beyond any limits that can be assigned.
[Footnote 200: La Place, _ibid._]
The other circumstance, on which the stability of our system depends,
is the small eccentricity of the planetary orbits, or their near
approach to circles. Were their orbits very eccentric, an opening
would be given to progressive change, that might so far increase, as
to prove the destruction of the whole. But neither the movement of all
the planets in the same direction, nor the small eccentricity of their
orbits, can be ascribed to accident, since that either of these should
happen by chance, in as many instances as there are planets, both
primary and secondary, is almost infinitely improbable. Again, that any
necessity in the nature of things should have either determined the
_direction_ of the planetary motions, or proportioned the _quantity_
of them to the intensity of the central force, cannot be admitted, as
these are things unavoidably conceived to be quite independent of one
another. It remains, therefore, that we consider the laws, which make
the disturbances in our system correct themselves, and by that means
give firmness and permanence to it, as a proof of the consummate wisdom
with which the whole is constructed.
387. The geological system of Dr Hutton, resembles, in many respects,
that which appears to preside over the heavenly motions. In both, we
perceive continual vicissitude and change, but confined within certain
limits, and never from a certain, mean condition, which is such, that,
in the lapse of time, the deviations from it on the one side, must
become just equal to the deviations from it on the other. In both, a
provision is made for duration of unlimited extent, and the lapse of
time has no effect to wear out or destroy a machine, constructed with
so much wisdom. Where the movements are all so perfect, their beginning
and end must be alike invisible.
NOTE XXI. § 122.
_Changes in the apparent
Level of the Sea._
388. In speaking of the natural epochs marked out by the phenomena
of the mineral kingdom, we have supposed a greater simplicity, and
separation of effects from one another, than probably takes place in
nature. We have, for instance, abstracted, in speaking of the waste
and degradation of the land, from that elevation which may have been
carried on at the same time. This appeared necessary to be done, in
order to simplify as much as possible the view that was to be given of
the whole; but there can be no doubt, that, while the land has been
gradually worn down by the operations on its surface, it has been
raised up by the expansive forces acting from below. There is even
reason to think, that the elevation has not been uniform, but has been
subject to a kind of oscillation, insomuch, that the continents have
both ascended and descended, or have had their level alternately raised
and depressed, independently of all action at the surface, and this
within a period comparatively of no great extent.
It will be easily understood, that the facts we are going to state,
each taken singly, prove nothing more than a change of the line in
which the surface of the sea intersects the surface of the land,
leaving it uncertain to which of the two the change ought really to be
ascribed. Taken in combination, however, these facts may determine what
each of them separately cannot ascertain. I shall first, therefore,
mention some of the principal observations relative to the change above
mentioned, and shall then compare them, in order to discover whether it
is most probable that this change has been produced by the motion of
the land or of the sea.
389. If we begin with examining the coasts of our own island, we shall
find clear evidence every where, that the sea once reached higher up
upon the land than it does at present. The marks of an ancient sea
beach are to be seen beyond the present limits of the tide, and beds
of sea shells, not mineralized, are found in the loose earth or soil,
sometimes as high as thirty feet above the present level of the sea.
Some of these on the shores of the Frith of Forth are very well known,
and have been often mentioned. Indeed, on the shores of that frith,
many monuments appear, which would seem to carry the difference
between the present and the ancient level of the sea, to more than
forty feet. The ground on which the Botanic Garden of Edinburgh is
situated, after a thin covering of soil is removed, consists entirely
of sea sand, very regularly stratified, with layers of a black
carbonaceous matter, in thin lamellæ, interposed between them. Shells I
believe are but rarely found in it, but it has every other appearance
of a sea beach. The height of this ground above the present level of
the sea is certainly not less than forty feet.
390. On almost every part of the coast where the rocks do not rise
quite abrupt and precipitous from the sea, similar marks of the
lowering of the sea, or the rising of the land, may be observed. On the
shores opposite to ours, the same appearances are remarked. The author
of the Lettre Critique to M. de Buffon, tells us, that he had found
the bottom of a bason at Dunkirk, which he had reason to think was dug
about 950 years ago, ten feet and a half above the present low water
mark, though it must have been originally under it. The bottom of this
bason is in the native chalk. From this, the same author concludes,
that the sea at Dunkirk lowers its level at the rate of an inch nearly
in seven years. The observation was made in 1762, (Lettre à M. le
Comte de Buffon, &c. p. 55.)[201]
[Footnote 201: In the county of Suffolk, near Wood Bridge, at the
distance of seven or eight miles from the sea, are the Crag-pits, in
which prodigious quantities of sea shells are discovered, many of them
perfect and quite solid, (Pennant's Arctic Zoology, Introd. p. 6.)
Lincolnshire affords various proofs of the same kind; but some other
circumstances in the appearance of that coast, just about to be taken
notice of, indicate changes of a more complicated nature.]
391. The shores of the Low Countries, and of Holland, have been often
instanced in proof of the same kind of changes, and it has been
supposed, that, independently of those artificial barriers which at
present exclude the waters of the ocean from overflowing a great part
of this tract, nature herself has brought it nearer to the surface
than it had formerly been. It is indeed certain, that those countries,
to a very great extent inland, have either been under the sea at some
period, by no means remote if compared with the great revolutions of
the globe, or that they are entirely alluvial, and of the same sort
with the Deltas formed at the mouths of rivers. The relative changes,
however, of the sea and land on this tract, have been differently
represented, and I am unwilling, on that account, to found any
argument on them.
392. If we proceed farther to the north, to the shores of the Baltic
for instance, we have undoubted evidence of a change of level in the
same direction as on our own shores. The level of this sea has been
represented as lowering at so great a rate as 40 inches in a century.
Celsius observed, that several rocks which are now above water,
were not long ago sunken rocks, and dangerous to navigators; and he
particularly took notice of one, which, in the year 1680, was on the
surface of the water, and in the year 1791 was 20-1/2 Swedish inches
above it. From an inscription near Aspô, in the lake Melar, which
communicates with the Baltic, engraved, as is supposed, about five
centuries ago, the level of the sea appears to have sunk in that time
no less than 13 Swedish feet.[202] All these facts, with many more
which it is unnecessary to enumerate, make the gradual depression,
not only of the Baltic, but of the whole northern ocean, a matter of
certainty.
[Footnote 202: Frisii Opera, tom. iii. p. 274.]
393. Supposing these changes of level between the sea and land to be
sufficiently ascertained, the supposition which at first occurs is,
that the motion has been in the sea rather than in the land, and that
the former has actually descended to a lower level. The imagination
naturally feels less difficulty in conceiving, that an unstable
fluid like the sea, which changes its level twice every day, has
undergone a permanent depression in its surface, than that the land,
the _terra firma_ itself, has admitted of an equal elevation. In all
this, however, we are guided much more by fancy than reason; for, in
order to depress or elevate the absolute level of the sea, by a given
quantity, in any one place, we must depress or elevate it by the same
quantity over the whole surface of the earth; whereas no such necessity
exists with respect to the elevation or depression of the land. To
make the sea subside 30 feet all round the coast of Great Britain, it
is necessary to displace a body of water 30 feet deep over the whole
surface of the ocean. The quantity of matter to be moved in that way is
incomparably greater than if the land itself were to be elevated; for
though it is nearly three times less in specific gravity, it is as much
greater in bulk, as the surface of the ocean is greater than that of
this island.
394. Besides, the sea cannot change its level, without a proportional
change in the solid bottom on which it rests. Though there be reason to
suppose that such changes in the bottom do actually take place, yet
they are probably much slower and more imperceptible than those which
we are here considering. It is evident, therefore, that the simplest
hypothesis for explaining those changes of level, is, that they proceed
from the motion, upwards or downwards, of the land itself, and not
from that of the sea. As no elevation or depression of the sea can
take place, but over the whole, its level cannot be affected by local
causes, and is probably as little subject to variation as any thing to
be met with on the surface of the globe.
395. Other observations, however, made on different shores from the
preceding, give greater certainty to this conclusion, and make it
clear, that the motion or change which we are now treating of is not to
be ascribed to the sea itself.
The observations just mentioned prove, that the level of the North
Sea is lower now than it was heretofore; but it appears, that in the
Mediterranean, the opposite takes place. Very accurate observations
made by MANFREDI, render it certain, that the superficies of the
Hadriatic was higher about the middle of the last century, than toward
the beginning of the Christian era.
Some repairs that were carrying on in the cathedral church of Ravenna,
in the year 1731, afforded him an opportunity of observing, that the
ancient, and probably original, pavement, was four feet and a half
below the present, and nearly a foot under the level of the sea at high
water.[203] Now, when the church was built, this cannot have been the
position of the pavement, relatively to the level of the sea, for it
would have subjected the floor to be under water twice in twenty-four
hours, and must have done so the more unavoidably, because at that time
(the beginning of the fifth century) the walls of Ravenna were washed
by the sea. The fact that this pavement is under the high-water mark,
by the quantity just mentioned, was ascertained by actual levelling.
This result was confirmed by similar facts, observed by ZENDRINI at
Venice.
[Footnote 203: Commentarii Academiæ Bononiensis, tom. ii. pars 1ma, p.
237, &c. and pars 2da, p. 1. &c.]
396. Manfredi himself attributes all this to the elevation of the
surface of the sea, and has entered into a long calculation to
ascertain at what rate that surface may be supposed to rise, on account
of the earth and sand brought down by the rivers, and spread out over
the bottom of the sea. But as the fact of the rise of the level of the
sea is not general, and as the contrary is observed in the north seas,
as already proved, this hypothesis will not explain the apparent rise
in the level of the Hadriatic.
397. Though a local subsidence, or settling of the ground, could hardly
account for this change, the pavement being perfect in its level, and
the walls of the cathedral without any shake, yet a subsidence that
has extended to a great tract, as to the whole of Italy, if the mass
moved has continued parallel to itself, and changed its place slowly,
will agree very well with the appearances. The facts here stated are
also the more deserving of attention, that about Ravenna, the land, at
the same time that it has sunk in its level, has extended its surface,
and has encroached on the sea. Since the time of AUGUSTUS, the line
of the coast has been carried farther out by about three miles.[204]
This last is the undoubted effect of the degradation of the land by the
rivers; and here we have very clear evidence of the forces, both under
and above the surface, producing their respective effects at the same
time, so that while the surface is raised by earth brought down by the
rivers, every given point in the ground is depressed and let down to a
lower level.[205]
[Footnote 204: Manfredi, _ibid._]
[Footnote 205: On the coast of Dalmatia also, the rising of the level
of the sea has been remarked, particularly at the ruins of Diocletian's
palace of Spalatro.]
398. On the southern coast of Italy similar facts have been observed.
BREISLAC, in his _Topographia Fisica della Campagnia di Roma_,[206]
from certain appearances in the Gulfs of Baja and Naples, concludes,
that at the beginning of the Christian era, the level of the sea was
lower on that part of the coast than it is now. The facts which he
mentions are the following: _1mo_, The remains of an ancient road are
now to be seen in the Gulf of Baja at a considerable distance from
the land. _2do_, Some ancient buildings belonging to Porto Giulio are
at present covered by the sea. _3tio_, Ten columns of granite at the
foot of Monte Nuovo, which appear to have belonged to the Temple of
the Nymphs, are also nearly covered by the sea. _4to_, The pavement of
the Temple of Serapis is now somewhat lower than the high water mark,
though it cannot be supposed that this edifice when built was exposed
to the inconvenience of having its floor frequently under water. _5to_,
The ruins of a palace, built by Tiberius in the island of Caprea, are
now entirely covered by the sea.
[Footnote 206: Cap. vi. p. 300.]
Thus, it appears that the level of the sea is sinking in the more
northern latitudes, and rising in the Mediterranean, and it is evident
that this cannot happen by the motion of the sea itself. The parts
of the ocean all communicating with one another, cannot rise in one
place and fall in another; but, in order to maintain a level surface,
must rise equally or fall equally over the whole of its extent. If,
therefore, we place any confidence in the preceding observations, and
they are certainly liable to no objection, either from their own nature
or the character of the observers, we must consider it as demonstrated,
that the relative change of level has proceeded from the elevation or
depression of the land itself. This agrees well with the preceding
theory, which holds, that our continents are subject to be acted upon
by the expansive forces of the mineral regions; that by these forces
they have been actually raised up, and are sustained by them in their
present situation.
399. According to some other facts stated by the same ingenious
author, it appears, that on the coast of Italy the progress of the
sea in ascending, or of the land in descending, has not been uniform
during the period above mentioned, but that different oscillations
have taken place; so that, from about the beginning of the Christian
era, till some time in the middle ages, the sea rose to be sixteen
feet higher than at present, from which height it has descended till
it became lower than it is now, and from that state of depression it
is now rising again. Breislac infers this from two facts, which he
combines very ingeniously with the preceding, viz. the remains of some
ancient buildings, at the foot of Monte Nuovo, five or six feet above
the present level of the sea, in which are found the shells of some
of those little marine animals that eat into stone: And again, the
marble columns of the temple of Serapis, which are also perforated
by pholades, to the height of sixteen feet above the ground. All
these changes Breislac ascribes to the motion of the sea itself; a
supposition which, as we have seen, cannot possibly be admitted, since
nothing can permanently affect the level of the sea in one place, which
does not affect it in all places whatsoever.
400. Appearances, which indicate such alternations as have just been
mentioned in the level of the sea, are to be met with on some other
coasts. In England, on the coast of Lincolnshire, the remains of a
forest have been observed, which are now entirely covered by the
sea.[207] The submarine stratum which contains the remains of this
forest, can be traced into the country to a great distance, and is
found throughout all the fens of Lincolnshire. The stratum itself is
about four feet thick; it is covered in some places by a bed of clay
sixteen feet thick, and under it for twenty feet more is a bed of soft
mud, like the scourings of a ditch, mixed with shells and silt.
[Footnote 207: Phil. Trans. 1799, p. 145.]
Here then we have a stratum which must have been once uppermost on the
surface of the dry land, though one part of it is now immersed under
the sea, and another covered with earth, to the depth of sixteen feet.
A change of level in the sea itself will not explain these appearances:
they can only be explained by supposing the whole tract of land to
have subsided, which is the hypothesis adopted by the author of the
description in the Transactions, M. CORRIA DE SERRA; the subsidence,
however, is not here understood to arise from the mere yielding of
some of the strata immediately underneath, but is conceived to be a
part of that geological system of alternate depression and elevation
of the surface, which probably extends to the whole mineral kingdom.
To reconcile all the different facts, I should be tempted to think,
that the forest which once covered Lincolnshire, was immersed under the
sea by the subsidence of the land to a great depth, and at a period
considerably remote; that when so immersed, it was covered over with
the bed of clay which now lies on it, by deposition from the sea, and
the washing down of earth from the land; that it has emerged from this
great depth till a part of it has become dry land; but that it is now
sinking again, if the tradition of the country deserves any credit,
that the part of it in the sea is deeper under water at present than
it was a few years ago. This might also serve to reconcile, in some
measure, the phenomena of this submarine forest with the appearances
which indicate an extension of the land on the coast of Lincolnshire.
Indeed the extension of the land is no direct proof, either of its own
elevation, or of the depression of the sea, as we may conclude from the
instance of Ravenna already mentioned.
401. We have concluded from the facts stated above, that the level of
the sea rises in the Mediterranean, and sinks in the more northern
latitudes; and thence some have suspected, that the level of the sea
had in general a tendency to rise towards the equator, and to sink
towards the poles. This is the notion of Frisi, as has been already
remarked, and he suggests, that this rise of the sea may be owing to a
slight acceleration in the earth's diurnal motion. But there are facts
which show, that between the tropics the relative level of the sea
and land has sunk, and is lower at present than it was at some former
period, probably not extremely remote. The opinion of Frisi, therefore,
is unsupported by observation, and, as has been already shown, cannot
be justified from theory.
Between the tropics, islands are formed from the mere accumulation
of coral; and it is the peculiarity of those regions, to produce
rocks that have not passed through the usual process of mineral
consolidation.[208] The islets, however, which are thus formed, must
have their bases laid on a solid rock, though perhaps at a great
depth; and it is not probable, that after they are once raised above
the surface of the sea, they can still rise farther, except by some
elevation of the rock which serves as their foundation.[209] Now,
at Palmerston island, which comprehends nine or ten low islets, that
may be reckoned the heads of a great reef of coral rock, Captain
Cook informs us of his having seen, "far beyond the reach of the
sea, even in the most violent storms, elevated coral rocks, which,
on examination, appeared to have been perforated in the same manner
that the rocks are that now compose the outer edge of the reef. This
evidently shows," he adds, "that the sea had formerly reached so far;
and some of these perforated rocks were almost in the centre of the
island."[210]
[Footnote 208: Dr Foster, in his Voyage round the World, (vol. ii. p.
146,) gives an instance in the South Sea Islands, where the surface of
the island, though entirely a coral rock, was raised forty feet above
the level of the sea.]
[Footnote 209: A very curious account of the formation of such islands
is given by A. Dalrymple, Esq., in the Philosophical Transitions, vol.
lvii. p. 394.]
[Footnote 210: Cook's Third Voyage, vol i. p. 221.]
The same excellent navigator, giving an account of the peninsula at
Cape Denbigh, remarks: "It appeared to me, that this peninsula must
have been an island in remote times; for there were marks of the sea
having flowed over the isthmus."
402. We are here touching on one of those subjects, where we feel much
the want of accurate and ancient observations, and where it is not from
the infancy, but the maturity of science that any thing approaching
to certainty can be looked for. The utmost that we can expect at
present, is an anticipation, which future ages must certainly modify
and correct. The best thing, in the mean time, that can be done for the
advancement of this branch of geological knowledge, is to ascertain
with exactness the relative level of the sea, and of such points upon
the land as can be distinctly marked, and pointed out to succeeding
ages. This is not so easy as it may at first appear. Where every object
changes, it is difficult to find a measure of change, or a fixed point
from which the computation may begin. The astronomers already feel
this inconvenience, and when they would refer their observations to
an immoveable plane, that shall preserve its position the same in all
ages, they meet with difficulties, which cannot be removed but by a
profound mathematical investigation.
In geology, we cannot hope to be delivered from this embarrassment in
the same manner; and we have no resource but to multiply observations
of the difference of level; to make them as exact as possible, and
to select points of comparison that have a chance of being long
distinguished. The improvements in barometrical measurements, which
give such facility to the determination of heights, along with so
considerable a degree of accuracy, will furnish an accumulation of
facts that must one day be of great value to the geologist.
NOTE XXII. § 123.
_Fossil Bones._
403. The remains of organized bodies, at present included in the solid
parts of the globe, may be divided into three classes. The first
consists of the shells, corals,-and even bodies of fish, and amphibious
animals, which are now converted into stone, and make integrant parts
of the solid rock. All these are parts of animals that existed _before
the formation of the present land_, or even of the rocks whereof it
consists. These remains have been already treated of, and the evidence
which they furnish must ever be regarded as of the utmost importance
in the theory of the earth. The second class consists of remains,
which, by the help of stalactitical concretions, are converted into
stone. These are the exuviæ of animals, which existed on the very same
continents on which we now dwell, and are no doubt the most ancient
among their inhabitants, of which any monument is preserved. In
comparison of the first class, they must, nevertheless, be considered
as of very modern origin.
404. The third class consists of the bones of animals found in the
loose earth or soil; these have not acquired a stony character, and
their nature appears to be but little changed, except by the progress
of decomposition and of mouldering into earth. No decided line can be
drawn between the antiquity of this and the preceding class, as there
may be between the preceding and the first. In some instances, the
objects of this third class may be coeval with those of the second; in
general, they must be accounted of later origin, as they are certainly
not preserved in a manner so well fitted for long continuance.
405. The animal remains of the second class, are generally found in
the neighbourhood of limestone strata, and are either enveloped or
penetrated by calcareous, or sometimes ferruginous matter. Of this
sort are the bones found in the rock of Gibraltar, and on the coast
of Dalmatia. The latter are peculiarly marked for their number, and
the extent of the country over which they are scattered, leaving it
doubtful whether they are the work of successive ages, or of some
sudden catastrophe that has assembled in one place, and overwhelmed
with immediate destruction, a vast multitude of the inhabitants of
the globe. These remains are found in greatest abundance in the
islands of Cherso and Osero ; end always in what the Abbé FORTIS calls
an _ocreo-stalactitic earth_. The bones are often in the state of
mere splinters, the broken and confused relics of various animals,
concreted with fragments of marble and lime, in clefts and chasms of
the strata.[211] Sometimes human bones are said to be found in these
confused masses.
[Footnote 211: Travels into Dalmatia, p. 449.]
406. A very remarkable collection of bones in this state is found
in the caves of Bayreuth in Franconia. Many of these belong, as is
inferred with great certainty from the structure of their teeth, to
a carnivorous animal of vast size, and having very little affinity
to any of those that are now known. The bones are found in different
states, some being without any stalactitical concretion, and having
the calcareous earth still united to the phosphoric acid, so that
they belong to the third, rather than the second, of the preceding
divisions. In others, the phosphoric acid has wholly disappeared, and
given place to the carbonic.
The number of these bones, accumulated in the same place, is matter of
astonishment, when it is considered, that the animals to which they
belonged were carnivorous, so that more than two can never have lived
in the same cavern at the same time. The caves of Bayreuth seem to have
been the den and the tomb of a whole dynasty of unknown monsters, that
issued from this central spot to devour the feebler inhabitants of the
woods, during a long succession of ages, before man had subdued the
earth, and freed it from all domination but his own.
407. The fossil bones of the second and third class, but chiefly of the
third, have now afforded matter of conjecture and discussion for more
than a century. The facts with respect to them are very numerous and
interesting, but can be considered here only very generally.
The remains of this kind, consist of the bones only of large animals,
so that they have generally been compared with those of the elephant,
the rhinoceros, the hippopotamus, or other animals of great size The
bones of smaller animals have also been found, but much more rarely
than the other. It is usually remarked, that the bones thus discovered
in the earth are larger than those of the similar living animals.
Another general fact concerning these remains, is, that they are found
in all countries whatsoever, but always in the loose or travelled
earth, and never in the genuine strata. Since the year 1696, when the
attention of the curious was called to this subject, by the skeleton
of an elephant dug up in Thuringia, and described by Tentzelius,[212]
there is hardly a country in Europe which has not afforded instances
of the same kind. Fossil bones, particularly grinders and tusks of
elephants, have been found in other places of Germany, in Poland,
France, Italy, Britain, Ireland, and even Iceland.[213] Two countries,
however, afford them in greater abundance by far than any other part of
the known world; namely, the plains of Siberia in the old continent,
and the flat grounds on the banks of the Ohio in the new.[214]
[Footnote 212: Phil. Trans. vol. xix. p. 757.]
[Footnote 213: A grinder of an elephant found in Iceland, is described
by _Bartholinus_, Acta Hafniens. vol. i. p. 83.]
[Footnote 214: The fossil bones on the Ohio are described in two papers
by Mr P. Collinson, Phil. Trans. vol. lvii. p. 464 and 468.]
408. When the bones in Siberia were first discovered, they were
supposed to belong to an animal that lived under ground, to which
they gave the name of the _mammouth_; and the credit bestowed on this
absurd fiction, is a proof of the strong desire which all men feel
of reconciling extraordinary appearances with the regular course of
nature. Much skill, however, in natural history was not required to
discover that many of the bones in question resembled those of the
elephant, particularly the grinders and the tusks of that animal.
Others resembled the bones of the rhinoceros; and a head of that kind,
having the hide preserved upon it, was found in Siberia, and is still
in the imperial cabinet at Petersburgh.
Pallas has described the fossil bones which he found in the museum at
Petersburgh, on his being appointed to the superintendence of it, and
enumerates, not only bones that belong, in his opinion, to the elephant
and rhinoceros, but others that belong to a kind of buffalo, very
different from any now known, and of a size vastly greater.[215] He has
also described, in another very curious memoir, the bones of the same
kind that he met with in his travels through the north-east parts of
Asia.
[Footnote 215: Novi Comment Petrop. tom. xiii. (1768,) p. 436, and tom.
xvii. p. 576, &c.]
The fossil bones found on the banks of the Ohio, resemble in many
things those of Siberia; like them they are contained in the soil or
alluvial earth, and never in the solid strata; like them too they are
no otherwise changed from their natural state, than by being sometimes
slightly calcined at the surface; they are also of great size, and in
great numbers, being probably the remains of several different species.
409. Two inquiries concerning these bones have excited the curiosity
of naturalists; first, to discover among the living tribes at present
inhabiting the earth, those to which the fossil remains may with the
greatest probability be referred; and, secondly, to find out the cause
why these remains exist in such quantities, in countries where the
animals to which they belong, whatever they be, are at present unknown.
The solution of the first of these questions, is much more within our
reach than the second, and at any rate must be first sought for.
On the authority of so eminent a naturalist as Pallas, the bones from
Siberia may safely be referred to the elephant, the rhinoceros, and
buffalo, as mentioned above, though perhaps to varieties of them with
which we are not now acquainted. With respect to the bones of North
America, the question is more doubtful, for they have this particular
circumstance attending them, viz. that along with the thighbones,
tusks, &c. which might be supposed to belong to the elephant, grinders
are always found of a structure and form entirely different from
the grinders of that animal.[216] Some naturalists, particularly M.
DAUBENTON, referred these grinders to the hippopotamus; but Dr W.
HUNTER appears to have proved, in a very satisfactory manner, that they
cannot have belonged to either of the animals just mentioned, but to a
_carnivorous_ animal of enormous size, the race of which, fortunately
for the present inhabitants of the earth, seems now to be entirely
extinct.[217] The foundation of Dr Hunter's opinion is, that in these
grinders the enamel is merely an external covering; whereas, in the
elephant, and other animals destined to live on vegetable food, the
enamel is intermixed with the substance of the tooth.[218]
[Footnote 216: See Mr Collinson's papers, above referred to. Phil.
Trans. vol. lvii.]
[Footnote 217: Phil. Trans. vol. lviii. p. 3, &c.]
[Footnote 218: A fossil grinder in the collection of JOHN MACGOWAN,
Esq. of Edinburgh, answers nearly to Mr Collinson's description, and is
very well represented by the figure which accompanies it. This grinder
weighs four pounds one-fourth avoirdupois; the circumference of the
_corona_ is eighteen inches; the coat of enamel is one-fourth of an
inch thick; there are five double teeth; in Mr Collinson's specimen
there are only four.]
410. Though this argument appears to be of considerable weight, yet
CAMPER, who was greatly skilled in comparative anatomy, and who had
studied this subject with particular attention, was of opinion, that
these grinders belong to a species of elephant. This opinion he states
in a letter to Pallas, who had found grinders and other bones of this
same animal, on the western declivity of the Ural mountains.[219]
Camper denies that the animal is carnivorous, because the _incisores_,
or canine teeth, are wanting; and he argues farther, from the weight of
the head, which may be inferred from the weight of the grinders, that
the neck must have been short, and the animal must have been furnished
with a _proboscis_. He afterwards abandoned the latter hypothesis, and
gave it as his opinion that the _incognitum_ was neither carnivorous,
nor a species of the elephant.[220]
[Footnote 219: Acta Acad. Petrop. tom. i. (1777,) pars posterior, p.
213, &c.]
[Footnote 220: _Ibid._ tom. ii. (1784,) p. 262.]
411. Nevertheless, CUVIER, in a _mémoire_ read before the National
Institute of Paris, maintains, that the fossil bones of the new
Continent, as well as most of those of the old, belong to certain
species of the elephant; of which, at least, two do not now exist, and
are only known from remains preserved in the ground. He distinguishes
them thus:[221]
[Footnote 221: Mémoires de l'Institut National, Sciences Physiques,
tom. ii. p. 19, &c.]
_Elephas mammonteus_,--_maxillâ obtusiore, lamellis molarium tenuibus,
rectis_.
_Elephas Americanus_,--_molarities multicuspidibus, lamellis post
detritionem quadric-lobatis_.
The latter species, which is meant to include the _animal incognitum_,
is said to have lived, not only in America, but in many parts of the
old Continent. Yet some late inquiries into the structure of the teeth
of graminivorous animals, and particularly of the elephant, make it
very improbable that the _incognitum_ has belonged to this genus.[222]
The grinders of the elephant have been found to consist of three
substances, enamel, bone, and what is called the _crusta petrosa_,
applied in layers, or folds contiguous to one another; and no vestige
of this structure appears in the grinders of the unknown animal of the
Ohio.[223] At the same time, Dr Hunter's assertion, that this animal
was carnivorous, is rendered doubtful, not only by the want of _canine_
teeth, but also from the resemblance between its grinders and those of
the wild boar, which Mr Home has observed to be considerable.[224] The
grinder of the boar is similar to that of the elephant, in the extent
of the masticating surface, but not at all in the internal structure;
and the same is true of the tooth of the _animal incognitum_, so that
a considerable probability is established, that it and the boar are of
the same genus, and both destined to live occasionally either on animal
or vegetable food.
[Footnote 222: See Mr Home's Observations on the Teeth of Graminivorous
Animals, Phil. Trans. 1799. Also, an Essay on the Structure of the
Teeth, by Dr Blake.]
[Footnote 223: In a paper inserted in the fourth volume of the American
Philosophical Transactions, an account is given of two different
grinders that are found at the Salt Licks near the Ohio. One of them
resembles the grinder of the elephant, and may have belonged to the
_Elephas Americanus_ of Cuvier; the other agrees pretty nearly with the
grinder of Dr Hunter's _animal incognitum_. The author of the paper
thinks that the _animal incognitum_ was not wholly carnivorous, as the
incisores, or canine teeth, are never found. At the Great Bone Lick,
bones of smaller animals, particularly of the buffalo kind, have been
discovered. The saline impregnation of the earth at these Licks must
no doubt have contributed to the preservation of the bones. Trans.
American Phil. Soc. vol. iv. (1799,) p. 510, &c.]
[Footnote 224: Observations on the Grinding Teeth of the wild boar and
_animal incognitum_. Phil. Trans. 1801, p. 319.]
412. Another _animal incognitum_ found in South America has been
described by Cuvier, and appears to be of a different genus from the
_incognitum_ of the North. Thus, if we include the two _incognita_ of
America, the _elephas mammonteus_, the unknown buffalo of Pallas, and
the great animal of Bayreuth, we have at least five distinct genera,
or species of the animal kingdom, which existed on our continents
formerly, but do not exist on them now. The number is probably much
greater: Pallas mentions fossil horns of a gazelle, of an unknown
species; and horns of deer are often found, that cannot be referred to
any species now existing. Those extinct races have been remarkable for
their size: some of the ancient elephants appear to have been three
times as large as any of the present.[225]
[Footnote 225: Camper, Nov. Acta Petrop. tom. ii. (1784) p. 257.]
413. The inhabitants of the globe, then, like all the other parts of
it, are subject to change. It is not only the individual that perishes,
but whole _species_, and even perhaps _genera_, are extinguished. It
is not unnatural to consider some part of this change as the operation
of man. The extension of his power would necessary subvert the balance
that had before been established between the inhabitants of the earth,
and the means of their subsistence. Some of the larger and fiercer
animals might indeed dispute with him, for a long time, the empire of
the globe; and it may have required the arm of a Hercules to subdue
the monsters which lurked in the caves of Bayreuth, or roamed on the
banks of the Ohio. But these, with others of the same character, were
at length exterminated: the more innocent species fled to a distance
from man; and being forced to retire into the most inaccessible parts,
where their food was scanty, and their migration checked, they may have
degenerated from, the size and strength of their ancestors, and some
species may have been entirely extinguished.
But besides this, a change in the animal kingdom seems to be a part of
the order of nature, and is visible in instances to which human power
cannot have extended. If we look to the most ancient inhabitants of the
globe, of which the remains are preserved in the strata themselves,
we find in the shells and corals of a former world hardly any that
resemble exactly those which exist in the present. The species, except
in a few instances, ate the same, but subject to great varieties. The
vegetable impressions on slate, and other argillaceous stones, can
seldom be exactly recognised; and even the insects included in amber
are different from those of the countries in which the amber is found.
414. Supposing, then, the changes which have taken place in the
qualities and habits of the animal creation, to be as great as those
in their structure and external form, we can have no reason to wonder
if it should appear that some have formerly dwelt in countries from
which the similar races are now entirely banished. The power of living
in a different climate, of enduring greater degrees of cold or of
heat, or of subsisting on different kinds of food, may very well have
accompanied the other changes. Though one species of elephant may
now be confined to the southern parts of Asia, another may have been
able to endure the severer climates of the north; and the same may
be true of the buffalo or the rhinoceros. In all this no physical
impossibility is involved; though whether it is a probable solution of
the difficulty concerning the origin of these animal remains, can only
be judged of from other circumstances.
415. If we consider attentively the facts that respect the Siberian
fossil bones, there will appear insurmountable objections to every
theory that supposes them to be exotic, and to have been brought into
their present situation from a distant country.
The extent of the tract through which these bones are scattered, is a
circumstance truly wonderful. Pallas assures us,[226] that there is not
a river of considerable size in all the north of Asia, from the Tanais,
which runs into the Black Sea, to the Anadyr, which falls into the Gulf
of Kamtchatka, in the sides or bottom of which bones of elephants and
other large animals have not been found. This is especially the case
where the rivers run in plains through gravel, sand, clay, &c.; among
the mountains, the bones are rarely discovered. The extent of the tract
just mentioned exceeds four thousand miles; and how the bones could
be distributed over all that extent, by any means but by the animals
having lived there, it seems impossible to conceive. No torrent nor
inundation could have produced this effect, nor could the bones brought
in that way have been laid together so as to form complete skeletons.
[Footnote 226: De Reliquiis Animalium exoticorum, per Asiam Borealem
repertis.--"Nov. Comment Petrop. tom. xvii. (1772,) p. 576.]
416. One fact recorded by the same author, seems calculated to remove
all uncertainty. It is that of the carcase of a rhinoceros, almost
entire, and covered with the hide, found in the earth in the banks of
the river Wilui, which falls into the Lena below Jakutsk.[227] Some of
the muscles and tendons were actually adhering to the head when Pallas
received it. The head, after being dried in an oven, is still preserved
in the museum at Petersburgh. The preservation of the skin and muscles
of this natural mummy, as Pallas calls it, was no doubt brought
about by its being buried in earth that was in a state of perpetual
congelation; for the place is in the parallel of 64°, where the ground
is never thawed but to a very small depth below the surface.
[Footnote 227: Pallas _ubi supra_, p. 586. Also, Voyages de Pallas,
tom. iv. p. 131.]
But by what means can we account for the carcase of a rhinoceros being
buried in the earth, on the confines of the polar circle? Shall we
ascribe it to some immense torrent, which, sweeping across the deserts
of Tartary, and the mountains of Altai, transported the productions of
India to the plains of Siberia, and interred in the mud of the Lena
the animals that had fed on the banks of the Barampooter or the Ganges?
Were all other objections to so extraordinary a supposition removed,
the preservation of the hide and muscles of a dead animal, and the
adhesion of the parts, while it was dragged for 2000 miles over some
of the highest and most rugged mountains in the world, is too absurd
to be for a moment admitted. Or shall we suppose that this carcase has
been floated in by an inundation of the sea, from some tropical country
now swallowed up, and of which the numerous islands of the Indian
Archipelago are the remains? The heat of a tropical climate, and the
putrescence naturally arising from it, would soon, independently of all
other accidents, have stripped the bones of their covering. Indeed this
_instantia singularis_, as in every sense it may properly be called,
seems calculated for the express purpose of excluding every hypothesis
but one from being employed to explain the origin of fossil bones.
It not only excludes the two which have just been mentioned, but it
excludes also that of Buffon, viz. that these bones are the remains of
animals which lived in Siberia, when the arctic regions enjoyed a fine
climate, and a temperature like that which southern Asia now possesses.
From the preservation of the flesh and hide of this rhinoceros, it is
plain, that when the body was buried in the earth, the climate was
much the same that it is now, and the cold sufficient to resist the
progress of putrefaction.
Pallas takes notice of the inconsistency of the state of this skeleton,
with the hypothesis of Buffon; but he does not observe that the
inconsistency is equally great between it and his own hypothesis, the
importation of the fossil bones by an inundation of the sea, and that
flesh or muscle must have been entirely consumed long before it could
be carried by the waves to the parallel of 64°, from any climate which
the rhinoceros at present inhabits.
417. The presence of petrified marine objects in places where some of
the fossil bones are found, is no proof that the latter have come from
the sea, though it is produced as such both by Pallas himself, and
afterwards by Kirwan. These marine bodies are the shells and corals
that have been parts of calcareous rocks, from which being detached
by the ordinary progress of disintegration, they are now contained in
the beds of sand or gravel where the animal remains are buried. They
have nothing in common with these remains; they are real stones, and
belong to another, and a far more remote epochs. Such objects being
found in the same place where the bones lie, argues only that the
strata in the higher grounds, from which the gravel has come, are
calcareous; and nothing can show in a stronger light the necessity of
distinguishing the different condition of fossil bodies, united by the
mere circumstance of contiguity, before we draw any inference as to
their having a common origin. If the marine remains were in the same
condition with the bones; if they were in no respect mineralized; then
the conclusion, that both had been imported by the sea, would have
great probability; but without that, their present union must be held
as casual, and can give no insight into the origin of either.
418. On the whole, therefore, no conclusion remains, but that these
bones have belonged to species of elephants, rhinoceros, &c. which
inhabited the very countries where their remains are now buried, and
which could endure the severity of the Siberian climate. The rhinoceros
of the Wilui certainly lived on the confines of the Polar Circle, and
was exposed to the same cold while alive, by which, when dead, its body
has been so long, and so curiously preserved.
These animals may also have lived occasionally farther to the south,
among the valleys between the great ranges of mountains that bound
Siberia on that side. Fossil bones are but rarely found in these
valleys, probably because they have been washed down from thence into
the plains. We must observe, too, that those animals may have migrated
with the seasons, and by that means avoided the rigorous winter of the
high latitudes. The dominion of man, by rendering such migration to
the larger animals difficult or impossible, must have greatly changed
the economy of all those tribes, and narrowed the circle of their
enjoyments and existence. The heaps in which the fossil bones appear to
be accumulated in particular places, especially in North America, have
a great appearance of being connected with the migrations of animals,
and the accidents that might bring multitudes of them into the same
spot.
What holds of Siberia and of North America, is applicable, _a
fortiori_, to all the other places where animal remains are found
in the same condition. Thus we are carried back to a time when many
larger species of animals, now entirely extinct, inhabited the earth,
and when varieties of those that are at present confined to particular
situations, were, either by the liberty of migration, or by their
natural constitution, accommodated to all the diversities of climate.
This period, though beyond the limits of ordinary chronology, is
posterior to the great revolutions on the earth's surface, and the
latest among geological epochs.
NOTE XXIII. § 128.
_Geology of_ KIRWAN _and_ DE LUC.
419. The two champions of the Neptunian system, who have distinguished
themselves most by their hostility to Dr HUTTON, are DE LUC and KIRWAN.
They have carried on their attack nearly on the same plan, and have
employed against their antagonist the weapons both of theology and
science. With a spirit as injurious to the dignity of religion, as to
the freedom of philosophical inquiry, they have disregarded a maxim
enforced by the authority of Bacon, and by all our experience of the
past; "_Tanto magis hæc vanitas inhibenda venit et coërcenda, quia,
ex divinorum et humanorum male-sana admixtione, non solum educitur
philosophia phantastica, sed etiam religio hæretica. Itaque salutare
admodum est, si mente sobriâ, fidei tantum dentur quæ fidei sunt_."[228]
[Footnote 228: The whole passage is deserving of attention, and
it seems as if the prophetic spirit of Bacon had addressed it to
the cosmologists of the present day. "_Pessima enim res errorem_
APOTHEOSIS, _et pro peste intellectús habenda est, si vanis accedat
veneratio. Huic autan vanitati nonnulli ex modernis summâ levitate ita
indulserunt, ut, in primo capitolo_ GENESEOS, _et aliis Scripturis
Sacris, philosophiam naturalem fundari conati sunt:_ Inter VIVA
quærentes MORTUA.--"Nov. Organum, lib. i. aphor. 65]
Proceeding, accordingly, in direct opposition to rules that, have never
yet been violated with impunity, and mistaking the true object of a
theory of the earth, they carry back their inquiries to a period prior
to the present series of causes and effects, where, having neither
experience nor analogy to direct them, they pretend to be guided by
a superior light. They would have us to consider their geological
speculations as a commentary on the text of MOSES; they endeavour to
explain the action of creative power, and, with indiscreet curiosity,
would tear off the veil which the hand of the prophet has so wisely
respected. But the veil cannot be torn off, and all that is behind it
must be to man as that which never has existed.
420. M. De Luc has nevertheless treated very diffusely of the history
of the solar system, previous to the establishment of the present laws
of nature, and has dwelt on it with great complacency, and singular
minuteness of detail. His tenth letter to LA METHERIE has the following
title:
"On the History of the Earth, from the time when that planet was
penetrated by _light_, till the appearance of the sun; a portion of
time which includes the origin of heat, and of the figure of the
earth; of its primeval strata, of the ancient sea, of our continents,
as the bottom of that sea, of the great chains of mountains, and of
vegetation."[229]
[Footnote 229: Journal de Physique, tom. 37. (1790,) partie 2de, p.
332. As I may not have done justice to this extraordinary title, it may
be right to present it in the original. "Sur l'Histoire de la TERRE,
depuis que cette planette fut penetrée de LUMIERE, jusqu'à l'apparition
du SOLEIL; espace de tems qui renferme les ORIGINES de la _chaleur_,
et de la _figure_ de notre globe; de ses _couches primordiales_, de
_l'ancienne mer_, de nos _continens_, comme fond de cette mer, de leurs
grandes chaînes de _montagnes_, et de la _vegetation_."]
I must confess that I am unacquainted with every thing of this letter
but the title; and could not easily be prevailed on to follow any man
who professedly goes out of nature in search of knowledge; who pretends
to give the history of our planetary system when there was no sun,
and to enumerate the events which took place between the existence of
that luminary, and the existence of light. The absurdity of such an
undertaking admits of no apology; and the smile which it might excite,
if addressed merely to the fancy, gives place to indignation when it
assumes the air of philosophic investigation.
421. It sets, however, in a strong light, the inconsistencies that
may be observed in the intellectual character of the same individual,
to consider that the author of this strange and inconsistent
reverie, is, nevertheless, an excellent observer, and well skilled in
experimental inquiries. It will hardly be believed that he who writes
the history of the earth before the formation of the sun, is versed
in the principles of inductive reasoning; and that he has added much
to the stock of geological knowledge, having observed accurately, and
described with great perspicuity and candour. His _Lettres Physiques_
are full of valuable and just observations, though accompanied with
reasonings that do not seem always entitled to the same praise; and
in another work he has succeeded where many men of genius had failed,
and has made considerable improvements in a branch of the mathematics,
without borrowing almost any assistance from the principles of that
science.[230]
[Footnote 230: Essai sur les Modifications de l'Atmosphere.]
422. Some of the same observations apply to Mr Kirwan. His Geological
Essays have also for their object to explain the first origin of
things; and to say that he has not succeeded, in an attempt where no
man ever can succeed, implies no reproach on the execution of his
work, whatever it may do on the design. We have indeed no criterion by
which the execution of it can be estimated: what would in any other
place be a blemish, may be here deserving of praise; and if the work
is full of confusion and perplexity, these are qualities inherent in
the subject which it is intended to describe. It were, no doubt, to
be wished, that after emerging into the regions of day, Mr Kirwan had
been as successful in copying the beauty and simplicity of nature, as
in representing the disorder and inconsistency of the chaotic mass.
But his cosmology is without unity in its principles, or consistency
in its parts; the causes introduced, are, for the most part, such as
will account for one set of appearances just as well as for another;
or, if any of them is likely to prove inadequate to the effect ascribed
to it, a new and arbitrary hypothesis is always ready to come to its
assistance. The information given is seldom exact: a multitude of facts
brought together, without the order and discussion essential to precise
knowledge; and an infinity of quotations, amassed without criticism or
comparison, afford proofs of extensive reading, but of the most hasty
and superficial inquiry. Thus we have seen passages from ULLOA and
FRISI, produced in support of opinions, which, when fairly stated, they
had the most direct tendency to overthrow.
423. In one respect, the geological writings of Kirwan are far inferior
to De Luc's: They are evidently the productions of a man who has not
seen nature with his own eyes; who has studied mineralogy in cabinets,
or in books only; but who has seldom beheld fossils in their native
place. With the balance in his hand, and the external characters of
WERNER in his view, he has examined minerals with diligence, and has
discovered many of those marks which serve to ascertain their places,
in a system of artificial arrangement. But to _reason_ and to _arrange_
are very different occupations of the mind; and a man may deserve
praise as a mineralogist, who is but ill qualified for the researches
of geology.
424. The same hurry and impatience are visible in the manner in which
his argument against Dr Hutton is usually conducted. He has seldom been
careful to make himself master of the opinions of his adversary; and
what he gives as such, and directs his reasonings against, have often
no resemblance to them whatsoever. Without any intention to deceive
others, but deceived himself, he usually begins with misrepresenting Dr
Hutton's notions, and then proceeds to the refutation of them. In this
imaginary contest, it will readily be supposed, that he is in general
successful: when a man has the framing both of his own argument, and
that of his antagonist, he must be a very unskilful logician if he does
not come off with the advantage.
425. It is but justice, however, to the Neptunists, to acknowledge,
that they are not all liable to the censure of beginning their
researches from a period antecedent to the existence of the laws of
nature. This absurdity does not, so far as I know, infect the system of
Werner. That mineralogist has not proposed to explain the first origin
of things, though he has supposed, at some former period, a condition
of the globe very unlike the present, viz. the entire submersion of the
solid under the fluid part.
NOTE XXIV. § 129.
_System of_ BUFFON.
426. The affinity of Dr Hutton's theory to that of Buffon, is nothing
more than what arises from their making use of the same agents, viz.
fire and water, in producing the present condition of the earth's
surface. In almost all other respects the two theories are extremely
different. The order in which those agents are employed in them, is
directly opposite, as has already been remarked; Buffon introducing the
action of fire first, and of water only in the second place, to waste
and destroy mineral bodies, and afterwards to dispose them anew, and
arrange them into strata. He makes no provision for the consolidation
of these strata, nor any for their angular elevation; he has no means
of explaining the unstratified rocks; nor any, but one extremely
imperfect, for explaining the inequalities of the earth's surface.
Again, Buffon mistook, in some degree, the true object of a theory of
the earth; and though he did not go back, like the geologists just
named, to a time when the laws of nature were not fully established,
he begins from a condition of things too unlike the present to be the
basis of any rational speculation. He does not, indeed, undertake to
examine the state of our planetary system before the sun existed; for
from such extravagance, even when most disposed to indulge his fancy,
he would surely have revolted. But he treats of the world, when the
earth and the planets had just ceased to be a part of the sun, and were
newly detached from the body of that luminary.[231]
[Footnote 231: According to Buffon, the granite is the true solar
matter, unchanged but by its congelation.]
This hypothesis concerning the origin of the planets, contrived chiefly
to account for the circumstance of their motion being all in the
same direction, and in other respects not only unsupported, but even
inconsistent with the principle of gravitation, has nothing in common
with a theory, confined as Dr Hutton's is, within the field which must
for ever bound our inquiries, and not venturing to speculate about the
earth, when in a condition totally different from the present.
427. In what relates to the future, the two systems are not more like
than in what relates to the past Buffon represents the cooling of our
planet, and its loss of heat, as a process continually advancing, and
which has no limit, but the final extinction of life and motion over
all the surface, and through all the interior, of the earth. The death
of nature herself is the distant but gloomy object that terminates
our view, and reminds us of the wild fictions of the Scandinavian
mythology, according to which, _annihilation_ is at last to extend
its empire even to the gods. This dismal and unphilosophic vision
was unworthy of the genius of Buffon, and wonderfully ill suited to
the elegance and extent of his understanding. It forms a complete
contrast to the theory of Dr Hutton, where nothing is to be seen beyond
the continuation of the present order; where no latent seed of evil
threatens final destruction to the whole; and where the movements are
so perfect, that they can never terminate of themselves. This is surely
a view of the world more suited to the dignity of NATURE, and the
wisdom of its AUTHOR, than has yet been offered by any other system of
cosmology.
428. I have often quoted Buffon in, the course of these
_Illustrations_, and most commonly for the purpose of combating his
opinions; but I am very sensible, nevertheless, of the obligations
under which he has laid all the sciences connected with the natural
history of the earth.
The extent and variety of his knowledge, the justness of his
reasonings, the greatness of his views, his correct taste, and manly
eloquence, qualified him, better, perhaps, than any other individual,
to compose the History of Nature. The errors into which he Has fallen,
are almost all the unavoidable consequences of the circumstances in
which he was placed; and if their amount is estimated by the proportion
that they bear to the general excellence of the work, they will be
reckoned but of small account. Buffon began to write when many parts
of natural history had made but little progress; when the quantity
of authentic information was small, and when scientific and correct
description was hardly to be found. Many of the greatest and most
important facts in geology were quite unknown, and scarcely any part
of the mineral kingdom had been accurately surveyed; and, with such
materials as this state of things afforded, it is not wonderful if
some parts of the edifice he erected have not proved so solid and
durable as the rest. Had he appeared somewhat later; had he been
farther removed from the time when reasonings _a priori_ usurped the
place of induction; and had he been as willing to correct the errors
into which he had been betrayed by imperfect information, as he was
ingenious in defending them, his work would probably have reached as
great perfection, as it is given for any thing without the sphere of
the accurate sciences to attain. If he had examined the natural history
of the earth more with his own eyes, and been as careful to delineate
it with fidelity as force; if he had listened with greater care to
the philosophers around him; had he attended to the demonstrations of
NEWTON more, and despised the arrangements of LINNÆUS less; he would
have produced a work, as singular for its truth as for its beauty, and
would have gone near to merit the eulogy pronounced by the enthusiasm
of his countrymen, MAJESTATI NATURÆ PAR INGENIUM.
NOTE XXV. § 130.
_Figure of the Earth._
429. That the earth is a spheroidal body, compressed at the poles,
or elevated at the equator, is a fact established by many accurate
experiments; and though these experiments do not exactly coincide, as
to the degree of oblateness which they give to that spheroid, they
agree sufficiently to put it beyond all dispute, that the earth, though
solid, has nearly the same figure which it would assume if fluid, in
consequence of its rotation on its axis.
Now, it is not at all obvious, to what physical cause this phenomenon
is to be ascribed. The earth, as it exists at present, has none of the
conditions that render the assumption of the figure of equilibrium in
any way necessary to it. Constituted as it is, its parts cohere with
forces incomparably too great to obey the laws of statical pressure,
or to assume any one figure rather than another, on account of the
centrifugal tendency which results from its revolution on its axis.
There is no necessity that its superficies should be every where level,
or perpendicular to the direction of gravity, nor that every two
columns, standing on the same base, any where within it, and reaching
from thence to any two points of the surface, should be of such weights
as precisely to balance one another. Neither of these, indeed, is at
all conformable to fact. They are, however, the very suppositions on
which the determination of the spheroid of equilibrium is founded; and
as they certainly do in no degree belong to the earth, it seems strange
that the result deduced from them should be in any way applicable to
it. This coincidence remains, therefore, to be explained; and it must
greatly enhance the merit of any geological system, if it can connect
this great and enigmatical phenomenon with the other facts in the
natural history of the earth.
430. To establish such a connection, has, accordingly, been a favourite
object with geologists, whether they have embraced the Neptunian or
Volcanic theory: both have thought that they were entitled to suppose
the primeval fluidity of the globe, the one by water, and the other
by fire; and in whatsoever way that fluidity was produced, the result
of it could be no other than the spheroidal figure of the whole mass,
agreeably to the laws of hydrostatics. If in this fluid state the earth
was homogeneous, the spheroid would be accurately elliptical, and the
compression at the poles would be 1/230 of the radius of the equator;
if the fluid was denser toward the centre, the flattening would be
less: and in either case, the body, as it acquired solidity, may be
supposed to have retained its spheroidal figure with little variation.
But though the fluidity of the earth will account for the phenomenon
of its oblate figure, it may reasonably be questioned, whether this
fluidity can be admitted, in consistency with other appearances.
According to what is established above, none of the appearances in the
mineral kingdom indicate more than a partial fluidity in any former
condition of the earth. The present strata, made up as they are of the
ruins of former strata, though softened by heat, have not been rendered
fluid by it, and have even possessed their softness in parts, and in
succession, not altogether, nor at the same time.
The unstratified, and more crystallized substances, were cast in the
bosom of others, which were solid at the time when they were fluid. In
all this, therefore, there is no indication of a fluidity prevailing
through the whole mass, or even over the whole surface of the earth,
and therefore nothing that can explain the spheroidal figure which it
has acquired. The supposition, then, of the entire body of the earth,
or even of its external crust, having been fluid, though it might
account for the compression at the poles, does not connect that fact
with the other facts in the natural history of the globe, and fails,
therefore, in the point most essential to a theory. It is liable, also,
to other objections, whether it be conceived to have proceeded from
fire or from water; whether it has happened on the principles of Buffon
or of Werner.
431. First, let us suppose that the fluidity of the earth, or of the
external crust of it, at least to a certain depth, proceeded from a
solution of the whole in the waters of the ocean; and, waving all the
objections that have been stated to this hypothesis, on account of
the absolute insolubility of many mineral substances in water, let us
suppose them all soluble in a certain degree, and let us compute the
quantity of the menstruum, which, on the suppositions most favourable
to the system, must have been required to this great geologico-chemical
operation.
The siliceous earth, though not soluble in water _per se_, yet, after
being dissolved in that fluid by means of an alkali, was found by Dr
Black, in his analysis of the Geyser water, to remain suspended in a
quantity of water, between 500 and 1000 times its own weight. This is
one of the facts most favourable to the Neptunian theory; and that
every advantage may be given to that theory, we shall take the least of
the numbers just mentioned, and suppose that siliceous earth may be
dissolved or suspended in 500 times its weight of water.
Taking this for the extreme degree of insolubility of mineral
substances, (though there are many of which the insolubility is
absolute, or, to speak in the language of calculation, infinitely
great,) we may suppose the insolubility of all the rest, or the
quantities of water in which they are dissolved, to be ranged in a
descending scale from 500 to 0, the extreme degree of deliquescence.
Then, taking the arithmetical mean between these extremes, it will give
us 250, as the proportion of water in which mineral substances may at
an average be dissolved. But this average is much less than the truth;
for the quantity of siliceous earth is great in comparison of any of
the rest, and the mineral substances that are extremely soluble in
water are but in a small quantity; therefore, when we suppose mineral
bodies, at a medium, to be soluble in 250 times their own weight of
water, we make a supposition extremely favourable to the Neptunian
system.
432. This is the proportion between the _weight_ of the solvent, and
of the substances held in solution: to have the proportion of their
_bulks_, we may suppose the specific gravity of mineral bodies in
general to be to that of water as 5 to 2, and then we have the ratio
of bulks, that of 250 × 5 to 2 × 1, or of 625 to 1. It follows, then,
that minerals in general cannot be supposed soluble in less than 625
times their bulk of water.
433. Again, it must be allowed to the Neptunists, that the fluidity
of the whole earth is not necessary to account for its assuming the
spheroidal figure. It is sufficient if the whole of that crust or shell
of matter was fluid, which is contained between the actual surface
of the terrestrial spheroid, and the surface of the sphere inscribed
within it; that is, of the sphere which has for its diameter the polar
axis of the earth. The whole of the minerals which compose this shell,
must at least have been dissolved in water, and have formed the chaotic
mass of Mr Kirwan. The volume of the water required for this was not
less than 625 times the bulk of the spheroidal shell that has just been
mentioned.
But, assuming the difference between the polar axis and the equatorial
diameter to be 1/300 of the latter, which is the supposition most
agreeable to the phenomena, it is easy to show that the magnitude of
the above spheroidal shell, or the difference between the solid content
of the earth, and the sphere inscribed in it, is greater than 1/151 and
less than 1/150 of the whole earth; so that the earth is less than 151
times the spheroidal shell.
The volume of the water, therefore, necessary to hold in solution the
materials of this shell, is to the volume of the whole earth as 625
to 151, or in a greater ratio than that of four to one: and such,
therefore, at the very least, is the quantity of water which Mr Kirwan
supposes, after it ceased to act in its chemical capacity, to have
retired into caverns in the interior of the earth. Thus the Neptunists,
in their account of the spheroidal figure of the earth, are reduced
to a cruel dilemma, and are forced to choose between a physical and a
mathematical impossibility.
If we would inquire whether the opinion of the igneous origin of
minerals, as commonly received by the Vulcanists, is capable of
affording a better solution of this difficulty, the theory of M. de
Buffon is the first that presents itself.
434. That philosopher considers the existence of the spheroidal figure
as a proof that the whole of the earth must have been originally fluid;
and as the fluidity of the whole can only be ascribed to fusion, he has
supposed that the earth was originally a mass of melted matter struck
off from the sun by the collision of a comet; and that this mass, when
made to revolve on its axis, put on a spheroidal figure, which it has
retained, though now cooled down to congelation.
This system need not be considered in detail; the foundation of it is
laid in such defiance of the principles of geometry and mechanics, that
the architect, notwithstanding all the fertility of his invention, and
all the resources of his genius, was never able to give any solidity to
the structure.
But it will be said, that we may take a part of the system, without
venturing on the whole, and may suppose that the earth, or at least
the external crust of it, has been fluid by fire, though we do not
inquire into the cause of this fire, or into the manner in which it was
produced.
It is indeed true, that, when this is done, we have not the same sort
of absurdity to encounter that we met with in the Neptunian system, and
that the Volcanic theory does not, like it, come into direct collision
with an axiom of geometry. There are, nevertheless, great objections
to it; for though all the phenomena of the mineral kingdom attest a
fluidity of igneous origin, yet it is a fluidity that was never more
than partial; and though it has been over all the earth, has been over
it in succession only. Besides, we are not entitled to assume the
existence, and again the disappearance of such a great quantity of
heat, without assigning some cause for the change.
435. Since, then, neither the hypothesis of the Neptunists or the
Vulcanists, affords any good explanation of the figure of the earth, or
such a one as can connect it with the other appearances in its natural
history, it remains to inquire, whether the system that supposes a
partial and successive fluidity, like Dr Hutton's, has any resource for
explaining this great phenomenon.
Of this subject Dr Hutton has not treated; and when I was first made
acquainted with his system, it appeared to me a very serious objection
to it, that it did not profess to give an explanation of so important
a fact as the oblate figure of the earth. On considering the matter
more closely, however, I found that there were principles contained
in it from which a very satisfactory solution (and, I think, the only
satisfactory solution) of that difficulty might be deduced. This
solution I shall endeavour to explain, in as far, at least, as is
necessary for the purpose of general illustration.
It is laid down in Dr Hutton's theory, that the surface of the earth is
perpetually changed by the _detritus_ of the land; and that from the
materials thus afforded, new horizontal strata are perpetually formed
at the bottom of the sea. If this be true, and if the alternations of
decay and renovation have been often repeated, it is certain, that the
figure of the earth, whatever it may have originally been, must be
brought at length to coincide with the spheroid of equilibrium.
436. Here it is necessary to remark, that the expressions, _figure of
the earth_, and _surface of the earth_ are each of them occasionally
taken in two different senses.
The surface of the earth, in its most obvious sense, is that which
bounds the whole earth, and includes all its inequalities; it is a
surface extremely irregular, rising to the tops of the mountains,
descending to the bottoms of the valleys, and having the continuity
of its curvature often interrupted, or suddenly changed. This may be
called the _actual_ surface, and the figure bounded by it, the _actual_
figure, of the earth.
The surface of the earth, in another sense, is one that is every where
horizontal, and is the same which water assumes when at rest.
This superficies is determined by the circumstance of its being
constantly perpendicular to the direction of gravity; it is the surface
marked out by levelling, and may be supposed to be continued from the
sea, through the interior of the land, till it meet the sea again. The
figure bounded by this horizontal surface, may properly be called the
_statical_ figure of the earth.
When it is said that the figure of the earth is an oblate spheroid, it
is the statical, not the actual figure which is meant; and the degrees
of the meridian which astronomers measure, are also referred to the
superficies of the former.
437. Suppose now a body like the earth, but with its actual figure
infinitely more irregular, having a sea circumfused around it, the
water will descend into the lowest situations, and will so arrange
itself, that its surface shall be perpendicular every where to the
plumb-line, or to the direction of gravity, in which state only it can
remain at rest. The figure of the superficies which the sea must thus
take will be of a continuous curvature, and will return into itself;
though it may, if the actual figure is very irregular, be far either
from a sphere or a spheroid. If, however, we suppose the solid parts
of this mass subject to be dissolved or worn away, and carried down to
the ocean, there will be a tendency to give to the whole body the same
figure that it would have assumed, if it had been entirely fluid, and
subject to the laws of hydrostatics. This tendency is the result of
two principles.
438. Let us suppose the body just described to have no rotation, so
that the particles of it are actuated only by the forces of cohesion
and of attraction.
It is then clear, that every particle taken away by attrition from the
parts above the level of the sea, and deposited under the surface of
it, makes the general figure more compact, bringing the remoter parts
nearer to the centre of gravity of the whole; so that, in time, if the
body is homogeneous, all the points of the surface will become equally
distant from that centre. Thus the _actual_ figure changes continually,
and approaches nearer to the _statical_.
While this change is going forward in the actual figure, there is
another produced on the statical, that tends very much to accelerate
the final coincidence, of the two.
The effect of the inequalities of the land, that rise above the
horizontal surface, is, by their attraction, to render the parts of
that surface immediately under them, more convex, _cæteris paribus_,
than the rest. Again, where there are parts of extraordinary depth in
the sea, that is, where the solid and denser parts are far removed from
the surface of the ocean, the curvature of the superficies of the sea
is thereby diminished, and that superficies is rendered less convex
than it would be if the sea were shallower. These propositions are both
capable of strict mathematical demonstration. Hence the taking away of
any particle of matter from the top of a mountain tends to diminish the
curvature of the horizontal surface under the mountain, where it is
greatest; and the deposition of the same particle at the bottom of the
sea, tends to increase the curvature of this superficies where it is
least. The general tendency, therefore, being to increase the curvature
where it is least, and to diminish it where it is greatest, must be
to bring about an uniform curvature throughout, that is, a spherical
figure. Thus, by the waste and subsequent stratification of the land,
the direction of gravity is continually altered; it is more and more
concentrated, and the figure brought nearer to that which a fluid would
assume.
439. If now we suppose the body to revolve on its axis, all other
things remaining as before, the surface bounding the sea will become
different from what it was in the former case, and will be more swelled
out toward the middle or equatorial regions. The land above the level
of the sea will still, as before, be worn down and deposited in the
bottom of the sea, so as to form strata nearly parallel to its surface:
the tendency, therefore, is to render the real figure of the planet
nearer to the statical. At the same time the _statical_ figure is
changed, as explained above; so that the two figures mutually approach,
and the limit, or ultimate figure to which they tend, is one over which
the ocean might be diffused every where to the same depth, for then the
causes of change would entirely cease. But this figure is no other than
the spheroid of equilibrium, which, therefore, is the effect which the
waste and reconsolidation of the land would necessarily produce, if the
process were continued indefinitely, without interruption. In this, as
in many other instances, when a body is subject to the action of causes
by which its form is _gradually_ changed, the figure best adapted
to resist those changes, is the figure which the changes themselves
ultimately produce.
Also, whatever be the irregularities of density, the tendency to a
change of figure will not cease till the body is moulded into that
particular spheroid which admits of being covered with water every
where to the same depth.[232] Thus it appears, that a solid of an
irregular figure, and of irregular density, provided it be in part
covered with water; and be at the same time subject to waste above the
surface of the sea, and reconsolidation under it, has a tendency to
acquire, in time, the same figure that it would have acquired had it
been entirely fluid.
[Footnote 232: In the same manner as a transition is thus made from an
irregular figure to a spheroid of equilibrium, so, if the actual figure
were at first more simple than the spheroid, it would still be changed
into this last by degrees.
Let us conceive, for instance, that the earth is at rest, and is a
perfect sphere of solid matter, surrounded by an ocean every where of
equal depth, for example, of one mile. Then, if a rotatory motion be
communicated to it, so that it shall revolve on its axis in twenty-four
hours, in consequence of the centrifugal force, the water circumfused
about the sphere will immediately rise up under the equator, and will
become part of a spheroidal surface, (not elliptical, but nearly so,)
the equatorial diameter of which is greater than the polar axis, in the
ratio of 588 to 577 By this means the water will be accumulated at the
equator to the depth of nearly 2.5 miles, and form a zone surrounding
the earth, and extending about 37° on each side of the equator. The
remainder of the surface will be left dry, forming two vast circumpolar
continents, that reach 53° on every side of the poles, and that are
elevated in the middle more than four miles above the level of the sea.
Such would be the state of our globe, on the hypothesis above laid
down; and, if there were no waste or destruction of the land, this
order of things would be permanent, and neither the solid nor fluid
part of the mass could ever acquire any other figure than that which
has been described. But, if the same laws be supposed to regulate the
action of the atmosphere in those circumstances, that do actually
regulate it according to the present constitution of the globe, the
vapours raised up from the surface of the sea, would be carried by the
winds over the land, where they would be condensed and precipitated in
rain. Thus, all the agents of destruction would be let loose on the two
great circumpolar continents; rivers would be formed; the land would
become deeply intersected by ravines; those ravines would gradually
open into wide valleys; the masses of greatest resistance would be
shaped into hills and mountains: and from a superficies originally
smooth and uniform, the same inequalities would be produced which at
present diversify the surface of the earth.
While the parts of the sphere without the spheroid are thus continually
diminished, the loose earth and sand washed down from them, will be
deposited at the bottom of the sea, and will form strata parallel
to the surface of the superincumbent water. The actual and statical
figure are thus brought nearer one another; and, at the same time the
statical is changed, on the principle already explained, (the change in
the direction of gravity,) and is made continually to approximate to a
state, which when it has attained, no farther change can take place,
viz. an oblate elliptic spheroid, of which the surface is perpendicular
to the direction of gravity, having the equatorial diameter to the
polar axis in the ratio of 230 to 229.]
440. In the preceding reasonings, we have supposed the process of decay
and subsequent stratification to be carried on without interruption,
till the whole of the land is covered by the sea. This supposition is
useful for explaining the nature of the forces which have determined
the figure of the earth; but there is no reason to think that it has
ever been realized in its full extent, the elevation of strata from the
bottom of the sea interrupting the progress, and producing new land
in one place, as the old decays in another. The very same land also,
which is wasted at its surface, may perhaps be lifted up by the forces
that are placed under it; or it may be let down, undergoing alterations
of its level, from causes that we do not perceive, but of which the
action is undoubted, (§ 388.) But notwithstanding these interruptions,
the general tendency to produce in the earth a spheroidal figure may
remain, and more may be done by every revolution, to bring about the
attainment of that figure than to cause a deviation from it. This
figure, therefore, though never likely to be perfectly acquired, will
be the _limiting_ or _asymptotic_ figure, if it may be so called, to
which the earth will continually approach.
441. If the preceding conclusions are just, and if the figure of
equilibrium is only an asymptotic figure, to which that of the earth
may approximate, but cannot perfectly attain, we are not to be
surprised if considerable deviations from it are actually observed.
This has accordingly happened, insomuch, that the results deduced from
the most accurate measurement of degrees of the meridian, differ from
one another, in the oblateness they give to the earth, by nearly one
half of the quantity to be determined. When we compare the degrees
measured in France, and in some other countries of Europe, with
those measured in Peru, we obtain for the compression at the poles,
less than 1/300 of the radius of the earth. But when we compare the
degrees measured in France with one another, and with those lately
measured in England, we find that they are best represented by a
spheroid that has its compression 1/150 of its semi-axis.[233] There
is reason to think, therefore, that the meridians are not elliptical;
and other observations seem to show, that they are not even similar
to one another; or that the earth is not, strictly speaking, a solid
of revolution; so, also, the comparison of the degree measured at
the Cape of Good Hope, with those measured on the opposite side of
the equator, creates a suspicion, that the northern and southern
hemispheres are not perfectly alike, and that the earth is not equally
compressed at the Arctic and the Antarctic poles. These irregularities,
though they do not affect the general fact of the earth's compression
at the poles, show that the true statical figure is but imperfectly
attained; and though this may be accounted for, without having recourse
to the principles involved in our theory, it is in a manner very
unsatisfactory, and, by help of suppositions, not at all consistent
with the original fluidity ascribed to the whole mass, or to the
exterior crust of the earth.
[Footnote 233: Exposition du Systéme du Monde, par La Place, p. 61, 2d
edit.]
442. As the principles here laid down explain how a solid body may
attain very nearly the figure which a fluid would acquire in order to
preserve its parts in equilibrium; and since the oblate figure belongs
to other of the planets as well as the earth, and the globular to all
the great bodies of the universe, this suggests an analogy that goes
deep into the economy of nature, and extends far beyond the limits
within which the mineralogist is wont to confine his speculations.
443. That no very irregular figure is found among the planetary bodies,
may therefore be considered as a proof of the universality of that
system of waste and reconsolidation that we have been endeavouring to
trace in the natural history of the earth. A farther proof of the same
arises from considering, that for every given mass of matter, having a
given period of rotation, there are two different spheroids that answer
the conditions of establishing an equilibrium among its parts, the one
near to the sphere, and the other very distant from it, and so oblate
as to have a lenticular form. Thus the earth, supposing it homogeneous,
might either be in equilibrium, by means of the figure which it
actually has, or of one in which the polar was to the equatorial
diameter as 1 to 768. The same is true of the other planets; and yet we
no where find that this highly compressed spheroid is actually employed
by nature. The reason, no doubt, is, that in so oblate a spheroid, the
equilibrium between the gravitating and the centrifugal force is of
the kind that does not re-establish itself when disturbed; so that the
parts let loose, and not kept in their place by firm cohesion, would
fly off altogether. In such a body, the waste at the surface would
lead to an entire change of form, and therefore the constitution here
supposed could not be permanent.
444. In the system of Saturn, we have a great deviation from the
general order, which, nevertheless, has led to a very unexpected
verification of some of the conclusions deduced above. A principle
extremely like that which is the basis of all the foregoing reasonings,
led one of the greatest philosophers of the present age to discover
the revolution of Saturn's ring on its axis, and even to determine
the velocity of that revolution, such as it has been since found by
observation. LA PLACE, laying it down as a maxim, that nothing in
nature can exist, where there are causes of change, not balanced or
compensated by other causes,[234] concluded, that the-parts of the
ring must be held from falling down to the body of the planet by some
other force than their mere cohesion to one another. Were it otherwise,
every particle detached from the ring, by any means, must descend in a
straight line, almost perpendicular to the surface of Saturn; and the
final destruction of the ring must be inevitable. The only force that
could balance this effect of gravitation, seemed to be a centrifugal
force, arising from the rotation of the ring on an axis passing through
its centre, and perpendicular to its plane. La Place proceeded to
inquire what celerity of rotation was adequate to this effect, and
found that one of ten hours and a quarter would be required, which
is almost precisely the time afterwards determined by Dr HERSCHEL
from actual observation. If, with this rotation, the ring is a solid
annulus generated by the rotation of a very flat ellipsis about a given
point in its greater axis, coinciding with the centre of Saturn, it may
be so constituted, that the attraction of Saturn, combined with the
centrifugal force, may produce a force perpendicular to its surface,
and may enable detached parts to remain at rest, animals, for instance,
to walk on its surface, and fluids to be _in equilibrio_. The system of
Saturn is thus fortified against the lapse of time, as effectually as
that of the earth itself; and the means by which this is accomplished,
seem to prove, that the weapons which time employs, are in both cases
the same, viz. the slow wearing and decomposition of the solid parts.
This slow wearing may have produced the figure by which its action is
most effectually resisted.
[Footnote 234: La Place, _ubi supra_, p. 242.]
445. Thus Dr Hutton's theory of the earth comes at last to connect
itself with the researches of physical astronomy. The conclusion to be
drawn from this coincidence is to the credit of both sciences. When two
travellers, who set out from points so distant as the mineralogist and
the astronomer, and who follow routes so different, meet at the end of
their journey, and agree in their report of the countries through which
they have passed, it affords no slight presumption, that they have kept
the right way, and that they relate what they have actually seen.
NOTE XXVI. § 133.
_Prejudices relating to
the Theory of the Earth._
446. Among the prejudices which a new theory of the earth has to
overcome, is an opinion, held, or affected to be held, by many, that
geological science is not yet ripe for such elevated and difficult
speculations. They would, therefore, get rid of these speculations, _by
moving the previous question_, and declaring that at present we ought
to have no theory at all. We are not yet, they allege, sufficiently
acquainted with the phenomena of geology; the subject is so various and
extensive that our knowledge of it must for a long time, perhaps for
ever, remain extremely imperfect. And hence it is, that the theories
hitherto proposed have succeeded one another with so great rapidity,
hardly any of them having been able to last longer than the discovery
of a new fact, or a fact unknown when it was invented. It has proved
insufficient to connect this fact with the phenomena already known, and
has therefore been justly abandoned. In this manner, they say, have
passed away the theories of Woodward, Burnet, Whiston, and even of
Buffon; and so will pass, in their turn, those of Hutton and Werner.
447. This unfavourable view of geology, ought not, however, to be
received without examination; in science, presumption is less hurtful
than despair, and inactivity is more dangerous than error.
One reason of the rapid succession of geological theories, is the
mistake that has been made as to their object, and the folly of
attempting to explain by them the first origin of things. This mistake
has led to fanciful speculations that had nothing but their novelty to
recommend them, and which, when that charm had ceased, were rejected
as mere suppositions, incapable of proof. But if it is once settled,
that a theory of the earth ought to have no other aim but to discover
the laws that regulate the changes on the surface, or in the interior
of the globe, the subject is brought within the sphere either of
observation or analogy; and there is no reason to suppose, that man,
who has numbered the stars, and measured their forces, shall ultimately
prove unequal to this investigation.
448. Again, theories that have a rational object, though they be false
or imperfect in their principles, are for the most part approximations
to the truth, suited to the information at the time when they were
proposed. They are steps, therefore, in the advancement of knowledge,
and are terms of a series that must end when the real laws of nature
are discovered. It is, on this account, rash to conclude, that in the
revolutions of science, what has happened must continue to happen,
and because systems have changed rapidly in time past, that they must
necessarily do so in time to come.
He who would have reasoned so, and who had seen the ancient physical
systems, at first all rivals to one another, and then swallowed up by
the Aristotelian; the Aristotelian physics giving way to those of Des
Cartes; and the physics of Des Cartes to those of Newton; would have
predicted that these last were also, in their turn, to give place to
the philosophy of some later period. This is, however, a conclusion
that hardly any one will now be bold enough to maintain, after a
hundred years of the most scrupulous examination have done nothing
but add to the evidence of the NEWTONIAN SYSTEM. It seems certain,
therefore, that the rise and fall of theories in times past, does not
argue, that the same will happen in the time that is to come.
449. The multifarious and extremely diversified object of geological
researches, does, no doubt, render the first steps difficult, and
may very well account for the instability hitherto observed in such
theories; but the very same thing gives reason for expecting a very
high degree of certainty to be ultimately attained in these inquiries.
Where the phenomena are few and simple, there may be several different
theories that will explain them in a manner equally satisfactory;
and in such cases, the true and the false hypotheses are not easily
distinguished from one another. When, on the other hand, the phenomena
are greatly varied, the probability is, that among them, some of
those _instantiæ crucis_ will be found, that exclude every hypothesis
but one, and reduce the explanation given to the highest degree of
certainty. It was thus, when the phenomena of the heavens were but
imperfectly known, and were confined to a few general and simple facts,
that the Philolaic could claim no preference to the Ptolemaic system:
The former seemed a possible hypothesis; but as it performed nothing
that the other did not perform, and was inconsistent with some of our
most natural prejudices, it had but few adherents. The invention of the
telescope, and the use of more accurate instruments, by multiplying
and diversifying the facts, established its credit; and when not only
the general laws, but also the inequalities, and disturbances of the
planetary motions were understood, all physical hypotheses vanished,
like phantoms, before the philosophy of NEWTON. Hence the number, the
variety, and even the complication of facts, contribute ultimately to
separate truth from falsehood; and the same causes which, in any case,
render the first attempts toward a theory difficult, make the final
success of such attempts just so much the more probable.
This maxim, however, though a general encouragement to the prosecution
of geological inquiries, does not amount to a proof that we are yet
arrived at the period when those inquiries may safely assume the form
of a theory. But that we are arrived at such a period, appears clear
from other circumstances.
450. It cannot be denied, that a great multitude of facts, respecting
the mineral kingdom, are now known with considerable precision; and
that the many diligent and skilful observers, who have arisen in the
course of the last thirty years, have produced a great change in the
state of geological knowledge. It is unnecessary to enumerate them
all; FERBER, BERGMAN, DE LUC, SAUSSURE, DOLOMIEU, are those on whom
Dr Hutton chiefly relied; and it is on their observations and his own
that his system is founded. If it be said, that only a small part of
the earth's surface has yet been surveyed, and described with such
accuracy as is found in the writers just named, it may be answered,
that the earth is constructed with such a degree of uniformity, that a
tract of no very large extent may afford instances of all the leading
facts that we can ever observe in the mineral kingdom. The variety of
geological appearances which a traveller meets with, is not at all in
proportion to the extent of country he traverses; and if he take in a
portion of land sufficient to include primitive and secondary strata,
together with mountains, rivers, and plains, and unstratified bodies in
veins and in masses, though it be not a very large part of the earth's
surface, he may find examples of all the most important facts in the
history of fossils. Though the labours of mineralogists have embraced
but a small part of the globe, they may therefore have comprehended a
very large proportion of the phenomena which it exhibits; and hence a
presumption arises, that the outlines, at least, of geology have now
been traced with tolerable truth, and are not susceptible of great
variation.
451. When the phenomena of any class are in general ambiguous, and
admit of being explained by different or even opposite theories; if few
of those exclusive facts are known, which admit but of one or a few
solutions, then we have no right to expect much from our endeavours to
generalise, except the knowledge of the points where our information
is most deficient, and to which our observations ought chiefly to be
directed. But that many of the exclusive and unambiguous instances are
known, in the natural history of the globe, I think is evident from the
reasoning in the foregoing pages, where so many examples have occurred
of appearances that give the most direct negative to the Neptunian
system, and exclude it from the number of possible hypotheses, by
which the phenomena of geology can be explained. The abundance of such
instances is an infallible sign, that the mass of knowledge is in that
state of fermentation, from which the true theory may be expected to
emerge.
452. Another indication of the same kind, is the near approach that
even the most opposite theories make, in some respects, to one another.
There are so many points of contact between them, that they appear
to approximate to an ultimate state, in which, however unwillingly,
they must at last coincide. That ultimate form, too, which all these
theories have a tendency to put on, if I am not deceived, is no other
than that of the Huttonian theory.
453. The first example I shall take from the system of Saussure. It
is to be regretted, that this excellent geologist has no where given
us a complete account of his theory. Some of the leading principles
of it are, however, unfolded in the course of his observations, and
enable us to form a notion of its general outline. It was evidently far
removed from the system of subterraneous heat, and seems, especially
in the latter part of the author's life, to have been very much
accommodated to the prevailing system of WERNER. Nevertheless, with
so little affinity between their general views, Saussure and Hutton
agree in that most important article which regards the elevation of
the strata. Saussure plainly perceived the impossibility of the strata
being formed in the vertical situations which so many of them now
occupy; and he takes great pains to demonstrate this impossibility,
from some facts that have been referred to above. He also believed that
this elevation had been given to strata that were originally level, by
a force directed upwards, or by the _refoulement_ of the beds, not by
their falling in, as is the opinion of De Luc and some other of the
Neptunists.
Now, whoever admits this principle, and reasons on it consistently,
without being afraid to follow it through all its consequences, must
unavoidably come very close to the Huttonian theory. He must see,
that a power which, acting from below, produced this great effect
can never have belonged to water, unless rarefied into steam by the
application of heat. But if it be once admitted that heat resides in
the mineral regions, the great objection to Dr Hutton's system is
removed; and the theorist, who was furnished with so active and so
powerful an agent, would be very unskilful in the management of his
own resources, if he did not employ it in the work of consolidating as
well as in that of raising up the strata. A little attention will show,
that it is qualified for both purposes; though insuperable objections
must, no doubt, offer themselves, where the effects of compression
are not understood. We may safely conclude, then, that the accurate
and ingenious Geologist of Geneva ought to have been a _Plutonist_,
in order to give consistency to the principles which he had adopted,
and to make them coalesce as parts of one and the same system. If he
embraced an opposite opinion, it probably was from feeling the force of
those objections that arise from our discovering nothing in the bowels
of the earth like the remains left by combustion, or inflammation, at
its surface. The secret by which these seeming contradictions are to be
reconciled, was unknown to this mineralogist, and he has accordingly
decided strongly against the action of fire, even in the case of those
unstratified substances that have the greatest affinity to volcanic
lava.
454. The theoretical conclusions of another accurate and skilful
observer, Dolomieu, furnish a still more remarkable example of a
tendency to union between systems professedly hostile to one another.
This ingenious mineralogist, observing the interposition of the basalt
between stratified rocks, so that it had not only regular beds of
sandstone for its base, but was also covered with beds of the same
kind, saw plainly that these appearances were inconsistent with the
supposition of common volcanic explosions at the surface. He therefore
conceived, that the volcanic eruption had happened at the bottom of
the sea, (the level of which, in former ages, had been much higher
than at present,) and that the materials afterwards deposited on the
lava, had been in length of time consolidated into beds of stone. It
is evident, that this notion of submarine volcanoes, comes very near,
in many respects, to Dr Hutton's explanation of the same appearances.
If the only thing to be accounted for were the phenomenon in question,
it cannot be denied that Dolomieu's hypothesis would be perfectly
sufficient; but Dr Hutton, to whom this phenomenon was familiar, and
who, like Dolomieu, conceived the basalt to have been in fusion, was
convinced that the retreat of the sea was not a fact well attested by
geological appearances, and if admitted, was inadequate to account for
the facts usually explained by it. He conceived, therefore, that such
lava as the preceding had flowed not only at the bottom of the sea,
but in the bowels of the earth, and having been forced up through the
fissures of rocks already formed, had heaved up some of these rocks,
and interposed itself between them. This agrees with the other facts in
the natural history both of the basaltes and the strata.
It is plain, that, in this, there is a great approach of the two
theories to one another: both maintain the igneous origin of basaltes,
and its affinity to lava; both acknowledge that this lava cannot have
flowed at the surface, and that the strata which cover it have been
formed at the bottom of the sea. They only differ as to the mode in
which the submarine or subterraneous volcano produced its effect, and
that difference arises merely from the one geologist having generalized
more than the other. Dolomieu sought to connect the basalt with the
lavas that proceed from volcanic explosions at the surface; Dr Hutton
sought not only to connect these two appearances with one another, but
also with the other phenomena of mineralogy, particularly with the
veins of basaltes, and the elevation of the strata.
455. In another point, the coincidence of Dolomieu's opinions and Dr
Hutton's is still more striking. The former has remarked, that many
of the extinguished volcanoes are in granite countries, and that,
nevertheless, the lavas that they have erupted contain no granitic
stones. There must be, therefore, says he, something under the granite,
and this last is not, at least in all cases, to be considered as the
basis of the mineral kingdom, or as the body on which all others rest.
In this system, therefore, granite is not always a primordial rock, any
more than in Dr Hutton's.
But Dolomieu makes a still nearer advance to the Huttonian theory; for
he supposes, that under the solid and hard crust of the globe, there is
a sphere of melted stone, from which this basaltic lava was thrown up.
The system of subterraneous heat is here adopted in its utmost extent,
and in that form which is considered as the most liable to objection,
viz. the existence of it at the present moment, in such a degree as to
melt rocks, and keep them in a state of fusion. In this conclusion, the
two theories agree perfectly; and if they do so, it is only because
the nature of things has forced them into union, notwithstanding the
dissimilitude of their fundamental principles.
This ought to be considered as a strong proof, that the phenomena
known to mineralogists are sufficient to justify the attempts to form
a theory of the earth, and are such as lead to the same conclusions,
where there was not only no previous concert, but even a very marked
opposition. I have already observed, that there is a greater tendency
to agree among geological theories, than among the authors of those
theories.
456. Another circumstance worthy of consideration is, that in the
search which the Neptunists have made, for facts most favourable to the
aqueous formation of minerals, we find hardly any of a kind that was
unknown to the author of the system here explained. The appearances on
which WERNER grounds his opinion with respect to basaltes, and by which
he would exclude the action of fire from any share in the formation of
it, are all comprehended in the alternation of that rock with beds, or
strata obviously of aqueous origin. Now these appearances were well
known to Dr Hutton, and are easily explained by his theory, provided
the effects of compression are admitted. From this, and the other
circumstances just observed, I am disposed to think, that the great
facts on which every geological system must depend, are now known, and
that it is not too bold an anticipation to say, that a theory of the
earth, which explains all the phenomena with which we are at present
acquainted, will be found to explain all those that remain to be
discovered.
457. The time indeed was, and we are not yet far removed from it,
when one of the most important principles involved in Dr Hutton's
theory was not only unknown, but could not be discovered. This was
before the causticity produced in limestone by exposure to fire was
understood, and when it was not known that it arose from the expulsion
of a certain aerial fluid, which before was a component part of the
stone. It could not then be perceived, that this aerial part might be
retained by pressure, even in spite of the action of fire, and that in
a region where great compression existed, the absence of causticity was
no proof that great heat had not been applied. The discoveries of Dr
BLACK, therefore, mark an era, before which men were not qualified to
judge of the nature of the powers that had acted in the consolidation
of mineral substances. Those discoveries were, indeed, destined to
produce a memorable change in chemistry, and in all the branches of
knowledge allied to it; and have been the foundation of that brilliant
progress, by which a collection of practical rules, and of insulated
facts, has in a few years risen to the rank of a very perfect science.
But even before they had explained the nature of carbonic gas, and its
affinity to calcareous earth, I am not sure but that Dr Hutton's theory
was, at least, partly formed, though it must certainly have remained,
even in his own opinion, exposed to great difficulties. His active and
penetrating genius soon perceived, in the experiments of his friend,
the solution of those difficulties, and formed that happy combination
of principles, which has enabled him to explain the most enigmatical
appearances in the natural history of the earth.
As we are not yet far removed from the time when our chemical knowledge
was too imperfect to admit of a satisfactory explanation of the
phenomena of mineralogy, so it is not unlikely that we are approaching
to other discoveries that are to throw new light on this science. It
would, however, be to argue strangely to say, that we must wait till
those discoveries are made before we begin any theoretical reasonings.
If this rule were followed, we should not know where the imperfections
of our science lay, nor when the remedies were found out, should we
be in a condition to avail ourselves of them. Such conduct would not
be caution, but timidity, and an excess of prudence fatal to all
philosophical inquiry.
458. The truth, indeed, is, that in physical inquiries, the work of
theory and observation must go hand in hand, and ought to be carried
on at the same time, more especially if the matter is very complicated,
for there the clue of theory is necessary to direct the observer.
Though a man may begin to observe without any hypothesis, he cannot
continue long without seeing some general conclusion arise; and to this
nascent theory it is his business to attend, because, by seeking either
to verify or to disprove it, he is led to new experiments, or new
observations. He is led also to the very experiments and observations
that are of the greatest importance, namely, to those _instantiæ
crucis_, which are the _criteria_ that naturally present themselves
for the trial of every hypothesis. He is conducted to the places where
the transitions of nature are most perceptible, and where the absence
of former, or the presence of new circumstances, excludes the action
of imaginary causes. By this correction of his first opinion, a new
approximation is made to the truth; and by the repetition of the same
process, certainty is finally obtained. Thus theory and observation
mutually assist one another; and the spirit of system, against which
there are so many and such just complaints, appears, nevertheless, as
the animating principle of inductive investigation. The business of
sound philosophy is not to extinguish this spirit, but to restrain and
direct its efforts.
459. It is therefore hurtful to the progress of physical science to
represent observation and theory as standing opposed to one another.
Bergman has said, "Observationes veras quàm ingeniosissimas fictiones
sequi præstat; naturæ mysteria potius indagare quàm divinare."
If it is meant by this merely to say, that it is better to have facts
without theory, than theory without facts, and that it is wiser to
inquire into the secrets of nature, than to guess at them, the truth
of the maxim will hardly be controverted. But if we are to understand
by it, as some may perhaps have done, that all theory is mere fiction,
and that the only alternative a philosopher has, is to devote himself
to the study of facts unconnected by theory, or of theory unsupported
by facts, the maxim is as far from the truth, as I am convinced it is
from the real sense of Bergman. Such an opposition between the business
of the theorist and the observer, can only occur when the speculations
of the former are vague and indistinct, and cannot be so embodied as to
become visible to the latter. But the philosopher who has ascended to
his theory by a regular generalization of facts, and who descends from
it again by drawing such palpable conclusions as may be compared with
experience, furnishes the infallible means of distinguishing between
_perfect science_ and _ingenious fiction_. Of a geological theory that
has stood this double test of the analytic and synthetic methods, Dr
Hutton has furnished us with an excellent instance, in his explanation
of granite. The appearances which he observed in that stone led him to
conclude, that it had been melted, and injected while fluid, among the
stratified rocks already formed. He then considered, that if this is
true, veins of granite must often run from the larger masses of that
stone, and penetrate the strata in various directions; and this must
be visible at those places where these different kinds of rock come
into contact with one another. This led him to search in Arran and
Glentilt for the phenomena in question; the result, as we have seen,
afforded to his theory the fullest confirmation, and to himself the
high satisfaction which must ever accompany the success of candid and
judicious inquiry.
460. It cannot, however, be denied, that the impartiality of an
observer may often be affected by system; but this is a misfortune
against which the want of theory is not always a complete security.
The partialities in favour of opinions are not more dangerous than
the prejudices against them; for such is the spirit of system, and so
naturally do all men's notions tend to reduce themselves into some
regular form, that the very belief that there can be no theory, becomes
a theory itself, and may have no inconsiderable sway over the mind
of an observer. Besides, one man may have as much delight in pulling
down, as another has in building up, and may choose to display his
dexterity in the one occupation as well as in the other. The want
of theory, then, does not secure the candour of an observer, and it
may very much diminish his skill. The discipline that seems best
calculated to promote both, is a thorough knowledge of the methods of
inductive investigation; an acquaintance with the history of physical
discovery; and the careful study of those sciences in which the rules
of philosophising have been most successfully applied.
FINIS.
* * * * *
Transcriber Note
Minor typos corrected. Some differences in application of accents and
formatting were left as printed.
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