Chapter IX: Section III (3)

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.