Chapter X: Section III (4)

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.