Illustrations of the Huttonian Theory of the Earth

by John Playfair

260. From this wedge-form of the whinstone masses, and in general from the irregularity of their surfaces, another conclusion follows, similar to the preceding, and one which has been already mentioned. Where the surface of the interposed mass is greatly inclined to the horizon, the strata which rest on this inclined plane, are nevertheless as exactly parallel to that plane, and to one another, as if they were really horizontal. It is certain, therefore, that they were not deposited on the same inclined plane on which they now rest; for, if so, they would have been still nearly horizontal, and by no means parallel to the inclined side of the whinstone. This follows from the nature of aqueous deposition, as already explained.

We have a remarkable instance of the phenomenon here referred to, in the rock of Salisbury Craig, of which mention has been so often made, and in which almost every circumstance is united, that can serve to elucidate the natural history of basaltic rocks. The north end of that rock is in the figure of a wedge, with its inclined side considerably steep, and covered by strata of grit, perfectly regular, and parallel to the surface on which they lie. The inspection of them will convince any one, that they were not deposited by the water, on a bottom so highly inclined as that on which they now rest. They are of a structure very schistose; their layers very thin; so that any inaccuracy of their parallelism would be readily observed. The appearances of the horizontal deposition of these strata, are indeed so clear, and so impossible to be misunderstood, that the followers of the Huttonian system would not risk much, if they were to leave the whole theory of whinstone to the decision of this single fact, and should agree to abandon that theory altogether, if the Neptunists can shew any physical or statical principle, on which the deposition now described can possibly have been made; or will point out the rule, by which nature has given a structure so nicely stratified to arenaceous beds deposited on a surface so highly inclined. If no such principle can be pointed out, though we cannot conclude that the Huttonian Theory is true, we certainly may conclude that the Neptunian is false.

261. Proofs of the igneous formation of whinstone, still more direct, are derived from the induration of the contiguous strata; from their disturbance when interfered by veins of whinstone; and from the charring of the coal which happens to be in contact with these veins. These are considered above at § [66], [67], &c.; and it is particularly taken notice of at [§ 66], that pieces of sandstone are sometimes found as if floating in the whinstone, and, at the same time, greatly altered in their texture. One of the best and most unequivocal instances of this sort which I have seen, is to be found on the south side of Arthur's Seat, near Edinburgh. The rock which composes the upper part of the hill, on that side, is a whinstone breccia, such as we have many examples of, and, I believe, very much resembling what is called a lava brecciata by the volcanic geologists. The stony fragments included in this compound mass, are for the greater part rounded; and some of them are of whinstone, others of porphyry, strongly characterized by rectangular maculæ of feldspar, and many seem to be of sandstone, but so considerably altered, as to leave it at least disputable whether they really are so or not. In one part, however, where the face of the rock is nearly perpendicular, a narrow ridge is seen standing out from the rest, and of a different colour, being more entirely covered with moss than the rock round about it, and, as may be presumed from that circumstance, less liable to decomposition. On examination I found, that this ridge does not consist of whinstone, but of a very hard and highly consolidated sandstone. It appears to be the edge of a stratum, of the thickness of about nine or ten inches, and of the height of fifteen or sixteen feet. It is not perfectly straight, but slightly waved, its general direction being nearly vertical; and it is on both sides firmly embraced by the whinstone. When broken, it appears that this sandstone resembles in colour, and in every thing but its greater consolidation, and more vitreous structure, the common grit found at the bottom of the hill, and over all the adjacent plain.

262. If all these circumstances are put together, there appears but one conclusion that can be drawn from them. We have here the manifest marks of some power which could lift up this fragment of rock from its native place, distant at least several hundred yards from its present situation, place it upright on its edge, encompass it with a solid rock, of a nature quite heterogeneous to itself, and bestow on it, at the same time, a great addition of solidity and induration. If the mass in which this stone is now imbedded, be supposed to have been once in fusion, and forcibly thrown up from below, invading the strata, and carrying the fragments along with it, the whole phenomena now described admit of an explanation, and all the circumstances accord perfectly with one another; but, without this supposition, they are so many separate prodigies, which have no connection with one another, nor with any thing that is known. It is indeed impossible, that the effects of motion and heat can be more clearly expressed than they are here, or the subject in which these powers resided more distinctly pointed out.

263. The preceding facts being susceptible but of one interpretation, are on that account extremely valuable. The phenomena of Salisbury Craig, near the same place, are almost equally free from ambiguity. The basaltic rock which forms that precipice, rests on arenaceous or marly strata; and these, in their immediate contact with the former, afford an instance of what is mentioned [§ 67], namely, the conversion of the strata in such situations into a kind of petrosilex, or even jasper. The line which separates the one rock from the other, is, at the same time, so well defined, as, in the eyes even of the most determined Neptunist, to exclude all idea of insensible gradation.

264. The same rock affords some remarkable instances of the disturbance of the strata contiguous to the whinstone. The beds of the former are bent upwards in several places; and, at one in particular, form an arch, with its convexity downward, so as to make it evident, that the force which produced this bending was directed from below upwards.

265. It is, however, where whinstone takes the form of veins, intersecting the strata, that the induration of the latter is most conspicuous. The coast of Ayrshire, and the opposite coast of Arran, exhibit these veins in astonishing variety and abundance. The strata are, in many instances, so reticulated by the veins, and intersected at such small distances, that it seems necessary to suppose, that the fissures in them were hardly sooner made than filled up. This at least is true, if the veins are to be accounted all of the same formation; and, in the greatest number of instances by far, there is no mark of the one being posterior to the other.

266. The induration of the sides of these veins, in some cases, has been such, that the sides have become more durable than the vein itself; so that the whinstone has been worn away by the washing of the waves, and has left the sides standing up, with an empty space, like a ditch, between them. One of these I remarked on the south side of Brodick Bay, in Arran, which, where it met the face of an abrupt cliff was not less than forty or fifty feet in depth.

267. I shall pass over whatever argument might be drawn in favour of our system, from the slender ramifications of the veins, and the varieties of their sizes, from a few inches to many fathoms in diameter, and also from the connection which they often appear to have with the great tabular masses of basaltes; and shall only add a few remarks on the charring of coal in the vicinity of veins or masses of whinstone. The connection between the charring of coal and the presence of whinstone, was first observed by Dr Hutton; and, as far as opportunities of verifying the observation have yet occurred, appears to be a fact no less general than it is curious and interesting. In the coal mines of Scotland, it certainly holds remarkably, particularly in those about Saltcoats in Ayrshire, where a whinstone dike is known to stretch across the whole of the coal country, and to be every where accompanied with blind or uninflammable coal. At Newcastle, dikes of the same kind are met with, and one, in particular, in what is called the Walker Colliery, has proved the action of subterraneous fire, to the satisfaction of mineralogists nowise prejudiced in favour of the Huttonian system.

The coal found under basaltes, in the Island of Sky, has been already mentioned, [§ 140]. To what was said concerning the fibrous structure of the parts of that fossil in immediate contact with the whin, it may be added, that it is also charred in those parts, so as to have hardly any flame when it is burnt, though further down it is of the nature of ordinary coal. Indeed, if there be any truth in Mr Kirwan's general remark, that it is common to find wood coal under basaltes, it must be understood to arise from this, that the coal in contact with the basaltes is frequently charred, and its fibrous structure, by that means, rendered more visible.