Illustrations of the Huttonian Theory of the Earth

by John Playfair

In one instance, mentioned by Mr Strange, this seems actually to have taken place. In the Veronese, a country remarkable for a mixture of limestone strata, containing marine objects, with volcanic or basaltine hills, he assures us, that he had seen a mass of stone, which had evidently concreted from fusion, in which the marine fossil bodies, originally, as he supposes, contained in the strata, were perfectly distinguishable, though variously disfigured.[136] It may be, that in this, as in the foregoing examples, it was not real basaltes, or real lava, which contained the shells, but the conterminal rock; but, supposing it to be as Mr Strange represents it, there appears to be no inconsistency between the phenomenon, and the igneous origin of the rock in which the shells were included. Here, however, it should be remarked, that the presence of great pressure, to prevent the conversion of the shells into quicklime, seems absolutely necessary; and that the phenomenon of these basaltic petrifactions, requires the application of heat to have been deep under the surface of the earth.

[136] Phil. Trans. 1775, p. 25.

256. The phenomena we have been considering, have been selected as the most unfavourable to the igneous origin of basaltic rocks; and we have seen, that when duly examined, they are not at all inconsistent with it. We are now to take a view of some appearances, that seem quite irreconcilable with the aqueous formation of these rocks.

Where whinstone rocks are found in masses, bounded by the strata, and insulated among them, they subject the Neptunian system to great difficulties. For, supposing it true that this stone may be produced by the precipitation and crystallization of mineral substances dissolved in water, yet it seems unaccountable, that this effect has been so local and limited in extent, as often to be confined to an irregular figure of a few acres, while, all round, the substances deposited have had no tendency to crystallization, and have been formed into the common secondary strata. The rock of Salisbury Craig, for instance, is a mass of whinstone, having a perpendicular face eighty or ninety feet high toward the west, and extending from north to south with a circular sweep about 900 yards. The whole of this rock rests on regular beds of secondary sandstone, not horizontal, but considerably depressed toward the north-east: the rock is loftiest in the middle, and decreases in thickness toward each end, terminating at its northern extremity in a kind of wedge. It is covered at top, toward that extremity, with regular beds of sandstone, perfectly similar to those on which it is incumbent; and it is not improbable, that this covering formerly extended over the whole.

Now, what cause can have determined the column of water, which rested on the base at present occupied by this rock, to deposit nothing but the materials of whinstone, while the water on the south, west, and north, was depositing the materials of arenaceous and marly strata? Wherefore, within this small space, was the precipitate every where chemical, to use the language of Werner, while close to it, on either side, it was entirely mechanical? Why is there, in this case, no gradation? and why is a mere mathematical line the boundary between regions where such different laws have prevailed? Whence also, we may ask, has the basaltic deposit been abruptly terminated toward the west, so as to produce the steep face which has just been mentioned? The operation of currents, or of any motion that can take place in a fluid, will furnish no explanation whatever of these phenomena; yet they are phenomena far from being peculiar to a single hill; they are among the most general and characteristic appearances in the natural history of whinstone mountains; and a geological theory which does not account for them, is hardly entitled to any consideration.

257. The basaltic rock, just described, is also covered, at least partly, with strata perfectly similar to those that lie under it. Now, it appears altogether unaccountable, that after the water had done depositing the materials of the whin on the spot in question, the former order was so quickly resumed, and a deposition of sand, and of the other materials of the strata, took place just as before. All this is quite unintelligible; and the principles of the Neptunian system seem here to stand as much in need of explanation, as any of the appearances which they are intended to account for.

258. The unequal thickness, and great irregularity in the surface of the whinstone mass, here treated of, and of many rocks of the same kind, is also a great objection to the notion of their aqueous formation. This seems to have been perceived by Werner, in the instance of the rocks formerly mentioned; and he endeavours to explain it, by supposing, that much of these rocks has been destroyed by waste and decomposition, so that an irregularity of their surface, and want of correspondence has been given to them, which they did not originally possess. In the instance of Salisbury Craig, however, we have a proof, that the great irregularity of surface, and the inequality of thickness, do not always arise from these causes. The thinnest part of that rock, toward its northern extremity, is still covered by the strata in their natural place, and has been perfectly defended by them from every sort of wearing and decay. The cuneiform shape, therefore, which this rock takes at its extremities, and the great difference of its thickness at them and in the middle, is a part of its original constitution, and can be attributed to nothing casual, or subsequent to its consolidation.

The same may be said of many other basaltic rocks, where an inequality of thickness, most unlike to what belongs to aqueous deposits, is known to exist in beds of whinstone that are still deep under the surface. Thus the toadstone of Derbyshire, even where it has a thick covering of strata over it, has been found, by the sinking of perpendicular shafts, to vary from the thickness of eighteen yards to more than sixty, within the horizontal distance of less than a furlong. Nothing of this kind is ever found to take place in those beds of rock which are certainly known to originate from aqueous deposition, and no character can more strongly mark an essential difference of formation.

259. We have had frequent occasion to consider the characters of those masses of whinstone which are so often found interposed between stratified rocks. These have been found in general very adverse to the Neptunian system; and two of them which yet remain to be mentioned, are even more so than any of the rest.

Where a bed or tabular mass of whinstone is interposed between strata, and wherever an opportunity offers of seeing its termination, if the strata under it are not broken, it may be remarked, that they do not abut themselves bluff and abrupt against the whin. On the contrary, if we mark the course of the stratum which covers the whinstone, and of that which is the base of it, we shall find they converge toward one another, the interposed mass growing thinner and thinner, like a wedge. When the latter terminates, the two former come in contact, and have no stratum interposed between them. Thus the roof and base of the whinstone rock are contiguous beds, that appear as if they had been lifted up and bent, and separated by an interposed mass. Had the whole been an effect of simultaneous deposition, the regular strata must have been abruptly terminated by the whin, like two courses of different forts of masonry where they meet with one another.