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 × √½ = 86 × 5/7 = 61 miles nearly.
[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.