37. Supposing the appearances which clearly indicate submersion under water to reach no higher than ten thousand feet above the present level of the sea, and of course the surface of the sea to have been formerly higher by that quantity than it is now; it necessarily follows, that a bulk of water has disappeared, equal to more than a seven hundredth part of the whole magnitude of the globe.[13] The existence of empty caverns of extent sufficient to contain this vast body of water, and of such a convulsion as to lay them open, and give room to the retreat of the sea, are suppositions which a philosopher could only be justified in admitting, if they promised to furnish a very complete explanation of appearances. But this justification is entirely wanting in the present case; for the retreat of the ocean to a lower level, furnishes a very partial and imperfect explanation of the phenomena of geology. It will not explain the numberless remains of ancient continents that are involved, as we have seen, in the present, unless it be supposed that the ancient ocean, though it rose to so great a height, had nevertheless its shores, and was the boundary of land still higher than itself. And, as to that which is now more immediately the object of inquiry, the position of the strata, though the above hypothesis would account in some sort for the change of their place, relatively to the level of the sea; yet, if it shall be proved, that the strata have changed their place relatively to each other, and relatively to the plane of the horizon, so as to have had an angular motion impressed on them, it is evident that, for these facts, the retreat of the sea does not afford even the shadow of a theory.
38. Now, it is certain, that many of the strata have been moved angularly, because that, in their original position, they must have been all nearly horizontal. Loose materials, such as sand and gravel subsiding at the bottom of the sea, and having their interstices filled with water, possess a kind of fluidity: they are disposed to yield on the side opposite to that where the pressure is greatest, and are therefore, in some degree, subject to the laws of hydrostatics. On this account they will arrange themselves in horizontal layers; and the vibrations of the incumbent fluid, by impressing slight motion backward, and forward, on the materials of these layers, will very much assist the accuracy of their level.
It is not, however, meant to deny, that the form of the bottom might influence, in a certain degree, the stratification of the substances deposited on it. The figure of the lower beds deposited on an uneven surface, would necessarily be affected by two causes; the inclination of that surface, on the one hand, and the tendency to horizontality, on the other; but, as the former cause would grow less powerful as the distance from the bottom increased, the latter cause would finally prevail, so that the upper beds would approach to horizontally, and the lower would neither be exactly parallel to them, nor to one another. Whenever, therefore, we meet with rocks, disposed in layers quite parallel to one another, we may rest assured, that the inequalities of the bottom have had no effect, and that no cause has interrupted the statical tendency above explained.
Now, rocks having their layers exactly parallel, are very common, and prove their original horizontally to have been more precise than we could venture to conclude from analogy alone. In beds of sandstone, for instance, nothing is more frequent than to see the thin layers of sand, separated from one another by layers still finer of coaly, or micaceous matter, that are almost exactly parallel, and continue so to a great extent without any sensible deviation. These planes can have acquired their parallelism only in consequence of the property of water just stated, by which it renders the surfaces of the layers, which it deposits, parallel to its own surface, and therefore parallel to one another. Though such strata, therefore, may not now be horizontal, they must have been so originally; otherwise it is impossible to discover any cause for their parallelism, or any rule by which it can have been produced.
39. This argument for the original horizontality of the strata, is applicable to those that are now farthest removed from that position. Among such, for instance, as are highly inclined, or even quite vertical, and among those that are bent and incurvated in the most fantastical manner, as happens more especially in the primary schisti, we observe, through all their sinuosities and inflections, an equality of thickness and of distance among their component laminæ. This equality could only be produced by those laminæ having been originally spread out on a flat and level surface, from which situation, therefore, they must afterwards have been lifted up by the action of some powerful cause, and must have suffered this disturbance while they were yet in a certain degree flexible and ductile. Though the primary direction of the force which thus elevated them must have been from below upwards, yet it has been so combined with the gravity and resistance of the mass to which it was applied, as to create a lateral and oblique thrust, and to produce those contortions of the strata, which, when on the great scale, are among the most striking and instructive phenomena of geology.
40. Great additional force is given to this argument, in many cases, by the nature of the materials of which the stratified rocks are composed. The beds of breccia and pudding-stone, for instance, are often in planes almost vertical, and, at the same time, contain gravel-stones, and other fragments of rock, of such a size and weight, that they could not remain in their present position an instant, if the cement which unites them were to become soft; and therefore they certainly had not that position at the time when this cement was actually soft. This remark has been made by mineralogists who were not led to it by any system. The judicious and indefatigable observer of the Alps, describing the pudding-stone of Valorsine, near the sources of the Arve, tells us, that he was astonished to find it in beds almost vertical, a situation in which it could not possibly have been formed. "That particles," he adds, "of extreme tenuity, suspended in a fluid, might become agglutinated, and form vertical beds, is a thing that may be conceived; but that pieces of stone, of several pounds weight, should have rested on the side of a perpendicular wall, till they were enveloped in a stony cement, and united into one mass, is a supposition impossible and absurd. It should be considered, therefore, as a thing demonstrated, that this pudding-stone was formed in a horizontal position, or one nearly such, and elevated after its induration. We know not," he continues, "the force by which this elevation has been effected; but it is an important step among the prodigious number of vertical beds that are to be met with in the Alps, to have found some that must certainly have been formed in a horizontal situation."[14]
[14] Voyages aux Alpes, tom. ii § 690.
41. Nothing can be more sound and conclusive than this reasoning; and had the ingenious author pursued it more systematically, it must have led him to a theory of mountains very little different from that which we are now endeavouring to explain. If some of the vertical strata are proved to have been formed horizontally, there can be no reason for not extending the same conclusion to them all, even if we had not the support of the argument from the parallelism of the layers, which has been already stated.
42. The highly inclined position, and the manifold inflections of the strata, are not the only proofs of the disturbance that they have suffered, and of the violence with which they have been forced up from their original place. Those interruptions of their continuity which are observed, both at the surface and under it, are evidences of the same fact. It is plain, that if they remained now in the situation in which they were at first deposited, they would never appear to be suddenly broken off. No stratum would terminate abruptly; but, however its nature and properties might change, it would constitute an entire and continued rock, at least where the effects of waste and detritus had not produced a separation. This, however, is very far from being the actual condition of stratified bodies. Those that are much inclined, or that make considerable angles with the horizontal plane, must terminate abruptly where they come up to the surface. Their doing so is a necessary consequence of their position, and furnishes no argument, it may be said, for their having been disturbed, different from that which has been already deduced from their inclination. There are, however, instances of a breach of continuity in the strata, under the surface, that afford a proof of the violence with which they have been displaced, different from any hitherto mentioned. Of this nature are the slips or shifts, that so often perplex the miner in his subterraneous journey, and which change at once all those lines and bearings that had hitherto directed his course. When his mine reaches a certain plane, which is sometimes perpendicular, sometimes oblique to the horizon, he finds the beds of rock broken asunder, those on the one side of the plane having changed their place, by sliding in a particular direction along the face of the others. In this motion they have sometimes preserved their parallelism, that is, the strata on one side of the slip continue parallel to those on the other; in other cases, the strata on each side become inclined to one another, though their identity is still to be recognized by their possessing the same thickness, and the same internal characters. These shifts are often of great extent, and must be measured by the quantity of the rock moved, taken in conjunction with the distance to which it has been carried. In some instances, a vein is formed at the plane of the shift or slip, filled with materials of the kinds which will be hereafter mentioned; in other instances, the opposite sides of the rock remain contiguous, or have the interval between them filled with soft and unconsolidated earth. All these are the undeniable effects of some great convulsion, which has shaken the very foundations of the earth; but which, far from being a disorder in nature, is part of a regular system, essential to the constitution and economy of the globe.