36. We have seen of what materials the strata are composed, and by what power they have been consolidated; we are next to inquire, from what cause it proceeds, that they are now so far removed from the region which they originally occupied, and wherefore, from being all covered by the ocean, they are at present raised in many places fifteen thousand feet above its surface. Whether this great change of relative place can be best accounted for by the depression of the sea, or the elevation of the strata themselves, remains to be considered.

Of these two suppositions, the former, at first sight, seems undoubtedly the most probable, and we feel less reluctance to suppose, that a fluid, so unstable as the ocean, has undergone the great revolution here referred to, than that the solid foundations of the land have moved a single fathom from their place. This, however, is a mere illusion. Such a depression of the level of the sea as is here supposed, could not happen without a change proportionally great in the solid part of the globe; and, though admitted as true, will be found very inadequate to explain the present condition of the strata.

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.

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]

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.

The production of the appearances now described, belongs, without doubt, to different periods of time; and, where slips intersect one another, we can often distinguish the less from the more ancient They are all, however, of a date posterior to that at which the waving and undulated forms of the strata were acquired, as they do not carry with them any marks of the softness of the rock, but many of its complete induration.

The same phenomenon which is thus exemplified on a great scale in the bowels of the earth, is often most beautifully exhibited in single specimens of stone, and is accompanied with this remarkable circumstance, that the integrity of the stone is not destroyed by the shifts, whatever wounds had been made in it being healed, and the parts firmly reunited to one another.[15]

43. Though such marks of violence as have been now enumerated are common in some degree to all the strata, they abound most among the primary, and point out these as the part of our globe which has been exposed to the greatest vicissitudes. At their junction with the secondary, or where they emerge, as it were, from under the latter, phenomena occur, which mark some of those vicissitudes with astonishing precision; phenomena of which the nature was first accurately explored, and the consequences fully deduced, by the geologist whose system I am endeavouring to explain. He observed, in several instances, that where the primary schistus rises in beds almost vertical, it is covered by horizontal layers of secondary sandstone, which last are penetrated by the irregular tops of the schistus, and also involve fragments of that rock, some angular, others round and smooth, as if worn by attrition. From this he concluded, that the primary strata, after being formed at the bottom of the sea, in planes nearly horizontal, were raised, so as to become almost vertical, while they were yet covered by the ocean, and before the secondary strata had begun to be deposited on them. He also argued, that, as the fragments of the primary rock, included in the secondary, are many of them rounded and worn, the deposition of the latter must have been separated from the elevation of the former by such an interval of time, as gave room for the action of waste and decay, allowing those fragments first to be detached, and afterwards wrought into a round figure.[16]

44. Indeed, the interposition of a breccia between the primary and secondary strata, in which the fragments, whether round or angular, are always of the primary rock, is a fact so general, and the quantity of this breccia is often so great, that it leads to a conclusion more paradoxical than any of the preceding, but from which, nevertheless, it seems very difficult to withhold assent. Round gravel, when in great abundance, agreeably to a remark already made, must necessarily be considered as a production peculiar to the beds of rivers, or the shores of continents, and as hardly ever formed at great depths under the surface of the sea. It should seem, then, that the primary schistus, after attaining its erect position, had been raised up to the surface, where this gravel was formed; and from thence had been let down again to the depths of the ocean, where the secondary strata were deposited on it. Such alternate elevations and depressions of the bottom of the sea, however extraordinary they may seem, will appear to make a part of the system of the mineral kingdom, from other phenomena hereafter to be described.

45. On the whole, therefore, by comparing the actual position of the strata, their erectness, their curvature, the interruptions of their continuity, and the transverse stratification of the secondary in respect of the primary, with the regular and level situation which the same strata must have originally possessed, we have a complete demonstration of their having been disturbed, torn asunder, and moved angularly, by a force that has, in general, been directed from below upwards. In establishing this conclusion, we have reasoned more from the facts which relate to the angular elevation of the strata, than from those which relate to their absolute elevation, or their translation to a greater distance from the centre of the earth. This has been done, because the appearances, which respect the absolute lifting up of the strata are more ambiguous than those, which respect the change of their angular position. The former might be accounted for, could they be separated from the latter, in two ways, viz. either by the retreat of the sea, or the raising up of the land; but the latter can be explained only in one way, and force us of necessity to acknowledge the existence of an expanding power, which has acted on the strata with incredible energy, and has been directed from the centre toward the circumference.

46. When we are assured of the existence of such a power as this in the mineral regions, we should argue with singular inconsistency, if we did not ascribe to it all the other appearances of motion in those regions, which it is adequate to produce. If nature in her subterraneous abodes is provided with a force that could burst asunder the massy pavement of the globe, and place the fragments upright upon their edges, could she not, by the same effort, raise them from the greatest depths of the sea, to the highest elevation of the land? The cause that is adequate to one of these effects is adequate to them both together; for it is a principle well known in mechanical philosophy, that the force which produces a parallel motion, may, according to the way in which it is applied, produce also an angular motion, without any diminution of the former effect. It would, therefore, be extremely unphilosophical to suppose, that any other cause has changed the relative level of the strata, and the surface of the sea, than that which has, in so many cases, raised the strata from a horizontal to a highly inclined, or even vertical situation: it would be to introduce the action of more causes than the phenomena require, and to forget, that nature, whose operations we are endeavouring to trace, combines the possession of infinite resources with the most economical application of them.

47. From all, therefore, that relates to the position of the strata, I think I am justified in affirming, that their disturbance and removal from the place of their original formation, by a force directed from below upwards, is a fact in the natural history of the earth, as perfectly ascertained as any thing which is not the subject of immediate observation. As to the power by which this great effect has been produced, we cannot expect to decide with equal evidence, but must be contented to pass from what is certain to what is probable. We may, then, remark, that of the forces in nature to which our experience does in any degree extend, none seems so capable of the effect we would ascribe to it, as the expansive power of heat; a power to which no limits can be set, and one, which, on grounds quite independent of the elevation of the strata, has been already concluded to act with great energy in the subterraneous regions. We have, indeed, no other alternative, but either to adopt this explanation, or to ascribe the facts in question to some secret and unknown cause, though we are ignorant of its nature, and have no evidence of its existence.

We are therefore to suppose, that the power of the same subterraneous heat, which consolidated and mineralized the strata at the bottom of the sea, has since raised them up to the height at which they are now placed, and has given them the various inclinations to the horizon which they are found actually to possess.

48. The probability of this hypothesis will appear greatly increased, when it is considered, that, besides those now enumerated, there are other indications of movement among the bodies of the mineral kingdom, where effects of heat more characteristic than simple expansion are clearly to be discovered. Thus, on examining the marks of disorder and movement which are found among the strata, it cannot fail to be observed, that notwithstanding the fracture and dislocation, of which they afford so many examples, there are few empty spaces to be met with among them, as far as our observation extends. The breaches and separations are numerous, and distinct; but they are, for the most part, completely filled up with minerals of a kind quite different from the rock on each side of them, and remarkable for containing no vestiges of stratification. We are thus led to consider the unstratified minerals, the second of the divisions into which the whole mineral kingdom, viewed geologically, ought to be distinguished. These minerals are immediately connected with the disturbance of the strata, and appear, in many instances, to have been the instruments of their elevation.