[30] See Lehman, Traités de Phys. &c. tom. iii. p. 359. Note.
104. Before we take leave of the rivers and the plains, we must remark another fact, often observed in the natural history of the latter, and clearly evincing the former existence of immense bodies of strata, in situations from which they have now entirely disappeared. The fact here alluded to is, the great quantity of round and hard gravel, often to be met with in the soil, under such circumstances, as prove, that it can only have come from the decomposition of rocks, that once occupied the very ground over which this gravel is now spread. In the chalk country, for instance, about London, the quantity of flints in the soil is every where great; and, in particular situations, nothing but flinty gravel is found to a considerable depth. Now, the source from which these flints are derived is quite evident, for they are precisely the same with those contained in the chalk beds, wherever these last are found undisturbed, and from the destruction of such beds they have no doubt originated. Hence a great thickness of chalk must have been decomposed, to yield the quantity of flints now in the soil of these countries; for the flints are but thinly scattered through the native chalk, compared with their abundance in the loose earth. To afford, for example, such a body of flinty gravel as is found about Kensington, what an enormous quantity of chalk rock must have been destroyed?
105. This argument, which Dr Hutton has applied particularly to the chalk countries, may be extended to many others. The great plain of Crau, near the mouth of the Rhone, is well known, and was regarded with wonder, even in ages when the natural history of the globe was not an object of much attention. The immense quantity of large round gravel-stones, with which this extensive plain is entirely covered, has been supposed, by some mineralogists, to have been brought down by the Durance, and other torrents, from the Alps; but, on further examination, has been found to be of the same kind that is contained in certain horizontal layers of pudding-stone, which are the basis of the whole plain. It cannot be doubted, therefore, that the vast body of gravel spread over it, has originated from the destruction of layers of the same rock, which may perhaps have risen to a great height above what is now the surface. Indeed, from knowing the depth of the gravel that covers the plain, and the average quantity of the like gravel contained in a given thickness of rock, one might estimate how much of the latter has been actually worn away. Whether data precise enough could be found, to give any weight to such a computation, must be left fer future inquiry to determine.[31]
106. In these instances, chalk and pudding-stone, by containing in them parts infinitely less destructible than their general mass, have, after they are worn away, left behind them very unequivocal marks of their existence. The same has happened in the case of mineral veins, where the substances least subject to dissolution have remained, and are scattered at a great distance from their native place. Thus gold, the least liable to decomposition of all the metals, is very generally diffused through the earth, and is found, in a greater or less abundance, in the sand of almost all rivers. But the native place of this mineral is the solid rock, or the veins and cavities contained in the rock, and from thence it must have made its way into the soil. This, therefore, is another proof of the vast extent to which the degradation of the land, and of the rock, which is the basis of it, has been earned; and consequently, of the great difference between the elevation and shape of the earth's surface in the present, and in former ages.
107. The veins of tin furnish an argument of the same kind. The ores of this metal are very indestructible, and little subject to decomposition, so that they remain very long in the ground without change. Where there are tin veins, as in Cornwall, the tin-stone or tin ore is found in great abundance in such vallies and streams as have the same direction with the veins; and hence the streaming, as it is called, or washing of the earth, to obtain the tin-stone from it. Now, if it be considered, that none of this ore can have come into the soil but from parts of a vein actually destroyed, it must appear evident that a great waste of these veins has taken place, and consequently of the schistus or granite in which they are contained.
108. These lessons, which the geologist is taught in flat and open countries, become more striking, by the study of those Alpine tracts, where the surface of the earth attains its greatest elevation. If we suppose him placed for the first time in the midst of such a scene, as soon as he has recovered from the impression made by the novelty and magnificence of the spectacle before him, he begins to discover the footsteps of time, and to perceive, that the works of nature, usually deemed the most permanent, are those on which the characters of vicissitude are most deeply imprinted. He sees himself in the midst of a vast ruin, where the precipices which rise on all sides with such boldness and asperity, the sharp peaks of the granite mountains, and the huge fragments that surround their bases, do but mark so many epochs in the progress of decay, and point out the energy of those destructive causes, which even the magnitude and solidity of such great bodies have been unable to resist.
109. The result of a more minute investigation, is in perfect unison with this general impression. Whence is it, that the elevation of mountains is so obviously connected with the hardness and indestructibility of the rocks which compose them? Why is it, that a lofty mountain of soft and secondary rock is no where to be found; and that such chains, as the Pyrenees or the Alps, never consist of any but the hardest stone, of granite for instance, or of those primary strata, which, if we are to credit the preceding theory, have been twice heated in the fires, and twice tempered in the waters, of the mineral regions? Is it not plain that this arises, not from any direct connection between the hardness of stones, and their height in the atmosphere, but from this, that the waste and detritus to which all things are subject, will not allow soft and weak substances to remain long in an exposed and elevated situation? Were it not for this, the secondary rocks, being in position superincumbent on the primary, ought to be the highest of the two, and should cover the primary, (as they no doubt have at one time done,) in the highest as well as the lowest situations, or among the mountains as well as in the plains.
110. Again, wherefore is it, that among all mountains, remarkable for their ruggedness and asperity, the rock, on examination, is always found of very unequal destructibility, some parts yielding to the weather, and to the other causes of disintegration, much more slowly than the rest, and having strength sufficient to support themselves, when left alone, in slender pyramids, bold projections, and overhanging cliffs? Where, on the other hand, the rock wastes uniformly, the mountains are similar to one another; their swells and slopes are gentle, and they are bounded by a waving and continuous surface. The intermediate degrees of resistance which the rocks oppose to the causes of destruction, produce intermediate forms. It is this which gives to the mountains, of every different species of rock, a different habit and expression, and which, in particular, has imparted to those of granite that venerable and majestic character, by which they rarely fail to be distinguished.
111. The structure of the vallies among mountains, shows clearly to what cause their existence is to be ascribed. Here we have first a large valley, communicating directly with the plain, and winding between high ridges of mountains, while the river in the bottom of it descends over a surface, remarkable, in such a scene, for its uniform declivity. Into this, open a multitude of transverse or secondary vallies, intersecting the ridges on either side of the former, each bringing a contribution to the main stream, proportioned to its magnitude; and, except where a cataract now and then intervenes, all having that nice adjustment in their levels, (99.) which is the more wonderful, the greater the irregularity of the surface. These secondary vallies have others of a smaller size opening into them; and, among mountains of the first order, where all is laid out on the greatest scale, these ramifications are continued to a fourth, and even a fifth, each diminishing in size as it increases in elevation, and as its supply of water is less. Through them all, this law is in general observed, that where a higher valley joins a lower one, of the two angles which it makes with the latter, that which is obtuse is always on the descending side; a law that is the same with that which regulates the confluence of streams running on a surface nearly of uniform inclination. This alone is a proof that the vallies are the work of the streams; and indeed what else but the water itself, working its way through obstacles of unequal resistance, could have opened or kept up a communication between the inequalities of an irregular and alpine surface?