Illustrations of the Huttonian Theory of the Earth

by John Playfair

323. In order to give uniform declivities to the rivers, the lakes must not only be filled up or drained, but the cataract, wherever there is one, must be worn away. The latter is an operation in all cases visible. The stream, as it precipitates itself over the rocks, hurries along with it, not only sand and gravel, but occasionally large stones, which grind and wear down the rock with a force proportioned to their magnitude and acceleration. The smooth surface of the rocks in all waterfalls, their rounded surface, and curious excavations, are the most satisfactory proofs of the constant attrition which they endure; and, where the rocks are deeply intersected, these marks often reach to a great height above the level on which the water now flows. The phenomena, in such instances, are among the arguments best calculated to remove all incredulity respecting the waste which rivers have produced, and are continuing to produce. They suffer no doubt to remain, that the height and asperity of every waterfall are continually diminishing; that innumerable cataracts are entirely obliterated; that those which remain are verging toward the same end, and that the Falls of Montmorenci and Niagara must ultimately disappear.

324. Though there can be no doubt of the justness of the preceding conclusions, when applied to lakes in general, some apparent exceptions occur, in which the progress of draining and filling up seems to have been suspended, or even to have gone in a contrary direction. These exceptions consist of the lakes which appear to have received a greater quantity of materials than was sufficient to have filled them up. Such, for example, is the Lake of Geneva, which receives the Rhone descending from the Valais, one of the deepest and longest valleys on the surface of the earth. Now, if this valley, or even a large proportion of it, had been excavated by the Rhone itself, as our theory leads us to suppose, the lake ought to have been entirely filled up, because the materials brought down by the river seem to be much greater than the lake, on any reasonable supposition concerning its original magnitude, can possibly have received. What, then, it may be said, has become of all that the Rhone has brought down and deposited in it? The lake, at this moment, retains, in some places, the depth of more than 1000 feet; and yet, of all that the Rhone carries into it, nothing but the pure water issues. If it has been continuing to diminish, both in superficial extent and in depth, from the time when the Rhone began to run into it, what must have been its original dimensions?

I cannot pretend to remove entirely the difficulty which is here stated; yet I think the following remarks may go some length in doing so.

325. It is certain, that from the present state of the Lake of Geneva, and of the ground round it, we can hardly draw any inference as to its original dimensions. Saussure has traced, with his usual skill, the marks of the course of the Rhone, on a level greatly above the present; and, by observations on the side of Mont Saleve, has found proofs of the running of water, at least 200 toises above the present superficies of the lake. But, if ever the superficies of the lake stood at this height, or at this height nearly, though we can conjecture but little concerning the state of the adjacent country, which no doubt was also on a higher level, the lake may very well be supposed to have been of far greater dimensions than it is now. It may have occupied the whole space from Jura to Saleve, and included the Lake of Neufchâtel; so that it may have been of magnitude sufficient to receive the spoils of the Valais, which, as the surface of its waters lowered, may have been washed away and carried down to the sea. Thus it may have afforded a temporary receptacle for the debris of the Alps, and may have served for an entrepot, as it were, where those debris were deposited, before they were carried to the place of their ultimate destination.

326. But the great depth which the lake has at present, still remains to be explained, because no mud or gravel could be carried beyond the gulf, of a thousand feet deep, which was here ready to receive it. The reality of this difficulty must be acknowledged; and some cause seems to act, if not in the generation, yet certainly in the preservation of lakes, with which we are but little acquainted We can indeed imagine some causes of that kind to occur in the course of the degradation of the land, which may produce new lakes, or increase the dimensions of the old. The wearing away of a stratum, or body of strata, may lay bare, and render accessible to the water, some beds of mineral substances soluble in that fluid. The district, for instance, in Cheshire, which contains rock-salt, extends over a tract of fourteen or fifteen miles, and is covered by a thick stratum of clay, more or less indurated, which defends the salt from the water at the surface, and preserves the whole mass in a state of dryness. Should this covering be broke open by any natural convulsion, or should it be worn away, as it must be in the progress of the general detritus, the water would gain admission to the saline strata, would gradually dissolve them, and form of course a very deep and extensive lake, where all was before dry land. This event is not only possible, but it should seem, that in the course of things it must necessarily happen.

327. Something of this kind may have taken place in the track of the Rhone, and may have produced the Leman Lake. It is not impossible, that, at a very remote period, the Rhone descended from the Alps without forming any lake, or at least any lake of which the remains are now existing; and this supposition, which is more probable than that of [§ 325], we shall soon find to be conformable to appearances of another kind. The river may have wore away the secondary limestone strata over which it took its course after it left the schistus of the mountains; and, in doing so, may have reached some stratum of a saline nature, and this being washed out, may have left behind it a lake, which is but modern compared with many of the revolutions that have happened on the surface of the earth.[166]

[166] There are salt springs at Bex, near Aigle, about ten miles from the head of the lake: saline strata, therefore, are probably at no great distance.

This explanation is no doubt hypothetical; but it is proposed in one of those cases, in which hypothetical reasonings are warranted by the strictest rules of philosophical investigation. It is proposed in a case, where the causes visible to man seem inadequate to the effect, and where we must therefore have recourse to an agent that is invisible. If the operations ascribed to this agent are conformable to the analogy of nature, it is all that can in reason be required.

328. Another circumstance may also influence the generation and preservation of lakes; but it is also one with which we are but little acquainted. The strata, and indeed the whole body of mineral substances which forms the basis of our land, have been raised up from the bottom of the sea, by a progress that should seem in general to have been gradual and slow. Appearances, however, are not wanting, which show, that this progress is not uniform; and that both rising and sinking in the surface of the land, or in the rocks which are the base of it, have happened within a period of time, which is by no means of great extent. In this progress, the elevations and depressions may not be the same for every spot. They may be partial, and one part of a stratum, or body of strata, may rise to a greater height, or be more depressed, than another. It is not impossible, that this process may affect the depth of lakes, and change the relative level of their sides and bottom.

329. All lakes, however, do not involve the difficulty which the preceding conjectures are intended to remove. The great lakes of North America do not, for instance, receive their supply from very large rivers. Of course, it is not from a tract great in comparison of themselves, that the waste and detritus is brought down into them; and it seems not at all wonderful, that, without being filled up, they have been able to receive it. The same, in a degree at least, is true of many other lakes.