perhaps be supposed that these lakes may have been sufficiently deep to swallow up all the debris of the valley, without being chocked up. But, rather than admit such suppositions, why not grant that the same unknown cause which has scooped out the lake, has also scooped out the valley which is only a continuation of it? 4. But if facts had proved that the waters degrade the rocks, scoop them out, and perpetually remove their debris, we might perhaps be induced to admit that unknown causes, of which we are absolutely ignorant, and of which we can form no idea, have given to the original rivers the means of surmounting all these obstacles. Now, observation would seem absolutely to prove the contrary. We have remarked, that rapid rivers which, in the bottom of valleys, fall in cascades, from rock to rock, which beat with violence against the walls which contain them, do not in any degree alter these rocks, and that, far from corroding their surface, they allow it to be covered with a rich coating of mosses, confervæ, &c. which could neither maintain itself, nor be formed at all, were the least portion of the surface of these rocks continually or even only frequently removed. A much more striking fact is that which some of the great rivers present, such as the Nile, the Orinoco, &c. which flow in the equatorial regions. These powerful rivers, when they have arrived at places where they are contracted, and, as it were, jammed in between two rocky walls, form impetuous cataracts. Their waters, endowed by the celerity of this fall with the greatest erosive power that can be attributed to this fluid, must necessarily have corroded, or at least worn, the rocks which they have thus beat against since the creation of our present Continent. Now, so far from removing the surface, they cover it with a brownish varnish of a peculiar nature. It appears, therefore, well established, that water _alone_ does not scoop those rocks, whose aggregation is complete, or which are solid; and that it does not wear them in any way, whatever be its quantity of motion. We say _water alone_; and we must insist on this distinction, in order to make the preceding facts agree with other facts, which might seem contradictory. We often see furrows scooped out on the walls that bound the narrows of rivers; we also see rocks rounded, and entirely destitute of moss. But let the facts be examined with attention, and we shall find that this erosion always takes place in the parts of their course, where, on account of the nature of the neighbouring soil, the torrents carry with them, in their risings (or floods), debris and detached stones from their banks; and it is by means of these stones that they wear the rocks which are in their bed.