Essay on the Theory of the Earth

by baron Georges Cuvier

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.

It is very easy to appreciate these circumstances. It is remarked, that this erosion has never taken place at the sources of powerful springs. All the pebbles which had to be carried off have been so long ago, and the mosses which grow abundantly on the rocks at the level of the water, and in the bed of these torrents, have nothing more to fear from the destructive action of these solid bodies. The case is the same with the parts which immediately succeed a lake, or a great excavation, capable of arresting all the hard bodies carried off by the river. There the mosses appear in abundance; because they are not subjected to the action of any other substance than of the water alone.

The present rivers do not therefore appear to have any erosive power upon the rocks which are completely aggregated, when they act by themselves, and when no other cause, such as frost, decomposition, &c. has disintegrated the rock. The absence of these foreign circumstances is proved by the vegetation or the enamel which then cover the rocks exposed to the action of the water.

These rivers, in proportion as they remove from the rocks in the neighbourhood of the lofty mountains in which they took their rise, often gain in volume what they have lost in velocity; but the power dependent upon size rarely compensates that which they owed to rapidity; and although these large rivers still retain a transporting power, sufficient to carry along with them the obstacles which oppose themselves to their progress, they are far from presenting results of action so striking as those of torrents. They stir up, when flooded, or when they change place, the earth and mobile sand which cover their bottom, especially towards their edges, and transport them to some distance; but they scarcely move pebbles larger than an egg, which occur in their bed, and which have been brought there in other times, and under other circumstances. On thus transporting the comminuted and mobile mineral matters, they deposite them again in places where their current is relaxed by some cause, and thus raise the bottom of their bed in these places; they seek a new passage in the midst of the barriers which they have themselves constructed. The principal current is then borne, sometimes against one bank, and sometimes against the other; and when it comes to beat upon the foot of a steep part, composed of moveable soil, as the banks commonly are, in such cases, they really erode it, and make it fall into the river; and transport to another part of its course, the earth resulting from the destruction of the bank, and give rise to new obstacles. Hence the new deposites, which border rivers in all points where their current is slackened, and principally toward their mouth. It is sufficient for our present purpose to have referred to facts remarkable for their number, for the importance which they have had in regard to the modern changes of the configuration of the globe; and, lastly, in regard to agriculture and civilisation;—facts of easy observation, and which tend to prove, that the action of rivers, whose fall is not sufficiently rapid to entitle them to be considered as torrents, is not to scoop out their bed, either in the valleys or in the plains through which they flow, but rather to raise them, and to tend, consequently, rather to level and flatten the earth than to furrow it, more than it has been since the Continents have assumed the configuration which they now possess.