Thus the beds of rivers are to be considered as the passages through which both the lighter and heavier bodies of the land are gradually travelling; and it is through them that those moveable bodies are from time to time protruded towards the sea shore. But, in the course of rivers, it often happens that there intervenes a lake; and this must be considered as a repository for heavy bodies which had been transported by the force of running water, in the narrow bed through which it was obliged to pass; for, being arrived in the lake, the issue of which is above the level of its bottom, the moving water loses its force in protruding heavy bodies, which therefore it deposits. Thus the bottom of the lake would be filled up, before the heavy materials which the river carries could be made to advance any farther towards the sea.
Reasoning upon these principles, we shall find, that the general tendency of the operations of water upon the surface of this earth is to form plains of lakes, and not, contrarily, lakes of plains. For example, it was not the Rhône that formed the lake of Geneva; for, had the lake subsisted in its present state, while the Rhône had transported all the matter which it is demonstrable had passed through that channel from the Alps, the bed of the lake must have been made a plain through, which the river would continue to pass, but in a changing channel, as it does in any other plain. We are therefore led to believe, that the passage of the Rhône through the lake, in its present state, is not a thing of long existence, compared with the depredations which time had made by that river upon the earth above the lake. But how far there are any means for judging, with regard to the causes of that change which must have taken place, and produced the present state of things about this lake, can only be determined by those who have the proper opportunity of examining that country.
If lakes are not in the natural constitution of the earth, when this is elevated from the sea into the place of land, they must be formed by some posterior operation, which may be now considered.
There are in nature, that is, in the natural operations of the globe, two ways by which a lake may properly be formed in a place where it had not before existed. One of these is the sliding or overshooting of a mountain or a rock, which, being undermined by the river, and pressed by its weight, may give way, and thus close up the defile through which the river had worn for itself a passage. The other is the operation of an earthquake, which may either sink a higher ground, or raise a lower, and thus produce a lake where none had been before. To which, indeed, may be added a third, the dissolution of saline or soluble earthy substances which had filled the place.
So many must have been those alterations upon the surface of the earth which we inhabit, and so short the period of history by which, from the experience of man, we have to judge, that we must be persuaded we see but little of those operations which make any sensible change upon the earth; and we should be cautious not to form a history of nature from our narrow views of things; views which comprehend so little of the effects of time, that they may be considered as nothing in the scale by which we are to calculate what has passed in the works of nature.
To form an idea of the quantity of the solid land which has been carried away from the surface of the earth, we must consider our land, with the view of a mineralist, as having all the soil and travelled materials removed, so as we might see the terminations of all the strata, where these are broken off and left abrupt. Now, the generality of those strata are declined from the horizontal plane in which they had been formed, and shew that the upper extremity had been broken off and carried away; and the quantity of that which has been carried away, since the time of the formation of those strata, so far as may be judged from the nature and situation of what remains, must be concluded as very great. This is best to be observed in mountainous countries, where not only the causes of this destruction of the land are more powerful, but the opportunities of investigating the effects more frequent, from the washing away of the loose soil or covering.
The correspondent angles of the valleys among mountains is a subject of this nature, in which may be perceived a visible waste of the solid mountain which has those correspondent angles. I am happy to have an authority so much better than my own observations to give on this occasion, where the question relates to what is common or general in these appearances. It is that of M. de Luc, Lettres Physique et Morales, tom. 2. p. 221. «Mais avant de finir sur les montagnes primordiales, il faut que je revienne à ces angles saillans et rentrans alternativement opposés, qui lorsque Mr. Bourguet les annonça, firent un si grand bruit parmi les naturalistes qu'on ne douta plus que toutes les montagnes ne fussent l'ouvrage de la mer. Voici ce que c'est que ce phénomène prétendu démonstratif.
«Lorsqu'on voyage dans les vallées, on va ordinairement en tournoyant; et quand un angle saillant oblige à courber la route, on trouve assez souvent un angle rentrant qui lui fait face, et la vallée conserve à peu près la même largeur. M. Bourguet ayant fait cette remarque, et considérant que les bords opposés d'une rivière qui serpente, offrent la même opposition des angles saillans et rentrans, en conclut en général, que les montagnes avoient été formées par les courans de la mer.
«Si toutes les montagnes, et les Alpes par exemple, avoient tous les autres caractères qu'exige une telle formation celui-là sans doute ne paroîtroit pas les contredire; et l'on ne peut même disconvenir, qu'au premier coup d'oeil, ces zig-zags ne ressemblent beaucoup aux effets des eaux courantes. Cependant ce caractère appartient bien plus aux eaux qui se frayent une route, qu'à celles qui font des dépôts. Un rivière qui creuse son lit, se détourne à la rencontre d'un obstacle, et ronge le côté opposé; c'est ce qui produit ses méandres. Mais on ne voit point les mêmes causes de zig-zags dans les courans au sein de la mer; à moins qu'il n'y ait déjà des montagnes.
«En effet si l'on considère les montagnes et les collines qui par leurs couches et les corps étrangers qu'elles renferment, montrent sans équivoque qu'elles sont l'ouvrage des eaux, on les trouvera le plus souvent rangées sans ordre. Quelquefois elles ne paroissent que des monceaux posés çà et là; comme dans une grand partie du Piémont. Ou si elles sont sous la forme de chaînes continues, on y trouve peu de parallélisme, c'est-à-dire de ces angles rentrans opposés aux angles saillans: tel est le Jura.