The natural tendency of the operations of water upon the surface of this earth is to form a system of rivers every where, and to fill up occasional lakes. The system of rivers is executed by wearing and wasting away the surface of the earth; and this, it must be allowed, is perfect or complete, at least so far as consistent with another system, which would also appear to be in nature. This is a system of lakes with which the rivers are properly connected. Now, as there are more way than one by which a lake may be formed, consistent with the Theory, the particular explanation of every lake must be left to the natural history of the place, so far as this shall be found sufficient for the purpose.
There are many places which give certain appearances, from which it is concluded, by most intelligent observators, that there had formerly existed great lakes of fresh water, which had been drained by the discharge of those waters through conduits formed by some natural operation; and those naturalists seem to be disposed to attribute to some great convulsion, rather than to the slow operation of a rivulet, those changes which may be observed upon the surface of the earth. Let us now examine some of those appearances, in order to connect them with that general system of moving water which we have been representing as every where modifying the surface of the earth on which we dwell.
It is the P. Chrysologue De Gy, who gives the following description. Journal de Physique, Avril 1787.
«La principaute de Porrentrui l'emporte encore en ce genre sur le reste du Jura à ce qu'il paroît. On pourra en juger sur les circonstances locales que je vais rapporter. Une partie de cette principauté est divisée en quatre grandes vallées, d'environs quatre lieues de long, sur trois quarts-d'heure ou une heure de large, séparées par autant de chaînes de montagnes fort élevés et large en quelques endroits d'une lieue et demie. Les extrémités de chacune de ces vallées sont plus élevées que le milieu, et on ne peut pas en sortir par ces extrémités sans beaucoup monter. Mais ces vallées ont des communications entr'elles par une pente assez douce à travers ces masses énormes de montagnes qui les separent, et qui sont coupées au niveau du milieu des vallées sur 300, 400, 500 toises de hauteur et dans toute leur largeur. On pourroit assez justement comparer ces vallées à des berceaux posés les uns à côté des autres, dont les extrémités, remplies en talus, seroient plus élevés que les cotés, et dont ces côtés seroient coupés jusqu'au fond, pour laisser une passage de l'un à l'autre. Je connois sept à huit passages semblables à travers ces hautes montagnes, dans une quarré d'environ quatre à cinq lieues; et dont quatre aboutissent à la vallée de Mouthier-Grand-Val. Ces passages sont évasés dans le dessus, d'environ une demi-lieue par endroits; mais leurs parois, en talus, se rejoignent dans le fond où coule un ruisseau. On a pratiqué des routes sur quelques-uns de ces talus, mais les roches sont quelquefois si resserrées et si escarpées, qu'on a été obligé de construire un canal sur le ruisseau, pour y faire passer la route. C'est-là que l'on voit à son aise, la nature de ces rochers primitives, leur direction, leur inclinaison, et tous leurs autres accidens qui demanderaient chacun une dissertation particulière trop longue pour le moment, et il faut les avoir vues pour se faire une juste idée des sentimens de grandeur, de surprise, et d'admiration qu'elles inspirent, et que l'on ne peut pas exprimer par des paroles. Cependant, les sources de ruisseaux, ou si l'on veut des rivières qui traversent ces montagnes, sont beaucoup plus basses que les sommités des montagnes elles-mêmes, ces sources ne font donc pas la cause de ces effets merveilleux. Il a fallu un agent plus puissant pour creuser ces abîmes.»
M. de la Metherie has taken a very enlightened view of the country of France; and has given us a plan of the different ridges of mountains that may be traced in that kingdom, (Journal de Physique, Janvier 1787). Now there is a double purpose in natural history to which such a plan as this may be applied; viz. first, to trace the nature of the solid parts, on which the soil for vegetation rests; and, secondly, to trace the nature of the soil or cultivated surface of the earth, on which depends the growth of plants.
With regard to the first, we may see here the granite raising up the strata, and bringing them to the light, where they appear on each side of those centrical ridges. What M. de la Metherie calls Monts Secondaires, I would call the proper strata of the globe, whether primary or secondary; and the Monts Granit, I would consider as mineral masses, which truly, or in a certain sense, are secondary, as having been made to invade, in a fluid state, the strata from below, when they were under water; and which masses had served to raise the country above the level of the ocean.
But this is not the subject here immediately under consideration; we are now tracing the operations of rivers upon the surface of the earth, in order to see in the present state of things a former state, and to explain the apparent irregularity of the surface and confusion of the various mineral bodies, by finding order in the works of nature; or a general system of the globe, in which the preservation of the habitable world is consulted.
For this last purpose also the mineral map of M. de la Metherie is valuable. It gives us a plan of the valleys of the great rivers, and their various branches, which, however infinitely ramified, may be considered as forming each one great valley watered, or rather drained, by its proper river. But the view I would now wish to take of those valleys, is that of habitable and fertile countries formed by the attrition of those rivers; and to perceive the operation of water wearing down the softer and less solid parts, while the more hard and solid rocks of the ridges, as well as scattered mountains, had resisted and preserved a higher station.
In this map, for example, let us suppose the first and second ridge of our author's plan to be joined at the mouth of the Loire, and retain the water of that river, as high as the summit of its surrounding ridges; this great valley of the Loire, which at present is so fine and fertile a country, would become a lake; in like manner as the proper valley of the Rhône, above St Maurice, would be drowned by shutting up that gap of the mountains through which the Rhône passes in order to enter the plain of Geneva.
This is the view that P. Chrysologue takes of those small valleys formed between the ridges of the Jura. But this is not perhaps the just view of the subject; for though by closing the gap by which the Loire or Rhône, passes through the inclosing ridge, the present country above would certainly be overflowed by the accumulated waters, yet it is more natural to suppose, that the great gap of the Loire, or the Rhône, had been formed gradually, in proportion as the inclosed country had been worn down and transported to the sea. We have but to consider, that the attrition of those transported materials must have been as necessary for the hollowing out of those gaps in the solid rock of the obstructing mountains, as the opening of those gaps may have been for the transporting of those materials to the sea. But it is perhaps impossible, from the present appearance of things, to see what revolutions may have happened to this country in the course of its degradation; what lakes may have been formed; what mountains of softer materials may have been levelled; and what basons of water filled up and obliterated.