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. It should also be considered, that we may err greatly in the estimate we make of the materials actually carried down and deposited in any lake. To judge of their entire amount, we should know the original form of the inequalities on the earth's surface; of the quantity of depression which existed, independently of the rivers; and though, in general, these original inequalities may be overlooked, and the present considered as made by the running of water, yet, in particular instances, this may be far from true. The Valais, for example, which we consider as the work of the Rhone, may, when the Alps rose out of the sea, have included many depressions of the surface, which the river joined together, and, from being a series of lakes, formed into one great valley. * * * * *