[Footnote 29:  Journal de Physique, Mai 1787.]

Those French philosophers, who have seen this plain, have little doubts of this having been a lake, that is to say, they easily admit of the original continuity of those ridges of mountains in which the gaps are now found, through which the river passes.  But upon those principles it must be evident, that the river has hollowed out that plain, at the same time that it had formed the gaps in those ridges of the granite mountains.  The only solid part, or original stratum, which M. de Bournon has described as having seen in this plain, is a decomposing gres or sandstone; but there is reason to suppose, that there had been both calcareous and argillaceous or marly strata filling the hollow of that space which is inclosed by the granite mountains; consequently, no difficulty in conceiving that the river, which must wear away a passage through those mountains, should also hollow out the softer materials within, and thus form the plain, or rather a succession of plains, in proportion as the level of the water had been lowered with the wearing mountains.

If we are allowed to make this step, which I think can hardly be refused, we may proceed to enlarge our view, by comprehending, first, the Vallais of the Rhone, secondly, the countries of the Seine and Rhone, above the mountains through which those two rivers in conjunction have broke, below Lyons; and, lastly, that country of the Rhone and Durance which is almost inclosed by the surrounding mountains, meeting at the mouth of the Rhone.  But this reasoning will equally apply to the countries of the Garonne, the Loire, and the Seine.

One observation more may now be made with regard to the courses of great rivers, and the fertile countries which they form in depositing the travelled soil; it is this.  That though those rivers have hollowed out their beds and raised their banks; though they are constantly operating in forming fertile soil in one place and destroying it in another; and though, in many particular situations, the fertile countries, formed at the mouths of those rivers, are visibly upon the increase, yet the general progress of those operations is so slow, that human history does not serve to give us information almost of any former state of things.  The Nile will serve as an example of this fact.

The river Nile, which rises in the heights of Ethiopia, runs an amazing tract through desert countries, and discharges its waters near the bottom of the Mediterranean sea, fertilizes a long valley among barren countries with which it is surrounded, and thus lays the foundation of a kingdom, which, from its situation and the number of people it can maintain and easily bring together for any manner of action, is perhaps the strongest that can well be imagined.  Accordingly, it has been of old a great kingdom, that is to say, a powerful state within itself; and has left monuments of this power, which have long been the admiration of the world.  The most ancient Grecian Histories mention these monuments as being no better known, with regard to their dates and authors, than they are at this day.