pper end of the plain (as M. de Bournon has
described)[29] "Par une gorge tres etroite et tortueuse," and goes out in
like manner at the under End.

[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 a