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 Rhone, above St Maurice, would be drowned by shutting up that gap of the mountains through which the Rhone 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 Rhone, 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 Rhone, 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.

This general view of the valley of the Loire, and all its branches, is perhaps too extensive to be admitted in this reasoning from effect to cause; we must approximate it by an intermediate step, which will easily be acknowledged as entering within the rule.  It is in Forrez, near the head of the Loire.  There we find the plain of Mont Brison, 40,000 toises or 22 miles long and half as wide, surrounded by a ridge of granite mountains on every side.  Here the river, which is a small branch of the Loire, enters at the upper 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.