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. 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 très étroite 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 _grès_ 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.