402 



ILLUSTRATIONS OF THE 



the rock on either fide of them. The flow work- 



I 



ing of water, on the other hand, or the powers 

 that we fee every day in adlion, arc quite fuffi- 

 cient for this efFed:, if time only is allowed them, 

 360. Some valleys are fo particularly con- 

 llruded, as to carry with them a ftill (Ironger 



refutation of the exiftence of a debacle. 



Thefe 



are the longitudinal valleys, which have the 

 openings by which the water is difcharged, 

 nbt at one extremitv, but at the broadfide. 



F 



Such is that on the eaft iide of Mont Blanc, 

 deeply excavated on the confines of the granite 

 and fchiftus rock, and extending parallel to the 



beds of the latter, from the Col de la Segne to 

 the Col de Ferret ; its opening is nearly in the 

 middle, from which the Dorea iflfues, and takes 

 its courfe through a great valley, nearly at right 

 angles to the chain of the Alps, and to the 

 valley juft mentioned. From the ftrudure of 

 thefe valleys, SauiTure has argued very jullly 

 againft Buffon's hypothelis, concerning the for- 

 mation of valleys by currents at the bottom of the 

 fea *. It affords indeed a complete refutation of 

 that hypothelis ; and it affords one no lefs com- 

 plete of the fyitem which SauiTure himfelf feems 

 on fome occafions fo much inclined to fupport. 



For if it be faid, that this valley was cut out by the 



current 



* Voyages aux Alpes, torn. ii. § 920 



f , 



