BEDS OF LAKES. 



251 



of a softer bed, placed between two hard rocks, and thus form small 

 longitudinal valleys. I have observed several instances of such val- 

 leys in the Alps, which may probably have been furrowed by moun- 

 tain torrents in the course of ages. Some valliesj as Les Echelles, 

 near Cbambery, are closed at one end by a perpendicular wall of 

 rock ; through this rock a tunnel has been cut for the road : but it is 

 impossible to conceive, that any action of water courses could have 

 formed such a valley. There is only a feeble stream that flows from 

 it.* Malham Cove, at the head of the valley of the Aire, in York- 

 shire, is a perpendicular wall of limestone 200 feet high : at its feet 

 the river rises ; but no conceivable action of the river could have 

 originally formed this valley. Whatever extension we may reasonably 

 grant to the action of rivers, it will not be found sufficient for the 

 excavation of valleys, except in particular situations. 



The third theory, which attributes the formation of valleys to the 

 elevation of mountain ranges, appears to assign a cause, that will ex- 

 plain, in a simple manner, the formation of many valleys ; but on ex- 

 amination, it will be found inadequate to explain the phenomena of 

 other valleys, without the concurrence of inundations or the action 

 of water. 



If the crust of the globe were broken, and raised in parallel ridg- 

 es, they might form mountain ranges, with valleys between them, 

 like what are observed bordering the central range of the Alps ; the 

 arched stratification of many of the calcareous mountains, and the 

 vertical position of the beds, favour this hypothesis. 



In some instances, where the beds of a mountain are raised from 

 an horizontal, to a nearly vertical position, they would leave a chasm 

 proportionate to the part that had been raised ; and this might form 

 the bed of a lake. The steep escarpments, which the calcareous 

 mountains in Switzerland and Savoy present to the lakes which they 

 border, indicate that the beds of the lakes were formed in the hol- 

 lows that had been left by the elevation of the mountains. The beds 

 of the mountains on the side opposite to the escarpments, general- 

 ly slope down to the lakes; hence M. De Luc inferred, that these 

 mountains had sunk down, and left the chasm which forms the bed 

 of the lake. Indeed it is highly probable, that when the beds of rock 

 were broken and elevated in one part, the beds adjoining would sink 

 down, leaving vast chasms, which were soon filled with water, and 

 formed lakes. It seems quite certain, that the lakes in the valleys 

 of mountainous countries, could never have been excavated by the 

 rivers that flow into them. The great lakes of North America are 

 situated upon a vast extent of table land, about 800 feet above the 

 sea ; but the country is so level, that the rivers which flow into the 



* For a particular account of the structure of this valley, see Travels in the 

 Tarantaise, vol. i. p. 169. I there ascribe its original formation to subsidence. 



