Valleys of the Bhine and Ehone. 155 



the ancient water-line. Their most remarkable feature, how- 

 ever, is the miniature valley which has been carved out, in 

 each case, by the stream, in consequence of the changed re- 

 lations of the lake. It appears that the stream, on no longer 

 being received into the body of still water, has begun to fall, 

 as a sort of cascade, over the steep front of the delta. The 

 matter being soft, has readily given way, and been carried 

 forward into the diminished lake. The stream has thus worn 

 a way back towards the mountains, exactly as the Niagara 

 river is believed to do with the rock over which it descends. 

 It therefore appears to be demonstrated, that, while a delta 

 is formed by the connection of a recipient body of water with 

 a river producing detritus, the 'cutting down of such a delta 

 is the consequence of the withdrawal of that body of water. 



The same explanation serves for the peculiar forms of 

 valley alluvia already described. These, indeed, are neither 

 more nor less than deltas confined between parallel ranges 

 of hills. 1 conceive, that in every such case, there has been 

 a recipient body of water up to the highest point where any 

 such formation is found. Into this the detritus of the stream 

 is thrown, and there deposited in an equable slope. While 

 such continues to be the relation of the objects, nothing like 

 the formation of a trough or valley in the alluvium can take 

 place. For that process, it is necessary that the body of still 

 or recipient water be withdrawn, in which case the detrital 

 sheet formerly lying under still water is left under a sub- 

 aerial exposure ; the stream once received into the still water, 

 now runs over the exposed alluvium : on that it exercises, of 

 course, a wearing power, while seeking a line of descent con- 

 formable to its new circumstances. A hollow or valley is 

 consequently formed, the removed matter being always car- 

 ried forward into the receded body of still water. Thus a 

 body of water might retire down a valley, so as to allow of 

 fresh formations being deposited, to be in their turn cut down 

 by the stream, until final rest was obtained in a lake of the 

 present era, or in the sea at its present level. 



In speculating on such bodies of water, it seems natural, 

 in the first instance, to think of inland lakes, and to suppose 

 that these have been discharged by the giving way of their 



