Conglomerate and Gravel Deposits. 399 



tant rocks, from their banks, but to transport which, by the 

 rivers, physical obstacles oppose themselves. Such obstacles 

 commonly present themselves in the shape of lakes, the beds of 

 which it is impossible the rivers could have cut. Into these the 

 rapid and detritus-bearing rivers deposit their gravel and sand, 

 so that such rivers constantly tend to fill up lakes so situated. 

 The detritus, thus driven into a lake, will always be deposited 

 in a peculiar form, variously modified according to the depth of 

 the lake, and the pebble or sandy nature of the detritus. 



In cases where rivers discharge pebbles into lakes, that of the 

 Drance torrent for instance, which deposits its pebbles in the 

 lake of Geneva, the advance is gradual and local. It is obvi- 

 ous that the stratification resulting from these causes must have 

 a peculiar figure; and supposing a lake, nearly filled by these 

 means, to be examined after drainage, the beds of gravel, sand 

 or clay, would be very irregular, and not be disposed hori- 

 zontally. 



To take examples from the Alps; the present transport of 

 river-formed pebbles from a large part of these mountains is 

 prevented by numerous lakes on their north and south sides. 

 On the north the Rhone deposits its mountain detritus in the 

 lake of Constance, and the Rhone its transported pebbles and 

 sands in the lake of Geneva. Between these the lakes of 

 Zurich, Lucerne, &c. receive the gravels of other alpine rivers. 

 On the south the Lago Maggiore receives the alpine detritus of 

 the Ticino, the lake of Como that of the Adda, and the lakes 

 of Garda, &c. perform the same office to other rivers. From 

 these circumstances it will be evident that the river-formed 

 pebbles of a large portion of the Alps cannot travel by the 

 rivers into either the ocean or the Mediterranean: it might at 

 first sight be supposed that the Po could transport the river- 

 formed pebbles of a large portion of the Alps into the Adriatic; 

 but the Po becomes a sandy-bended river before it receives the 

 Ticino. 



It may also be supposed that though the Rhone can trans- 

 port no alpine detritus beyond the spot where it enters the lake 

 of Geneva, yet that, after it has quitted that lake, it can carry 

 all the pebbles borne down by the Arve from the district of 

 Mont Blanc. I have often stood at the junction of the two 

 rivers, and could not perceive that there were marks of any 

 great transport of pebbles by the Arve, though it held, as is 

 common to most alpine glacier waters, a considerable quantity 

 of sand in mechanical suspension. The banks of both the Arve 

 and the Rhone afford abundance of rounded alpine pebbles, 

 and it would be no easy matter to 6ay, how much of the peb- 

 ble bed of the Arve was derived directly from the Alps, and how 

 much from its banks near Geneva. But supposing the Arve 

 did bring down abundantly pebbles from the Alps, their progress 



