168 Mr. De la Beche's Notes on the Formation 



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 Rhine 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 rivei'- 

 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-bedded 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 say, 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 would seem to be checked at the gulf known as the 

 Perte du Rhone. 



As a general fact, it may be fairly stated that rivers, where 

 their courses are short and rapid, bear down pebbles into the 

 seas near them, as is the case with the torrents in the Mari- 

 time Alps ; but that when their courses are long, and changed 

 from rapid to slow, they deposit the pebbles where the force 

 of the stream diminishes, and finally transport mere sand or 

 mud to their mouths, as is the case with the Rhine, Rhone, 

 Po, Danube, &c. 



IV. Dh' 



