of extensive Conglomerate and Gravel Deposits. 167 



From these sources the river obtains the materials for its 

 gravel. The greater the velocity of the water, the sooner vi\\i 

 the angular fragments be ground by attrition into pebbles. 

 Rivers are most rapid in high mountain ranges, having to find 

 their way from a high to a much lower level in comparatively 

 short distances. Now as the decomposition and the fall of 

 rocks is greatest amid high mountains, and as the rivers are 

 most rapid in the same situations, the greatest quantity of river 

 gravel is there produced. 



In low situations, where rivers lose their rapidity, gravels 

 are rarely formed, but sands or mud are common. In times 

 of flood, gravels formed in their beds, in the high lands, are 

 brought down into their beds in the plains ; but even these do 

 not appear to travel far. It is asserted, but has not been proved, 

 that rivers carry their gravels to considerable distances ; but 

 I cannot avoid suspecting that pebbles derived from great 

 gravel plains, or from cliffs of loosely aggregated conglome- 

 rate, such as the Nagelfluhe of Switzerland, cut away by the 

 rivers, and thus carried into their beds, have been sometimes 

 mistaken for gravels transported from great distances by the 

 rivers. There is no want of gravel, composed of pebbles 

 from the high Alps, in the bed of the Rhone, where that river 

 quits the lake of Geneva, or in the bed of the Ticino, where it 

 quits the Lago Maggiore; and I presume no person would 

 imagine that the gravels have been brought down by either 

 river from the Alps, as all such pebbles must have been quietly 

 deposited in the bottoms of the respective lakes. In both in- 

 stances the gravels have been derived from conglomerates 

 formed by more general causes, cut through by the rivers after 

 they have quitted the lakes. Innumerable other instances 

 might be produced. The same observation applies to rivers 

 cutting great gravel plains, where they obtain pebbles, de- 

 rived originally from distant rocks, from their banks, but to 

 the transport of 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 gravels and sand, so that such rivers con- 

 stantly 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 

 pebbly or sandy nature of the detritus. 



In cases where rivers discliarge 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 ob- 

 vious that the stratification resulting from these causes must 



liave 



