224 R. PUMPELLY — SECULAR ROCK-DISINTEGRATION. 



which is nevertheless of importance. In the range of ray observation it is 

 the most important of conglomerate-producing processes. In the arid in- 

 terior of our continent a large district is set with mountain ranges due to 

 uplift along fault lines, while the intervening valleys are due to down- 

 throw ; and there is a continual transfer of material from the mountains to 

 the valleys. This material is detached from the mountain masses by pro- 

 cesses chiefly mechanical, and involving little rock decay. In the valleys it 

 is assorted by running water, and the coarser part is accumulated as gravel 

 in alluvial cones along the mountain bases, where it needs only cementation 

 to become conglomerajte. 



