Studies in the Sierra 



417 



water-torrents. They are frequently seen descending the sum- 

 mit peaks, flowing in regular channels, the surfaces of which 

 they erode by striking off large chips and blocks, as well as by 

 wearing off sand and dust. 



A considerable amount of grinding also goes on in the body 

 of the avalanche itself, reducing the size of the masses, and 

 preparing them for the action of other agents. Some ava- 

 lanches hurl their detritus directly into the beds of streams, 

 thus bringing it under the influence of running water, by 

 which a portion of it is carried into the ocean. 



The range of rock avalanches, however produced, is re- 

 stricted within comparatively narrow bounds. The shattered 

 peaks are constant fountains, but the more powerful moun- 

 tain-shaking avalanches are confined to the edges of deep 

 canons in a zone twelve or fifteen miles wide, and gradually 

 merge into land-slips along their lower limits. 



Large rock avalanches pour freely through the air from a 

 height of hundreds or thousands of feet, and on striking the 

 bottom of the valley are dashed into a kind of coarse stone 

 foam. Or, they make the descent in several leaps, or rumble 

 over jagged inclines in the form of cascades. But in any case 

 they constitute currents of loose-flowing fragments. Land- 

 slips, on the contrary, slip in one mass, and, unless sheer cHffs 

 lie in their paths, may come to rest right-side up and undivided. 

 There is also a marked difference in their geographical distri- 

 bution, land-slips being restricted to deeply eroded banks and 

 hillsides of the lower half of the range, beginning just where 

 rock avalanches cease. Again, the material of land-slips is 

 chiefly fine soil and decomposing boulders, while that of rock 



avalanches is mostly of un- 

 weathered angular blocks. 



Let Figure i represent a 

 section across a valley in 

 which moraine matter, A, 

 is deposited upon the in- 

 cHned bed - rock, B B B. 

 Now, strong young mo- 

 raine material deposited in 

 this way, in a kind of rude 



Fig. I. 



