78 



Sierra Club Bulletin 



which a portion of the avalanche - stream, F, falls, and is 

 spouted to the left of its original course. Because the larger 

 boulders composing the spouted portion of the current move 

 faster, their momentum carries them farther toward H, giving 

 rise to the talus E, while the finer material is deposited at D. 

 Again, the blocks sufficiently large to bound out beyond the de- 

 flecting spout form the rough talus C, while the smallest frag- 

 ments of all — namely, the fine dust derived from chafing — float 

 out far beyond, and settle in thin films silently as dew. 



In portions of canon walls where diagonal cleavage is de- 

 veloped, inclines such as A B (Fig. 5) are common. If two 

 boulders in falling from the heights above should strike glan- 

 cingly at A, the greater mass or more favorable form of boul- 

 der B might cause it to bound sufficiently far to reach the sec- 

 ond incline, which would carry it toward D ; while the smaller 

 boulder, C, falling short, might fall under the guidance of a 



earthquake sufficiently severe to produce large avalanches oc- 

 curred at least three centuries ago, and no other of equal power 

 has occurred since. By this earthquake alone, thousands of 

 acres of noble soil-beds were suddenly and simultaneously de- 

 posited throughout all the deep cafions of the range. Though 



third incline, and be shed off 

 toward E, the two boulders 

 finally coming to rest a hun- 

 dred yards or more apart. 

 By these means the most del- 

 icate decompositions of stone- 

 torrents are effected, the va- 

 rious resulting soils being 

 delivered at different shoots 

 and spouts, like the bran, 

 shorts, and fine flour of a 

 grist-mill. The ages of the 

 oldest trees growing upon 

 these soils furnish data by 

 which some approximation 

 to the time of their forma- 

 tion may be made. 



Fig. 5. 



The first post-glacial 



