106 GEOLOGY 



the loose material are coarse, like bowlders, their surfaces are 

 affected like those of larger bodies of rock. 



In view of these considerations, the breaking of rock by changes 

 of temperature should be greatest on the bare slopes of isolated 

 elevations of crystalline rock, and where the atmosphere is rela- 

 tively free from moisture. All these conditions are not often 

 found in one place, but the disrupting effects of changing temper- 

 atures are best seen where several of them are associated. 



Fig. 66. Serrate mountain peaks with abundant talus. Cascade Mountains, 



Washington. 



The importance of this method of rock-breaking is rarely appre- 

 ciated except by those familiar with high and dry regions. 

 Mountain climbers know that most high peaks are covered with 

 broken rock to such an extent as to make their ascent dangerous 

 to the uninitiated. High serrate peaks, especially of crystalline 

 rock, are, as a rule, literally crumbling to pieces (Fig. 65). The 

 piles of talus which lie at the bases of steep mountain slopes (Fig. 66) 

 are often hundreds of feet in height, and their materials are often 

 in large part the result of the process here under discussion. Masses 



