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PHYSICAL GEOLOGY 



Comparison of Effects of Chemical and Mechanical Weathering. 

 — Chemical decomposition of rocks is slow and long continued as 

 compared with mechanical disintegration, which is a rapid process. 

 By the former the rocks are broken up into fine particles, and by the 

 latter into larger and smaller fragments. Chemical action is not only 

 long continued, but is also more universal than mechanical action, 

 being important under all climates, except in desert regions and on 

 mountains where mechanical disintegration is so rapid that sufficient 

 time is not permitted for conspicuous chemical action. Chemical 

 decomposition tends to smooth surfaces, while mechanical disinte- 

 gration tends to roughen them. Where the mechanical predominates, 

 the slopes are stronger and tend to form cliffs. 



Results of Weathering 



Some of the most conspicuous features of scenery are produced by 

 weathering. These features are seldom due to a single agent, but 

 more often to two or more acting in conjunction. 



Fig. 13. — Pinnacle Peak, Canadian Rockies. The ragged outlines are due 

 largely to frost work. (Photo. M. H. Smith.) 



The rough and jagged peaks so characteristic of high mountains 

 have been sculptured largely (1) by frost and (2) by changes in daily 

 temperature (p. 31) (Fig. 13). The debris derived from such peaks 



