EOCKS are broken down into soil material through the processes 

 of weathering (Figs. 3, 4, and 5). These may be divided into (1) 

 physical agencies that break the rock into smaller pieces without 

 affecting it chemically, and (2) chemical agencies that change the 

 composition of the minerals forming the rock and in so doing exert 



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 I 



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. 



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Fio. 3. Irregular weathering of rock due to Joints and stratification. Note talus at base. 

 (Chamberlain tind Salisbury, Courtesy Henry Holt A Co.) 



a marked influence upon its physical character. The work of the 

 physical agencies is disintegration, while that of the chemical agen- 

 cies is decomposition. Each is accompanied and aided by the other 

 in its work and the changes tend to produce more stable forms under 

 existing conditions. As an illustration, feldspars are not very stable 

 minerals under ordinary conditions, and hence break down into 

 substances that are more stable. The chemical changes produce 

 hydrous aluminum silicate, carbonates and free silica which are 

 much more stable than the feldspar from which they are derived. 

 Physically, the clay is much more stable than the original mineral 



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