SOIL-FORMATION, HEAT AND COLD 



19 



power of water for many substances, and renders it a 

 more destructive agent generally. This action can not 

 be discussed separately, but must be kept in mind in 

 the consideration of those other agencies of decompo- 



Fig. 8. Two types of rock disintegration. The forms reflect the different 

 hardness and composition of the rocks 



sition. Especially important are alterations of tempera- 

 ture, by which compounds whose rates of solution are 

 differently affected by temperature may be successively 

 acted upon. 



Heat acts mechanically in two ways to break up 

 rocks: (1) Through expanison and contraction due to 



