SOIL-FORMING PROCESSES 23 



the effect of animals, will be almost the sole agents. 

 In humid regions, however, the forces are more varied 

 and practically the full quota will be at work. Chemical 

 decay will accompany the disintegration, and the resultant 

 product will be finer and more minutely divided. The 

 separate minerals will show also the change of color and 

 loss of luster due to the decomposition of some of their 

 essential elements. The same rocks, then, will behave 

 differently under different climatic conditions. A granite, 

 for instance, is a very insoluble rock as compared with a 

 limestone, and in a humid region where chemical agencies 

 are dominant it would be markedly more resistant. If, 

 however, these two rocks are placed under arid conditions 

 where the physical forces are potent, particularly as re- 

 gards change of temperature, the comparison is different. 

 The limestone, being homogeneous, is not affected by 

 atmospheric changes; but the stresses set up in granite 

 due to differential contraction and expansion must ulti- 

 mately reduce it to fragments. 



As weathering is confined to the very surface of the 

 earth, the exposure or position of a rock will determine 

 the kind and the rate of decay. If the rock is very deep 

 below the surface, only hydration may occur ; while if 

 it exists as an exposed ledge, the full force of the weather- 

 ing agents will be sustained. If the debris of the decayed 

 rocks is not removed, this serves as a blanket for the 

 protection of the rocks below. The tran sportive powers 

 of weathering are important in maintaining a clean sur- 

 face for action. 



The texture of the rock is also a factor. Other things 

 being equal, a coarsely crystalline rock will disintegrate 

 and decompose more rapidly than one of finer grain. 

 The coarser the gram, the larger the amount of interstitial 



