SOIL 187 



tary soil, or from the rock bed at a distance and 

 brought down by running streams or other agents, and 

 called drift or transported soil. 



To produce soil many agents in nature are at work, 

 the chief of which are water, wind, plants, and animals. 



The wearing off of the rock by water and air is called 

 weathering. The oxygen of the air combines readily 

 with other elements in the rocks to form new com- 

 pounds, which break away from the surface. This 

 action is very similar to the rusting of iron. All are 

 familiar with the small flakes of rust thus produced. 

 Similar flakes are formed 011 the surface of the rocks. 

 This is a slow process, to be sure, but in the course of 

 the ages that this process has been at work, great 

 changes have been effected. The air, as wind, sweeps 

 up the smaller particles of sand and earth and whips 

 them together and against the surface of projecting 

 rock, thereby grinding the smaller particles finer and 

 also wearing off the surface of the larger, harder 

 rocks. 



Water has great power of dissolution and dissolves 

 the more soluble compounds out of the rocks, thereby 

 weakening and eventually breaking them up into 

 smaller pieces. Pure water dissolves but slowly, but all 

 soil water usually has taken on carbon dioxide as it 

 came down through the air, or acids from the soil 01 

 roots of plants, which make it a most powerful disin 

 tegrating agent. 



Wherever water seeps through the soil into the. 



