THE SOIL 



5 



conformable with the face of the land over which it was 

 riding, the sharper outstanding points were cut away and 

 the deeper river canons filled in. Desolate and rugged rocky 

 wastes were thrown down and spread over with rich soil." 



The joint action of air, moisture, and frost was still 

 another agent of soil making. This action is called weath- 

 ering. Whenever you have noticed the outside rocks of a 

 spring house, you have noticed that tiny bits are crumbling 



FIG. 2. GROUND ROCK AT END OF A GLACIER 



from the face of the rocks, and adding little by little to the 

 soil. This is a slow way of making additions to the soil. It 

 is estimated that it would take 728,000 years to wear away 

 limestone rock to a depth of thirty-nine inches. But when 

 you recall the countless years through which the weather 

 has striven against the rocks, you can readily understand that 

 its never-wearying activity has added immensely to the soil. 

 In the rock soil formed in these various ways, and 

 indeed on the rocks themselves, tiny plants that live on 



