30 THE PRINCIPLES OF SOIL MANAGEMENT 



and brick sidewalks. In mountainous regions wnere 

 vegetation has gained a foothold in the crevices, the 

 tops serve the wind as a lever to pry rocks apart. The 

 overturning of trees is a familiar example of the process. 

 Animal life also has a part in the mechanical breaking 

 down of rocks. Burrowing animals are most active. 

 The gopher, the prairie dog, the badger, the rabbit, 

 moles, etc., all burrow in the ground and, in the aggre- 

 gate, move large masses of material. Cray-fish and 

 earth-worms are even more widespread, and the latter 

 by their large numbers have a capacity which is likely 

 to be underrated because it is largely out of sight. 

 Ants are another very active form of animal life in 

 effecting soil formation. They burrow into crevices of 

 rocks and into soil formations, and deposit the material 

 from the passages at the surface mixed with their acid 

 saliva. Like the earth-worms, they handle immense 

 amounts of material. 



V. GEOLOGICAL CLASSIFICATION AND CHEMICAL 

 COMPOSITION OF SOILS 



All soil material may be divided into two groups, 

 depending upon the extent to which it has been moved 

 in the process of formation. Those materials which have 

 not been subject to any appreciable transportation 

 are termed (a) Sedentary. Those which have been 

 carried to their present position that is, have been 

 appreciably moved are termed (6) Transported. There 

 are several agencies of transportation, such as gravity, 

 water, ice and wind. These give rise to subdivisions. 



