CHAPTER II 



GEOLOGICAL FORMATION AND CLASSIFICATION OF SOILS 



47. Geological Study of Soils. — The geological 

 study of a soil concerns itself with the past history of 

 that soil, the material out of which it has been pro- 

 duced, together with the agencies which have taken a 

 part in its formation and distribution. Geologically, 

 soils are classified according to the period in the earth's 

 history when formed, and also according to the agen- 

 cies which have distributed them. Agricultural geology 

 is of itself a separate branch of agricultural science. 

 The principles of soil formation and soil distribution 

 should be understood, because thev have such an im- 

 portant bearing upon soil fertility. In this work, only 

 a few of the more important topics of agricultural 

 geology are treated in a general way. 



48. Formation of Soils. — Many geologists believe 

 the surface of the earth to have been at one time solid 

 rock. It is now almost universally held that soils 

 have been formed from rock by various agents ; as, (1) 

 heat and cold, (2) water, (3) gases, (4) micro-organisms 

 and vegetable life. The disintegration of rock is 

 usually effected by the combined action of these vari- 

 ous agents. The process of soil formation is a slow 

 one and the various agents have been at work for an 

 almost indefinite period. 



