CHAPTER IV. 



ORIGIN OF SOILS. 



The soil is the solid outer covering of the earth, which, by 

 being disintegrated into particles, and provided with organic 

 matter and nitrogen, has become capable of sustaining the growth 

 of cultivated plants. 



The chemical composition and physical character of the soil 

 are closely related to the material of its origin and its mode of 

 formation. 



Geology teaches that ages ago the surface of the earth was a 

 mass of rock, formed by the solidification of molten material. 

 Since then mountain chains have been elevated and razed, and 

 succeeded by new mountains which have been likewise eroded and 

 succeeded by others. Lakes have been filled up or drained, 

 rivers have eroded channels and deposited sediment. Land now 

 dry has been deposited under water. Great changes of climate 

 have occurred. During one period the vegetation was tropical in 

 character. At another time, the climate was colder, and 

 immense sheets of ice covered the northern part of 

 North America. Races of plants and animals have appeared 

 and disappeared. The agencies of the air, and water, 

 have broken up rocks, carried the particles away, and laid 

 them down, perhaps to be formed into rock, elevated into land, 

 and to go through another series of decomposition and rock 

 formation. This process has occurred over and over again. In 

 this way a variety of rocks and a great many soils have been 

 formed. 



Soils Formed from Rocks. Soils are formed by air, water, heat, 

 cold, and plant life, which are termed weathering agencies, acting 

 upon rocks. The term rock in the geological sense, means any 

 layer of the earth's crust, whether hard or soft. Thus loose 

 sand and clay are rocks to the geologist as truly as sandstone 

 or granite. The soil chemist, however, does not consider un- 

 consolidated surface deposits as rocks. A deposit formed by 

 wave action, and afterwards elevated so as to become a soil, is 

 not considered as a rock, but as a transported soil. 



