THE SOIL 



25 



of Oklahoma, Arizona, and New Mexico, nurserymen frequently 

 place a stick of dynamite in this hard pan layer and blast open- 

 ings in it where young trees are to be planted. Unless this is 

 done, the trees grow very slowly and frequently die. 



Origin of Soils. All soils have been derived directly or indi- 

 rectly by the disintegration of rocks, generally through the pro- 

 longed action of heat, cold, air, water, frost, and ice. In some 

 cases, however, this was brought about by the action of low but 



The Mississippi flowing through an alluvial plain. 



tiny forms of vegetable and animal life. These rocks may be 

 roughly put into two classes: (1) igneous rocks; (2) aqueous 

 rocks. 



Igneous rocks are those which have been produced by the action 

 of fire. Granite and trap are the two best examples. 



Aqueous Rocks. These are produced by the action of water. 

 As examples of aqueous rocks, limestone and red sandstone may 

 be mentioned. 



Other Classes of Soils. With reference to their origin, soils 

 may be divided into : (1) sedentary soils; (2) transported soils, con- 

 sisting of (a) alluvial soils, (b) drift soils, (c) wind blown soils. 



