CHAPTER IIL 

 THE SOIL 



Soil is the layer of disintegrated rock, mixed with the 

 remains of plants (and animals), which covers a large portion 

 of the land. It also contains living organisms of various 

 kinds, and variable quantities of water and air. The depth of^ 

 soil varies greatly, being usually from six to twelve inches, but 

 sometimes is as great as several feet. Beneath it is the sub- 

 soil, which differs from it in being less oxidised and less rich 

 in organic matter. In many cases the line of separation 

 between the soil and subsoil is very clearly marked, often by 

 a difference in colour, the subsoil being generally the lighter 

 coloured. 



Soils, consisting so largely of disintegrated rock fragments, 

 naturally depend for their chemical nature mainly upon the 

 character of the rocks beneath. Rocks are often classified by 

 geologists, according to their origin, into three classes : 



(1) Igneous rocks — i.e., those which have resulted from the 

 cooling of intensely heated fluid matter. 



(2) Sedimentary rocks — i.e., those produced by the settling 

 out of particles suspended (or in some few cases dissolved) in 

 water. 



(3) Metamorphic rocks — i.e., those which have been essentially 

 altered in character since their deposition. 



Rocks are rarely homogeneous — i.e., alike in all parts — but 

 are generally made up of several components mingled together, 

 often lying side by side in separate crystals. These com- 

 ponents, which have a more or less definite molecular struc- 



83 O 



