50 OUTLINES OF FORESTRY. 



V. THE FORMATION OF SOIL. 



THE soil, in which the plant grows and which 

 forms its cradle, is composed chiefly of mineral 

 matters derived from the originally crystalline 

 rocks which were formed by the gradual cooling 

 of the earth's crust. The soil, however, also con- 

 tains a small quantity of vegetable mould or 

 humus, obtained by the growth and subsequent 

 decay of successive generations of plants. 



Before soil can be formed, the hard crystalline 

 rocks must be broken up, or, as it is technically 

 called, disintegrated. 



This disintegration is effected to some slight 

 degree by the roots of plants, but it is mainly 

 brought about by the action, in one way or an- 

 other, of water. 



Soils are divided by Gray into gravelly, sandy, 

 clayey, calcareous, loamy, and peaty. 



Gravelly soils are such as have coarse pebbles 

 or fragments of quartz, lime, or feldspar spread 

 through more finely divided mineral matter. 



Sandy soils are usually formed of fine particles 



