312 THE SOIL. 



of a higher order of plants, which were in time succeeded 

 by others, and those by others, until, finally, the changes 

 effected in the soil by the action of the chemical forces, and 

 by the deposit of vegetable matter, have enabled it to pro- 

 duce the vegetation required for the uses of man. 



Classification of soils. 



Some soils were formed mainly of the rocks on which they 

 now lie as those of the granite region of New England 

 and these take their names from these rocks, as granitic soil, 

 limestone soil, sandstone soil, &c. 



Others have been formed by the deposit, by means of 

 great floods, or the gradual silting of rivers. The latter 

 of these (as the flat lands of the Mississippi Yalley) are 

 called alluvial soils ; and the former (comprising those soils 

 of varied composition in which occur clay, gravel, boulders, 

 &c.) are called diluvial soils. 



Another classification, which is much more definite, is the 

 following : 



1. PURE CLAY consists of about 60 per cent, of silica and 

 40 per cent, of alumina and oxide of iron, usually chemi- 

 cally combined. 



2. STRONGEST CLAY SOIL consists of pure clay, mixed 

 with 5 to 15 per cent, of silicious sand. 



3. CLAY LOAM consists of pure clay, mixed with 15 to 30 

 per cent, of fine sand. 



4. LOAMY SOIL deposits from 30 to 60 per cent, of sand. 



