i.] CLASSIFICATION OF SOILS 31 



is indeed based upon climate, since the rocks from 

 which all soils are formed possess very similar char- 

 acteristics, and may be mineralogically identical in very 

 distant places. Such a classification marks off the 

 earth's surface into zones of soils, which correspond 

 with the great natural zones of vegetation, in agreement 

 with the general fact which we may find illustrated 

 within our narrower range of conditions, that vegetation 

 tnd crops vary in response to the climate rather than to 

 the chemical or physical character of the soils on which 

 they grow. 



In such a "genetic" classification of soils Sibirtzef 

 distinguishes the following great groups or types : 



1. Laterite soils developed in hot, tropical climates 

 with a high rainfall, by the very rapid weathering 

 Df the more primitive and volcanic rocks under the 

 prevailing conditions of heat and moisture. Laterite 

 soils, which characteristically are bright red in colour, 

 ire distinguished by a very high percentage of hydrated 

 Dxides of iron and alumina. Laterites are found in 

 India, Ceylon, and the other East Indian islands, 

 China, Central Africa and America. 



2. Wind-blown loess soils formed of fine particles 

 Df rock disintegrated by heat and cold and transported 

 by wind. As a rule these soils are rich in carbonates 

 :>f lime and magnesia, but are poor in humus. They 

 are often deep and characteristically uniform and 

 stoneless. They are chiefly developed in Central Asia, 

 to some extent in Northern Africa and America. 



3. Semi-arid soils of the dry steppe formed in 

 :limates possessing violent contrasts of heat and cold 

 and a small rainfall (10 to 15 inches). In these soils 

 :he soluble salts arising from the decomposition of the 

 rock have been imperfectly washed out and usually 

 accumulate at a small distance below the surface. 



