42 



SOILS AND MANUEES 



posed minerals of which the bulk of the soil is composed. 

 The natural fertility of soils, so far as the supply of plant 

 food is concerned, depends .upon the rapidity with which 

 these changes take place, and it is the object of the various 

 operations of cultivation, ploughing, fallowing, etc., to 

 promote them as well as to improve the physical properties 

 of the soil. 



Classification of Soils. For agricultural purposes soils 

 are most conveniently classified according to the nature of 

 the predominant constituent. Names such as sandy soil, 

 clay soil, calcareous soil, etc., are self-explanatory. Those 

 of intermediate texture, between sand and clay, are called 

 loamy soils. They are sometimes further qualified as sandy 

 loams or clay loams. In a similar manner we may have 

 calcareous sands, clays or loams, humous clays, and so on. 



The following tabular ^statement shows the principal 

 formations of the stratified rocks with their more impor- 

 tant divisions and sub -divisions arranged in geological 

 order and the nature of the materials 1 of which the different 

 strata are composed : 



Geikie. 



