242 GEOLOGY AND CHEMISTRY OF SOILS. 



are subjected to heat, there is a partial decomposition, and the 

 alkalies are ready to act upon the crop. 



The above enumeration contains the most important varie- 

 ties of soil as derived from the rocks. They will be readily 

 recognized by the practical geologist, and it is hoped that the 

 farmer may derive some idea of their character and proper- 

 ties. 



This geological classification, which is based chiefly on that 

 proposed by Prof Hitchcock, makes us acquainted with the 

 soils as they stand related to the rocks. This is always use- 

 ful and interesting, especially to the scientific agriculturist ; 

 but it is not so practical as the chemical mode of classifica- 

 tion. It is to be hoped that the reader will, at least, examine 

 this method and compare it with that which follows, that he 

 may, as already remarked, obtain from both what could not 

 be derived from either by itself. 



II. Chemical Classijication and Dcsci-iption of Soils. 



We regard the geological classification of soils, as pre- 

 senting the most enlarged view of the subject; but a more 

 simple and practical method is to arrange soils according to 

 their prevailing earths. These earths are silica, alumina, 

 lime and magnesia. Hence those soils in which silex niostly 

 predominates, are called siliceous or sandy soils. Those in 

 which clay is in the greater proportion, are called aluminous 

 or clayey soils. Those in which the carbonate of lime is the 

 chief ingredient, calcareous soils ; and when the lime is chalk, 

 chcdky soils. Magnesia, also, sometimes exists in sufficient 

 quantities to give a name to the soil in which it is found. 

 There is another class called loamy, which answers nearly to 

 the more fertile alluvions, but results from a long course of 

 cultivation, when large quantities of animal and vegetable 

 matters are employed. The 2)eaty soils are also sufiiciently 

 definite to form a distinct class. A short description of these 

 soils, including the characters by which they may be recog- 



