PART THIRD. 



OP SOILS AND SUBSOILS. 



I. THE CLASSIFICATION OF SOILS. 



It has been already shown that soils were composed of 

 various materials, of which the principal were silica, alu- 

 mina, and lime. 



Silica, the most abundant of the incombustible materials 

 of soils, exists in the form of sand or gravel, more or less 

 pure ; sometimes, also, as flint stones or slaty fragments. 



Alumina is the basis of clay, that which is so called 

 being a compound of about four parts of alumina, and six 

 of sand. When water is put upon a portion of earth and 

 agitated, it is clay which muddies the water, and is, for 

 some time, held in solution by it, while the sand subsides 

 to the bottom. 



Lime sometimes is visibly present in soils, existing as a 

 fine earth, or white gravel, or shells ; or it may exist in 

 considerable quantity, though not detected by the eye ; 

 but it is easily perceived by the effervescence it makes, if 

 some vitriol or dilute muriatic acid be poured upon a por- 

 tion of the soil. 



Soils are characterized from these their principal con- 

 stituents, thus : If silica prevail, they are siliceous ; if 

 clay prevail, they are argillaceous or clayey; if lime pre- 

 vail, they are calcareous ; if vegetable matter prevail, they 

 are peaty. 



In valuation practice, these distinctions should be so clear 

 and apparent, that the eye alone might be able to dis- 

 criminate. But in our very best soils this is not always 

 possible, and the term loam has been used to designate 

 soils in which it is difficult to say which mineral charac- 

 teristic predominates: the admixture of sand and clay 



