CHEMICAL CLASSIFICATION OF SOILS. 243 



nized, their general mode of improvement, and their natural 

 adaptation to the various crops cultivated by the farmer, may 

 not be inappropriate. 



1. Siliceous soils. In the silicious soils, from whatever 

 class of rocks they are derived, silex or silica is the predomi- 

 nant earth. These soils originate generally either from the 

 disintegration of silicious rocks, from glacial action, or from 

 streams and rivers which pass over sandstone rocks. 



Properties. Siliceous soils are either gravelly or sandy, 

 or a mixture of both ; they are always of a loose texture, per- 

 mitting the water to pass easily through them. 



They absorb but little moisture from the atmosphere, and 

 part with it readily, on the application of heat. Hence in 

 seasons of drought, they become mealy, and their vegetation 

 is scorched and dried up. As sandy and gravelly soils do 

 not generally combine with manure or vegetable matter, 

 which is introduced into them, they easily part with it, and 

 hence they have been denominated hungry soils. If the sub- 

 soil is gravelly or sandy, they are subject to leaching, and the 

 vegetable matter passes through them almost as fast as it is 

 rendered soluble in water. 



Sandy and gravelly soils are generally warm and quick, 

 and from their want of adhesiveness, easily tilled. They dif- 

 fer from absolute barrenness to a high degree of fertility. 

 When wholly without cohesion in their parts, they are entirely 

 barren, and can only be made fertile by the admixture of 

 other substances. This is the case often with the coarser 

 gravels. When fine or sandy, and mixed with aluminous 

 earth, or magnesia and a suitable proportion of organic mat- 

 ter, they become very fertile, especially if they have a tena- 

 cious sub-soil. 



Mode of improvement. A sandy or gravelly soil may be 

 improved by mixing clay or peat compost with them, so as to 

 increase their adhesiveness, their power of absorbing water and 



