244 GEOLOGY AND CHEMISTRY OF SOILS. 



of retaining manure. The stones should not be all removed, 

 as''they aid in retaining heat and moisture. 



These soils are naturally better fitted for rye, barley and 

 Indian corn than for wheat; but from their porous charac- 

 ter, they are particularly fitted for those crops which are cul- 

 tivated for the tubers of their roots, such as potatoes, turnips, 

 beets, etc. For the tuberous roots, however, they must pos- 

 sess somewhat the characteristics of loam. They are also 

 well adapted to timothy, clover and red-top. 



2. Aluminous or clai/ soils are those in which clay in some 

 of its varieties predominates. They vary in composition. 

 Silica constitutes more than one half of their substance. 

 These soils originate generally from the tertiary beds of clay, 

 but are often formed by the disintegration of argillaceous 

 rock, and by the agency of rivers, especially near their 

 ttiouths, where the tides and waves throw back aluminous 

 matter, which is either contained in the water in a finely di- 

 vided state, or worn ofi" from the cliffs of clay near the shores. 



Aluminous soils are stiff and heavy, generally destitute 

 of stones and very tenacious of water ; of which they absorb 

 large quantities, and yield it up with difficulty. When wet, 

 they have the appearance of mortar, and adhere to the plough, 

 when it passes through them. When dry, they break up in- 

 to lumps when ploughed, or contract upon the surface, leav- 

 ing small fissures crossing each other in various directions ; 

 hence, they are subject to the extremes of wet and drought. 

 The clay soils differ in texture according to the quantity of 

 other earths. A large quantity of silirrous earth renders them 

 less cohesive ; and if vegetable and animal substances are 

 added, they often become similar to loams. They are natu- 

 rally cold, especially when they are light colored, in which 

 case they are not easily heated by the sun's rays. They 

 are capable of uniting chemically with vegetable acids 

 and earths, a circumstance of great practical importance, as 

 it renders them very retentive of manures, so that in this re- 



