50 MIXED EARTHS. 



case the ingredients of the earth which result from 

 them will maintain their original character ; but 

 if these are decomposed, their relations may be 

 changed. Soils do not always contain those sub- 

 stances alone which are found in the rocks of their 

 immediate neighborhood, for by the action of dilu- 

 vial currents the loose matter of different rocks is 

 frequently intermingled, while sometimes the foreign 

 entirely covers the native material. Alluvial land 

 is formed by the deposits of streams, and has in its 

 composition the substances inherent in the various 

 rocks through which it has passed. Peaty soils are 

 mostly the result of the decay of vegetable matter, 

 and have but little of the components found in the 

 original rocks. The fertility of soils depends in a 

 great measure upon its admixture and subdivision. 

 An earth, if it may be properly so called, composed 

 entirely of pebbles, would not support vegetation. 

 Coarse sand may be knit together by the roots of a 

 few plants, but in a fine state of division and de- 

 composition it can nourish almost any plant. The 

 benefit resulting from the mixture of soils is shown 

 by the fact that those lands formed from the decom- 

 position of conglomerate rock are very fertile. Dr. 

 Hitchcock, in his " Geological Survey of Massachu- 

 setts," says that soils consist, " first, of their earthy 

 and metallic ingredients, which are mostly silicates ; 

 secondly, the acids, alkalies, and salts which existed 

 originally in them, or are introduced by cultivation ; 



