ORIGIN AND FORMATION OF SOILS. 131 



salts of ammoTiia and of soda, however, it is taken up in 

 considerable quantity. Solution of nitrate of aiiimonia 

 dissolves lime and magnesia and their carbonates with 

 great ease. In general, up to a certain limit, a saline so- 

 lution acquires increased solvent power by increase in the 

 amount and number of dissolved matters. This import- 

 ant fact is one to which we shall recur at another time. 



Action of Oxygen. — This element, the great mover of 

 chemical changes, which is present so largely in the at- 

 mosphere, has a strong tendency to unite with certain 

 bodies which are almost universally distributed in the 

 rocks. On turning to the analyses of minerals, p. 110, we 

 notice in nearly every instance a quantity of protoxide of 

 iron, or protoxide of manganese. The green, dark gray, 

 or black minerals, as the micas, amphibole, pyroxene, 

 chlorite, talc, and serpentine, invariably contain these prot- 

 oxides in notable proportion. In the feldspars they exist, 

 indeed, in very minute quantity, but are almost never en- 

 tirely wanting. Sulphide of iron (iron pyrites), in many 

 of its forms, is also disposed to oxidize its sulphur to sul- 

 phuric acid, its iron to sesquioxide, and this mineral is 

 widely distributed as an admixture in many rocks. In 

 trap or basaltic rocks, as at Bergen Hill, tnetalHe iron is 

 said to occur in minute proportion,* and in a state of fine 

 division. The oxidation of these substances materially 

 hastens the disintegration of the rocks containing them, 

 since the higher oxides of iron and of manganese occupy 

 more space than the metals or lower oxides. This fact is 

 well illustrated by the sulphate of protoxide of iron (cop- 

 jieras, or green-vitriol), which, on long keeping, exposed to 

 the air, is converted from transparent, glassy, green crys- 

 tals to a bulky, brown, opaque powder of sulphate of 

 sesquioxide of iron. 



WeatherinjHf. — The conjoined influence of water, car- 



This statement rests on the authority of Professor Henry Wurtz, of New York. 



