ORIGIN AND FORMATION OF SOILS. 131 
salts of ammonia and of soda, however, it is taken up in 
considerable quantity. Solution of nitrate of ammonia 
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, metallic 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- 
peras, 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. 
Weathering.—The conjoined influence of water, car- 
© This statement rests on the authority of Professor Henry Wurtz, of New York. 
