IMPORTANT ELEMENTS AND MINERALS AT 
augite (Fig. 8). These occur in many of the lavas 
and granitic rocks, and when so found, are commonly 
dark colored; indeed, they are often jet-black grains. 
Chemically they are complex silicates, in which iron 
is often present. They decay with ease, frequently 
forming reddish or yellowish stains of iron rust, as the 
iron molecules combine to form iron oxide. The de- 
cayed products of these minerals are common in the 
clays, and much of the iron coloring-matter of the soil 
is formed by their disintegration. 
Ores of Iron. — Besides the carbonate of iron (side- 
rite, p. 44), there are several oxides of iron and the 
sulphide (iron pyrites) which are common in the 
earth. . | 
Wherever any of the iron-bearing minerals (such as 
hornblende) decay, an oxide of iron is one of the results. 
The abundance of these oxides is attested by the red 
and yellow colors of soils and rocks. Many springs 
bring a form of iron oxide to the surface, where it is 
deposited from solution as a soft, yellow, iron rust. 
Iron ores also occur in nearly all the rocks, and some- 
times in distinct beds, or veins, which are worked as a 
source of iron. 
Of the iron ores the oxides are the most common. 
Magnetite (Fe,0,), a black mineral, usually crystallme 
and heavy, is present in many of the volcanic rocks, 
as well as in ore beds and elsewhere. When it rusts 
