28 ELEMENTARY. GEOLOGY 
In one or another of these forms, in company with 
water, silicon finds its way into nearly all the rocks 
of the earth. In some, like the sandstones, it con- 
stitutes a large part of the rock; while in others, such 
as coal, it is present only in minute quantities — the 
ash of the coal being in part a compound of silicon. 
Aluminum (Al). — This is a metallic element which 
was a chemical curiosity forty years ago, but is now 
in somewhat common use as a light, silver-white 
metal. Like silicon, it combines with oxygen, forming 
an oxide, alumina (Al,0;), which we know as the 
gem sapphire. Aluminum is most common in the 
earth in combination with both oxygen and silicon, 
forming a great group of minerals, the silicates of 
alumina (see Feldspar, p. 40). Nearly 8% of the crust 
is aluminum. 
These three common elements, oxygen, silicon, and 
aluminum, constitute the bulk of our clay and soil, and 
indeed of the rocks themselves. More than four-fifths 
of the crust is made of these substances; and when 
combined with other elements, they form a great 
variety of minerals, numbering many hundreds. The 
other elements are relatively uncommon, though they 
enter into the composition of many important minerals. 
Iron (Fe) and Manganese (Mn). — Iron, with which 
we are so familiar, is rarely found as a free element 
in nature. Usually it is combined with oxygen; and,’ 
