56 MINERALOGY. 



CHAPTER VII. 



EARTHY MINERALS. 



111. Composition. A large proportion of the earthy 

 minerals are silicates of the earths alumina, lime, mag- 

 nesia, etc. These earths, as you learned in Part II., are 

 oxyds of metals, and, united with silica (that is, silicic 

 acid), they form salts called silicates. The different 

 kinds of glass are artificial silicates, many of them very 

 beautiful, but surpassed greatly in beauty by many of 

 the natural silicates found in the rocks. Many of these 

 silicates are very compound, the silica being united with 

 several bases at the same time. Some examples of 

 double salts (salts in which one acid is united with two 

 bases) were brought to your notice in the alums, and 

 some other salts, in the preceding chapter. But many 

 of the earthy minerals are much more compound than 

 this. Some have not only several bases, but more than 

 one acid. Thus, in that beautiful azure-blue mineral, la- 

 pis lazuli, we have both silicic acid and sulphuric acid. 

 Some of the earthy minerals, on the other hand, have a 

 very simple composition. Thus the splendid sapphire 

 and common emery are both the pure earth alumina, 

 that is, oxyd of aluminum ; and then we have, as you 

 will see, many varieties of quartz, which is pure silica, or 

 silicic acid. 



112. Silica. This substance constitutes about 45 per 

 cent, of the crust of the earth, some rocks being entirely 

 composed of it, but more of them having it in combina- 

 tion or in mixture. In the granite we have it in both 

 these states, for the quartz is pure silica; and while this 

 is mixed with two other minerals, mica and feldspar, in 

 these minerals, as you will see in another part of this 



