Building Materials, Their Use and Origin 425 



plain, there abounds silica. Plant life contains silica, as is 

 shown by analysis of the ash. The glistening and stiffness 

 of many grasses, such as bamboo, are due to silica. Animal 

 life is not without it, and certain microscopic forms contain a 

 large percentage of silica in their composition. 



Quartz in granite is pure silica; the other constituents of 

 granite, felspar, mica, and hornblende, contain more or less 

 silica. Much felspar is a compound of silica, alumina, and 

 potassium. Mica and hornblende are similarly made up, 

 though they contain in addition lime, magnesia, and iron oxid. 

 Felspar and mica are dissolved in water, and the silica and 

 alumina are carried away and deposited to form clay. 



Clay is thus seen to contain silica. One form of clay, called 

 kaolin, contains a very large proportion of alumina; it is the 

 purest variety of clay and most useful in making the finest 

 pottery and chinaware. But the other varieties contain a 

 large amount of silica. 



TABLE OF COMMON SUBSTANCES IN ROCKS AND SHALE 



This indicates that silica is not only the most widely dis- 

 tributed, but perhaps the most abundant mineral in the crust 

 of the earth. Silicon, the chemical element, constitutes about 



