!] 



ROCK-FORMING MINERALS 



21 



yellow, clay containing fragments of quartz and flakes 

 of mica. 



The following analyses show the change that takes 

 place in passing from orthoclase felspar to kaolin ; in 

 the third column the analysis of kaolin is recalculated 

 to show what arises from ioo parts of felspar, on the 

 assumption that none of the alumina is removed by 

 solution : 



Mica is essentially a double silicate of alumina and 

 potash, with some oxide of iron : the potash being 

 replaced by magnesia in black mica or biotite. Mica 

 splits up into minute flakes as the rock weathers, but 

 these flakes are fairly resistant to chemical change, and 

 may be detected in most sands and sandstones. Ulti- 

 mately, however, they pass into hydrated silicates of 

 alumina, and are rarely to be detected in the soils 

 resting upon sedimentary rocks. 



Hornblende and Augite, though differing in crystalline 

 shape, are chemically identical, and consist of silicates of 

 varying proportions of lime, magnesia, alumina, ferrous 

 and ferric oxides ; manganese and the alkali metals are 

 generally also present. They constitute, with plagioclase 

 felspar and magnetic oxide of iron, the chief part of the 

 rocks that are sometimes roughly termed " greenstone" 

 basalts, diorites, etc., of both volcanic and plutonic 

 origin. They decompose under the action of carbonic 



