THE OEIGIN OF SOILS 



23 



that class; those rich in lime, e.g., augite, hornblende, 

 etc., are found chiefly in the volcanic rocks. 



The following tables show the average chemical composi- 

 tion of some of the commoner rock-forming minerals and 

 the proportion in which, it is estimated, they enter into 

 the composition of the crust of the earth. 



AVERAGE CHEMICAL COMPOSITION OF MINERALS. 



AVERAGE MINERALOGICAL COMPOSITION OF THE CRUST OF THE EARTH, 



Felspars . . . ... 



Quartz ..... 



Micas ..... 



Talc 



Carbonates of lime and magnesia 

 Amphiboles (Hornblende) 3 ^ 

 Pyroxenes (Augite) 

 Olivine } 



Clay 



All other minerals 



100 



1 Calculated as Fe 2 O 3 . 



2 Very variable. 



8 Arnphibole and Pyroxene are the names of the groups to which 

 hornblende and augite respectively belong. Each group includes 

 many minerals besides the types mentioned. 



