SOIL-FORMING ROCKS 9 



of silicates, of which a few are easily chief. (5) These 

 silicates crystallize into a multitude of minerals, of 

 which again a few are chief. (6) These minerals are 

 aggregated in various ways to form rocks." 



Hundreds of analyses of rocks have been made in 

 this country and abroad and from these Clark finds the 

 mineralogical composition of igneous rocks of the earth's 

 crust to be as follows: 



Feldspars 59.5 



Hornblend and pyroxine 16.8 



Quartz 12.0 



Biotite mica 3.8 



Titanium minerals 1 .5 



Apatite 0.6 



94.2 

 This leaves 5.8 per cent to be distributed among the 

 more rare minerals. 



III. IMPORTANT SOIL-FORMING ROCKS J THEIR PROPER- 

 TIES AND OCCURRENCE 



A rock is an aggregate of minerals. Moreover, it 

 usually exhibits a considerable degree of consolidation, 

 and forms an essential portion of the earth's structure. 

 Very few minerals occur in nature in large pure masses. 

 They are usually grouped together in different combi- 

 nations, and, while it is essential to trace the changes 

 of each mineral, it is also necessary to give attention to 

 the groups of minerals rocks since the association 

 of minerals determines very largely the processes by 

 which rocks are transformed into soil and the character- 

 istics of the resulting soil. 



