I.] 



ROCK-FORMING MINERALS 



17 



Rock-form ing Minerals. 



In the solid crust of the earth D'Orbigny has 

 estimated that the chief minerals are present in the 

 following proportions felspars, 48 per cent. ; quartz, 

 35 per cent. ; micas, 8 per cent. ; talc, 5 per cent ; 

 carbonates of lime and magnesia, 1 per cent. ; 

 hornblende, augite, etc., 1 per cent. ; other minerals 

 and weathered products, 2 per cent. 



The following table shows the composition of these 

 chief minerals, with a few others that play some part in 

 the formation of soil : 



Quartz, the crystalline form of silica, is found 

 massive and in veins in the primitive rocks, and in 

 fragments of all sizes in the granites, gneisses, and 

 similar rocks. From the waste of these crystalline 

 rocks are derived the sandstones of all geological ages, 

 and directly or indirectly the sands now existing. 

 In a sandstone rock the grains of quartz are bound 

 together by a cement, which may be oxide or car- 

 bonate of iron, as in the Lower Greensand of Surrey 



C 



