ROCKS AND MINERALS 59 



ored red and yellow on account of the presence of iron 

 and other impurities. Pnre kaolin is white, is in- 

 soluble in acids, and is incapable of supplying any 

 nourishment to plants. Clay soils are fertile on ac- 

 count of the other minerals and organic matter mixed 

 with the clay and are usually well stocked with pot- 

 ash because of the incomplete removal of the potash 

 from the disintegrated feldspar. It is to be observed 

 that the term clay used chemically means alu- 

 minum silicate, while physically it is any substance, the 

 particles of which are less than 0.005 mm. in diameter. 



66. Other Rocks and Minerals. — In addition to 

 the rocks and minerals which have been discussed, 

 there are many others that contribute to soil forma- 

 tion, as limestone, which is calcium carbonate ; dolo- 

 mite, a double carbonate of calcium and magnesium ; 

 serpentine, a silicate of magnesium ; and gypsum, cal- 

 cium sulphate. 



Chemical Composition of Rocks 1u 



cfl 



J3 . «i 



V 



I* — ' 



.2 x 



</) o 



o « u 



V 



Si go Sn i° £0 So to" 



■T.'tri << p-W tJrtZ »4 V SS n.fc !>£ 



Quartz 95—100 .... 



Feldspar 55-67 20-29 0-12 i-to i-ii 



Kaolin 46 39 14 



AiTDaLl LG •••••• .... . a * * .... .... ^'Z .... I ..-)') .... 



Mica 40-45 12-37 5-12 1-5 



Hornblende.. 40-55 0-15 



Granite 60-80 10-15 4-5 2-3 • • • 



