DISINTEGRATION OF ROCKS AND MINERALS 55 



composed and is acted upon by water the potash is re- 

 moved and water of hydration is taken up, forming 

 the product kaolin, which is hydrated silicate of alu- 

 mina, Al 4 (SiO 4 ) 3 .H 2 O. Impure varieties of clay are 

 colored red and yellow on account of the presence of 

 iron and other impurities. Pure kaolin is white, is 

 insoluble in acids, and is incapable of supplying any 

 nourisment to plants. Clay soils are fertile on account 

 of the other minerals and organic matter mixed with 

 the clay and are usually well-stocked with potash be- 

 cause of the incomplete removal of the potash from 

 the disintegrated feldspar. It is to be observed that 

 the term clay used- chemically means aluminum sili- 

 cate, while physically it is any substance, the particles 

 of which are less than 0.005 mm - in diameter. 



66. Disintegration of Rocks and Minerals. In ad- 

 dition to the rocks and minerals which have been 

 mentioned, there are many others that contribute to 

 soil formation as limestone which is calcium carbon- 

 ate, dolomite a double carbonate of calcium and mag- 

 nesium, serpentine a silicate of magnesium, and gyp- 

 sum or calcium sulphate. All rocks and minerals are 

 subject to disintegration and change in chemical com- 

 position and physical properties. The process of soil 

 formation has resulted in numerous chemical and 

 physical changes. These changes are still taking place, 

 and as a result plant food is constantly being made 

 available. 



