I.] ROCK-FORMING MINERALS 19 



potash felspar. The lime or the alkali is removed in 

 solution, some of the silica is also removed ; the alumina 

 remains as a hydrated silicate, A1 2 3 , 2Si0 2 , 2H 2 0, 

 called kaolinite. Owing to this disintegration of 

 felspar, the crystalline rocks in which felspar is present 

 weather rapidly, the other materials, quartz, mica, 

 hornblende, become loosened from the matrix, and 

 the whole rock becomes rotten. The granite of Corn- 

 wall and Devon is generally covered to a considerable 

 depth, as much as 100 feet in some cases, with a 

 layer of kaolinite, in which the unchanged quartz and 

 mica are embedded; the kaolinite, freed by washing 

 from the quartz, mica, etc., forms the " china clay " or 

 kaolin of commerce. In the same way the basalts and 

 other kindred rocks give rise to a red clay, consisting 

 of kaolinite and the red iron oxides resulting from 

 the oxidation of the magnetite and the hornblende, 

 augite, etc., which contain ferrous silicates. From the 

 decomposition of the felspars, augite, hornblende, etc., 

 all our clays arise; as these minerals also generally 

 contain potash, they are the source of the potash 

 required by crops, which is always more abundant as 

 clay predominates in the soil. 



Daubree caused 3 kilos of fragments of felspar 

 to revolve in an iron cylinder with 3 litres of water, 

 so that they practically performed a journey of 460 

 kilometres, with the result that 2-72 kilos of mud were 

 formed, of which 36 grams were clay, and in the 

 water there were 126 grams of potash in solution as 

 silicate. 



Senft examined the action of water charged with 

 carbonic acid upon two granites, one (A) composed of 

 orthoclase, quartz, and potash mica, the other of (B) 

 plagioclase, quartz, and magnesia f mica, and obtained 

 in solution — 



