I.] ROCK-FORMING MINERALS 19 



durability and the former greater extension of the 

 chalk, flints are found in many districts remote from 

 the chalk, even in the drift beds of the Channel Islands. 

 When first won from the chalk, flints possess a clear 

 black translucent structure, and are easily fractured and 

 crushed ; when weathered, either in flint gravels or 

 on the surface of the soil, they become yellow or brown 

 in colour, more opaque, and much harder, so that 

 weathered flints are always preferred for road-making. 

 The surface also becomes covered with a white incrusta- 

 tion, extending to a depth of T V of an inch or more ; 

 this is, however, only incipient weathering, probably 

 due to the freezing of the small amount of water that 

 soaks in at the surface. 



The Felspars constitute the most important group of 

 minerals found in the crystalline rocks : they are 

 double silicates of alumina and some other base, potash, 

 soda, or lime, of the general formula R 2 0, A1 2 3 , 6Si0 2 , 

 where R 2 may be either K.,0, Na 2 0, or CaO. In 

 granites and gneisses the common felspar is orthoclase 

 or potash felspar ; in the volcanic rocks plagioclase 

 felspars predominate, in which the base is lime, gener- 

 ally with some admixture of soda and potash. 



The felspars are all distinguished by the ease with 

 which they are attacked by water containing carbonic 

 acid, those containing lime more readily so than the 

 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 



