34 THE WORLD'S LUMBER ROOM. 



Of the rocks containing felspar, the one with which 

 we are perhaps most familiar is granite (Fig. 10), which 

 consists of the three minerals, felspar, mica, and quartz, 

 crystallised together in very varying proportions, and in 

 crystals of very various sizes. Quartz is nearly pure silica ;* 



Fig. io. SECTION OF CORNISH GRANITE, SHOWING FELSPAR, ETC. 



it is so hard as to scratch glass, is but little affected 

 by heat or cold, or by the gases of the atmosphere, 

 and absorbs so little moisture that even frost .cannot 

 greatly injure it. 



Hence a granite which contains a large proportion of 

 quartz is one of the most durable rocks there is. It is 

 the felspar t which admits the enemy ; and when its potash, 

 soda, lime, or magnesia, have been converted into soluble 

 carbonates and washed away, the silicate of alumina, which 



* Flint is impure silica. f See Chap. viii. 



