GRANITE. 



this result through the rusting of its iron. The fel- 

 spar sometimes also appears to suffer this change 

 from the alkali in its composition ; thus producing the 

 well-known porcelain clays. Hence the colour of 

 granite is no rule on this subject ; many of the pale 

 ones being as durable as the red. But those which 

 contain hornblende in place of mica are the most 

 durable; while this property is most perfect as they 

 approach the nearer to greenstone, combining, then, 

 strength also with durability. This is a question of 

 importance; from t lie extensive use of this rock in 

 architecture, and the popular notion that granite is 

 necessarily a rock of the highest durability. The 

 works of antient nations are sometimes indebted to 

 climate for their duration ; and it must also be recol- 

 lected, that, under this general term, are compre- 

 hended many substances, essentially differing in the 

 proportion, mixture, and nature, of their ingredients. 



Considering the resemblance between granite an 

 gneiss, it is remarkable that the latter should so ^' 

 much more powerfully resist decomposition; as is ^- ^ 

 very evident in Scotland at least, almost universally. / ./ 

 The soils produced by granite vary according to the ^^ 

 nature and proportions of the ingredients, especially 

 of the felspar and mica in the original rock. Where 

 quartz predominates, the soil is necessarily barren. M/^ 



Metallic veins are found in granite; and, to us > 

 those of Cornwall are the most remarkable, contain- 

 ing principally tin and copper. Lead, arsenic, cobalt, 

 zinc, wolfram, bismuth, silver, and iron, also occur; 

 and to these may be added, if less common, manga- 

 nese, titanium, uranium, and gold. 



