VARIETIES OF GRANITE. 



73 



base, which contains black specks of mica. The 

 remainder of the mass is quartz. 



When hornblende is the substitute for mica, the 

 rock is called syenite^ from the celebrated ancient 

 quarries in Syene, in Egypt. This often passes into 

 the syeniiic greenstone, a rock of the trap family. 

 A compound of quariz, feldspar, mica, and horn- 

 blende is called syeniiic granite. Talcose granite is a 

 mixture of feldspar, quartz, and talc, called by the 

 French prutogine. This produces, by its decompo- 

 sition, the China or porcelain clay, more than 

 12,000 tons of which are annually exported from 

 Cornwall, England, for the potteries.* Felspaihic 

 granite is that in which the feldspar is the principal 

 ingredient, and the quartz, and particularly the mi- 

 ca, very rare. This kind of rock is frequently 

 nearly white; feldspar being generally white, though 

 it is sometimes flesh or rose-coloured. All these 

 granites pass into certain kinds of trap, which lends 

 great plausibility to the prevailing theory, that the 

 granites are of igneous origin. It has been shown 

 that the minerals which compose the granitic as 

 well as volcanic rocks, consist almost exclusively 

 of seven elements, namely, silica, alumina, magne- 

 sia, lime, soda, potash, and iron ; and these may ex- 

 ist in about the same proportions in a porous lava, 

 a basalt, or a crystalline granite. " The ordinary 

 granite of Aberdeenshire," says Dr. MacCulloch, " is 

 the usual ternary compound of quartz, feldspar, and 

 mica; but sometimes hornblende is substituted for 

 the mica. But in many places a variety occurs 

 which is composed simply of feldspar and horn- 

 blende; and in examining more minutely this dupli- 

 cate compound, it is observed in some places to 

 assume a fine grain, and at length to become undis- 

 tinauishable from the greenstones of the trap fam- 

 ily. It also passes in the same uninterrupted man- 



* Boase on Primary Geology. 



