VARIETIES OP 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 syenitic greenstone, a rock of the trap family. 
A compound of quartz, feldspar, mica, and horn- 
blende is called syenitic granite. Talcose granite is a 
mixture of feldspar, quartz, and talc, called by the 
French protogine. 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.* Felspathic 
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. MacCuUoch, " 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- 
tinguishable from the greenstones of the trap fam- 
ily. It also passes in the same uninterrupted man- 
* Boase on Primary Geology. 
G 
