552 
ELEMENTS OF GEOLOGY. 
surface, the other far beneath it. The platonic formations 
also agree with the volcanic in having veins or ramifications 
proceeding from central masses into the adjoining rocks, and 
causing alterations in these last, which will be presently de¬ 
scribed. They also resemble trap in containing no organic 
remains; but they differ in being more uniform in texture, 
whole mountain masses of indefinite extent appearing to 
have originated under conditions precisely similar. 
The two principal members of the Plutonic family of rocks 
are Granite and Syenite, each of which, with their varieties, 
bear very much the same i^elation to each other as the tra¬ 
chytes bear to the basalts. Granite is a compound of feldspar, 
quartz, and mica, the feldspars being rich in silica, which forms 
from 60 to 70 per cent, of the whole aggregate. In Syenite 
quartz is rare or wanting, hornblende taking the place of 
mica, and the proportion of silica not exceeding 50 to 60 per 
cent. 
Granite and its Varieties. —Granite often preserves a very 
uniform character throughout a wide range of territory, 
forming hills of a peculiar rounded form, usually clad with a 
scanty vegetation. The surface of the rock is for the most 
part in a crumbling state, and the hills are often surmounted 
by piles of stones like the remains of a stratified mass, as in 
the annexed figure, and sometimes like heaps of boulders, 
Fig. 605. 
for which they have been mistaken. The exterior of these 
stones, originally quadrangular, acquires a rounded form by 
the action of air and water, for the edges and angles waste 
away more rapidly than the sides. A similar spherical 
structure has already been described as characteristic of ba¬ 
salt and other volcanic formations, and it must be referred to 
analogous causes, as yet but imperfectly understood. Al¬ 
though it is the general peculiarity of granite to assume no 
definite shapes, it is nevertheless oc'casionally subdivided by 
fissures, so as to assume a cuboidal, and even a columnar, 
structure. Examples of these appearances may be seen near 
the Land’s End, in Cornwall. (See Fig. 606.) 
