Ch. XXXIII.;, GENERAL ASPECT OF GRANITE. 703 



comprehended under the name of granite, which is, then, understood 

 to embrace a large family of crystalline and compound rocks, usually 

 found underlying all other formations ; whereas we have seen that 

 trap very commonly overlies strata of different ages. Granite often 

 preserves a very uniform character throughout a wide range of terri- 

 tory, 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 



Fig. 733. 



Mass of granite near the Sharp Tor, Cornwall. 



sometimes like heaps of boulders, 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 struc- 

 ture has already been described as characteristic of basalt and other 

 volcanic formations, and it must be referred to analogous causes, as 

 yet but imperfectly understood. 



Although it is the general peculiarity of granite to assume no defi- 

 nite shapes, it is nevertheless occasionally 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. 734.) 



The plutonic 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 

 described. 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 con- 

 ditions precisely similar. They also differ in never being scoriaceous 

 or amygdaloidal, and never forming a porphyry with an uncrystalline 

 base, or alternating with tuffs. Nor do they form conglomerates, 

 although there is sometimes an insensible passage from a fine to a 

 coarse-grained granite, and occasionally patches of a fine texture are 

 imbedded in a coarser variety. 



Felspar, quartz, and mica are usually considered as the minerals 

 essential to granite, the felspar being most abundant in quantity, and 

 the proportion of quartz exceeding that of mica. These minerals are 



