14 GRANITIC ROCKS OF CORNWALL. [Ch. II. 



loped in the large crystals of a different kind of mineral. And 

 it is a very common occurrence for one, or even more, of these 

 minerals to be developed in large crystals, in a granular basis 

 of the whole, so as to constitute a porphyritic granite. This 

 character is generally imparted by the felspar, and rarely by 

 the quartz or mica. 



When the crystalline minerals are united in moderate- 

 sized granules, the rock may be called common granite, since 

 it occurs more universally than any other kind : indeed there 

 is, perhaps, no primitive range of any considerable extent in 

 which it is altogether wanting. In the characteristic speci- 

 mens of this variety the felspar is most abuncjant, and the 

 quartz exceeds the mica : the respective proportions of these, 

 however, are liable to great fluctuations. Other varieties of 

 this rock, depending on the size of the component parts, may 

 be noticed; which, relatively to the common, may be termed 

 small and large grained : the former exhibits every degree of 

 diminution, until the ingredients can scarcely be distinguished 

 by the naked eye ; and this kind, like the common, is of too 

 frequent occurrence to need any particular references. The 

 latter, or large-grained variety, however, when its minerals 

 extend to any great size, is more rare : it is not to be found 

 in Cornwall, but near Aberdeen in Scotland, and in Siberia. 

 Each constituent attains a very large size, and is very crystal- 

 line, more particularly the mica, in Siberia, where it occurs 

 in large hexagonal tables, easily separating into transparent 

 laminae or plates, which have been applied to economical 

 purposes. 



When either of these kinds of granite contains, in addition 

 to the essential minerals, any other substance in such quantity 

 as to alter the appearance of the rock, then it may be 

 regarded as a distinct variety; but when the proportion is 

 very small, the presence of the adventitious mineral may be 

 overlooked in the classification : though this circumstance 

 ought to be noticed, because, in all probability, it will be 



