Ch. II.] CHARACTERISTICS OF CORNISH GRANITE. 15 



found to increase in importance in other parts of the same 

 range. 



Such are the ordinary varieties of true granite, rocks which 

 are of such frequent occurrence, and for the most part in such 

 general use, that they are familiar to the most cursory observer. 

 So far, then, the nature of these granitic rocks cannot be 

 mistaken: those that remain to be described are far more 

 numerous, and more partially distributed, and can therefore 

 only be found in certain localities. On this account it is pro- 

 posed to describe the granitic rocks of several districts, by 

 which means their history will be better developed, and the 

 details be rendered more interesting. 



In Cornwall the granitic rocks occur at the surface, in the 

 form of eight insulated masses, four of which are of much 

 greater extent than the others ; each of these masses presents 

 some peculiarities, but all of them contain several varieties of 

 true granite. 



The most general feature of the Cornish granite is the 

 abundance of its felspar, which not only forms the greater 

 part of this rock in the ordinary granular varieties, but is 

 frequently superadded in the form of large porphyritic crystals, 

 constituting no inconsiderable proportion of the whole mass. 

 Shorl, however, is the characteristic mineral of this district; 

 indeed it is seldom altogether absent from these granitic rocks 

 for any extent, though it is often in such minute particles as 

 to require a magnifying glass for its detection. 



When shorl forms a portion of true granite, the rock may 

 be sufficiently distinguished by the term shorlaceous granite ; 

 but when the shorl usurps the place of mica, or is combined 

 only with quartz, then the compound requires a peculiar 

 name: that of shorl-rock has been applied to some of its 

 varieties, and it is proposed to use it here in a more extended 

 sense. 



Shorl-rock is sometimes composed of a crystalline granular 

 mixture of felspar, quartz, and shorl ; and this granitic shorl- 

 rock, assuming various appearances, according to the size and 



