Ch. III.] COMPARED WITH THOSE OF CORNWALL. 27 



mountain consists of granite, which is composed of a very 

 equal mixture of reddish felspar and white quartz, with very 

 little mica, nearly resembling the granite of Cairn Gorm. 

 The granite is traversed by veins or courses of porphyry, 

 varying from three or four feet to more than fifty in breadth : 

 these are all very erect, and, in a general view, appear to be 

 perpendicular ; and are so numerous, that they form a con- 

 siderable portion of the mountain. This porphyry is of 

 various colours and composition ; its basis consists of that sub- 

 stance called compact felspar, and its imbedded minerals are 

 commonly felspar and hornblende; and, according to the 

 proportion and the manner in which these are severally 

 united, the porphyry assumes a great variety of aspects. 

 Thus, by an uniform mixture of the basis and hornblende, 

 the transition from perfect porphyry into greenstone is 

 effected ; and this greenstone sometimes resembles well-cha- 

 racterised basalt. Specimens of the junction between granite 

 and this kind of basalt may be here obtained in great variety 

 and abundance: the line of junction is, in all cases, clear and 

 well defined ; but does not admit of ready separation, even 

 after long exposure to weather." 



Thus we learn that, in Scotland as in Cornwall, the granitic 

 rocks traverse the main mass of granite, in regular beds, veins, 

 or courses, which only differ from each other in size : and 

 that these pass into each other, and into the characteristic 

 granite of the district, by the most gradual mineral transitions, 

 so that it is not easy to detach one from the other. 



In some parts of the granitic ranges of Scotland, quartz so 

 predominates in the composition of the rock, that it deserves 

 particular notice. 



When this is not accompanied by the displacement of either 

 of the other ingredients, it may be considered merely as a 

 quartzose variety of granite; but very often it is only asso- 

 ciated with felspar or mica, and in many instances both of 

 these minerals are wanting. 



These forms of granite have received distinct names from 



