GRANITE. 



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erals, varying in different proportions, and are rather modes of the 

 same rock than different species. They pass by gradation into each 

 other, as one or other of their constituent minerals becomes, more 

 or less, abundant; they alternate with each other in various situa- 

 tions, and may be regarded as contemporaneous. It may, howev- 

 er, for the convenience of description, be proper to treat of each 

 separately. 



^ : Rocks of the first Class. 



Granite is considered as the foundation rock, on which slate rocks 

 and all secondary rocks are laid. From its great relative depth, 

 granite is not frequently met with, except in alpine situations, where 

 it appears to have been forced through the more superficial covering 

 of the globe. Where granite rises above the surface, the beds of 

 other rocks in the same district generally rise towards it, and their 

 angles of elevation increase as they approach nearer to it.* Granite 

 is composed of the three minerals described in the third chapter, — • 

 quartz, felspar, and mica, — which are more or less perfectly crystal- 

 lized, and closely united together. 



The three minerals of which granite is composed vary, much, in 

 their proportions in different granitic rocks, and often, in specimens 

 from the same rock, the crystals are large, or small, or equally inter- 

 mixed, in one part, and in another part, quartz or felspar greatly pre- 

 dominates. Some granites are composed of small grains, and have 

 large crystals of felspar interspersed; these are denominated porphy- 

 ritic granites. Stones of this kind are common in the foot-pavements 

 of London. f 



Felspar constitutes by far the largest part of granite : the more 

 common colours are white and red ; it is sometimes in a soft or de- 

 composing state, and appears earthy. In some granites, the crys- 

 tals of felspar are distinctly formed. Quartz, generally, occurs in 

 small irregular shaped grains, which have a vitreous lustre. The 

 mica, in granite, occurs, most commonly, in small shining scales, 

 which are, generally, either black, or whitish and silvery. It, some- 

 times, occurs in large hexagonal plates ; but this is, more common- 

 ly, the case in the granite that forms veins in granitic mountains; such 

 veins, with large plates of mica, are frequent near Aberdeen, in 



* Some writers derive the name from geraniies, a word used by Pliny to de- 

 note a particular kind of stone ; others, with more probability, suppose that the 

 name originated from its granular structure, or the grains of which it is com- 

 posed. 



t Specimens of Cornish and Scotch granites are not difficult to procure in Lon- 

 don, as they are commonly used for paving-stones. In the former, the felspar is 

 Avhite; the mica appears like glistening scales, which have a tarnished semi-me- 

 tallic lustre. The quartz has a vitreous appearance, and is of a light grey colour. 

 In Scotch granite, the felspar has more commonly a reddish-brown colour. The 

 mica is, not unfrequently, black and splendent, and may be divided into thin scales 

 by the point of a penknife: this distinguishes it from hornblende, which is some- 

 times intermixed with this granite. 



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