GRANITE. 235 



by the cross section of the quartz prisms. These 

 are frequently triangular, occasionally hexagonal 

 and flattened ; and in a few rare instances, the 

 two minerals form alternating laminae. The fel- 

 spar is generally predominant ; often in a great 

 degree. The sizes of the prisms vary materially, 

 and hence a great variety of aspect. In some 

 rare instances, where cavities exist in the veins, 

 the quartz is found with pyramidal summits. 

 Throughout a whole vein of this rock, however 

 large, it will generally be found that a common 

 polarity pervades the felspar ; or, practically, that 

 the corresponding laminae reflect light at the 

 same angle over a wide extent of irregular surface. 



C. Quartz, and hornblende. 



This appears to be merely an incidental 

 variety. 



D. Felspar, and hornblende. 



a. Large grained, or the hornblende crystallized. 



b. An uniform granular mixture ; the respective 

 ingredients varying- materially in their sizes and pro- 

 portions, sovas to produce a great variety of aspect. 



c. Intimately mixed, so as to be nearly undis- 

 tinguishable. 



Var. b. often resembles the greenstones of 



