704 



MINERAL COMPOSITION OF GRANITE. [Oh. XXXIII. 



Fig. 734. 



Granite having a cuboidal and rude columnar structure, Land's End, Cornwall. 



united in what is termed a contused crystallization ; that is to say, 

 there is no regular arrangement of the crystals in granite, as in gneiss 

 (see fig. 756, p. 733), except in the variety termed graphic granite, 

 which occurs mostly in granitic veins. This variety is a compound 

 of felspar and quartz, so arranged as to produce an imperfect laminar 

 structure. The crystals of felspar appear to have been first formed, 

 leaving between them the space now occupied by the darker-colored 

 quartz. This mineral, when a section is made at right angles to the 



Fig. 735. 



Fig. 736. 



Graphic granite. 

 Fig. 735. Section parallel to the lamince. 

 Fig. 736. Section transverse to the lamina?. 



alternate plates of felspar and quartz, presents broken lines, which 

 have been compared to Hebrew characters. The variety of granite 

 called by the French Pegmatite, which is a mixture of quartz and 

 common felspar, usually with some small admixture of white silvery 

 mica, often passes into graphic granite. 



