562 



MINERAL COMPOSITION OP GEANITE. [Ch. XXXIIl 



and causing alterations in these last, wliich will be presently described. 

 They also resemble trap in containing no organic remains; but they 

 differ in being more uniform in texture, v/hole mountain masses of inde- 

 finite extent appearing to have originated under conditions precisely 

 similar. They also differ in never being scoriaceous or amygdaloidal, 

 and never forming a porphyry with an uncrystalline base, or alternating 

 with tuffs. Nor do they form conglomerates, although there is sometimes 

 an insensible passage from a fine to a coarse grained granite, and occa- 

 sionally patches of a fine texture are imbedded in a coarser variety. 



Felspar, quartz, and mica are usually considered as the minerals 

 essential to granite, the felspar being most abundant in quantity, and 

 the proportion of quartz exceeding that of mica. These minerals are 

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

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

 fig. '704, p. 590), except in the variety termed graphic granite, which 

 occurs mostly in granitic veins. This variety is a compound of felspar 

 and quartz, so ai-ranged as to produce an imperfect laminar structure. 

 The crystals of felspar appear to have been first formed, leaving betvveen 



683. 



Fi-. 684. 



Graphic granite. 

 Fig. 683, Section parallel to the laminss. 

 Fia;. 684. Section transverse to the laminaa. 



them the space now occupied by the darker-colored quartz. This min- 

 eral, when a section is made at right angles to the 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. 



As a general rule, quartz, in a compact or amorphous state, forms 

 a vitreous mass, serving as the base in which felspar and mica have 

 crystallized ; f9r although these minerals are much more fusible than 

 silex, they have often imprinted their shapes upon the quartz. This 

 fact, apparently so paradoxical, has given rise to much ingenious specu- 

 lation. We should naturally have anticipated that, during the cooHng 

 of the mass, the flinty portion would be the first to consolidate ; and 

 that the different varieties of felspar, as well as garnets and tourmalines, 

 being more easily liquefied by heat, would be the last. Precisely tha 



