120 



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LYELL'S ELEMENTS OF GEOLOGY. 



Mineral Composition of Granitic Rocks. 



form in texture, whole mountain masses of indefinite extent ap- 

 pearing to have originated under conditions precisely similar. 

 But they differ in never being scoriaceous or amygdaloidal, in 

 never forming a porphyry with an uncrystalline base, and never 

 alternating with tuffs. Nor do they form conglomerates, al- 

 though there is sometimes an insensible passage from a fine to a 

 coarse grained granite, and occasionally patches of a fine tex- 

 ture are imbedded in a coarser variety. 



Felspar, quartz, and mica are usually considered as the mine- 

 rals essential to granite, the felspar being most abundant in quan- 

 tity, 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. 107.), except in the variety termed 



Fig. 107. 



Oneiss. {See description, 132.) 



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 



Fig. 108. Fig. 109. 



Orapliic granite. 

 Fig. 108. Section parallel to the laminae. 

 Fig. 109. Section transverse to the laminse. 



appear to have been first formed, leaving between them the space 

 now occupied by the darker coloured quartz. This mineral, 

 when a section is made at right angles to the alternate plates of 



