GROUP C. 



NOMENCLATURE. 



EOCKS in which orthoclase or sanidine 1 constitute the dominant felspathie 

 constituent are for the most .part of acid composition. The granites 

 represent the granitic texture, some of the quartz-felsites represent a kind 

 of intermediate texture, the liparites represent the trachytic texture. The 

 obsidians and pitchstones constitute the vitreous members of the same 

 group. While recognizing the fact that most of the orthoclase-bearing 

 rocks are of acid composition, it must not be forgotten that, occurring on 

 the outskirts, so to speak, of the main group, are rocks of intermediate and 

 even basic composition. Our classification based primarily on BUNSEN'S law 

 breaks down, therefore, if we endeavour to apply it in a perfectly rigid 

 manner. Various names have been given by different writers to different 

 varieties of rocks in which orthoclase is or is supposed to be , the dominant 

 felspar. Some of the most important of these are granite; granitite, granulito 

 pegmatite, aplite, eurite, felsite, quartz-felsite, felsite-porphyry, elvanite, 

 quartz-porphyry, granophyre, felsophyre, vitrophyre, trachyte, quartz - 

 trachyte, liparite, rhyolite, nevadite, syenite, augite-syenite, monzonite. 



Granite, This is a very old term said to have been used as far back as 

 1 596 by CLESALPINUS .As now used it implies that the rock is holo-crystalline 

 and that it contains orthoclase, quartz, and a third mineral, generally one 

 of the ferro-magnesian compounds. Plagioclase is very commonly present. 

 If we regard the term granite as connoting the presence of the granitic 

 texture and the minerals felspar (mostly orthoclase) and quartz we may 

 subdivide the group according to the nature of the other minerals which 

 play an important part in the composition of the rock. We thus have as 

 principal types 



MUSCOVITE (1) -GRANITE. 

 BTOTITE-GRANITE. 

 HORNBLENDE-GRANITE. 

 AUGITE-GRANITE. 



The above types, with the exception of the second, are rarely found in 



(1) The term muscovite is here used as synonymous with white mica and biotite as 

 synonymous with black mica. White magnesian micas are not known to occur in the granites, 



