THE GRANITE LIPAEITE GROUP 



61 



eral composition, or at least m the relative proportion of the 

 various essential constituents 



1. THE GBANITE-LIPAItlTE GROUP 



This group includes the most acid of all eruptive rocks , that 

 is, those which on analysis are found to yield the highest per- 

 centages of silica Their chief essential constituents are quartz 

 and potash feldspars, while the more basic ferruginous minerals 

 are m quantities proportionately small The group includes a 

 deep-seated or plutonic type, granite, and two effusive or vol- 

 canic types, quartz porphyry, and liparite or rhyolite They 

 may be described m detail as below: 



(1) THE GRANITES 



Mineral Composition. — The essential constituents of granite 

 are quartz and a potash feldspar (either orthoclase or micro- 

 clme), and plagioelase. Nearly always one or more minerals of 

 the mica, hornblende, or pyroxene group are present, and m 

 small, usually microscopic forms, the accessories magnetite, 

 apatite, and zircon, more rarely occur sphene, beryl, topaz, 

 tourmaline, garnet, epidote, allanite, fluorite, and pyrite De- 

 lesse^ has made the following determination of the relative pro- 

 portion of the various constituents m two well-known granites : 



Mineral Composition op Granite 



Egyptxan Ekd Gkanite 



Parts 



PoEpniEiTic Geanitb Vosass 



Paets 



Red orthoclase .... 

 White albite .... 

 Gray quartz .... 

 Black mica 



43 



9 



44 

 4 



White orthoclase 



Reddish oligoclase .... 



Gray quartz 



Mica 



Total 



28 

 7 



59 

 6 



Total . . 



100 



100 



Perkins^ gives the mineral composition of the '* Medium Stock 

 gray granite of Barre, Vermont, as follows • 



^Prestwich, Chemical and Physical Geology, Vol I, p 42 

 2 Report State Geologist of Vermont, 1901-2 



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