PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 1003 



Granite.] 



sometimes a foot or more in diameter; they are composed of the same minerals as 

 the main part of the rock, but mica makes up a very large proportion of each dark 

 mass." Specimens examined were taken from the excavation for a canal at Fort 

 Frances, made by the Canadian government several years ago. 



Mic. The following description is condensed from that of A. N. Winchell, pub- 

 lished in the American Geologist (loc. cit.). 



Orthoclas'i is quite abundant in irregular grains. They include biotite, apatite, 

 quartz and hornblende, and occasionally show Carlsbad twinning, and micropegma- 

 titic intergrowths. 



Biotite also is abundant. 



Andesine oligoclase is quite common. It is Carlsbaded, with albite twinning in 

 both parts. No other plagioclase could be detected by the method of Fouque. 



Microdine occurs quite freely, embracing kaolin, epidote, quartz and biotite. 



Hornblende appears rather sparingly, and sometimes shows zonal structure, 

 with inclusions of other minerals. 



Quartz is in irregular grains, with few inclusions, and occasionally as micro- 

 pegmatitic growth in orthoclase and in andesine-oligoclase. 



Epidote is abundant, frequently showing the crystal form, especially in the 

 midst of biotite. 



Kaolin is common in all of the feldspars, sometimes associated with secondary 

 quartz and heterogeneous occurrences of epidote and sphene. 



Apatite appears in needle-like forms and in short prisms, and sometimes is so 

 thickly crowded in highly altered areas as to suggest a secondary origin. 



Sphene is in small amount. 



Tourmaline, in hexagonal crystal form and in shapeless aggregates, is identifiable 

 in limited amount. 



Pyrite is rare. 



Calcite was unsatisfactorily identified in a few small grains. 



Zoisite is about as abundant as apatite, and is distinguished from epidote, with 

 which it is associated in origin and occurrence, by the smallness of its grains and its 

 blue-gray interference color. 



Muscovite was doubtfully distinguished. 



By the use of a specific gravity liquid the constituent minerals were separated 

 with the following approximate result: 



Mica, 12 per cent. 



Above 3., 9-29 



Between 3. and 2.661, 34.66 



Between 2.661 and 2.642, 25.02 



Between 2.642 and 2.58, 7.60 



Below 2.58, 11.43 



100.00 per cent. 



