Dresser — Geology of Brpme Mountain. 



351 



The feldspar is chiefly a plagioclase, which is twinned accord- 

 ing to the albite law in broad lamellae upon which it extin- 

 guishes symmetrically an angle of 40 degrees or more. It is 

 bytownite, or basic labradorite. A few rather large crystals of 

 microperthite are the only other feldspathic constituents seen. 



,' a 



!'• : '° : j NORDMARKOSE 



LAURDALOSE 



[ 1 PALAEOZOIC 



Map of Brome Mountain, Quebec, Canada. 



The hornblende is trichroic, the scheme of absorption being 

 c > b > a, with b nearly equal to c. The color ranges from chest- 

 nut to yellowish brown. The maximum extinction angle, cAc, 

 that was observed was 20 degrees. 



The principal variety of augite present is slightly dichroic. 

 Sections having b or c parallel to the plane of the polarizer of 



