PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 603 



Granite. Hornblende schist.] 



shows undulating extinction. The feldspar is apparently largely orthoclase, and 

 there is a small amount of plagioclase (probably oligoclase) present. Associated 

 with the muscovite is some chlorite and with the chlorite, and commonly included in 

 it, are a few small grains of epidote. 



The schist is composed largely of biotite, quartz and feldspar. The last named 

 mineral shows no twinning and commonly no cleavage; its species was not deter- 

 mined. Chlorite, muscovite and pyrite, are also present, as well as a little epidote. 



The sections do not show the contact of the two rocks. 



Three sections. 

 Age. Archean. u, s G 



No. 924. GRANITE. (Biotitic.) 



S. W. J sec - 35> T. 63-17 W., west side of the channel west of Avis island, Vermilion lake. 

 Ref. Annual Report, xv, pages 291, 389. 



. A rather fine-grained, gray granite, composed of quartz, feldspar and 

 biotite. 



M'ic. The section shows a granite whose essential minerals are feldspar, quartz 

 and biotite. The feldspar is more or less clouded and apparently both orthoclase and 

 acid plagioclase (oligoclase). Muscovite, chlorite and iron ore are also present. 



One section. 



Age. Archean. u. S- G> 



No. 925. HORNBLENDE SCHIST. 



Same locality as No. 924. 



Ref. Annual Report, xv, pages 291, 389. 



Meg. A fine-grained, schistose, greenish-gray rock, having along its cleavages 

 many dark glistening scales. 



Mic. The section shows a rock composed essentially of feldspar, quartz and 

 green hornblende. The feldspar is much kaolinized. Chlorite, epidote and pyrite 

 are rather common. 



A minute vein crosses one end of the hand sample and is included in the section. 

 The vein is composed of a colorless mineral at times somewhat clouded by impurities 

 or alteration products. This mineral has an index of refraction higher than quartz 

 and a double refraction considerably stronger than quartz. It has one pretty good 

 cleavage and sometimes a poorer cleavage or cross fracturing approximately at right 

 angles to the good cleavage. The mineral is biaxial, with a rather small optic angle, 

 and the character of the double refraction is positive. The acute bisectrix is appar- 

 ently perpendicular to the good cleavage. The hardness is apparently 6 or more. 

 The species of the mineral is not known, but it may be thomsonite. One section. 



Age. Archean. u. s. o. 



