PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 325 



Mica schist. Gneiss.] 



No. 340. MICA SCHIST. 



" A little further west from No. 339, on the north side of this little water, the rock appears as a micaceous 

 quartzyte, which also varies to No. 341.'' 

 Ref. Annual Report, is, page 89. 



Mcy. A very fine-grained, hard, dark greenish-gray micaceous rock, somewhat 

 schistose. 



Mic. The section shows the schistose structure of the rock finely. It is 

 composed of liotite, /elds/xtr, quartz, cJdorite and pyrite. The feldspar is very 

 abundant; it is quite clear, but often inclined to become cloudy; it rarely shows 

 twinning, and then the twinning lamellae are minute. The feldspar rarely shows 

 any trace of cleavage. A number of grains showing bisectrices were found, but 

 only one of these showed cleavage; in this grain the cleavage was not pronounced, 

 it gave a negative bisectrix and an extinction of 76, indicating andesine-oligoclase. 

 Quartz is not nearly as abundant as the feldspar, although it is present. 



One section. 



Age. Archean (Coutchiching). u. s. G. 



No. 341. MICA SCHIST. (Tuff?} 



Same locality as No. 340; a variation of No. 340. 

 Eef. Annual Report, ix, page 89. 



Meg. A dark, fine-grained rock, hardly schistose. 



Mic. This is a fragmental rock, and in the slide all the elements have a 

 prevailing elongation in one direction. The sparse quartz and abundant feldspar are 

 so nearly alike that they cannot be distinguished except by the closest scrutiny at 

 high power, and in convergent light. They are in angular small bits, and, with a 

 little pyrite, they are set in a loose and scant frame-work of biotite. The feldspars 

 are nearly always destitute of cleavage, and the most of them present a deceptive, 

 limpid appearance, with double refraction lower than quartz. Some of them have a 

 a shadowy extinction, indicative of dynamic deformation. Other grains of feldspar 

 are probably of another species, for they are much clouded with saussuritic particles; 

 so much so, that an effort to determine them is fruitless. 



Throughout much of the slide, instead of evident mica there is a greenish, 

 clouded, semi-isotropic (perhaps chloritic) ingredient. It is probably this element 

 that gives the rock its dark color. 



Two sections. 



Age. Archean (Coutchiching). N. H. w. 



No. 342. GNEISS. 



"A gneissoid quartzyte, and makes a high bluff, the beds in all cases dipping to the south." Near the 

 same place as Nos. 340 and 341. 



Annual Report, ix, page 89. 



