PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 483 



Granite.] 



magnetite, and secondary hornblende. Considerable secondary eozoon or graphic 

 quartz was observed in the feldspathic material, giving the rock there the structure 

 of a graphic granite. Some pyrite occurs associated with the magnetite which, in 

 places, is surrounded and penetrated by the pyrite. Considerable apatite was observed 

 in the section." 



One poor section examined. 



Age. Cabotian. u. s. G. 



No. 673. GRANITE. (Red. ) 



Top of Brule mountain; N. } sec. 21, T. 63-1 W. This rock is not well seen in situ. 

 Ref. Annual Report, x, pages 74, 75. 



Meg. A brick-red, granitoid rock of medium grain, composed of red feldspar, a 

 little quartz and considerable of a black mineral. 



Mic. A few small porphyritic feldspars, much reddened, are seen in the fine- 

 grained groundmass of the red rocks. Certain dark areas in the slide, which 

 probably represent original augite, are filled with secondary products, among which 

 magnetite is important. It seems quite probable that the slide of this number was 

 not made from the specimen of similar number; the slide shows a finer-grained rock 

 than the hand sample. 



One section examined. 



Aye. Cabotian. u. s. G. 



No. 674. GRANITE (ivith augite). 



"A rather fine grained deposit, with quartz, which occurs on the north face of Brule' mountain, forming a 

 perpendicular wall about twenty feet high, but is only seen (so far as examined) a little east of but near the trail. 

 It is 240 feet below the top of the mountain, and makes a more or less continuous jog or shoulder.'' Perhaps in 

 S. y z sec. 16, T. 63-1 W. 



Ref. Annual Report, x, page 75. 



Meg. A gray rock of medium grain, granitoid texture, and composed of gray 

 to flesh-colored feldspar, augite and quartz. 



Mir. The section is composed of feldspar, augite, quartz, magnetite, hornblende, 

 /i/iit/h', apatite and brownish, yellowish and greenish alteration products. 



The feldspar occurs in two ways: First, as nearly idiomorphic crystals which 

 appear to be oligoclase and orthoclase; and second, intergrown with quartz to form 

 micropegmatyte, which frequently surrounds the other feldspars. The low equal 

 extinction angles in sections cut normal to 010, or nearly so, and the presence of two 

 sections, each of which gives an extinction angle of 2, cut practically normal to c 

 indicate that the first form of feldspar is not more basic than oliyoclase or oligoclaw- 

 andesine. The feldspar of the micropegmatyte is usually altered to an almost opaque 

 cloudy mass, while the other feldspar is much less altered; none of it is reddened. 



The augite is older than the feldspar and sometimes shows an idiomorphic form. 

 It is usually light green in color, but some of it approaches to the clear cinnamon 



