PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 485 



Gabbro.] 



No. 676. GABBRO. 



"At a lower level another perpendicular wall of rock is found on the north side, a little west of the section line 

 between sections 16 and 17 (T. 63-1 W.), rising about seventy-eight feet. This rock resembles No. 674, but the 

 bluff cannot be said to consist entirely of this." Near the same place as No. 675. 



Ref. Annual Report, x, pages 75, 76. 



Met/. A somewhat crumbling, brownish-gray rock of medium grain, composed 

 of feldspar and a black mineral (largely augite). 



Mic, The section is composed essentially of plagioclase, anyite and ni(n/iic/ifi'. 

 The plagioclase is labmdorite. Yellowish stains are common throughout the section 

 and yellow and brownish alteration products are abundant. One large grain, 

 considerably altered and stained, appears to be olivine, and some of the areas of 

 brownish alteration products may also represent this mineral. In texture the rock 

 is quite similar to the gabbros, although there is frequently a tendency to an idiomor- 

 phic development of the feldspar, and in general the augite is of later date than that 

 mineral. 



One section examined. 



Af/e. Cabotian. u. s. G. 



No. 677. GABBRO. (Fine, yranulitic. ) 



In a hill about 100 feet east of the trail from the western lake in Brule river, and a little more than half 

 way from this lake to Little Trout lake, and south of Little lake; near the centre of the N. % sec. 8, T. 63-1 W. 

 Ref. Annual Report, x, pages 76, 81. 



Meg. A fine-grained, compact, dark-yellowish, gray rock appearing considerably 

 like a quartzyte. 



Mic. The section contains feldspar, augite, olivine and magnetite, all in abundance, 

 and biotite and apatite in small amounts. Under a low power the augite and olivine 

 present rounded outlines, but on careful examination with, a higher power the augite 

 is seen to have less rounded outlines than the olivine, and frequently the former 

 mineral forms with the feldspar a completely allotriomorphic aggregate. The feld- 

 spar was, however, in general the last mineral to crystallize. It uncommonly shows 

 polysynthetic twinning and is allotriomorphic in form except for a very few 

 individuals which have a tendency to a lath-shaped development. The species was 

 not determined with certainty, although it seems to be near labratlorife. Sections 

 cut nearly normal to a bisectrix failed to show any cleavage or albite twinning, and 

 sections cut normal to 010 and showing the albite twinning are scarce. Moreover, 

 the thickness of the section, causing the different grains to overlap, renders a careful 

 determination of the feldspars difficult. 



The olivine is less abundant and in larger grains than the augite. The 

 magnetite frequently shows crystal outline and a few of the larger grains are 

 moulded on or inclose olivine grains. The biotite usually is seen surrounding 



