490 THE GEOLOGY OP MINNESOTA. 



[Diabase. Apotrachyte. Granophyre. 



No. 684. DIABASE. (Altered.) 



Same locality as No. G81, which see. 



Ref. Annual Report, x, pages 7G, 77, 84, 96. 



Meg. A fine-grained very dark-gray diabase. 



Mic. The section shows numerous lath-shaped labraaorites in a groundmass 

 which is now completely altered to a confused greenish-gray mass in which mag- 

 netite is abundant. 



One section examined. 



Age. Cabotian. u. s. G. 



No. 685. APOTRACHYTE (with 



Near the south line of sec. 32, T. 64-1 W., on the portage from Little Trout lake to Misquah lake. 

 Ref. Annual Keport, x, pages 75, 77, 78. 



. A compact, very fine-grained, reddish-brown rock, containing porphyritic 

 crystals of red feldspar and smaller black porphyritic crystals apparently of augite. 



Mic. The groundmass of the section is composed of a very fine-grained aggregate 

 of quartz and reddened feldspar, and in places the quartz is in larger areas, so that 

 under crossed nicols there is an approach to the " patchy " areas so frequently seen 

 in the devitrified groundmass of acid rocks. The groundmass of this specimen may 

 be regarded as a product of devitrification. Magnetite is common. Some clouded 

 and reddened porphyritic feldspars occur, and some of these appear to be near 

 oligoclase. A few small porphyritic grains of dark green augite are seen, and there 

 are also much smaller green areas in the groundmass; some of these are nowchlur/fc 

 and others seem to be somewhat altered augites. 



One section examined. 



Age. Cabotian. u. s. G. 



No. 686. GRANOPHYRE. (Keratopltyre?) 



Prom the east side of Misquah lake; sec. 32, T. 64-1 W. 

 Ref. Annual Report, x, page 78. 



Meg. This rock is medium grained, brick red, basaltified and massive, forming 

 mountain-like hills several hundred feet in hight. The main structure dips at a high 

 angle toward the east. 



Mic. The reddened substance has an incipiently radiated arrangement, appear- 

 ing like spheruliths, the centres of which are less reddened, but this structure only 

 appertains to the coloring matter, since each of the spheruliths has a feldspathic 

 background, which extinguishes regularly over the whole radiated mass. There are 

 other feldspars which are not thus reddened, and others in which the reddened 

 (feldspathic?) substance is distributed in the manner of a micro-pegmatyte. Within 

 such a crystal, whose idiomorphic outlines are distinct, are a great many minute 



