480 THE GEOLOGY OP MINNESOTA. 



[Diabase. Rhyolyte. Granite. 



No. 665. DIABASE. (Lustre-mottled.) 



From the hill back of Grand Marais and 800 feet above lake Superior. This specimen was taken on the 

 old trail ("Iron trail") which ran from Grand Marais north and northwestward past South Devil Track lake. 

 The locality of the specimen is not certain, but it probably was obtained from near the northwest corner of sec. 

 17, T. 61-1 E. 



Ref. Annual Report, x, pages 65, 71. 



Meg. A quite fine-grained rock of a dark-brown color. It shows clearly the 

 lustre-mottled character due to augite plates of considerable size. 



Mic. The most noticeable feature of the section is the presence of large plates 

 of augite thickly studded with small lath-shaped plagioclases. The augite plates do 

 not usually touch each other, and between them the feldspar is more abundantly 

 developed and the lath-shaped individuals are not sharply separated from a back- 

 ground of more or less allotriomorphic plagioclases. Besides the augite and plagio- 

 clase there are considerable amounts of a black opaque substance which is largely 

 magnetite and hematite and which may in part represent an unindividualized base 

 and in part possibly olivine. This black substance occurs in ai;eas of irregular shape 

 in the spaces between the augite plates, and some of it represents decomposed augite. 

 Chlorite and apatite are also present. From extinction angles in sections normal to 

 010 the feldspars appear to be as basic as labradorite. 



One section examined. 



Age. Cabotian or Manitou. u. s. G. 



No. 666. EHYOLYTE. 



Prom the northeast side of South Devil Track lake; E. % sec. 31, T. 62-1 E. Many loose pieces of this 

 rock occur here, but no rock is seen in place. 

 Ref. Annual Report, x, page 71. 



Meg. A fine-grained, reddish-gray rock containing a few small porphyritic 

 quartzes and feldspars. 



Mic. The section shows rounded porphyritic quartzes in a fine-grained ground- 

 mass of quartz and the usual reddened feldspar, magnetite and hematite. It is possible 

 that this groundmass is a product of devitrification, although we do not think that 

 this is the case. In ordinary light many needle-like transparent areas are seen to 

 penetrate the groundmass in all directions; they remind one somewhat of apatite 

 needles, but in polarized light they are seen to be quartz which extinguishes in 

 connection with the surrounding quartz areas. 



One section examined. 



Age. Cabotian. u. s. G. 



No. 667. GRANITE (with augite). 



Beach on south shore of Devil Track lake at the Iron trail; S. W. % sec. 30, T. 62-1 E. This rock is not 

 in place, but from its abundance and its fresh angles it is evidently the country rock. 

 Ref. Annual Report, x, page 72. 



