PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 467 



Granite.] 



devitrified glass. A dissimilarity exists in the absence from No. 641 of olivine. 

 Irving's rock is, however, from near the same place, viz.: S. E. Jsec. 9, T. 51-12 W., 

 about four miles southwest from the mouth of Knife river. 



One section. 



Age. Cabotian. N. H. w. 



No. 642. GRANITE (fine and red). 



"From about one mile north of the centre of T. 62-1 E. on Mayhew's trail;" perhaps in section 17, near 

 the present Grand Marais and Rove Lake road. 

 Ref. Annual Report, x, pages 66, 67. 



Meg. A very fine-grained sample of the red rock; reddish brown on a fresh 

 fracture and brick-red on weathered surfaces. Small reddish feldspars mixed with a 

 greenish and a black mineral and small amounts of quartz. 



Mii; The section is composed of feldspar, quartz, magnetite and yellowish and 

 greenish alteration products. The feldspar, as is usual in the red rocks, is clouded 

 and reddened and frequently is developed in lath-shaped forms. It frequently shows 

 a zonal structure which is indicated by the disposition of the reddened' parts; some 

 of the crystals have a red exterior, then a band of transparent substance and a centre 

 of red. In one case such a crystal appeared to be entirely replaced by quartz, but 

 this is not the case with all of them. The feldspar shows practically no polysynthetic 

 twinning and appears to be orthoclase, although its species was not determined with 

 certainty. The quartz is sometimes in small grains, but is more usually intergrown 

 with the feldspar to form the micropegmatyte which is so characteristic of these 

 red rocks. One side of the slide shows an area which perhaps represents part of a 

 porphyritic feldspar now highly altered. 



One section examined. 



Age. Cabotian. 



Remarks. The original nature of the rock is not certain, but, if the quartz is 

 assumed to be largely original, the rock would best be called a very fine-grained 

 red granite. u. s. G. 



No. 643. GRANITE (loith augite). 



Hill range in the north tier of sections in T. 62-1 E. Probably near the northwest corner of section 3 on 

 the present Grand Marais and Rove Lake road. 

 Ref. Annual Report, x, pages 66, 67. 



Meg. There are two small hand specimens of this rock. One is of a brick-red 

 color and drusy texture; it is a medium-grained aggregate of reddish feldspar and 

 quartz, the compact mass of feldspar making the quartz appear as almost porphyritic. 

 There are a few feldspars, brighter red in color, which are sub-porphyritic. The 

 other specimen is of the same general character but is of a flesh color. The quartz 

 is not in such prominent grains and there are a few scattered grains of a black 

 mineral (augite). The slides examined were made from this second specimen. 



