292 THE GEOLOGY OF MINNESOTA. 



[Quartzyte. Diabase. 



the grain, but is invisible about the exterior, owing to the total reflection which 

 darkens the peripheries of the grain. The general contour is pear-shaped. Both 

 rutile and sphene are doubtless results of change from titaniferous magnetite con- 

 tained in the original rock. 



Two sections examined. 



Age. Cabotian. N. H. w. 



No. 265 A. QUARTZYTE. (Dark gray.) 



Birch island, Wauswaugoning bay; S. E. J sec. 36, T. 64-6 E. Darker and firmer parts, appearing some- 

 what like dikes. " Birch island is caused by four hardened belts in the quartzyte and slates, from five to ten 

 feet wide, which run east and west, making the slates darker and in spots basaltic, and yet showing in other 

 spots their bedded slatiness. These belts resemble dikes of igneous rock, and they run as a reef almost to the 

 shore northwardly." 



Ref. Annual Report, ix, page 64. 



Meg. A fine'-grained, hard, heavy, compact, dark-gray, impure quartzyte, look- 

 ing almost like a diabase. Quartz grains, and a few of feldspar, are seen embedded 

 in a darker background. 



No section. 



Age. Animikie. u. s. G. 



No. 265B. QUARTZYTE. 



Birch island. Shows peculiar fractures due to glaciation (?). 

 Ref. Annual Report, ix, page 64. 



Meg. Sample not found. 

 No section. 



Age. Animikie. u. s. G. 



No. 266. DIABASE. 



South shore of Pigeon point, east of Birch island; probably in S. J^ sec. 31, T. 64-7 E. From a dyke which 

 cuts the quartzyte and contains scattered pyrite. 

 Ref. Annual Report, ix, page 64. 



Meg. A rather-fine grained, dark-gray diabase. 



Mic. The rock is considerably altered. In structure it is ophitic. The original 

 minerals are plagioclase, augite and magnetite. The secondary minerals are magnetite, 

 a greenish chloritic substance and a dirty brownish opaque material. These altera- 

 tion products seem to have come from the augite, although some unindividualized 

 material may have been present. Minute needles of apatite are common, penetrat- 

 ing all the minerals of the rock except the magnetite. One small porphyritic plagio- 

 clase, having minute twinning lamellae which are seen only with a high power, is 

 present. 



One section. 



Age. Cabotian. u. s. G. 



No. 267. DIABASE. 



"Basaltic rock from the main vein, containing a calcite centre, and which is about twenty -five feet wide." 

 A dike. South shore of Pigeon point, east of Birch island; probably in S. % sec. 31, T. 64-7 E. 

 Ref. Annual Report, ix, page 64. 



