696 THE GEOLOGY OF MINNESOTA. 



[Peridotyte. Qnartzyte. 



Meg. Heavy with magnetite, but fresh. 



Mic. The rock consists essentially of olivine and augite, with a few scattered 

 grains of plagioclase and a notable amount of magnetite. 



The augite embraces the other minerals poikilitically, but it is not a common 

 mineral although it exists in some large grains.* 



The appearance of the olivine suggests at once the occurrence of hypersthene, 

 because it is conspicuously cleaved and frequently presents parallel extinction. But 

 a closer examination shows that the grains cleaved and not cleaved have the same 

 index of refraction, and a very high double refraction, even higher than ordinary 

 olivine. Sections that show the highest colors are the most distinctly cleaved, and 

 those that show no regular cleavage exhibit a bisectrix n p , the axial angle being 

 rather small. This form of olivine has the special name fayalite. 



One or two small grains of labradorite( ?) serve to show the thinness of the 

 slide, and hence to furnish an estimate of the double refraction of the fayalite. 



Some reddish yellow portions are apparently boiclingite. They are nearly dark 

 between the nicols. 



Age. Cabotian (modified jaspilitic Keewatin). 



Remark. This form of peridotyte might take the name of picryte or cumber- 

 laiidyte. This rock is illustrated by figure 11, plate II. N. H. w. 



No. 1338. QUARTZ YTE. (Gray.) 



Associated with the ore No. 1336. 



Eef. Annual Report, xvi, pages 85, 121; Bulletin vi, pages 118, 127, 422. 



Meg. Compact, vitreous, with magnetite. 



Mic. Quartz, with some augite and magnetite. One (thick) section. 



Age. Pewabic (modified jaspilyte of the Keewatin). N. H. w. 



No. 1339. QUARTZYTE. (Pyroxenic.) 



Chub lake. Pyroxenic portions of the strata associated with the ore. 

 Ref. Annual Report, xvi, pages 85, 121 ; Bulletin vi, pages 118, 127, 422. 



Meg. Dark with pyroxene. 



Mic. The rock is about equally divided between quartz and diallagic augite. 

 From the latter many minute fibres of octinolite( ?) radiate into the quartz, along 

 the margins where the quartz and augite unite. In another section some distinct 

 form of amphibole is developed, twinned like grunerite. So far as can be seen the 

 quartz with its numerous inclusions is latest in origin. Magnetite grains are in both 

 quartz and augite. Four sections. 



Age. Pewabic quartzyte (modified jaspilyte of the Keewatin). N. H. w. 



* This mineral has a higher refractive index than the olivine, which it surrounds, and hence cannot be hornblende, as 

 identified by Bayley (Ninteenth Annual Report, p. 197, fig. 3), and it cannot be hypersthene, since it is not perceptibly pleochroic, 

 and has not parallel extinction in sections cut in the zone 001: 100. Bayley 's figure, however, does not agree with his descriptive 

 text, where this mineral is described as augite. 



