638 THE GEOLOGY OP MINNESOTA. 



[Biotite schist. Magnetite. Peridotyte. 



No. 1039. BIOTITE SCHIST (vrith cordier ite). 



First island south of the last, in Illusion lake; sec. 13, T. 64-8. 



Ref. Annual Report, xv, pages 360, 395; Annual Report, xxi, page 146. 



Meg. Biotite gneiss, showing a contorted structure. 



Mir. Besides the biotite, which is idiomorphic, there is considerable quartz and 

 scattered fine magnetite. But most of the slide is abundantly strewn with cordierite. 

 The quartz embraces all the other minerals poikilitically, forming large plages as in 

 the aporhyolytes. It is rather difficult to distinguish the cordierite from the quartz 

 in numerous instances, the most available optic character being the Becke line, 

 which always moves toward the mineral having the higher refractive index when 

 the objective is slightly raised by the fine adjustment screw. There is a little 

 oligoclase-andesine which forms, with alb ite, a microperthite. The crystals of this 

 plagioclase are larger than the cordierite grains and enclose them, and are also 

 broadly zoned. They were later to form than the biotite phenocrysts which pierce 

 them. 



The fortunate juxtaposition of these white minerals whose general aspects are 

 so similar affords occasion and fine opportunity for the application of some of the 

 nicer microscopical methods. The photographic illustration of this slide (plate II, 

 figure 8) shows an oligoclase-andesine, somewhat zoned, containing the microper- 

 thitic albite, also quartz surrounding it and embracing numerous cordierite grains. 

 The order of the minerals in date of generation seems to have been cordierite (not 

 much earlier than biotite), biotite, oligoclase-andesine, quartz. One section. 



Age. Keewatin modified by the gabbro revolution. N. H. w. 



No. 1040. MAGNETITE (with hyperstliene). 



A little west of the meander corner of sections 28 and 20. N. E. % sec. 29, T. 64-7, Thomas lake. 

 Ref. Annual Report, xv, pages 360, 395. 



Mac. Appearing like an iron ore. 



Mic. The iron ore embraces masses of hypersthene, but between the ore and the 

 hypersthene is usually an olivine rim. One (thick) section. 



Age. Cabotian modification of Keewatin iron ore. N. H. w. 



No. 1041. PERIDOTYTE. ( Cumber laiulyte. ) 



The old mining place on Frascr lake, near the section line between sees. 23 and 24, T. 04 7. 

 Ref. Annual Report, xv, pages 360, 395. 



Mry. A dark, heavy rock, apparently containing magnetite. A part of the 

 banded muscovadyte accompanying the iron ore of the locality. 



Mir. The slide consists almost entirely of oliviue, which, having evident 

 cleavages at right angles, and ,. in the acute optic angle, is fuyalite. There are 

 many dark bauds of inclusions in this mineral, running parallel with the cleavage 



