568 THE GEOLOGY OF MINNESOTA. 



[Slate. Schist. 



No. 847. SLATE. 



Little Falls. 



Ref. Annual Report, xiii, page 39. Same as Museum No. 2681 (Annual Report, vii, page 78). 



Meg. A fine-grained, dark gray slate, having the slaty cleavage at a consid- 

 erable angle with the bedding. Scattered porphyritically through the rock are 

 small glistening flakes of some mineral whose species was not determined. 



No section. 



Age. Upper Keewatin(?) u. s. G. 



Remark. Later a section was made of this rock and the glistening flakes were 

 immediately seen to be biotite. It had been, owing to its manner of occurrence, 

 taken for ottrelite, but it is optically negative. N. H. w. 



No. 848. MICA SCHIST. 



Little Palls. 



Ref. Annual Report, xiii, page 39. Same as Museum No. 2690 (Annual Report, vii, p. 78). 



Meg. A fine-grained mica schist, apparently composed essentially of quartz 

 and biotite. 



Mic. The rock is composed of a fine-grained, granular aggregate of quartz, 

 feldspar and biotite, with a little magnetite. The rock might be called a granulyte. 

 The feldspar is clear, untwinned, and very rarely shows cleavage. It is easily 

 confounded with quartz, but in converged polarized light its biaxial character is 

 distinct. It is optically positive and has a lower index of refraction than quartz. 

 It is quite likely albite or cordierite. 



Two sections. % 



Age. Upper Keewatin(?) u. s. G. 



No. 849. MICA SCHIST. ( Staurolitic. ) 



Pike Rapids, Morrison county, near the mouth of Swan river. 



Ref. Annual Report, xiii, page 39. Same as Museum No. 2689 (Annual Report, vii, page 78). 



Meg. A mica schist of medium grain, composed of biotite and quartz and also 

 probably feldspar. There are a number of staurolite crystals, about an inch long, 

 scattered through the rock, and there are also small red garnets not larger than a 

 pin's head. 



Mic. The section was evidently made from one of the staurolite crystals, and is 

 thickly strewn with small grains of quartz (and perhaps also feldspar); it contains 

 also a few grains of magnetite and garnet. The section is quite thick and the ple- 

 ochroism of the staurolite is very marked; the color varies from a light yellow 

 (almost colorless) to a deep brownish yellow. 



One section. 



Age. Upper Keewatin(?) u. s. G. 



