PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 765 



Mica schist. Magnetite.] 



M ic. The fine hematite particles are formlessly grouped, with intervening spaces 

 which were probably occupied by some other substance, but which in the slide are 

 entirely empty except for a dust which is doubtless derived from the process of 

 grinding followed by careless washing. It is hence impossible to detect from this 

 slide the steps of last transition. One section. 



Age. Animikie. N. H. w. 



No. 1708. MICA SCHIST. (Hornfels with cordierite. ) 



Near Partridge river, a little west of Allen junction. 



Ref. Annual Report, xxi, page 155. (Compare Nos. 370H, 387H.) 



Meg. Black or gray, collected as a part of the "black slate" of the Animikie. 

 Fine grained and dense. 



Mic. The slide is thickly sprinkled with small isolated scales of biotite and a 

 little magnetite. Mingled with the biotite scales are a few of tourmaline, which 

 resemble the biotite, but are distinguished readily by noting the direction of greatest 

 absorption, which is parallel with the horizontal spider line, that in biotite being 

 parallel with the vertical line. The vertical axis of tourmaline is n p while with biotite 

 the elongated sections have n f perpendicular to their greater dimension. These 

 little crystals of tourmaline also show the characteristic basal cleavage. The 

 section in general is noteworthy for containing much cordierite in good preservation. 

 This mineral forms the largest crystalline element. It encloses poikilitically 

 all the other grains. Its pleochroism is very low (indeed, it is inperceptible), 

 as in cordierite from the schists of contacts. Small interlocking grains of a 

 feldspar whose index of refraction is lower than that of the cordierite, are 

 probably of alb'tte. Quartz shares in the fine interlocking matrix. There are 

 also many small globular grains of high refractive index, which are probably diop- 

 side, their small size preventing the usual high colors of double refraction. Three 

 sections. 



Age. Animikie. 



Eemark. By the kindness of Prof. A. Lacroix it has been possible to compare 

 this rock with the almost identical metamorphic Devonian schists of Canterets of the 

 upper Pyrenees, and with numerous other cordierite schists. The metamorphism 

 produced on the Devonian schists is due to granite contact, but in Minnesota this 

 metamorphism is to be attributed to the gabbro which lies near adjacent to the 

 outcrop. N - H - w - 



No. 1709. MAGNETITE. (Ore.) 



S. W. J4 N. W. J4 sec. 4, T. 62-11, near the north end of White Iron lake. 

 Ref. Annual Report, xxi, page 155. 



Meg. Associated with vitreous quartz and with hornblende. 



