138 THE GEOLOGY OP MINNESOTA. 



[Mica schiat. 



No. 44. MICA SCHIST. (" Black Rock." ) 



Duluth. From the top of the hill at the head of First Avenue East. Very fine grained, black, like a 

 basalt. Apparently this is what has been known later as " the black rock." 

 Compare Nos. 1966 and 1967. 

 Ref. Annual Report, ix, page 17. 



Meg. Even with a loop nothing can be seen that would distinguish this rock, 

 in the specimens at hand, from a very fine diabase. 



Mic. But under the objective, and especially between crossed nicols, it is seen 

 at once to be very different from any of the fine-grained rocks so far described. It 

 has no ophitic structure, but is finely granular, with many translucent areas resem- 

 bling quartz. These are of irregular shapes, but sometimes several contiguous darken 

 simultaneously, as if they had the same crystalline orientation. In other areas the 

 darkening comes on in spots and disappears in the same way, showing several inde- 

 pendent grains. In general, finally, this same mineral (quartz) seems to extend as 

 a cementing framework throughout the section, being invisible in other places on 

 account of the presence of brightly polarizing or of opaque grains of other minerals. 

 The interference figure, so far as seen, consists of a broad straight bar which crosses 

 the field, indicating a uniaxial mineral. 



Besides the foregoing are two other principal minerals which make up much of 

 the rock. One is entirely opaque and black, and on reflecting surfaces has the 

 metallic lustre of magnetite. It is scattered promiscuously as a powder, with no 

 definite crystalline form, or is aggregated into granular masses of some size, the 

 cement even then being the same quartz. The other is in very fine grains or scales 

 which, in ordinary transmitted light, give a brown-gray coloration when they are 

 frequent and superposed, somewhat resembling in this respect the color of biotite. 

 Between the crossed nicols they extinguish four times in a revolution. Sometimes 

 they show an elongation, but usually they are of irregular shapes and of sub-oval or 

 orbicular angular outlines. When they are lengthened the extinctions take place 

 parallel to the elongation. Sometimes they are aggregated in sufficient thickness 

 to give a polarization color of reddish-yellow or of light blue, but their principal 

 effect is to darken the matrix of quartz in which they are intimately embedded. 



While these are the principal minerals there are also calcite and r/m/o/< in small 

 amounts. 



One section examined. 



Age. Taconic (Animikie). 



It't'DHi >/.*. This rock is the same as rock No. l,of the series collected at Duluth 

 by Prof. A. Lacroix, in 1888, deposited at the College de France. The biotite has the 

 forms and relations to the quartz characteristic of "contact" mica as distinguished 

 from granitic mica and the mica of gneissic rocks, i. e., it embraces the quartzes, 



