PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 577 



Quartzyte. Jaspilyte]. 



amount of a black, dust-like material, which may be carbonaceous matter. Quartz 

 makes up the bulk of the rock. Calcite, chlorite and muscovite flakes are also common. 

 The " knots " are due to a colorless mineral which includes little of the black, dust- 

 like material. In the description of this rock in Bulletin ii this colorless mineral is 

 spoken of as chiastolite; but it does not seem to have a high enough index of refrac- 

 tion or a strong enough double refraction for chiastolite. Moreover, it was found to 

 be optically negative instead of positive. Its species was not determined, but it may 

 be some form of feldspar. In it small muscovite flakes have developed and also 

 numerous minute grains which are perhaps zoisite. 



Two sections. 



Age. Keewatin. u. s. o. 



No. 865. QUARTZYTE. 



Point in Vermilion lake near S. W. ^ sec. 34, T. 62-16 W. 

 Ref. Annual Report, xv, pages 278, 386. 



Meg. A tine-grained, light, brownish-gray, somewhat schistose rock containing 

 some fine micaceous scales. 



Mic. The section shows a rock composed largely of a fine-grained mosaic of 

 quartz. With the quartz are numerous small flakes of muscovite and chlorite. The 

 arrangement of these flakes gives the schistose structure to the rock. There are a 

 number of feldspar grains of irregular outline and of larger size than the usual quartz 

 grains. These are considerably clouded by the development of minute muscovite, 

 and perhaps also kaolinite flakes. 



One section. 



A<je. Keewatin. u. s. o. 



No. 866. JASPILYTE. 



Jasper ridge, Tower; sec. 29, T. 62-15 W. 



Ref. Annual Report, xv, pages 264, 266, 386; Bulletin vi, pages 79, 421. 



Mr;/. The usual banded, red, white and black jaspilyte. The bands vary from 

 a sixteenth of an inch to two inches in width, and some of the red bands are very 

 finely laminated by red and gray colors. The black bands are the narrowest and are 

 composed essentially of magnetite. The white bands are of finely crystallized quartz 

 and the red bands of quartz and hematite. 



Mic. The section is composed of a very fine-grained mosaic of quartz, with 

 which is much hematite. The latter is commonly in more or less rounded grains 

 which vary from minute, dust-like particles to those nearly the size of the quartz 

 grains. The hematite occurs both between and in the quartz grains. A few scattered 

 grains of magnetite occur throughout the rock. The finely-laminated nature of the 

 red bands is due to a concentration of hematite along certain parallel lines, but this 



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