PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 563 



Quart zyte.] 



Meg. A soft, fine-grained, pinkish to white or brown shale (also said to be at 

 times of a pea-green color). It is easily scratched with the finger nail. It shows 

 numerous small, silver-like, micaceous scales. In a closed tube the rock on heating 

 gives off water. 



Mic. The section is composed of three essential parts: (1) A few, usually 

 angular, quartz grains; (2) Much very fine, indistinct earthy matter; (3) A fibrous 

 and flaky mineral. This mineral has the general appearance of Muscovite, but not 

 the double refraction. There is a good cleavage and the extinction is parallel, or 

 nearly so, on this cleavage. Cleavage flakes show a bisectrix nearly perpendicular 

 to the cleavage; optic angle small, but distinctly biaxial; the mineral is optically 

 negative. It occurs also in fan-shaped or almost vermicular forms, and here the 

 fibrous or flaky character is distinct. The elongation is always positive. The double 

 refraction is apparently a little less than that of quartz. The species of the mineral 

 was not determined certainly, but it seems to agree best with kaolinite. 



One section. 



Age. Potsdam. u. s. G. 



No. 831. QUARTZYTE. (Spotted.-) 



Sioux Falls, South Dakota. 



Ref. Annual Report, x, pages 120, 121. 



Meg. A fine-grained, brown quartzyte, containing somewhat irregularly out- 

 lined spots of a flesh color. These spots vary from about one-sixteenth to one-fourth 

 inch across. 



Mic. The section is composed essentially of small, sub-angular, closely-packed 

 quartz grains in a sparse cement of red iron oxide. The spots seem to be due to areas 

 in which the red cement is lacking. 



One section. 



Age. Potsdam. u. s. G. 



No. 832. QUARTZYTE. (Hal) 



From Redstone, near New Ulm. 

 Ref. Annual Report, x, page 121. 



Meg. A fine-grained, red quartzyte. There are a few small spots of lighter 

 color in the rock. 



Mic. The section shows many sub-angular and rounded quartz grains and a 

 large amount of red and black iron oxide. There are also a number of grains, 

 probably originally feldspar, which are now clouded and stained red or are largely 

 made up of small kaolinic flakes. These small flakes are also quite common between 

 the quartz grains. Some of the quartzes show enlargements. One section. 



Age. Potsdam. u. s. G. 



