474 



GEOLOGY OF THE BLACK HILLS. 



tlf<(,ini)(»sc(l, lejiviiio- small empty holes in the rock, wliich appearance is not 

 (iftcn found with mica. There is no cementing material present. 



The next rock [20.')], also from Amphibious Creek, is much darker 

 than tile preceding, being of a brown color. In the section, it is seen that 

 the (piart/ is in i(nuided grains, cemented together by a material showing 

 the wave-like lines and traces of a former fluidity, as mentioned before. 

 In .some parts of the slide this is very dark and almost opaque from oxide 

 of iron, the edges of the quartz grains also being colored very dark brown 

 by the same pigment. Quite striking is the appearance of the brightly 

 polarizing cementing mass, which is penetrating silica, between the dark- 

 boidered (piartz grains. This rock is again rather a sandstone than a 

 quartzite. 



The specimen from the locality southeast of Terry Peak [20GJ has a 

 decomposed, earthy luster of reddish-brown color, plainly showing the 

 presence of nuuli oxide of iron. Under the microscope, it was observed 

 that the quartz grains were rounded, but with very little cementing mate- 

 rial between them, although large masses of opaque oxide of iron were 

 scattered abundantly through the rock. Needle-like microlites were seen 

 in the quartz, and in the rock some brown mica and occasionally a little 

 hornblende. 



The rock from Red Canon Creek [207] consists also of rounded grains 

 of quartz, held together by a very dark or black cement. Nothing else of 

 note was observed, except some magnetite. 



In [208], from the mouth of French Creek, the quartz is in grains a 

 litth^ rounded, but pure and clear, with very little or no cementing mass. 

 There is some mica, opaque and probably weathered. The rock is of a 

 light-reddish color. 



The quartz in [200], from Hall's Water, is very pure and crj^stalline, 

 containing microlites and cavities, the former being in the shape of short, 

 .stout crystals. The white mica is very clear and transparent, polarizing 

 finely, and showing lines of lamination. There is also some brown mica 

 j)resent, and a few garnets are easily recognized by their isotropic behavior 

 with crossed nicols. The rock is very light colored, with glistening spangles 



