480 GEOLOGY OF THE BLACK HILLS. 



structure, with the larger masses of mica and quartz, place the rock as mid- 

 way between mica-schist and clay- slate, but it belongs more to the schists 

 than the slates. It is therefore a mica-slate. 



The slate [65] from Rapid Creek is similar in appearance to the pre- 

 ceding, but much more homogeneous and slaty, cleaving easily into very 

 thin pieces. Under the microscope, was observed, as before, quartz grains 

 and dark. mica in a very fine groundmass of a schistose structure. There 

 are fewer large masses of mica than in [64], the rock approaching more 

 nearly the clay-slates. A little plagioclase was also noted. This is again 

 a mica-slate. 



The specimen [67], from the same locality, shows a precisely similar 

 character to [64] and [65], even to the presence of a small piece of 

 plagioclase. 



The rock from Rapid Creek [69] resembles the preceding ones, but is 

 much more compact and has a greenish color, differing also in the absence 

 of any macroscopical mica. Under the microscope, the difference is remark- 

 able; quartz in pure, clear grains predominates, between which are small 

 long-shaped crystals of white mica arranged in the same parallel direction, 

 so as to occasion the slightly schistose character of the rock. These mica 

 crystals are almost exactly like those in [1] from French Creek, but much 

 smaller. The large, brown masses are wanting, however, there being only 

 one such piece in the slide. Remarkable is the abundance of magnetite in 

 good sized grains, which have a beautiful submetallic luster. The magne- 

 tite seems to follow in part the lines of direction of the mica crystals. The 

 quartz contains microlites, but they were too minute for determination. 

 From the above observations, this rock is an exceedingly compact and fine- 

 grained mica-schist, as it does not have the fibrous groundmass of the slates 

 [64], [65] and [67]. 



The next, however, from lower Rapid Creek [71], is very much the 

 same as [64] and [65], except that the color is quite reddish and the 

 appearance weathered. In the section, it was noted that the groundmass 

 is quite fibrous in structure and that there is less quartz scattered through 

 it. The mica appears quite black, opaque, and negative in behavior in 

 polarized light. There is also a reddish-brown mineral in considerable 



