476 GEOLOGY OF THE BLACK HILLS. 



rounded, a few only being sharp and angular, and in some are cavities and 

 microlites. The glauconite bodies have a bright grass-green color, their 

 texture being tolerably uniform, with the exception of a few dark spots 

 and cracks in the larger ones. The grains vary in size from quite small to 

 very large oval, while the forms and shapes are also different, depending 

 upon the plane of section. Some are long and narrow and others round 

 or oval; the large ones are bean-shaped and quite symmetrical, a few being 

 broken and of angular outlines. They do not change color in polarized 

 light. A few columnar and banded bodies in the cementing mass are frag- 

 ments of shells curved in form. The cement is quite abundant and is 

 colored very dark-brown by oxide of iron, which is in numerous small, 

 round grains, seldom having a square outline. 



SECTION II. 

 MICA-SCHISTS AND SLATES. 



The rocks described in this section belong chiefly to the two classes of 

 schists and slates. Mica-schist is a crystalline mixture of mica and quartz, 

 both minerals being present generally in good sized crystals; and, even 

 when of minute size, their crystalline character can be distinctly seen under 

 the microscope. Examples of the above cases are the rocks [1], from 

 French Creek, which is a very fine-grained mica-schist, and [13] and [27], 

 which have a coarser structure, while [69], from Rapid Creek, is almost a 

 compact rock, showing, however, the crystalline mica and quartz under the 

 microscope. 



Mica-schist has the schistose structure more or less marked, accordingly 

 as the mica or quartz predominates. When the rock becomes more homo- 

 geneous in character and of a darker color, being micro-crystalline in struc- 

 ture, with a slaty cleavage and appearance, it is called mica-slate, the 

 "Thonglimmerschiefer" of Cotta and Zirkel. This is an intermediate rock 

 between mica-schist and clay-slate, differing from the former in closely 

 resembling a clay-slate, and from the latter in being distinctly made up of 

 crystalline quartz and mica, which can easily be observed under the micro- 

 scope in a thin section of the rock. The specimens from Rapid Creek 

 [64], [65], [67] and [71] are mica-slates answering to this description. 



