110 GEOLOGV OF Till: BLACK HILLS. 



canons cut by the various streams as they pass from the Hills to the Plains, 

 >v]K'ro by the gradual dip of" the strata one member after another is carried 

 down and out of sij^lit; and even in the canons it was not always possible 

 to study them closely in detail. 



The gray limestone immediately overlies the thin-bedded pink lime- 

 stones, and according to many measurements taken in different parts of the 

 Hills has a thickness of 150 to 200 feet. It is a very uniform, pure lime- 

 stone, and one of the most prominent members of the Carboniferous series. 

 It is well exposed in every examined section of the Carboniferous rocks. 

 Its fossils are numerous, but are not readily separable fi-om the matrix excei)t 

 where the rock has been so greatly weathered as to destroy their surface 

 markings. What few species were recognized appear to be equally charac- 

 teristic of the overlying silicious limestone. 



The silicious limestone is a hard, white or grayish, gritty rock, which, 

 from the oxidation of its iron and perhaps also by percolation from the 

 overlying rocks, is generally stained irregularly red or pinkish. It contains 

 much silicious matter concentrated in spots, either as flint, or as crystalline 

 quartz lining small cavities, or in concretionary or agate-like deposits. 

 From its silicious and uneven composition it decomposes very in-egularly, 

 often having a brecciated appearance and weathering with cavities or crevi- 

 ces, and even with caverns of considerable extent. The cavities are usually 

 lined with crystals of calcite. 



The most abundant fossils in the two limestones are Spin/era Rocky- 

 montana, Athijris subtil ita, Productus, Euoniphalus, and Sircptorhynchus. 



While the lower members of the formation are well marked, easily 

 traced, and readily distinguished from each other and from the underlying 

 Potsdam, the study of the upper portions was attended with no little embar- 

 rassment. In the absence of any paleontological assistance of value, the 

 upper limits of the formation had to be drawn from the lithological nature 

 of the deposits. While the conclusion here reached is open to question 

 and is liable to be set aside whenever fossils are brouffht to bear on the 

 problem, I am strongly disposed to think that none of the beds here included 

 in the Carboniferous will ever be relegated to the Trias. 



hnmediatcly overlying and conforming to the silicious limestone are 



