406 N. H. DARTON STRATIGRAPHY OF THE BLACK HILLS, ETC. 



shales lying on typical Dakota sandstone. Thirty feet above the Graneros 

 sandstone are the Mowrie beds. The Greenhorn limestone is not clearly 

 defined at the north end of the range, but it appears southward near 

 Horse creek, having a thickness of only 3 inches, separated from the 

 Mowrie beds by 450 feet of dark shales. The rock is a sandy limestone 

 containing numerous Inoceramus labiatus. Next above are 230 feet of 

 Carlile beds containing near the top concretions carrying Prionotropis 

 ivoolgari, the fossil which characterizes this horizon throughout the North- 

 west. The following section gives the details of the Benton beds west 

 of Horse Creek station, Wyoming : 



Section of Benton Beds West of Horse Creek Station, Wyoming 



Feet 

 Niobrara Limestone 



' Black shale 10 



Sandstone and sandy shale 20 



j Gray shales, with concretions containing Prionotropis near 



top 200 



Carlile. 



Greenhorn. . Sandy limestone, with Inoceramus labiatus. 



Shales, dark and fissile below 450 



Hard shales and thin bedded hard sandstones, weathering 



Graneros light gray, with fish scales (Mowrie beds) 80 



Dark shale 30 



Hard coarse sandstone, massive 25 



Dark shales, fissile to soft .... 150 



Dakota Sandstone, buff, coarse, mostly soft 20 + 



At the north end of the range the Mowrie beds are overlain by about 

 500 feet of dark shales extending to a veiy conspicuous sandstone layer 

 20 feet or more in thickness, which is very fossiliferous in the Muddy 

 Creek valle}\ The principal forms were determined by Mr T. W. Stan- 

 ton as Inoceramus fragilis and Cardium panpereulina. This sandstone is 

 overlain by several hundred feet of dark shales with some thin layers 

 of sandstones and horizons of concretions, representatives of the Carlile 

 beds. 



NIOBRARA FORMATION 



The chalky deposits of the Niobrara extend northward from Colorado, 

 but have greatly diminished thickness near the northern end of the Lara- 

 mie range. On Horse creek and the Chugwater the formation consists 

 of limestone and impure chalk, grading into limy shale, its basal mem- 

 ber being a soft, light gray, massive limestone filled with a typical Ino- 

 ceramus deformis. There are two other limestone beds above, separated 

 by limy shales, while at the top is the usual chalky bed which weathers 

 to a bright straw or pale ocher color and contains thin limestone masses 



