RESUME — NIOBRARA, PIERRE. FOX HILLS 447 



is a conpicuous feature in Colorado and for some distance northward 

 into Wyoming. 



Along the foot of the Rocky Mountain and Laramie front ranges the 

 formation usually presents three limestone layers— a lower massive bed 

 and two upper layers, separated by limy shales, the uppermost overlain 

 by impure limestones, which weather to a bright yellow color and al- 

 ways contain numerous flat masses of limestone consisting of colonies 

 of Ostrea congesta. The formation thins to the northward in Colorado 

 and Wyoming, becoming|about 400 feet thick northwest of Cheyenne. 

 On the slopes of the Black hills the average amount is about 200 feet, 

 but the amount is about 100 feet at the north end of the uplift. The 

 bright yellow color of the weathered beds is a conspicuous feature. 

 The formation is not characterized along the eastern slopes of the Big- 

 horn mountains, although doubtless it is there represented by some 

 gray shales, not distinguishable from those of the adjoining formation, 

 which carry Carlile fossils below and Pierre fossils above. 



PIERRE 



The great shale series of the Pierre formation occupies a vast area in 

 the central Great plains, and probably originally it was of greater extent, 

 for apparently it has been removed by erosion in the mountain uplifts 

 and in eastern Nebraska and southern and eastern Kansas. No special 

 investigation has been made of the Pierre stratigraphy, and, although 

 the beds appear to be very uniform in composition, probably a careful 

 study of the distribution of its numerous fossils would show widespread 

 stages. One of these is the upper horizon of concretions with Lucina 

 occidentalism giving rise to " tepee buttes," which appears to extend from 

 the Arkansas valley through Colorado to and all around the Black hills. 

 In places along the western margin of the area great variations in thick- 

 ness are presented, the shales becoming thicker and local sandstone beds 

 being included. West of Denver the formation appears to have a thick- 

 ness of over 7,700 feet, and it is considerably over 3,000 feet thick at 

 Florence and near Boulder. 



FOX HILLS 



The Fox Hills formation appears to be present everywhere between 

 the Pierre shale and the Laramie formation, merging into both forma- 

 tions and constituting beds of passage between them. In some districts 

 the Fox Hills beds begin abruptly, there being a sudden change from 

 the dark shales of the Pierre to sandstones or sandy shales of the Fox 

 Hills containing some distinctive species. It is probable that this change 



