546 N. H. DARTON FISH REMAINS IN ORDOVICIAN ROCKS 



of the limestone on the slopes below. In plate 74 are shown some prom- 

 inent outcrops of this member. In the canyons there are close, high 

 walls where the streams cross the formation, and a vertical cliff as the 

 rock rises in the slopes. 



The upper portion of the formation consists of limestones softer and 

 purer than those below, the bedding is thinner, color white to gray, and 

 parts of the rock are very compact or fine grained, often resembling 

 lithographic stone. There is considerable variation in the local features 

 of this member, and its thickness varies from 75 to over 100 feet. In its 

 basal beds corals occur, often in great abundance, especially along the 

 central side of the uplift. In the . greater part of the area there is in- 

 cluded, a short distance above the coralline beds, a layer of hard, massive 

 limestone with network of silica, similar to the great lower member of the 

 formation, but less marked in character and only from 15 to 25 feet thick. 

 Some shale and sandy limestone beds are also included. On the south 

 branch of Eock creek the upper member of the formation is an impure, 

 thin bedded, gray limestone which weathers to a reddish clay and contains 

 large numbers of fossils of the Eichmond fauna. 



The basal sandstone of the formation is a distinct member separating 

 the massive Bighorn limestone from the limestones and shales of the Dead- 

 wood formation. It is most extensively developed in the northern central 

 portion of the uplift, where its thickness usually is from 25 to 30 feet. 

 The rock is a moderately coarse grained, massive sandstone, mostly of 

 light gray color. It thins to the northwestward and is absent at some 

 localities in the vicinity of Shell creek and Little Bighorn river. It also 

 thins south of latitude 44 degrees and finally ends with the termination 

 of the Bighorn limestone a short distance north of Cheevers, a ranch 10 

 miles southeast of Bigtrails, excepting a small area on the West fork of 

 Powder river, 12 miles farther southeast, where it has a thickness of 4 

 feet. 



THICKNESS 



North of Powder river the Bighorn limestone rarely varies materially 

 from 300 feet in thickness, but in some localities the amount is slightly 

 greater, notably in the lower part of Shell Creek canyon, where there are 

 two beds of massive silicious limestone in the upper series. In Beartrap 

 canyon (due west of Mayoworth) and the ridges southwest the formation 

 shows rapid decrease in thickness, and finally it thins out southeast of 

 Bigtrails, where the Madison limestone lies directly on the eroded surface 

 of the Deadwood formation. In an outlying knob 8 miles southeast of 

 Bigtrails the limestone is 25 feet thick and separated from the Deadwood 



