ISLAND RIDGES IN THE PLAINS. 29 



"Woman Fork, from which point was g-ained our first view of tlie Bhick 

 Hills, there were seen, low down in the northeast horizon, several low ele- 

 vations of the older rocks, all trending- in the same northwest direction 

 and dipping- towards the east, like the monoclinal ridges just mentioned. 

 Though these ridges were noticed on several occasions along- the route, and 

 seen to extend far to the northwest, the character of our march was such as 

 to prevent any close or detailed examination of them. At the head of Old 

 Woman Fork, however, a ridge was examined and found to be composed of 

 the following strata, beginning at the top : 2. Gray or whitish limestone, in 

 places showing an interstratification of reddish brown calcareous clays. 

 Thickness GO to 75 feet: dip 25'^ southeast. The limestone contains in 

 places much flint, chalcedony, and agate, and man}^ fossils of well-known 

 Carboniferous types — Athyris suhtlUta, Spin/era camerata, Procluctus, &c. 1. 

 Reddish and whitish sandstone, similar in character to that observed pre- 

 viously, and probably of Silurian age. 



Surrounding and overlapping the monoclinal uplift were seen the soft 

 beds of the Tertiary. 



The unconforming relation which the siu'rounding horizontal Tertiary 

 bears to these old lines of uplift shows that the latter were upheaved, and 

 were carved by denuding agencies before the Tertiary beds were deposited, 

 and that these were spread over them like a sheet, concealing their irregu- 

 larities. They have again been brought to light by the erosion of the 

 Tertiary. 



Southwest of Rawhide Butte, between it and the Laramie Mountains, 

 and a short distance northwest from Fort Laramie, several points of 

 upheaval have been noticed by other observers, exposing granitoid rocks 

 with overlying Carboniferous and Cretaceous strata, and, as indicated by 

 the descriptions, they have a similar structure to those already noticed. 

 These uplifts, with Rawhide Butte and the associated ridges, have been 

 considered to form a line of connection between the upheaval of the Lara- 

 mie Mountains and that of the Black Hills. Though less continuous they 

 have much of the character of the hog-backs, or ridges of sedimentary 

 rocks, which are so marked a feature of the flanks of the Rocky Mountains. 

 But unlike the hog-backs, which are the remnants of antichnal folds of the 



