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



Eldridge as upper Wyoming and lower Wyoming. The geologists of 

 the Hayden survey perceived the distinction between the two, classifying 

 the upper series as Triassic in some cases and in others joining them to 

 the Morrison under the head of Jurassic. 



The upper series consists mainly of fine grained red sediments, with 

 beds of gypsum and thin beds of limestone, which I believe represent the 

 southern extension of the Chugwater formation described on previous 

 pages. Accordingly this formation is equivalent to the upper Wyoming 

 of Eldridge. In the Arkansas valley the Red beds have been designated 

 the Fountain formation by Cross and Gilbert and the Badito formation 

 by Hills. From my own investigations I have come to the conclusion 

 that the Fountain formation is the southern extension of the lower 

 Wyoming of Eldridge, for both have characteristics representing the 

 same conditions of sedimentation and with identical stratigraphic rela- 

 tions, some of which I find to be more important than heretofore recog- 

 nized. The continuity of the outcrops is somewhat obscured by overlap 

 of younger formations in the Arkansas-Platte divide and the various 

 protuberances of old crystalline rocks, but sufficiently significant char- 

 acteristics and stratigraphic relations continue throughout. 



In the Chugwater formation or upper Wyoming in northern Colorado 

 I have found that the Minnekahta limestone almost certainly is repre- 

 sented, and underlying the formation there is a southern extension of 

 the Tensleep sandstone, constituting the top of the lower Wyoming, 

 and traceable at least as far south as Colorado Springs. The upper 

 " Red Bed " series (upper Wyoming) is believed not to be present in the 

 Arkansas valley, but apparently it reappears in southeastern Colorado, 

 between the Morrison formation and the coarser grained red sandstones 

 believed to represent the Fountain formation. 



The age of this upper series of Red beds is not definitely known. It 

 usually has been regarded as Triassic for the reason that some known 

 Triassic beds west of the Rocky mountains consist of similar rocks. In 

 later reports the Hayden Survey classified the lower Wyoming beds as 

 Triassic and the upper Wyoming, together with the overlying Morrison, 

 as Jurassic. From fossils found in the Minnekahta limestone in the 

 Black Hills region, a horizon which appears to be continuous through 

 the lower portion of the upper Wyoming beds in Colorado, it is believed 

 that at least the lower third of these beds is Permian ; but even if this is 

 the case, the upper portion may be Triassic and represent either all or a 

 part of that period. At their top there is an unconformit}^ which may 

 represent Triassic time together with the Jurassic time, as Jurassic beds 

 are absent to the southward. In the region in which the upper Wyo- 

 ming beds appear to be absent, southward from the Colorado Springs 



