NORTH AMERICA AND THEIR VERTEBRATE FAUNA. 63 



with the Strata of the limestones to the northward, the difference in the lithological 

 characteristics being due to the varied physical conditions during sedimentation." 

 In 1907, Darton reported finding a fossiliferous limestone 150 feet below 

 the top of the Red Beds on the west slope of the Bighorn Mountains, 3 miles 

 north of Thermopolis, Wyoming.^ The Red Beds are here nearly 1,000 feet 

 thick and lie upon upper Carboniferous limestones and sandstones. This 

 limestone carries a Permo-Carboniferous fauna similar to that which has been 

 found in the basal portions of the Red Beds in adjacent regions. The occur- 

 rence of the fauna in the limestone mentioned leaves only 150 feet to repre- 

 sent the Triassic. In the vicinity of Laramie, Knight found fossils of upper 

 Carboniferous age. It was found that on both sides of the Laramie Moun- 

 tains the upper Carboniferous limestones and sandstones shade into red beds 

 toward the south. These are the same beds which were called lower Wyo- 

 ming by Eldridge on the Rocky Mountain front and Fountain by Gilbert and 

 Cross. The Chugwater Red Beds wrhich overHe the upper Carboniferous to 

 the north continue unchanged on the Rocky Mountain front and overlie the 

 lower Wyoming and the Fountain formations. Williston regarded the beds 

 here called Chugwater as Triassic, from the evidence of vertebrate fossils,'' 

 but the invertebrate fauna is apparently Permo-Carboniferous. 



In 1909, Darton" wrote: 



"The Pennsylvanian division, which consists mainly of shales and sandstones 

 in northern Wyoming, contains thick bodies of limestone, but on both sides of the 

 Laramie Mountains, south of the latitude of Laramie, this mostly gives place to 

 sandstone, largely of a red color." 



In the same paper he stated that "in the eastern Front Range in Wyo- 

 ming limestone predominates in the Casper formation to the north, but much 

 reddish sa^dstone is included ; farther south and in Colorado the formation 

 changes to red sandstones, constituting the 'lower red beds' or Fountain 

 formation " (plate 19, figs, i and 2). And again: ^ 



"Fossils found at various horizons in limestones in the Chugwater Red Beds 

 do not afford conclusive evidence as to age. Those in the lower Hmestones in the 

 Bighorn Mountains and other uplifts are believed to be 'Permian' in the sense in 

 which the term is used in the Mississippi Valley. Many fossils occur in the Hme- 

 stone 150 feet below the top of the formation, on the east bank of Bighorn River, 

 3 miles below Thermopolis. According to Dr. G. H. Girty, the principal species is 

 Natica lelia, usually considered diagnostic of the Triassic, but probably it is older. 

 BakewelUa and probably Pleurophorus may also be present, and an Aviculipecten 

 occurs resembling A. curiicardinalis, which is characteristic of the Permo-Carbon- 

 iferous of Utah. Dr. Girty is inclined to correlate the fauna with the Permo- 

 Carboniferous of the Wasatch Mountain section. The 150 feet of red shales and 

 sandstones which overlie this limestone may possibly represent part of the Tri- 



» Darton, Bull. Geol. Soc. Amer., vol. 17, p. 724, 1907. 



•> Williston, Jour. Geol., vol. 12, 1904. 



• Darton, Bull. Geol. Soc. Amer., vol. xix, p. 403, 1909. 



^ Darton, U. S. Geological Survey, Professional Paper No. 62. 



