PERMIAN. 495 



Formations similar to those described occur about the Bighorn Basin and Moun- 

 tains, but the divisions have been called by local names, as equivalent formations 

 in the two areas are not identified. 



In the Bighorn Mountains Darton^"'^^^*" distinguished the Amsden formation, the 

 upper part of which contains a distinct Pennsylvanian fauna and the lower part of 

 which in some areas is of Mississippian age. Succeeding the Amsden, he recognized 

 the Tensleep sandstone (Pennsylvanian), the Embar formation (Pennsylvanian and 

 Permian?), and the Chugwater formation (Triassic or Permian). The Tensleep is 

 a massive white sandstone, in part calcareous, and the Embar consists of limestones 

 with associated shaly and cherty beds. The characteristic fossil of the Embar is 

 Spiriferina pulchra, which occurs just below the " Permo-Carbonif erous " of the 

 Wasatch Mountains and the Permian of the Grand Canyon. The Embar thins out 

 northward and in the northern part of the Bighorn Range the Chugwater formation 

 rests on the Tensleep sandstone. 



The Chugwater formation thus occupies part or all of the stratigraphic interval 

 between the Pennsylvanian and the marine Jurassic (Sundance formation), in 

 northern Wyoming. It is a typical red-bed formation from 600 to 1,200 feet thick. 

 Darton^^"*^ describes it as follows, in part: 



The rocks are mainly soft, massive, red, fine grained sandstones, merging into red shale, 

 but extensive gypsum deposits and a few thin limestone beds are also included. * * * 



The general character of the Chugwater deposits does not vary greatly in different portions 

 of the region. Toward the top and bottom red shales predominate, while in the middle soft 

 sandstones and alternations of sandstones and shales are the principal features. Near the 

 base there is invariably a thin bed of purplish-gray limestone, generally varying in thickness 

 from 5 to 15 feet. Fifty feet or more higher there is another bed of limestone, mostly massive, 

 of dirty-gray or buff color, in which weathering develops a characteristic porous structure. 

 This bed has a thickness of 10 to 15 feet along the east side of the uplift and to the south, but 

 along the west-central and northwestern slopes of the mountain its thickness increases con- 

 siderably and it forms a conspicuous ledge. Toward the top of the formation there is always 

 a succession of thin-bedded, light-colored limestones separated by red shales. These lime- 

 stone bodies vary from two to four in number, and they are usually separated by from 10 to 

 50 feet of shales. Gypsum deposits occur in the Chugwater formation in irregular bodies in 

 the lower and upper portions, the most persistent bed or series of beds being not far from the 

 base. The formation attains its greatest thickness in the region northwest and southwest of 

 Buffalo, where it measures 1,230 feet. In the region west of Sheridan the amount is 1,100 feet 

 but it gradually diminishes to the north, being only about ,600 feet at Bighorn River. On the 

 west side and southern portion of the range the average is about 800 feet, but it diminishes 

 somewhat to the north, the amount being not over 700 feet near Horse Creek. 



Darton cites occurrences of fossils from the lower part of the Chugwater forma- 

 tion which are, however, poorly preserved or for other reasons indeterminate as to 

 age. In the opinion of Girty and Schuchert they are probably Permian. The 

 upper part at least of the Chugwater is regarded as probably of Triassic age, and 

 both Darton ^^'® and Fisher ^'^* doubtfully assign the whole formation to the Triassic. 



K 13. BLACK HILLS, SOUTH DAKOTA. 



The Permian of the Black Hills comprises some indeterminate part of the red 

 beds, which Darton has separated into the Opeche, Minnekahta, and Spearfish for- 



