CARBONIFEROUS SYSTEM 413 



about Leo. Three miles southeast of Leo tlie red shale, supposed to rep- 

 resent the basal member of the Amsden formation, is underlain by 110 

 feet of hard limestone. A fe\r miles farther east, near Shirley, tliis 

 lower limestone is ?5 feet thick, while on the south side of the Shirley 

 hills, at a point 9 miles west-northwest of Difficulty, there are about 400 

 feet of limestone under the red shale. The upper portion of tliis lime- 

 stone is white, fine-grained, massive, weathers light dove-colored, and 

 erodes in pinnacled forms, features which characterize the upper member 

 of the Madison limestone in the Bighorn mountains. 



Fossils. — The Madison limestone is sparingly fossiliferous, and Spirifer 

 centronatus, Seminula Juimilis, and Chonetes loganensis are the species 

 of most frequent occurrence. Dr G. H. Girty has determined the follow- 

 ing from the head of Sage creek, on the Shoshone reservation, northwest 

 of Lander: SchucherteUa inftata f, Chonetes iUinoisensis, Produclus sp., 

 Spirifer centronatus, S. striatns, var. madisonensis, and Seminula hu- 

 milus ?. From the Madison limestone on north slope of Eattlesnake 

 mountains a short distance west of Garfield peak the following were iden- 

 tified: Zaphreniis sp., Fenestella sp., SchucherteUa inftata ?. Chonetes 

 iUinoisensis, Productus lavicosta, Spirifer centronatus, Spiriferina soti- 

 divostris, Cleiothyris crassicardinalis ?, Camerotoechia sp., Mod iota ? sp., 

 Straparollus luxus, S. aff. o])tusus, Bellerophon sp., Prcetus sp., and an 

 ostracod undetermined. 



From near the base of the limestone 3 miles southeast of Leo many 

 fragments were collected, among which Doctor Girty has determined 

 Spirifer striatus var. madisonensis and a Cleiothyris ? sp. 



The fossils indicate that the formation is of Mississippian age, etpiiva- 

 lent to the Madison limestone of j\[ontana and the Pahasapa limestone of 

 the Black hills. 



AMSDEN FORMATION 



General relations. — The red shale, sandstone, and cherty limestone of 

 the Amsden formation extend along the northeast slope of the ^Yind 

 Eiver range, and they appear also on Eattlesnake mountains and on 

 North Platte river south of Alcova. In the Casper and Laramie moun- 

 tains where the formation is much less characteristic, it is included in the 

 Casper forniation. 



Otvl Creel mountains. — In the Owl Creek uplift the Amsden forma- 

 tion varies from 200 to 2.50 feet in thickness, and it outcrops along both 

 slopes of the range, excepting locally where it is cut by faults. It sur- 

 mounts cliffs of Madison limestone in Bighorn canyon and along tlie 

 south front of the mountain westward nearly to the Lander-Thermopolis 

 road, and is prominent on the east slope of Black mountain and the ridge 



