64 G. R. Mansfield— Trias sic and 



stone, formerly considered Triassic or Jurassic, the Twin 

 Creek limestone, the Preuss sandstone, and the Stump 

 sandstone. The Nugget sandstone in this region is 

 apparently conformable with the Wood shale below. 

 Schultz, 15 however, believes that in southwestern Wyom- 

 ing, at least, the Nugget sandstone will prove to be uncon- 

 formable on beds corresponding to the Wood shale. The 

 contact between the Nugget and the overlying Twin 

 Creek, though apparently uniform, is also believed to be 

 an unconformity. The Twin Creek limestone is over- 

 lain unconformably by the Preuss sandstone and another 

 unconformity separates the Stump sandstone from the 

 overlying Cretaceous (?) formations. The total thick- 

 ness of the system is about 6,500 feet. 



Changes in nomenclature. — Before 1914 the Twin 

 Creek limestone was the only recognized Jurassic forma- 

 tion in southeastern Idaho ■ although the overlying rocks 

 which had been correlated with the Beckwith formation 

 of southwestern Wyoming were considered in part Juras- 

 sic. The Twin Creek is a marine limestone with some 

 highly fossiliferous beds. The upper part has been 

 determined as Upper Jurassic. Possibly the lower part 

 may include Middle Jurassic rocks. In 1914 field work 

 in the rocks that had been classified as Beckwith forma- 

 tion developed the presence of marine Upper Jurassic 

 fossils in beds now assigned to the Stump sandstone, 

 about 1,300 feet stratigraphically above the top of the 

 Twin Creek limestone. The term Beckwith has been dis- 

 continued in southeastern Idaho and the rocks there 

 formerly included by it are now distributed among seven 

 formations of which the first two, the Preuss and Stump 

 sandstones, are Jurassic and the remaining live, under- 

 lain by a pronounced unconformity, are tentatively 

 assigned to the Cretaceous. 16 With the assignment of 

 the Nugget to the Jurassic the base of the system is 

 lowered at least to the top of the Wood shale. It is 

 possible that further field studies may justify again low- 

 ering the base to include the group now tentatively con- 

 sidered Triassic. 



15 Schultz, A. E., personal communication. 



16 Mansfield, G. E., and Eoundy, P. V. : Eevision of the Beckwith and 

 Bear Elver formations of southeastern Idaho, U. S. Geol. Survey, Prof. 

 Paper 98, pp. 75-84, 1916. 



