34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



a plain which was nearly level. The fossiliferous beds deposited in 

 this sea in eastern Wyoming constitute the Sundance formation. 

 These beds are uniformly thin over a large area. Even where the 

 Sundance has not been clearly differentiated from the rocks above 

 and below it there is little room for variation in its thickness within 

 the region represented by the sections here described. There are 

 many critical places for which no convincing descriptions are obtain- 

 able. Perhaps the most confusing of these are in eastern Utah, 

 where gypsum occurs at two horizons nearly 1,000 feet apart. As 

 the rocks of the lower horizon contain marine Jurassic fossils, they 

 have been correlated with the fossiliferous and gypsiferous rocks 

 elsewhere. But there remains the possibility that some of these beds 

 elsewhere may represent the upper, rather than the lower, gypsiferous 

 horizon. 



It is possible, as already suggested (p. 27), that the difficulties in 

 correlating these rocks with those of other regions may be due to 

 the presence of marine Jurassic rocks at two horizons, whereas only 

 one has been recognized. 



DETERMINATION OF AGE 



The Sundance fauna is described as similar to the Oxfordian 

 fauna of Europe. On this basis the Sundance formation has been 

 correlated with the Oxford, which in Europe represents a stage 

 referred by some geologists to the base of the upper third of the 

 Jurassic system and by others to Middle Jurassic. It has been said, 

 therefore, that the Sundance belongs in the lower part of the Upper 

 Jurassic or the upper part of the Middle Jurassic. If this correla- 

 tion is correct and if the overlying or Morrison beds are Lower Cre- 

 taceous in age, as many geologists believe, evidence of a hiatus 

 should be found at the top of the Sundance. But little evidence of 

 such hiatus has yet been found unless the absence of certain beds, 

 as indicated by the groups of sections, be accepted as evidence. The 

 structural relations are such as would be expected if the Sundance 

 were late Jurassic, formed near the close of the period. On the 

 other hand, if the Sundance is represented by the beds in the middle 

 of the La Plata group, and if any considerable part T)f the upper 

 La Plata sediments accumulated after the retreat of the sea, the 

 time of this accumulation must be represented by a hiatus at the top 

 of the Sundance where upper La Plata is not represented. 



In considering the evidence of a lapse of time between the Sun- 

 dance and the overlying Morrison, the physical conditions of this 



