604 GEOLOGY OF THE YELLOWSTONE NATIONAL PAKE. 



Many of the species that are considered distinct are closely related. 

 It should be remembered, also, that we know only fragments of the fauna 

 that must have existed at that time if it approached in size those that 

 are now living. Whitfield records only 43 species from all the Black Hills 

 country, and we now have about the same number from the Yellowstone 

 region. If more exhaustive collections were made in both districts, it is 

 probable that the list of common species would be considerably increased, 

 but even as the record stands it shows rather close relationship of faunas. 

 Possibly the lowest Jurassic beds in the Yellowstone region may be slightly 

 older than the lowest in the Black Hills, but the difference in age can not 

 be great — not great enough, as it seems to me, to put them in different divi- 

 sions of the Jura. Throughout all the Rocky Mountain region, wherever 

 marine Jurassic strata are found they are only a few hundred feet in thick- 

 ness and they rest directly on the Triassic "Red Beds" or on older forma- 

 tions. It does not seem possible that Upper Jurassic marine beds could 

 have been deposited in the Black Hills and Wyoming without leaving any 

 traces in the Yellowstone, Montana, and Utah — that is, the stratigraphic 

 relations and the geographic distribution of the marine Jurassic of the 

 Rocky Mountain region are in favor of the idea that all of these deposits 

 were made contemporaneously in a single sea. 



CRETACEOUS. 



Dakota (?) formation. — The collection shows that several horizons of the 

 Cretaceous are represented in the Park. The lowest of these, according 

 to the geologists, is a thin bed of limestone, not far -above the local base of 

 the Cretaceous section, that is filled with fresh-water gastropods and a few 

 Unios. This fauna at once suggests a comparison with the fresh-water 

 forms (Idoplacodes veternus and Vivipants gilli) from beds of supposed 

 Jurassic age overlying the marine Jura in Wind River Valley, Wyoming, 

 but these forms are not represented in the Park collections. 



The few species obtained do not show their generic characters very 

 distinctly; still it is evident that they are not closely related to the fresh- 

 water Bear River fauna of southwestern Wyoming nor to the few fresh-water 

 forms known from the Dakota of Nebraska, both of which seem to hold 

 about the same stratigraphic position as this bed. There is one other 

 possibility, and that is that the Lower Cretaceous Kootanie formation is 



