84 INVERTED KATE PALEONTOLOGY. [WHITE. 



facts before stated, that land and fresh and brackish water mollnsks are 

 comparatively valueless as indices of the passage of geological time, the 

 presence of no known forms in its strata forbid the reference of this group to 

 the Cretaceous period. 



CENOZOIC AGE. 



TERTIARY PERIOD. 



The collections of Tertiary fossils contain sixt^ species, exclusive of 

 those that were obtained from localities beyond the limits of the Plateau 

 Province. All of them are either brackish or fresh water species ; the only 

 truly marine forms of Tertiary age being those obtained at Bijou Basin, 

 Colorado, which have already been noticed, and, as they are also described 

 and catalogued on following pages, they will not be considered in the fol- 

 lowing general remarks, which are intended to apply mainly to that portion 

 of the Pro*vince which lies north of the Uinta Mountains. 



One-half of all these species were obtained from the Bitter Creek 

 Group, the lowest group of the Tertiary series. This difference in the rel- 

 ative abundance of species in the different groups is of course due primarily 

 to the conditions under which the species lived, but evidently in large part 

 also to the very much greater geographical extent, as well as greater thick- 

 ness, of this group than of any of the others. Among the primary condi- 

 tions referred to, an obvious one was, the continuance of the brackish 

 waters, so common in the last epoch of the Cretaceous period, into the first 

 epoch of the Tertiary in some localities, although they seem to have given 

 place to wholly fresh waters in other localities before the close of the Cre- 

 taceous period. Collections have been made from these brackish- water 

 Tertiary strata at Black Buttes. Point of 'Rocks, and Rock Spring, all in 

 the valley of Bitter Creek, Wyoming, and where they reach a thickness of 

 from five to seven hundred feet above the base of the Tertiary series. The 

 species are clearly distinct from all others, either of this or any of the other 

 Tertiary groups; but it is not to be denied that, although they are also all 

 specifically distinct from any of the species found in the underlying Point 

 of Rocks Group of the Cretaceous period, there is a prevalent similarity of 

 type between the fossils of the two groups that is apparent upon merely 

 casual inspection. 



