INTRODUCTORY REMARKS. LXI 



have been enjoyed by the Dinosauria in this country than is yet. known to 

 have been granted them in the Old World. 



But it may be asked, are we to regard all such fossils as of no use whatever 

 in the determination of the ages of strata"? Certainly not, because, even in case 

 future discoveries in this country and the Old World should never modify the 

 present conclusions in regard to the geological range of fossil plants and these 

 types of the Reptilia, so as to enable us to use them with more certainty as 

 a means of drawing parallels on opposite sides of the Atlantic, they will 

 undoubtedly be useful, when viewed in their specific relations, for the identi- 

 fication of strata within more limited areas. That is, when all or most of the 

 details of the stratigraphy of the whole Rocky Mountain region and the ver- 

 tical range of species have become well known, these fossils will perhaps be 

 found nearly as safe guides in identifying strata at one locality with those of 

 others there, as many other kinds. 



TERTIARY ROCKS OF 'J HE WIND RIVER AND WHITE RIVER GROUPS. 



Wind River group. — This group is not certainly known to exist at any 

 other localities than in the Wind River Valley and west of the Wind River 

 Mountains, Wyoming. It certainly has not been identified in the Upper 

 Missouri country proper, and is only mentioned here because two species 

 from it are described farther on. It probably has no near relations to either 

 the White River or the Fort Benton group, but has been supposed to be of in- 

 termediate age, and consequently placed provisionally between them in sections. 

 It consists of gray and ash -colored sandstones, with more or less argillaceous 

 layers, and has been estimated by Dr. Hayden to attain a thickness of fifteen 

 hundred feet or more. So far as explored in the Wind River region, it did 

 not seem to be very fossiliferous ; but this may have been because favorable 

 localities were not there met with. It is probably of Miocene age. The 

 only vertebrates found there were one species of Trionyx and one of Testudo; 

 while the invertebrates collected include one species of Viviparus resembling 

 V. trochiformis, a Helix?, and a fine large Macrocydis. The latter two of 

 these only are figured from this rock in this report (see plate 42). 



No marine or brackish-water types are known to occur in this forma- 

 tion, which is probably wholly or in part. equivalent to that since called the 

 Bridger group. 



White River group. — This group is one of the most interesting in all 



