295 



The base of the Eocene is usually regarded as containing a small per 

 cent of the marine mollusks yet living ; the beginning of the Miocene, 

 about IT per cent of yet existing species ; and the beginning of the Pliocene 

 about 36 per cent. If now plants have changed in species during the lapse 

 of geological time with about the rapidity that marine mollusks have 

 changed, the Fort Union beds ought to be arranged in the Lower Miocene. 

 This would harmonize quite well with the idea that the Green River beds 

 belong to the Oligocene. 



9. Relationship of Lance Creek Fauna to That of the Judith Rivek 



Epoch. 



Having demonstrated, as I think I have, that there was, between the 

 time of the deposition of the Lance Creek beds and those known as Puerco 

 and Fort Union, a nearly complete change in the fauna and a considerable 

 change in the flora, I will endeavor to show that the fauna of the former 

 beds is closely related to that of the Judith River, a formation now recog- 

 nized as being well down in the Upper Cretaceous and separated from the 

 lowermost Laramie by about 1.000 feet of marine Cretaceous strata 

 (Stanton, Wash. Acad. Sci., xi, p. 256). This close relationship of the two 

 faunas has been recognized, it may be truthfully said, by all paleontolo- 

 gists who have given attention to the subject. For a long time it misled 

 geologists and paleontologists into the conclusion that all the deposits in 

 question belonged to a single epoch. Mr. J. B. Hatcher, who had collected 

 extensively both in the Judith River region and in the Lance Creek beds, 

 and who had studied closely the vertebrates of both regions, writes (Bull. 

 U. S. Geol. Surv., 257, p. 101) : 



When considered in its entirety, the vertebrate fauna of these beds 

 [Judith River] is remarkably similar to, though distinctly more primitive 

 than, that of the Laramie [Lance Creek beds]. Almost or quite all of the 

 types of vertebrates are present, though, as a rule, they are represented 

 by smaller and more primitive forms. 



Doctor T. W. Stanton, paleontologist of the U. S. Geological Survey, 

 who examined in company with Professor Hatcher the Judith River basin, 

 and who has given especial attention to the invertebrate fauna, records in 

 the same bulletin (p. 121) his opinion: 



When full collections are compared it will usually be easy to distin- 

 guish between Judith River and Laramie from the brackish-water fossils 

 alone, but if the collections are meager and fragmentary it may not be 



