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if, as was probable, the main migrations took place from this 

 Continent. It is better at once to recognize this principle, 

 rather than attempt to bring into exact parallelism, formations 

 that were not strictly contemporaneous. 



The freshwater Eocene deposits of our Western Territories, 

 which are in the same region at least two miles in vertical 

 thickness, may be separated into three distinct subdivisions. 

 The lowest of these, resting unconformably on the Cretaceous, 

 has been termed the Vermilion Creek, or Wahsatch, Group. It 

 contains a well-marked mammalian fauna, the largest and most 

 characteristic genus of which is the ungulate Coryphodon, and 

 hence I have called these deposits the Coryphodon Beds. The 

 middle Eocene strata, which have been termed the Green 

 River and Bridger Series, may be designated as the Dinoceras 

 Beds, as the gigantic animals of this order are only found here. 

 The uppermost Eocene, or the Uintah Group, is especially 

 well characterized by large mammals of the genus Diplacodon, 

 and hence may be termed the Diplacodon Beds. The fauna of 

 each of these three subdivisions was essentially distinct, and the 

 fossil remains of each were entombed in different and successive 

 ancient lakes. It is important to remember that these Eocene 

 lake-basins all lie between the Bocky Mountains on the east 

 and the Wahsatch Bange ou the west, or along the high 

 central plateau of the Continent. As these mountain chains 

 were elevated, the enclosed Cretaceous sea, cut off from the 

 ocean, gradually freshened, and formed these extensive lakes, 

 while the surrounding land was covered with a luxuriant trop- 

 ical vegetation, and with many strange forms of animal life. 

 As the upward movement of this region continued, these lake- 

 basins, which for ages had been filling up, preserving in their 

 sediments a faithful record of Eocene life-history, were slowly 

 drained by the constant deepening of the outflowing rivers, and 

 they have since remained essentially dry land. 



The Miocene lake-basins are on the flanks of this region, 

 where only land had been since the close of the Cretaceous. 

 These basins contain three faunas, nearly or quite distinct. The 

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