From Eocene to Recent Time 



233 



agency, to earth shrinkage and crustal flexures, or to local- 

 ized weighting of the crust by sedimentary deposits, need 

 not now concern us. 



The lowest stage of the freshwater Eocene strata has 

 variously been called the Wasatch in western North 

 America, the London Clay in England, the Suessonian in 

 France, the Liburnian in Germany, the Hypsiprimnus beds 

 in Tasmania. 



But the frequent intercalation of marine beds in most 

 of these shows that terrestrial oscillations were proceeding. 

 The Wasatch beds of north-west America were evidently 

 formed in a great inland lake, called by Cope the Wasatch 

 Lake, having a length at least of about 500 miles, and a 

 maximum width of 300. These beds may be as much as 

 4000 feet thick, and have yielded a varied vertebrate fauna. 

 The corresponding London Clay was in part freshwater, 

 in part marine, and the same is true of deposits over mid 

 and east continental Europe. 



Above the Lower Eocene in central North America 

 are the extensive Mid-Eocene Green River and Bridger 

 beds of about 2000 feet average thickness, and that are 

 probably wholly lacustrine in origin. These are extremely 

 rich in vertebrate remains, especially mammalian. 



Fig. 33. 



Fig. 32. Amia calva. A surviving North American fish of the 

 group Holostei, the earliest known ancestors of which occur in 

 freshwater Eocene beds of Wyoming and adjacent states. 



Fig. 33. Skeleton of the same fish, that shows close structural 

 relation to some Telcosts. (After Brown Goode and Franque). 



