GEOGRAPHICAL DEVELOPMENT OF THE AMERICAS 109 



the land-connection with Europe must have been across 

 the North Atlantic, most likely from Greenland eastward. At 

 the present time a land-bridge in such high latitudes would be 

 of little service in bringing about a similarity of mammals in the 

 two continents, for the severity of the Arctic climate would be 

 as effective a barrier against the intermigration of all save the 

 Arctic mammals as the ocean itself; but in the mild and genial 

 Eocene climate the latitude of the bridge was of small conse- 

 quence. 



The second of the Eocene stages, the Wind River — Green 

 River, is found in two very different phases. The Wind River 

 phase occupies the basin of that stream, north of the Wind 

 River Mountains in central Wyoming, and in the Big Horn 

 Basin of the same state it very extensively overlies the Wasatch, 

 and in this phase the sediments are very like those of the latter, 

 flood-plain and wind accumulations. A widely distant area 

 of this stage occurs in the Huerfano Canon in Colorado. The 

 Wind River beds contain numerous mammals which were 

 clearly sequential to those of the Wasatch, of which they were 

 the more or less modified descendants. With two possible 

 exceptions, there were no new immigrants and the connection 

 with the Old World may have been already severed, as it as- 

 suredly was in the succeeding age, the Bridger, though diver- 

 gent development had not yet had time to produce the very 

 striking differences in the mammals of North America from 

 those of Europe, which characterized the Bridger. 



The Green River phase is a thick body of finely laminated 

 "paper shales," which seem to have been deposited in a very 

 shallow lake and occupy some 5000 square miles of the Green 

 River valley in southern Wyoming and northern Utah, where 

 they overlie the Wasatch, just as do the Wind River beds in 

 the Big Horn Basin. These fine-grained and thinly laminated 

 shales have preserved, often in beautiful perfection, countless 

 remains of plants, insects and fishes, but no traces of mammals, 

 other than footprints, have been found. 



