THE CENOZOIC AGE. 211 



etable and animal life that is found entombed in the Eocene beds 

 of the mountains. The record there is comparatively full — here 

 there is none whatever. At the present day nearly 500 species of 

 Nebraska plants grow in the mountains, and on the foot-hills. 

 The proportion of animals common to the two regions is still 

 greater. In Eocene times the differences in level and climate were 

 probably not near/so great as now. It is therefore highly probable 

 that the larger number of vegetable and animal forms that then 

 flourished around the shores of this old Eocene lake in the moun- 

 tains, also lived in Nebraska. Unfortunately, many species, also, 

 that then existed here did not range so far west, and therefore no 

 memorials of their presence have been preserved. 



Clarence King has recognized four groups of the Eocene, which 

 he has named as follows: 



1. Vermillion Creek Group. — This is the Wasatch Group of Hay- 

 den. Lowest Eocene, 5,000 feet thick. 



2. Green River Group. — Hayden and King. Middle Eocene, 

 2,000 feet thick. 



3. Fort Bridger Group. — Hayden and King. Lower and middle 

 horizon of the upper Eocene, 2,500 feet thick. 



4. Uinta Group. — King. Upper Eocene, shading into Miocene, 

 500 feet thick. 



In these groups we have the most complete memorials of the 

 higher land and fresh water life of the Eocene of the continent. 

 The Gulf Alabama Eocene beds are much less complete, as they 

 begin at a much higher horizon than the Vermillion beds. As the 

 Eocene is not present in Nebraska, I will omit the lithological and 

 physical description of these beds, referring only to such particulars 

 as may throw light on Nebraska's geological history during those 

 times. 



The Length of the Eocene Epoch was very great. This is inferred 

 from the 12,000 feet of sediments that were accumulated in the 

 bottom of the Rocky Mountain Eocene lakes. Many of the sedi- 

 ments of the Green River and Fort Bridger groups are of the 

 character that accumulate with extreme slowness. A large part, 

 too, of the upper beds, where they constitute the surface rocks, has 

 been removed by erosion. Their original thickness, therefore, 

 must have been much greater than at present. The estimates of 

 time, however, are made from the remnants of these beds. It has 

 been estimated that at the most rapid rate, not more than one-fourth 



