210 GEOLOGY. 



ward to the meridian of 107 30', with a north and south extension 

 not yet definitely known. This depression was immediately occu- 

 pied by an early Eocene lake, whose northern portion corres- 

 ponded with approximate accuracy to the present drainage basin of 

 / Green River. Southward it extended through portions of Utah, 

 Y__ New Mexico, Colorado, and probably Arizona." (Clarence King). 

 Along with this uprising of the western portion of the continent,, 

 there was an epoch of mountain making at the close of the Laramie 

 period, as already stated. The Wasatch and Uinta mountains- 

 were further folded and raised, and the Colorado range was greatly 

 elevated. This folding helped to make the depression towards the 

 Wasatch and on each side of the Uintas, which became the bed 

 of the great Eocene lake, referred to above. This emergence to- 

 wards the north, and on the west of the continent, the greater ele- 

 vation of its mass, and the retreat of the seas necessarily produced 

 great changes of climate. The mean temperature had gradually 

 become lower, and the extremes greater. The climate also became 

 drier. And yet it was warmer and moister than at present, as is- 

 evident from the vegetable and animal life of the time. All the 

 old Cretaceous forms had disappeared, or had been, by changes of 

 environment, transformed into the modern representatives. Thus 

 was inaugurated the Cenozoic Age. 



The Cenozoic Era, or Age of Mammals, comprises two periods^ 

 namely: First, The Tertiary; second, The Quaternary. 



TERTIARY PERIOD. 



Lyell divided the Tertiary into three divisions, which were 

 named from the number of species of fossil shells which they con- 

 tain, and which are living in existing seas. They are the Eocene, 

 Miocene and Pliocene. Other divisions are in use in the east and 

 south, but as Lyell's method is most convenient, and his divisions 

 the most characteristic of the west, they are followed in this work. 



Eocene Epoch. As already stated, there are no deposits of this 

 period in Nebraska. During the whole of it, Nebraska was an 

 extended land surface. The forces that had finally lifted the conti- 

 nent from the embrace of the sea, during the closing centuries of 

 the Cretaceous period, had extended their work to the region of the 

 plains, and made them dry land. During the whole of the Eocene,, 

 therefore, Nebraska was an extended land surface. What really 

 occurred here during this period, can only be inferred from the veg- 



