INTliODUGTION 



Among the many extinct animals discovered in the Tertiary deposits 

 of the Rocky Monntain region, none, perliaps, are more remarkable than 

 the huge mannnals of the order Dhwcerata. Their remains have hitherto 

 been found in a single Eocene lake-basin in Wyoming, and none are 

 known from aii\' other part of this country, or from the Old World. 

 lliese gigantic l)easts, wliich nearly equalled the elephant in size, roamed 

 in great numbers about the borders of the ancient tropical lake in which 

 many of them were entombed. 



This lake-basin, now drained by the Green River, the main tributary 

 of the Colorado, slowly filled up with sediment, but remained a lake so 

 long that the dejjosits formed iu it, during Eocene time, reached a vertical 

 thickness of more than a mile. The Wasatch Mountains on the West, 

 and the Uinta cliain on the South, were the main sources of this sediment, 

 and still protect it, but the Wind River range to the North, and other 

 mountain elevations, also sent down a vast amount of material into this 

 great fresh- water lake, then more than one liundred miles in extent. 



At the present time, this ancient lake-basin, now six to eight 

 thousand feet above the sea, shows evidence of a vast erosion, and 

 probably more than one-half of the deposits once left in it have been 

 washed away, mainly through the Colorado River. What remains forms 

 one of the most ])icturesque regions in the whole West, veritable 

 Maiivaiscs tcrrd's, or bad lands, where slow denudation has carved out 

 cliffs, peaks, and colunuis of the most fantastic shapes, and varied colors. 



