86 THE AGE OF MAMMALS 



13,000 to 14,000 feet, with corresponding elevation of the intermediate 

 basins and with a continuous erosion of their summits. The mountain 

 ranges which were involved in this grand movement and defined the basin 

 areas which are of such extraordinary interest to the palaeontologist, are as 

 follows: The Wasatch Range, which now extends north and south for 

 over 100 miles on the east side of the Great Salt Lake Basin. East of the 

 south half of the Wasatch Range, stretching away for 150 miles, are the Uinta 

 Mountains (now over 13,000 feet in height); at the angle of a junction be- 

 tween these ranges is a great outflow of igneous rock (trachyte) . Eastward 

 of Utah, the Elk Mountains, the San Juan, and the Front Range of Colorado, 

 including the famous Spanish Peaks, were uplifted. To the west the Pacific 

 coast ranges, in time of elevation, lagged far behind the Sierra Nevada and 

 even behind the Rocky Mountain ranges, remaining at sea level long after 

 the Rocky Mountain system was born. 



Mountain basin deposits. — The oldest basin is the San Juan (Fig. 21 , 

 PT) in northwestern New Mexico, which includes the Basal Eocene, Puerco 

 and Torre JON, and Lower Eocene depositions, all lying on top of the Laramie 

 or Upper Cretaceous. As displayed in the accompanying map (Fig. 21) the 

 grandest basin area (W) is the Wasatch, lying east of the Wasatch Range and 

 north of the Uinta Range, with an original width of 300 miles and north 

 and south extent of 500 miles. This deposition concludes with the Lower 

 Eocene Wasatch deposits, scattered over an area 450 miles north and south 

 and 250 miles east and west to a thickness of 1,500 to 2,500 feet. Superposed 

 on these deposits are the Green River shales, also Lower Eocene, and 

 above these the Bridger (B) formation. 



Mode of origin. — The underlying Wasatch is of fluviatile and flood 

 plain origin, while the Green River shales cover an area of more than 

 5,000 square miles, and represent a vast shallow lake. The Bridger is 

 regarded as fluviatile and flood plain, as is also the succeeding Washakie 

 (WK). In northern Wyoming, west of the Big Horn Mountains, is another 

 basin (BH) filled with a flood plain deposition of Wasatch age, and in west 

 central Wyoming is the Wind River Basin (WR) of Lower and Middle 

 Eocene age. East of the Front Range in southern Colorado, close to the 

 Spanish Peaks, is the Huerfano Basin (H) of Lower and Middle Eocene 

 age; and directly south of the Uinta Mountains is the great Uinta 

 Basin (U), extending east and west over 140 miles and containing deposi- 

 tions of Middle and Upper Eocene age. To the northwest, in Oregon, are 

 the John Day Basin (7) of Oligocene age and the Deep River (8) of 

 Middle Miocene age. 



The successive periods of deposition in these mountain basins is deter- 

 mined through the abundant remains of fossil mammals and other verte- 

 brates which they contain, and it is a remarkable fact, brought out through 

 palaeontology, that these depositions were partly contemporaneous, partly 

 successive, so that they give us a wonderful panorama of the entire succession 



