158 



THE AGE OF MAMMALS 



for a rich fauna, and of late most accurately surveyed by the American 

 Museum parties. 



Clarence King ^ believed that the Bridger and Washakie deposits (Fig. 

 35, p. 118) were formed in a great single or partly divided 'Washakie 

 Lake.' This long accepted lacustrine theory has gradually given way 

 before the arguments of Matthew - and Davis "' for the flood plain and 

 fluviatile theory. It is definitely shown (Osborn, Granger) that the Washa- 

 kie Formation began during the Upper Bridger, but continued on after the 

 Bridger ceased. In both formations, as shown by the studies of Sinclair * 



Fig. 59. — Middle Eocene of the Washakie Basin, Wyo., Haystack Mountain or ' Mam- 

 moth Buttes.' Lower : top of Middle Eocene or Uintatherium Zone. Upper : base of Upper 

 Eocene or Eobasileus Zone. Photograph by American Museum of Natural History, 1906. 



and the analyses of Johannsen, volcanic ash, ejecta, and erosion materials 

 are important ingredients, so that we may imagine that in Bridger times 

 this basin was surrounded by active volcanoes, as represented in Fig. 23, 

 which lent grandeur to the landscape. From his observations while col- 

 lecting fossil turtles in the Bridger in 1903, Hay ^ concluded that the Bridger 

 deposits were almost solely the result of fluviatile and flood plain action, 

 that this basin was a nearly level country, probably covered with vegeta- 

 tion and well forested. The occurrence of fossil remains in all portions of 

 the Bridger beds indicates that there existed no permanent sheet of 



1 Clarence King, Systematic Geology, Washington, 1878, p. 458. 



' Matthew, W. D., Is the White River Tertiary an Eolian Formation? Amer. Natural., 

 Vol. XXXni, 1899, pp. 403-408. 



' Davis, W. M., The Fresh-water Tertiary Formations of the Rocky Mountain Region. 

 Amer. Acad. Arts and Sci., Proc, Vol. XXXV, 1900, pp. .346-373. 



^ Sinclair, W. J., Volcanic Ash in the Bridger Beds of Wyoming. Bull. Amer. Mus. Nat. 

 Hist., no. 22, 1906, pp. 273-280. 



' Hay, O. P., The Fossil Turtles of the Bridger Basin. Amer. Geol., Vol. XXXV, June, 

 1905, pp. 327-329. 



