122 



THE AGE OF MAMMALS 



San Juan Basin of northern New Mexico, 1,500 feet, overlying the Torrejon 

 and Puerco series; (4) the 'Wasatch 'of the Big Horn of Wyoming, 2,391 

 feet (Loomis). Phase II. (1) The lower portion of the Huerfano 

 Formation near Spanish Peaks, Colorado. The Wind River Formation 

 (Hayden) of northern Wyoming, 500 feet. 



These formations all contsdn Cory phodon and Eohippus, and may be* col- 

 lectively known as Lower Eocene. Below them were either mammalif- 





Fig. 37. — In the heart of the Lower Eocene badlands on Gray Bull River, Big Horn 

 Basin, Wyo. Wasatch Formation. Zone of Coryphodon, primitive horses, tapirs, etc. Photo- 

 graph by American Museum of Natural History, 1896. 



erous beds of undoubted Basal Eocene age (Puerco and Torrejon) or de- 

 posits of equivalent age (e.g. Fowkes, Almy, Fig. 36, p. 119) resting on the 

 Upper Cretaceous. The fact of paramount interest is the great thickness 

 of these Lower Eocene depositions, amounting in western Wyoming to 

 4,000 or 5,000 feet. For the beds which intervene between this Coryph- 

 odon Zone and the summit of the Cretaceous, the thickness indicates 

 an enormous period of time, ample even for the transformation of the 

 diminutive ancestors of Pantolamhda into the bulky Coryphodon (Figs. 30 

 and 31). 



The materials of which these various deposits of the Coryphodon Zone 



