OF WESTERN JMOKTU AMERICA. 



41 



3a. WASATCH OF THE BIGHORN BASIN. 



(Figs. 1-3; PL I.) 



Loomis" examined the Wasatch of the Bighorn Basin when the 

 question of epicontinental versus lake deposition was uppermost in 

 the minds of all. By a careful analysis of the fauna, combined with 

 an exact study of the geologic section, he dismisses the lake theory 

 entirely. Geologically, as displayed in fig. 3, the section is 2,391 

 feet thick, divided into lower, middle, and upper levels, all showing 

 flood-plain rather than eolian characteristics, but indicating different 

 rates of deposition and consequent longer or shorter exposure of the 

 deposits to the sun and air. Only the middle or red beds are de- 

 cidedly fossiliferous, and they seem to have been exposed longest to 

 the air, leaving the bones of terrestrial animals on the flats ; they con- 

 tain the typical Wasatch, Coryphodon and EoJiippus fauna. Oc- 

 casionally truly aquatic animals, such as crocodiles, fishes, and turtles, 

 becoming stranded or inclosed in lagoons far from the river, mixed 

 their remains with those of the land animals. Loomis's approximate 

 analysis of the natural habitat of the total vertebrate fauna is: 

 Aerial, 3 per cent; terrestrial and arboreal, 77 per cent; amphibious, 

 12 per cent; aquatic, 10 per cent. 



Remains of EoJiipjms, typical of a plains or partly open country, 

 alone make up 32 per cent of the total fauna. To this should be 

 added the Perissodactyla-Lophiodontidae-Helaletinae (Heptodon) , and 

 some of the Condylarthra-Phenacodontidse, which are very light- 

 footed forms. The primitive Titanotheriidse (Lamhdotherium) of the 

 period may have been hard-ground dwellers, because their feet are 

 more slender and contracted than those of the modern tapir, while the 

 Amblypoda-Coryphodontidse were certainly marshy-land dwellers 

 and perhaps partly amphibious or stream dwellers, although this is far 

 from demonstrated. As to relative age, Loomis fixes very positively 

 the typical American Wasatch fauna, or chief EoJiippus and Cory- 

 fhodon zones of Tatman Mountain, as only 100 to 200 feet below the 

 beds of the Buffalo basin. The deposits in the Buffalo basin show, 

 1,000 feet below the summit, a decided approach if not actual syn- 

 chronism to the lower deposits of the Wind River valley in the presence 

 of Lamhdotherium and in the progressive evolution of the Equidse. 

 Thus there is a prolonged time overlap between the deposits of the 

 Bighorn and those of Wind River. (See fig. 1, p. 23.) 



a Am. Jour. Sci. May, 1907, 4th ser., vol. 23, pp. 356-364. 



