208 GEOLOGY. 



may have boon responsible for tho shifting of the areas of deposi- 

 tion. 



2. At a later stage of the period, as judged by the fossils, aggra- 

 dation was in progress over much of Utah, western Colorado, and 

 Wyoming. On the supposition that the sediments were all lacus- 

 trine, it was formerly suggested that a single great lake, perhaps formed 

 by the spread and union of several earlier ones, may have reached 

 from New Mexico on the south to the Wind River mountains on the 

 north, during this stage of the period, covering a large part of western 

 Colorado and eastern Utah, and having a length of about 500 miles, 

 and a maximum width of 300. Even if the formations be partly 

 subaerial, as their fossils and composition indicate, the preceding 

 suggestion seems to emphasize the essential continuity of sedimenta- 

 tion over a great area. 



The deposits of this time represent the Wasatch stage 1 of the Eocene 

 (perhaps corresponding to the Chickasawan, p. 199). The beds of 

 this stage have a maximum thickness of 4000 feet near the Wasatch 

 range, and are now 6000 to 7000 feet above the sea. At about the 

 same time, as indicated by the fossils, there was an area of sedimen- 

 tation in the Bighorn basin in northwestern Wyoming. Some defor- 

 mation of the Wasatch beds followed their deposition. 2 



The sites of other small areas of deposition believed to be refer- 

 able to the Wasatch stage are known east of the mountians in south- 

 ern Colorado 3 (Cuchara formation), and they doubtless occur at other 

 points as well. 



All Eocene formations of Wasatch age or older, are referred to 

 the Lower Eocene. 



3. The third recognized stage of the Eocene of the west is the 

 Bridger 4 (perhaps corresponding to the Claibornian) . During this 

 stage, there were several known areas of sedimentation, lacustrine 



1 Here belong the Vermilion group of King, op. cit., the Coryphodon beds of Marsh, 

 Am. Jour. Sci., Vol. 14, p. 354, 5th Ann. Rept. U. S. Geol. Surv., p. 252, and Mono. 

 X, U. S. Geol. Surv., p. 6, and the Bitter Creek group of Powell Geol. of the Uinta 

 Mountains, pp. 64 and 162. 



2 King, Geol. Expl. of the 40th Parallel, Vol. I, p. 754. 



3 Walsenburg folio, U. S. Geol. Surv. 



4 The Green River group of Hay den, 3d Ann. Rept. U.S. Geol. Surv. of the Terri- 

 tories, 1869, p. 191, and Powell, Geol. of the Uinta mountains, pp. 63 and 166; and the 

 Wind River group of Hayden, Am. Nat., 1878, p. 831, and the Dinoceras beds of 

 Marsh, are here included. 



