W. Cross — Denver Tertiary Formation. 281 



The geological record of events in Eocene time is very im- 

 perfect, especially for the area on the eastern slope of the 

 Rocky Mountains. Even in the Great Basin area the lowest 

 recognized Eocene, the Wasatch, is found to contain a peculiar 

 group of mammals (Coryphodon) and the record contains noth- 

 ing concerning the development of this type. This argument 

 may well close with the simple suggestion, offered as a possible 

 basis for future discussion, that the Denver and Willow Creek 

 Formations may represent earlier Eocene deposits than are else- 

 where known at the present time in the western region. 



Eocene deposits with which the Denver beds may be com- 

 pared are at present unknown. The small interior basin about 

 Florissant, Colorado, situated only sixty miles a little west of 

 south from Denver, has been quite thoroughly described by 

 Professor S. H. Scudder,* and its flora and fauna by Lesque- 

 reux, Cope and Scudder. From the very abundant plants, in- 

 sects and fishes of the Florissant beds it has been supposed that 

 they are equivalent with certain parts of the Green River 

 Eocene, but this reference is not thought fully justified, by 

 Cope.f In his " Cretaceous and Tertiary Floras " Professor 

 Lesqnereux describes 152 species of fossil plants from the Floris- 

 sant beds, and but one of these is included in the Golden Flora 

 as described by the same author. Such a difference seems 

 quite remarkable in view of the fact that the Laramie flora has 

 so much in common with various Tertiary horizons. 



It is usually assumed by those who have written concerning 

 the Tertiary deposits of the West, that in the Eocene period 

 the plains area east of the Rocky Mountains was almost en- 

 tirely a continental region, and that no seas or lakes existed 

 there to receive sediments equivalent to those of the Great 

 Basin. This assumption rests, however, on a very imperfect 

 and general knowledge of the district in question. The known 

 destruction of a large j)art of the Denver beds prior to the 

 Monument Creek epoch suggests that other deposits may have 

 existed which were either entirely destroyed or are now repre- 

 sented by as yet unidentified remnants, corresponding to the 

 Denver and Willow Creek beds. 



As an example of our lack of knowledge concerning even 

 the best known regions of the west, may be cited the discovery 

 by Mr. R. C. Hills of 8000 feet of Tertiary strata at the east- 

 ern base of the Sangre de Cristo range in the Huerfano river 

 basin of southern Colorado.^ These strata rest uncomfortably 

 on Laramie and Colorado Cretaceous. They are provisionally 



* Bulletin, U. S. G. and G. S., vol. vi, No. 2, p. 279, 1881. 



fU. S. G. S. of Ter., vol. iii, Book 1, pp. 3, 10, 1884. 



% Described in a paper entitled "The recently discovered Tertiary Beds of the 

 Huerfano River Basin, Colorado," read before the Colorado Scientific Society, De- 

 cember 3, 1888. To be published in the Society's " Proceedings " for 1888. 



