W. Cross — Denver Tertiary Formation. 279 



upturned edges of the older rocks "* and situated on the divide 

 between the waters of the South Platte and of the Arkansas 

 rivers. The strata abut against the Archaean near Palmer 

 Lake (on the Denver and Rio Grande R. R.) and extend out 

 upon the plains an unknown distance. Hayden referred them 

 provisionally to the Miocene, without definite data. Cope sub- 

 sequently sought for fossils in these beds and as a result is 

 inclined to confirm the assignment of Hayden. Remains of 

 an Oreodon type are thought to prove an age later than the 

 Eocene, while the occurrence of Megaceratojvs Color adoensis 

 indicates pre-Pliocene age.f 



The strata here referred to the Monument Creek are plainly 

 connected with the ones originally described by Hayden and 

 no further evidence as to their age can be given. 



It is clear that the Monument Creek beds are much later than 

 the Denver beds. The latter have been folded into a vertical 

 position along the line between Golden and Green Mountain, 

 but the Monument Creek strata, as noticed by Hayden, £ pass 

 to a contact with the Archaean and are but slightly inclined 

 (5°-15°), showing that the interval between the Denver and 

 Monument Creek epochs witnessed important orographical 

 changes. Two-thirds of the Denver beds in the area studied 

 were removed during the interval. 



Were it not for the presence of the fossil described by Prof. 

 Marsh as Bison altiGornis,% the whole weight of evidence 

 would be in favor of assigning the Willow Creek and Denver 

 Formations — assuming that they are post-Laramie — to the earli- 

 est Tertiary possible. On account of this fossil, however, Prof. 

 Marsh has stated that the strata containing it are "probably late 

 Pliocene." But the bison specimen figured by Prof. Marsh was 

 dug out of solid typical Denver sandstone at the same general 

 horizon which has yielded all the other Denver vertebrates 

 yet found. This conflict of evidence is not yet explained. 



The preceding discussion has been confined to the evidence 

 gathered in the district studied. A few more general consid- 

 erations may now be brought out in conclusion. 



It can scarcely be said that the reference of the Denver 

 and Willow Creek Formations to the Eocene is opposed to the 

 general doctrines concerning the succession of geologic events 

 at the close of the Mesozoic. The reference is rather in full 

 harmony with those doctrines, for they assume great disturb- 

 ances at this time, causing nonconformities when sedimenta- 



* Annual Report for 1869, pp. 39-42. 



f Ann. Kep. U. S. G. & G. S.. 1873, p. 430. 



% Bull. 3, 2d Ser. U. S. G. S. of Ter., 1875, p. 210. 



§ Am. Journ. Sci., Oct., 1887, p. 324. 



