248 GEOLOGY OF THE DENVER BASIN. 



The plant life <>t' the Denver epoch was materially different from that 

 of the Laramie, as shown by Mr. Knowlton in Chapter VII; but until the 

 flora of the Arapahoe is much better known we ran not tell how much 

 of the modification was produced during the interval immediately preceding 

 the Denver, and how much during- the earlier intervals. 



The vertebrates of the Arapahoe and Denver beds thus far identified 

 are so few compared with those of Wyoming and Montana that until the 

 distribution of the latter in the several fossil-bearing horizons has been 

 clearly established it can not be known whether the Denver fauna has 

 peculiarities distinguishing it from that of the Arapahoe. 



Post-Denver mtervai. — The Denver formation may be probably considered 

 as the uppermost member of the Cretaceous, now that the Fort Union 

 beds of Montana have been recognized as Eocene from their rich fossil 

 flora. If the Denver beds are so regarded, the time-interval succeeding 

 their deposition is a most important one as marking the boundary between 

 Mesozoic and Cenozoic times in the Rocky Mountains. In the Denver 

 Basin no undisputed Eocene strata have been preserved, if, indeed, they 

 were ever deposited in this region. But there are three known localities 

 where formations apparently the equivalents of the Denver beds rest upon 

 the typical Laramie, and are overlain by the lowest Eocene deposits of the 

 respective regions. These localities are: On the Animas River, in Colo- 

 rado and New Mexico, where the Puerco formation overlies the Animas 

 beds; on Grand River, in western Colorado, where the Ruby beds are 

 overlain by the Wasatch, and in Montana, where the Livingston formation 

 is overlain by the Fort Union. In these three localities one may hope 

 to find some evidence as to the orographic or other dynamic disturbances 

 which are commonly assumed to have characterized this interval. 



Field researches in these regions have not as yet been sufficiently 

 thorough to demonstrate the absence of phenomena indicating orographic 

 disturbance, but no evidence of important movements has been announced. 

 In fact, as far as the writer is aware there is no described case of 

 unconformity or dynamic disturbance which has hitherto been supposed to 

 belong to the post-Cretaceous interval which may not as well be referred 

 to the pre-Arapahoe movement. In all cases where the lowest recognized 



