AGE OF TJE ARAPAHOE AND DENVER. 207 



The reason for this assignment was the discovery that between the Laramie 

 and Arapahoe epochs there had occurred an orographic disturbance whose 

 magnitude was measured, for this locality, by the presence in the Arapahoe 

 strata of pebbles of highly indurated clastic rocks, sandstones, conglom- 

 erates, etc., clearly belonging to various geological horizons as far down as 

 the Trias, representing erosion of 14, (too feet of strata, according to the 

 section of the formations in question in the Denver region. The lithological 

 character of the Denver beds showed that the interval of unknown dura- 

 tion between the Arapahoe and Denver epochs had witnessed the occurrence 

 of volcanic eruptions on a gigantic scale, and also subsequent local erosion. 



Up to the time when these formations were thus identified, g-reat 

 orographic movements in the Rocky Mountains had been commonly sup- 

 posed to mark the ending of Mesozoic time, and to he in great measure 

 the cause of the wonderful changes that took place at this period, especially 

 in vertebrate life, as shown by the remains in the earliest known Eocene 

 deposits. The beginning of Tertiary time was also known to he widely 

 characterized by great volcanic outbreaks, recorded in the sediments of the 

 Green River, Florissant, and other Eocene basins. Hence it seemed 

 natural to place the Arapahoe and Denver beds in the Tertiary, as, perhaps, 

 the earliest lake deposits of Cenozoic time. Examination of the paleonto- 

 logic evidence available at the time showed either that it did not controvert 

 the assignment, or. as in the case of the fossil plants, was entirely untrust- 

 worthy because the floras of the distinct horizons involved could not then 

 be compared. 



The recent discoveries of fossil vertebrate remains are said by paleon- 

 tologists to show that the life of the epochs under discussion was much more 

 nearly allied to Mesozoic than to Cenozoic types, and in deference to this 

 opinion the post-Laranhe formations are classed in this report with the 

 Cretaceous. But such a course raises at once the question as to the nature 

 and position of the boundary between Mesozoic and Cenozoic time in the 

 Rocky Mountains, and broadens very materially the treatment which must 

 be given to the problem. 



Discoveries of allied formations. Within tllC past f('W VCafS a lllllllllCl' of local 



formations have been discovered in Colorado and Montana which are more 



