AGE OF THE ARAPAHOE AND DENVER. 213 



allowing the Denver to rest on the Laramie. There is qo known reason 

 for the assumption that the Denver beds overlapped the Arapahoe on the 



west, but it is plain from the fact that red and white sandstone and the 

 Dakota conglomerate are prominent among the first uoneruptive materials 

 to appear in Denver sediments, that the western shore of the Denver sea 

 consisted in part of the same stratified nicks that contributed to the 

 Arapahoe, which had been long concealed with granite and gneiss beneath 

 the volcanic Hood. 



suggestions from other facts. — While the equivalent of the Arapahoe has not 

 been identified in the sections on the Animas River, on the Grand River, 

 or in Montana, when- direct equivalents of the Denver beds lie between 

 the Laramie and the lowest recognized Eocene, it is a noteworthy fact 

 that the Laramie is not especially thick in these places. In other words, 

 it does not appear that the Laramie epoch of deposition continued in 

 these localities during the lone- time represented l>v the Arapahoe and 

 the preceding interval of elevation and erosion. As far as inference may 

 be drawn from this fact, it is to the effect that the pre-Arapahoe uplift 

 terminated Laramie deposition throughout the mountain district of Colo- 

 rado, and at least locally in Montana. Subsequent deposition would at 

 present seem to have been more local during the Arapahoe than in the 

 Denver epoch, in Colorado at least. 



EVIDENCE OF ALLIED FORMATIONS. 



The formations to lie mentioned as allied either to the Arapahoe or 

 Denver occur chiefly in Colorado, on all sides of the mountain area, and 

 in its larger elevated basins or parks. One of the uewly differentiated 

 series of strata occurs in .Montana, and indications point to the presence of 

 similar formations in the intermediate area of Wyoming. 



Valuable information concerning several of these formations has Keen 

 given by R. ( '. Hills in a presidential address before the Colorado Scientific 

 Society. 1 The present writer has personally examined several of the 

 deposits, and in 1892 published a general review of what was then known 



1 Orographic and structural features of Rocky Mountain geology: Proc. Colo. Sci. Soc, Vol. 

 Ill, Part III, 1891, pp. 359-458. 



