AGE OF THE ARAPAHOE AND DENVER. 211 



It will scarcely be questioned by anyone that the Denver epoch 

 deserves to be recognized in any adequate chronology of geologic events 

 in this region, but the full importance of the time in which these volcanic 

 outbursts took place appears only after considering the facts presented l>v 

 other deposits allied to those of the Denver sea. The great extent of the 

 volcanic products, which must be assumed from the practical exclusion of 

 other material from the Denver sediments, has been dwelt upon, and any 

 claim that this feature is only of local importance is further answered in the 

 deposits of other contemporaneous seas. 



Texture of the Denver strata. Tile filie grain of the Deliver claYS. tuffs, SUlld- 



stones, and most of the conglomerates shows that they were accumulated 

 very slowly and that the epoch of sedimentation was of no mean importance. 

 The strata of very hue grain, such as the clays, tuffs, and sandstones, 

 which may be compared in this respect with the beds of the Laramie, 

 exceed the latter in thickness, and indicate that the Laramie and Denver 

 epochs of sedimentation may have been of nearly equal duration. The 

 Arapahoe strata are likewise of very fine grain, and in neither case can 

 it be assumed that these deposits were much more rapidly accumulated than 

 those of the Laramie. 



conclusions from nthoiogic evidence. — The pebbles of the Arapahoe formation show 

 that in the interval between Laramie and Arapahoe deposition an oro- 

 graphic movement took place in this vicinity, so great that all the formations 

 of the Cretaceous, and some still older ones, were elevated to form land 

 masses adjacent to the Arapahoe sea or lake. As the Arapahoe conglomerate 

 does not present a record of progressive erosion of these thousands of feet 

 of Mesozoic beds it must be inferred that a long period of degradation not 

 represented in known sediments preceded the Arapahoe and prepared the 

 land surface which contributed from many horizons of older strata to 

 the early conglomerates of this formation. 



The Denver strata are most clearly characterized in their lithologic 

 constitution and bear witness to important events in the preceding interval 

 The volcanic phenomena of this epoch, while affording criteria for distin- 

 guishing it from the preceding, do not bear upon the question of geologic 

 age so strongly as the facts of the Arapahoe. 



