206 GEOLOGY OF THE PEXYEE BASIK 



considerable thickness now exposed are those of Green or Table mountains, 

 situated on the western shore-line. From the exposures on the plains we 

 learn that while other shores did contribute noneruptive materials during 

 the earlier part of the Denver epoch, the eruptive debris from the western 

 area was at all times strongly predominant, and nothing appears, on extend- 

 ing the view, to invalidate the conclusions reached. At points on Coal 

 ( 'reck farthest removed from the western shore-line, and on Cherry ('reck, 

 near tin- Arapahoe beds of the southern shore, are strata consisting exclu- 

 sive!}' of eruptive materials, and a, strong admixture of Archean debris is 

 surely accompanied by evidences of its secondary and local source in the 

 Arapahoe grits. While the influx of eruptive materials was always strong 

 enough in these earlier periods to make itself everywhere predominant, it 

 is not found that the sand washed in at times from the northern and south- 

 ern Arapahoe shores ever became noticeable in contemporaneous deposits 

 of Green or Table mountains. 



SECTION III.— AGE OF THE ARAPAHOE AND DENVER FORMATIONS. 



By Whitman Cross. 

 STATEMENT OF THE QUESTION. 



Former classification of the formations. Until tllC dlSCOVerieS wllicll are described 



in preceding chapters were made the strata of the Arapahoe and Denver 

 formations had been uniformly assigned by geologists to the Laramie, 

 under the accepted definition of the latter as the uppermost division of the 

 conformable Cretaceous series; and not only had they been assigned to 

 the Laramie, but no characteristics of any kind had been mentioned, or 

 apparently observed, by which these upper beds might lie even locally 

 distinguished from the lower, coal-bearing horizon. This correlation was 

 based on the presence of the true Laramie below the beds in question, 

 on the failure to notice their peculiar and distinguishing characteristics, 

 and on assumptions regarding the unity of the fossil flora, whose species 

 were, however, collected from widely separated horizons in the Golden 

 section. 



The assignment to the Tertiary. 111 tll6 earliest descriptions of tllCSC tol'llia- 



tions by Mr. Eldridge and the writer they were assigned to the Tertiary. 



