274 G. B. RICHARDSON THE MONUMENT CREEK GROUP 



[No. 5836] List of plants from cut at railroad crossing 1 mile southwest of 

 Falcon, Colorado ; estimated 1,000 feet above top of Laramie. 

 Asplenium sp., new. 

 HcmitcUa ? sp., new. 

 Pteris ? sp., new ? 

 Quercus sp., new. 

 FicKS ? New ? 

 Several fragmentary dicotyledons. 



These, with the bones, are all the determinable fossils that have been 

 reported from the Dawson arkose, and it shonld be observed that the 

 collections have been obtained from the lower part of the formation. 

 The flora from the vicinity of Falcon, occurring in about the middle of 

 the arkose, is distinctly different from the Denver flora, and it is quite 

 probable that future discoveries of fossils will prove the presence of 

 liigher Eocene groups in the Dawson arkose. Yet as a whole the arkosie 

 deposits constitute a distinct lithologic unit which can not readily be 

 subdi^dded. 



It is evident that the Dawson arkose, together with its associated un- 

 conformities, represents the time between the Laramie and the Oligocene. 

 To what extent the later Eocene is represented by the upper part of the 

 arkose remains to be determined. Part of the Eocene no doubt is repre- 

 sented by the unconformity at the base of the Castle Eock conglomerate. 



Relations of the Dawson Arkose to the Denvee and Arapahoe 



Formations 



The stratigraphic relations of the Dawson arkose to the Denver and 

 Arapahoe formations, which lie in close proximity south of Denver, are 

 generally concealed by a cover of Quaternary deposits, so that actual 

 conditions are obscure. It is not claimed for the recent work that final 

 correlations have been established; but, nevertheless, previously unsus- 

 pected relationships are indicated. 



Approaching the geologically mapped part of the Denver Basin from 

 the south, where detailed work had not previously been done, it was 

 found that the lower part of the Dawson arkose seems to pass along the 

 strike into the Arapahoe and Denver formations; that the Dawson and 

 Arapahoe can not be separated lithologically, even at the type locality of 

 the Arapahoe, on the bluffs of AVillow Creek; and that the Denver and 

 Dawson apparently merge into each other, or interdigitate, layers of 

 arkose typical of the Dawson being found, in a gulch 3 miles northeast of 

 Acequia, intercalated in andesitic Denver material. These conditions 



