STRATIGRAPHIC RELATIONS OF DAWSON AKROSE 275 



indicate that the Arapahoe and Denver are equivalent to the lower part 

 of the Dawson arkose. The marked difference in lithology between the 

 andesitic Denver and the arkosic Dawson may be accounted for by the 

 geographic distribution of the rocks which supplied the sediments, the 

 arkose being derived from the Pikes Peak granite and associated rocks, 

 whereas the Denver formation was derived, apparently simultaneously, 

 from a local source of andesite. 



In the Denver Monograph, Cross pointed out the presence of non- 

 volcanic material in the Denver formation in the southern part of the 

 area reported on by him ; but, since the "Monument Creek" was believed 

 to be of Miocene age, the equivalency of part of it to the Denver forma- 

 tion was not suspected. Abrupt changes in material from andesitic to 

 arkosic are of common occurrence at the southern end of the Denver 

 formation, and it should be observed that such contacts may mark only 

 local unconformities, which are of frequent occurrence in continental 

 deposits. 



The relations of the Dawson, Arapahoe, and Denver formations sug- 

 gested by the stratigraphy are borne out by paleontologic evidence. A 

 number of Ceratops bones have been found in the Denver and Arapahoe 

 formations; and, although the writer has not found any in the Dawson, 

 Emmons states^" that Professor Marsh observed vertebrate fossils char- 

 acteristic of the post-Laramie in Monument Park, which is 8 miles north 

 of Colorado Springs. This observation is important, for the lower part 

 of Hayden's Monument Creek is the only post-Laramie formation in 

 Monument Park ; and, although the vertebrates observed by Marsh are not 

 named, the only ones characteristic of the "post-Laramie," as that term 

 is used in the Denver Monograph, are Ceralopsia. This evidence implies 

 the correlation of the lower part of the Monument Creek of Hayden (the 

 lower part of the Dawson arkose) and the "post-Laramie" of the Denver 

 Basin (the Arapahoe and Denver formations). The evidence of the 

 plants is corroborative, for, as already mentioned. Dr. Knowlton states 

 that the leaves from the lower part of the Dawson arkose, listed above, 

 are undoubtedly Denver in age. 



In the light of present knowledge, therefore, it seems plausible that 

 the Arapahoe and Denver formations are the equivalent of the lower part 

 of the Dawson arkose. 



Correlation 



Precise correlation of the Dawson witli other Eocene deposits of the 

 Rocky Mountain region outside of the Denver Basin in the present state 



1" Denver Monograph, p. 31. 



