686 W. T. LEE STRATIGRAPHY OF COAL FIELDS OF NEW MEXICO 



No. Date. 



Dawson arkose is a name applied by this writer to the lower 

 part of rocks formerly called Monument Creek. He states that 

 "it is evident that in the Castle Rock region the Dawson arkose, 

 together with its associated unconformities, represents the time 

 between the Laramie and the Oligocene . . . the lower part 

 of the Dawson arkose seems to pass along the strike into the 

 Arapahoe and Denver formations ; that the Dawson and Ara- 

 pahoe cannot be separated lithologically, even at the type locality 

 of the Arapahoe on the bluffs of Willow Creek ; and that the 

 Denver and Dawson apparentlj^ merge into each other." It is 

 further stated that "the evidence [of certain vertebrates] 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 for- 

 mations]. The evidence of the plants is corroborative, for . . . 

 Doctor Knowlton states that the leaves from the lower part of 

 the Dawson arkose . . . are undoubtedly Denver in age." 

 From these statements it is evident that the Dawson is regarded 

 as Tertiary in age and that the Arapahoe and Denver formations 

 also are Tertiary. 



