MONUMENT CREEK FORMATION. 39 



same horizon have been discovered at various points within the area of 

 the Denver Basin, and, though not actually in place, in such positions as 

 to indicate that tliev must have come from the disintegration of beds in 

 the near vicinity. In addition to this, there is the analogy of the beds 

 forming the divide between the North and South Platte rivers, along 

 the Colorado- Wyoming boundary, where the Miocene' (Brontotherium) 

 beds are overlain by Pliocene (Pliohippus). Although these beds differ 

 somewhat from the Monument ('reek beds in lithological composition, 

 containing more argillaceous and calcareous material, this difference is 

 readily explainable by the different character of the rocks composing the 

 mountain masses to the westward, from the abrasion of which the sediments 

 composing the respective series were formed. In Wyoming, Paleozoic 

 and Mesozoic formations once arched entirely over the Archean nucleus of 

 the mountains and protected it from erosion, whereas iu Colorado this 

 Archean nucleus was never entirely submerged, but has always Keen 

 exposed to erosion. 



It is probable that when the plains region to the east of the Denver 

 Basin shall have been systematically surveyed remnants of these beds 

 will be discovered that will be sufficient to prove that both Miocene and 

 Pliocene lakes were continuous across the Denver Basin northward, as 

 was probably the case with those in which the Arapahoe and Denver 

 beds were deposited. With regard to their southern extension, there is 

 more uncertainty, as erosion in the Arkansas Basin seems to have Keen 

 deeper and more extensive than in that of the South Platte. Professor 

 Marsh is of opinion that the Miocene deposits show signs of thinning out 

 to the southward. This idea is negatively confirmed by the tact that no 

 Miocene beds have been found in Huerfano Park, where Eocene beds are 

 directly overlain by what are supposed to he Pliocene strata. 



LATER MOVEMENTS. 



< )t later orographic movements in this field, the record is too incom- 

 plete and fragmentary to afford anything more than a general indication or 

 suggestion. 



1 Tin- assignment of a Miocene age to the Brontotherium beds is on the authority of Prof. 0. C. 

 Marsh. Prof. W. B. Scott and some other paleontologists class them as Oligocene. 



