38 GEOLOGY OF THE DENVER BASIN. 



nes's was 1,200 feet, would indicate a removal of about 500 feet in the 

 intervening period. This amount is relatively small if the period is 

 bounded by the Cretaceous on the one hand and the Miocene on the other, 

 and thus affords a further, though confessedly not very strong, argument 

 against assigning a ( !retaceous age to the Denver and Arapahoe beds. 



MONUMENT CREEK FORMATION. 



This series of beds, of which only projecting tongues from the large 

 area forming the divide between the Platte and Arkansas waters extend 

 into the region mapped, consists in general of much coarser material than 

 the Denver beds, but.is most readily distinguished by the absence of the 

 andesitic debris which characterizes the latter. It has been less carefully 

 studied, and no fossil remains have been found in the portions examined. 

 It consists in general of conglomerates, sandstones, and arenaceous clays of 

 variegated colors, made up mostly of Archean debris. Two divisions have 

 been distinguished, marked by an apparent unconformity and period of ero- 

 sion. The lower division is capped by flows of rhyolitic tuff, which forms 

 the present protecting cap of many mesas, as do the basalts of the Denver 

 beds of Table Mountain. The upper division contains, in addition to the 

 Archean detritus, fragments of rhyolitic tuff and of other eruptive rocks. 



The assignment of a Miocene age to the beds of this series is based 

 on the discovery, by earlier explorers, of vertebrate remains of this period 

 at points which, while not so definitely located by them as to make it 

 possible to tract- the actual connection of the beds, appear to have been 

 sufficiently near the area under consideration to leave little doubt that 

 they must have come from the Monument Creek beds, and probably from 

 the lower series. 



< hi the other hand, there are grounds of probability for assigning the 

 upper division to the Pliocene period, though they were not considered 

 sufficiently definite to justify the distinguishing of the upper series by a 

 distinct color or name. These grounds are, first, the discovery by O. C. 

 Marsh in 1871 of a Pliohippus fauna in the beds capping the Arkansas- 

 Platte divide, south of the Smoky Hill River near the eastern boundary 

 of the State: second, the fact that fossils which probably belong to the 



