278 W. Cross — Denver Tertiary Formation. 



advanced for referring the horizontal strata of Table Mountain 

 to the same formation with the vertical coal- measure rocks. 



Both the Willow Creek and the Denver series lie between 

 undisputed Laramie and a Tertiary Formation, which, though 

 not yet closely studied, has been referred without question to 

 the Miocene. The previous classification of the beds, and the 

 fact that the fossils with a single exception are said to indicate 

 or to require a reference to the Cretaceous, make the first 

 problem to be considered, as follows : Do the Willow Creek 

 and Denver Formations belong to the Laramie Group, or are 

 they of later age ? 



The character of the evidence in the case has been sub- 

 mitted. The Denver beds are apparently separated from the 

 acknowledged Laramie by a series of intermediate beds with 

 important characteristics, which are visibly unconformable with 

 the Laramie, as are the Denver beds, in turn, with the inter- 

 mediate Willow Creek. But local unconformities, even greater 

 than the angular ones here seen, may well exist with a great 

 Group like the Laramie, and it is necessary to consider these 

 unconformities in the light of the lithological evidence, before 

 their real significance can be appreciated. 



The Willow Creek conglomerate shows pebbles "derived 

 from every Formation below it in the Denver field," and the 

 unconformity here recorded was therefore not a local one 

 within the Laramie, but extended down through almost the 

 entire Mesozoic section. To explain this requires the assump- 

 tion of a great folding of the strata adjacent to the foothills in 

 the interval preceding the Willow Creek. In a similar man- 

 ner the materials of the Denver beds testify positively to a 

 period of great volcanic activity in the interval between the 

 Denver and the Willow Creek epochs. 



These are the facts of primary importance and the only 

 question which can be raised is as to their interpretation. 

 Whatever fossils have been, or may in future be, found in the 

 Denver strata, the significance of these primary facts cannot be 

 ignored. The claim that the Denver beds are Laramie 

 involves the claim that the events indicated above took place 

 within the Laramie time. 



It seems to the writer that if stratigraphy and lithology can 

 give grounds for drawing boundary lines the evidence sub- 

 mitted warrants the separation of the Denver and of the Wil- 

 low Creek beds from the Laramie, and their reference to the 

 Tertiary. 



Above the Denver beds, and unconformable with them, as 

 has been shown, comes the Monument Creek Formation. 

 Hayden gave this name, in 1859, to " a series of variegated 

 sands and arenaceous clays, nearly horizontal, resting on the 



