474 CROSS AND HOWE — RED BEDS OF SOUTHWESTERN COLORADO 



Comparison with Peale's section. — The Hayden map of the Uncompahgre 

 Plateau distinguishes " Upper Carboniferous," Trias, Jura, " Lower Da- 

 kota,'' and " Upper Dakota " as the cartographic units of the sedimentary 

 section. The " Upper Dakota " is clearly the Dakota as commonly 

 denned. The "Lower Dakota" embraces a group of strata approxi- 

 mately, or perhaps closely, corresponding to the McElmo, and the 

 "Jura" must include the upper part of the La Plata with, perhaps, the 

 lower part of the McElmo. The " Trias " is represented by the map as 

 the main formation of the region and as resting throughout the Plateau 

 on granite, gneiss, or schist. Near the Dolores the Carboniferous rocks 

 intervene. 



According to the descriptions of Peale, this Triassic formation clearly 

 embraces the greater part of the strata referred by Spencer to the La 

 Plata formation and at least the upper part of the Dolores. His general 

 description is as follows (35, page 80) : 



"A massive yellow, white, or pink sandstone forms the top of the series. Toward 

 the western part of our district (i. e., the area under discussion) this sandstone is 

 calcareous. In many places the sandstones are markedly cross-stratified. The 

 color is subject to much change locally, passing from white, through orange and 

 pink, into deep red. Below the massive sandstone are blood-red shales, followed 

 in most places by massive brick-red sandstone." 



This description seems obviously to apply to a section embracing the 

 lower La Plata and upper Dolores beds. 



Below the Triassic of Peale occur "shales and blood-red sandstones, which on 

 the Dolores, change gradually into gypsiferous shales and sandstones. The latter, 

 I have considered as belonging to the Permian. It is difficult to draw any line 

 between the Trias and Permian, and I have been obliged to do so arbitrarily." 

 (35, page 80.) 



The strata referred to by Peale as probably Permian are called " Upper 

 Carboniferous " on the published map. It is apparent that the Cutler 

 formation, if present in the lower Dolores valley, must embrace the beds 

 called Permian in Peale's report. 



In view of the fact that an examination of the Uncompahgre plateau 

 during the coming season is contemplated, it seems undesirable to sug- 

 gest at the present time any further correlation. It was Peale's belief 

 that the absence of the Carboniferous and the varying thickness of 

 Mesozoic beds between the gneisses and the Dakota Cretaceous in the 

 Uncompahgre plateau and the adjacent Gunnison valley to the east was 

 evidence of an island never covered by sediments until the Dakota epoch. 

 It seems probable, however, that the erosion of the pre-Dolores interval 

 removed the Paleozoic beds, and that the pre-La Plata erosion in turn 

 removed some, if not all, of the Dolores strata in this district. Peale's 

 overlap may prove to be wholly explainable in this way. Faulting and 



