PERMIAN. 481 



The assignment of Permian age to the Cutler formation rests on its apparent 

 conformity to distinct upper Pennsylvanian strata and on its unconformity with the 

 overlying Triassic. No fossils have yet been found in it. Its recognition outside 

 of the San Juan district is difficult. Cross ^^^^ presents the known facts and suggests 

 the probable correlation of the Cutler with appropriate divisions of the sections 

 observed by his predecessors. 



The Eed Beds of the Rocky Mountain province have proved to be perhaps the naost difficult 

 portion of the Paleozoic and Mesozoic section to analyze and reduce to definite groups and for- 

 mations. In the first place, the red color has been found unreliable as a guide in correlation. 

 It extends at some places into the fossiliferous Pennsylvanian rocks and may penetrate upward 

 into Jurassic beds. Fossils have been discovered in Red Bed strata ia many different locali- 

 ties — here of Permian, there of Triassic affinities. Unfortunately the two lands of fossil evidence 

 have seldom been found in one section, affording a means of division. Triassic fossils occur in 

 some places very near the top of the Red Beds and in others near their base, in proximity to 

 known Pennsylvanian strata. This apparent range of Triassic fossils has been a prominent 

 factor ia forming what seems to be the general opinion that the larger part of the Red Beds is 

 to be assigned to the Trias. 



As far as I know, no one has presented evidence of a stratigraphic break within the Red 

 Beds or suggested that the conflicting evidence as to age might be explained by such a break. 

 The Ouray unconformity, however, establishes for southwestern Colorado a definite line between 

 Triassic and pre-Triassic portions of the Red Beds, which can be traced on a lithologic basis 

 where no visible unconformity exists. From eviden'ce which will be presented in following 

 pages it seems certain that the Dolores formation may be positively recognized through its 

 fossils and lithologic character in Utah and Arizona and very possibly in New ^Mexico. As a 

 step toward a better imderstanding of the Red Beds I desire to make certain suggestions as to 

 the correlation of the Cutler and Dolores formations with strata in other parts of Colorado, in 

 Wyoming, and particularly in the plateau province of Utah, Arizona, and New Mexico. The 

 discussion is based on a study of the literature, on recent published discoveries, and on unpub- 

 lished information. Naturally much of the correlation suggested is tentative. 



Four principal factors have had influence in this tentative correlation: 



(a) The lithologic character of formations compared. — This is the most important on the 

 whole, since it is the only one applicable in all places. 



(6) Fossil evidence. — This guide is becormng more and more important as new discoveries 

 are made. Unfortunately, collectors of fossils and even paleontologists are not always alive to 

 the importance of careful stratigraphic studies and accurate measurement and description of 

 sections in the localities where fossils are obtained. Their work is therefore often shorn of 

 much of the value that should attach to it. 



(c) The influence of post- Triassic elevation and erosion. — The stratigraphic break at the base 

 of the La Plata sandstone [Jurassic] is one of great magnitude throughout the western portion 

 of Colorado. In several places the La Plata is seen in angular unconformity with the entire 

 Paleozoic and older Mesozoic section. It is possible that synchronous elevations, in gentle folds 

 or low domes, occurred in the plateau district, but have not been recognized. 



(d) The influence of the pre-Triassic elevation and erosion. — ^WhUe it appears probable that 

 the pre-Dolores uplift was by no means so extensive as the post-Dolores disturbance, it is plain 

 that its influence must be looked for in the plateau country as well as in the Rocky Mountains. 



The type locality of the Cutler formation (Cutler Creek, Ouray quadrangle, 

 Colo.) is in longitude 107° 45', latitude 38°. The plateau province stretches thence 

 westward, cut by the tributaries of Grand River known as Dolores and San Juan 



48011°— 12 31 



