470 CROSS AND HOWE — RED BEDS OF SOUTHWESTERN COLORADO 



The sandstones are almost wholly quartzose, and their normal color 

 adjacent to the San Juan mountains is white or gray. But in several 

 localities the lower sandstone is more or less strongly orange or yellow- 

 ish in color, and even red tints have been observed. This coloration is 

 most pronounced to the west of the San Juan, in the Dolores valley. 

 The possibility that the La Plata may be of strong red or orange color is 

 the principal ground for bringing the formation into the present dis- 

 cussion. 



The Mc Elmo formation. — This formation includes the upper part of the 

 Gunnison formation of Eldridge, described by him as consisting of vari- 

 ously colored clays and marls with subordinaate limestone. About the 

 San Juan mountains sandstones or sandy shales become prominent, al- 

 though the marls are everywhere a very characteristic feature. Eldridge 

 pointed out that the Gunnison beds seemed lithologically and strati- 

 graphically equivalent to the Morrison formation of the eastern border 

 of the mountains. The discovery by Riggs (37) of vertebrate remains 

 of Morrison types in the Grand River valley indicates clearly that cer- 

 tain portions of the McElmo and Morrison are lo be correlated, but that 

 the formations are coextensive remains to be proven. 



The McElmo beds range from 300 to 900 feet in thickness in different 

 parts of the San Juan area. Where thickest, they consist largely of 

 sandstone. The formation is ordinarily not highly colored except in 

 its marl and clay strata, but in certain localities the sandstones are 

 dull red. 



Part II. Correlation of the Formations 



INTRODUCTION 



The Red beds of the Rocky Mountain province have proved to be 

 perhaps the most difficult portion of the Paleozoic and Mesozoic section, 

 to analyze and reduce to definite groups and formations. In the first 

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

 It extends at some places into the fossiliferous Pennsylvania]! rocks and 

 may penetrate upward into Jurassic beds. Fossils have been discovered 

 in Red bed strata in many different localities — here of Permian, there of 

 Triassic affinities. Unfortunately the two kinds 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 ap- 

 parent range of Triassic fossils has been a prominent factor in forming 

 what seems to be the general opinion that the larger part of the Red 

 beds is to be assigned to the Trias. 



