472 CROSS AND HOWE — RED BEDS OF SOUTHWESTERN COLORADO 



tions illustrate the general principle that in the mountain belt of repeated 

 disturbances, of rapid erosion, of land areas, and shore lines, the strati- 

 graphic section will exhibit much greater variation in composition and 

 thickness of its members than in the adjacent region, which has been 

 one prevailingly of deposition. 



The general conditions under which correlation of the San Juan for- 

 mations with those of the Plateau section must be made are as follows : 

 Adjacent to the mountains there is a broad zone of gentle westward slope 

 in which Cretaceous beds occur. The main streams flowing west and 

 south cut valleys into and in some places through the Cretaceous into 

 underlying formations. Nearer the canyon of the Colorado the valleys 

 widen and broad platforms and terraces of Jurassic and Triassic beds 

 appear, the Cretaceous being restricted to the divides and isolated mesas. 

 The Paleozoic formations appear at first only in isolated exposures in 

 the deeper canyons, but far to the southwest rise to form the broad plain, 

 called the Colorado plateau, on the south side of the Grand canyon. 

 Thus the older the formation the greater are the gaps between districts 

 of good exposures, and the greater the likelihood that in the covered 

 tracts unsuspected complications have entered into the problem. 



The discussion following will first take up the evidence afforded by 

 recent observations in certain districts and later consider the correlation 

 from a more general standpoint. 



FORMATIONS OF DOLORES VALLEY AND UNCOMPAHQRE PLATEAU 



The region in general. — It is natural to begin this discussion of correla- 

 tion with a consideration of the formations occurring in the Uncom- 

 pahgre plateau and the bounding valleys of the Dolores and Grand 

 rivers, a district traversed by A. C. Peale, of the Hayden Survey, in 1875 

 (35). The heart of the plateau is about 80 miles northwest of Ouray, 

 and Cretaceous formations occupy the greater part of the intervening 

 country. The San Miguel river, however, affords a section extending 

 somewhat below the Dakota sandstone for the entire distance from the 

 Telluride quadrangle to its union with the Dolores river, and the Dolores 

 itself -penetrates locally through Jurassic and Triassic into Carboniferous 

 formations. 



Observations of A. C. Spencer.— In 1899 Dr A. C. Spencer, who had been 

 my assistant in western San Juan work for 3 years, made a reconnais- 

 sance trip to Paradox and Sinbad valleys, which are tributary to the 

 Dolores. His route of travel was from Placerville, on the San Miguel 

 river, following down that stream for some 15 miles, and thence across 

 to the East Paradox valley. This route allowed Spencer to trace the 

 Dakota, McElmo, and La Plata formations from the Telluride quadrangle 



