482 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



rivers and their branches. Regarding the general geologic conditions bearing on 

 correlation in this region Cross says: 



The general conditions under which correlation of the San Juan formations 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 compli- 

 cations have entered into the problem. 



Cross also describes the occurrence of Triassic and Jurassic strata and says: 

 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, afl'ords a 

 section extending somewhat below the Dakota sandstone for the entire distance from the Tehu- 

 ride quadrangle to its union with the Dolores River, and the Dolores itself penetrates locally 

 through Jurassic and Triassic into Carboniferous formations. 



********* 



Below the Dolores beds Spencer found coarser Red Beds, often conglomeratic, with pebbles 

 3 inches or more in diameter, and several hundred feet of such strata were noted. No oppor- 

 tunity was found to measure a section showing the full thickness of these coarser Red Beds, but, 

 as observed by Peale, they are underlain by fossiliferous Pennsylvanian Carboniferous in Sinbad 

 Valley, where there is also much structural complexity obscuring the relations. 



Spencer's observations seem to show that the section of the lower Dolores Valley embraces 

 strata to be correlated with the Cutler [Permian ?], Dolores [Triassic], La Plata [Jurassic], and 

 McElmo [Jurassic ?] formations of the San Juan region. 



Peale reconnoitered southwestern Colorado in connection with the Hayden 

 Survey (1875) and mapped as "upper Carboniferous" certain beds which he classed 

 as "probably Permian." These presumably represent the Cutler. Cross ^*°'' 

 restudied the section in 1905 and describes the remarkable Permian (?) conglomer- 

 ates in contact with and near granite outcrops on West Creek. 



Where the sediment was seen resting on granite, the lower layers consisted of coarse, angu- 

 lar gravel, scarcely bedded at all. At a distance of a few feet above the granite bedding planes 

 become niore distinct, through alternation of finer and coarser material aided by color differ- 

 ences, the finer-grained material being reddish. At 25 feet above the granite the characteristic 

 alternation of grit and conglomerate began. But the distribution of pebbles and bowlders is 

 very irregular, the latter are often subangular, and the whole is so little consolidated that the 

 disintegrated beds seem like surface gravels. 



Cross gives a detailed section of conglomerate and grit 879 feet thick and 

 continues the discussion of correlation as follows : 



The pebbles of the conglomerate strata of this section are principally of granite and the 

 most abundant variety is the very coarse textured one with large orthoclase crystals occurring 

 in the nearest exposures of the Uncompahgre Plateau. Among the pebbles are some of green- 

 stone schist which indicate that the pre-Cambrian complex furnishing this material is similar 

 to that from which the Cutler conglomerate of the San Juan was derived. Similar greenstones 

 were observed in the conglomerates of Grand River valley. 



Below the measured section there may be several hundred feet of similar strata, for the 

 somewhat deeper cutting of Dolores River does not reveal the base of the succession of grits 



