560 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



In the Red Beds paper were given the observations of Spencer, who traced the La Plata 

 sandstones to Paradox and Sinbad valleys, west of the Dolores, and of Gane, who followed 

 them down the San Juan VaUey to the Colorado Canyon. Both noted the prevalence of orange 

 or pink color in the lower country. 



On the strength of these observations and a study of literature it was concluded that " the 

 La Plata formation is seemingly equivalent to the White Cliff sandstone. Its local assumption 

 of red color has led to confusion with the Vermilion Cliff in certain districts and a reference to 

 the Trias." This correlation is considered to be amply substantiated by the recent observa- 

 tions. * * * 



The continuity of the La Plata sandstone from the San Juan Mountain flanks down the 

 Dolores and San Miguel valleys, around the La Sal Mountains to Grand River valley, may be 

 said to be perfectly plain and incontestable. In tMs distance the most notable change in the 

 formation is its increased thickness. .The massive texture and even grain of many strata, 

 cross-bedding, variation in color, and other marked features are but emphasized by the greater 

 volume. The intermediate strata are most variable in character, yet everywhere the two great 

 massive sandstone members are separated by beds distinguishable through their thin bedding, 

 darker color, and richness in calcareous cement or development of limestone. 



From the district covered by our reconnaissance the ledges of gray, pink, or orange La 

 Plata sandstone can be seen stretching to the west and south into the belt traversed by Green 

 and Colorado rivers, where Powell has described the White Cliff sandstone. This great unit in 

 the plateau country section was never described accurately nor in detail for any given locality, 

 but there seems to be no ground for questioning the assertion of PoweU that this formation is 

 continuous with persistent characters from northeastern Utah to the great esplanade bordered 

 by the White Cliffs in southern Utah, facing the Grand Canyon. The characters of the White 

 Cliff repeatedly emphasized are its massiveness, "oblique lamination" or "false stratification," 

 and its white, golden, orange, or light-red colors, which are so brilliant in the desert air. 



The upper boundary of the White Cliff is the basal marine limestone of the Flaming Gorge 

 group of Powell. The lower boundary is less clearly defined in the statements of Powell and 

 others, but, with recognition of the stratigraphic break soon to be mentioned and the marked 

 color line apparently everywhere present at the summit of the Triassic sandstones, it may be 

 hoped that no great difficulties in drawing the base of the White Chff will be experienced when 

 the attempt is seriously made. 



The far-reaching unconformity below the fresh-water Jurassic beds of central Colorado, by 

 which they overlap all older Mesozoic and Paleozoic beds and in many places rest on the pre- 

 Cambrian granites and schists, is well illustrated by the Hayden maps. This overlap is par- 

 ticularly well exhibited in the southern Elk Mountains and some of its details are shown in the 

 Anthracite -Crested Butte folio. 



The fact that no Paleozoic formations are present in the Uncompahgre Plateau was recog- 

 nized by Peale and expressed on the Hayden map. If, however, the greater part of Peale's 

 "Triassic" in that area be now referred to the La Plata Jurassic, as has been done in the preced- 

 ing discussion, the question arises as to whether evidence of erosional unconformity between the 

 La Plata and the underlying Dolores Triassic exists in that area or not. Our observations on 

 this point were quite limited but tend to show that such a break does occur. It is certain that 

 in the vicinity of the Unaweep Canyon the dark-red Triassic strata are much thinner than in 

 the Dolores VaUey to the west, and this decreased thickness appears to be principally due to 

 erosion of the massive red sandstone forming the upper part of the Triassic. 



On the north side of West Creek, which is the western stream flowing out of Unaweep 

 Canyon, Messrs. Emmons and Kay found the La Plata to rest on granite near the shore line of 

 the Permian (?) beds, which will be discussed in another part of this article. At the head of 

 West Side Creek a few miles south of the Unaweep the pink La Plata sandstone rests on thin- 

 bedded sandstones and shales belonging to the lower part of the Dolores formation, as shown 

 by the presence of the fossiliferous "Saurian conglomerate." Near the head of Atkinson Creek 

 on the western side of Uncompahgre Plateau, the La Plata seems to rest on gneiss. * * * 



