98 WHITMAN CROSS 



Triassic age of the Vermilion Cliff sandstone has not been directly- 

 questioned, neither has it been fully proven, and it will appear in the 

 course of this discussion that there is some slight basis for the sug- 

 gestion that it is lower Jurassic. 



As for the Shinarump group, Walcott long ago found Permian 

 fossils (24) in the lower beds referred to it by Powell and possibly 

 Jurassic fossils (4) in the upper part, while a Triassic vertebrate 

 fauna occurs near the middle of what Ward refers to the Shinarump 

 (26). The question as to the real character, scope, and correlation of 

 this group is then plainly one requiring further study and considera- 

 tion. 



The standpoint from which this review is written is that of several 

 years' experience in the stratigraphic section of southwestern Colorado, 

 with some information as to the gradual changes exhibited by the 

 formations as they pass from the mountain slopes into the adjoining 

 vast area of plain and caayon — the Plateau Province. It may be 

 well to present in the outset the facts of major importance bearing on 

 the topic under discussion and indicate frankly the tentative con- 

 clusions I have reached as to their application. 



It has been established that the Dolores formation of the San 

 Juan region of Colorado is of Triassic, and probably of upper 

 Triassic age. An angular unconformity has been found below the 

 Dolores by which the whole upper Paleozoic red-bed series and a 

 part of the Pennsylvanian strata are locally cut out. No middle or 

 lower Triassic beds have been found and none demonstrably Permian. 

 The stratigraphic break below the Dolores is thus shown to be of 

 much importance. 



The Dolores formation has been traced from the mountains into 

 the heart .of the Plateau district along two lines of approach. The 

 most important fact established is that the fossiliferous basal member 

 of the formation extends west and northwest from the San Juan 

 Mountains as far at least as Grand River, in Utah, where the angular 

 unconformity below it is very marked, and 1,500 to 2,000 feet of 

 probable Paleozoic beds are gone at some places. An overlap of the 

 basal Dolores conglomerate from Permian ( ?) beds directly to the pre- 

 Cambrian complex occurs on the western side of the Uncompahgre 

 Plateau, in Colorado, where it was observed by Peale in 1875. The 



