518 INDEX TO THE STEATIGRAPHY OF NORTH MIERICA. 



the top. The Triassic was thus supposed by him to embrace everything present in the great 

 section north of the Grand Canyon between the upper Aubrey (Pennsylvanian) and the marine 

 Jurassic. 



Dutton and other writers have referred the White Cliff sandstone to the Jurassic, and this 

 reference is no doubt correct, since the continuity has been estabhshed between this very well 

 marked formation and the La Plata sandstone of Colorado, which is in angular unconformity 

 Avith the Triassic and the entire Paleozoic section.'^ The Triassic age of the Vermilion Cliff 

 sandstone has not been directly questioned, neither has it been fully proved, and it will appear 

 in the course of this discussion that there is some slight basis for the suggestion that it is lower 

 Jurassic. 



As for the Shinarump group, Walcott long ago found Permian fossils * in the lower beds 

 referred to it by Powell and possibly Jurassic fossils " in the upper part, while a Triassic verte- 

 brate fauna occurs near the middle of what Ward refers to the Shinarump. <^ The question as 

 to the real character, scope, and correlation of this group is, then, plainly one requiring further 

 study and consideration. 



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 demon- 

 strably 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 fossU- 

 iferous 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 Uncqmpahgre Plateau, in Colorado, where it was observed by Peale 

 in 1875. The fossiliferous lower strata of the Dolores have also been traced down the San Juan 

 Valley nearly to the Glen Canyon of the Colorado, below the Henry Mountains. 



While nothing similar to the Dolores fossiliferous conglomerate has been described from 

 the original area of the Shinarump group in Utah or Arizona, the discovery by Ward" on the 

 Little Colorado River of the vertebrate fauna characteristic of the Dolores shows plainly that a 

 correlation of great importance is to be anticipated when the requisite studies have been made. 



J 17-18. RICHMOND BASIN, VIRGINIA. 



The stratigraphy of the Richmond Basin was first worked out in detail by 

 Shaler and Woodworth.''^^ They say: 



A casual observer traversing the Richmond area on any one of the section lines described 

 in this report can not but note that there are several leading lithological features displayed in 

 bands by which the strata may be thoroughly grouped. When one seeks to trace out these 

 bands along the strike of the strata the futility of the task is at once apparent. It is only on the 

 broadest possible lines with a considerable error in the placing of boundaries between distin- 

 guishable groups of strata that a mapping of the basin can at present be carried out. 



The accompanying table of divisions is an expression of the present knowledge concerning 

 the lithological and biologic characters of the area. 



"■ Cross, Whitman, and Howe, Ernest, Red Beds of southwestern Colorado and their correlation: Bull. Geol. Soc. 

 America, vol. 16, 1905, pp. 447-498. 



b Walcott, C. D., The Permian and other Paleozoic groups of the Kanab Valley, Arizona: Am. Jour. Sci., 3d ser., 

 vol. 20, 1880, pp. 221-225. 



"Ward, L. F., Geology of the Little Colorado Valley: Am. Jour. Sci., 4th ser., vol. 12, 1901, pp. 401-413. 



