12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



logic character and their stratigraphic relations over an area of con- 

 siderable size, where the rocks can be traced at the outcrop or where 

 exposures are so close together that correlation by lithology or other- 

 wise is satisfactory. From these observed relations the governing 

 physiographic conditions may be judged. If judged correctly, the 

 physiographic criteria help in correlating the rocks in a field 

 examined later or perhaps in correcting the correlations made with- 

 out their help. To apply this principle in the present study, it is 

 necessary to review the information relating to the La Plata sand- 

 stone and its age equivalents and to test by the new criterion the 

 correlations which have been made from time to time. 



In the accompanying groups of sections I have indicated by means 

 of the names attached to some of them the correlations which have 

 been made in the published descriptions. The symbols and connect- 

 ing lines indicate my personal inclinations as to correlation which in 

 several instances differs from that made by the different authors. 

 The data presented have been gleaned chiefly from the literature, 

 but much unpublished information is used which has been gathered 

 in part by myself and in part by several of my associates on the 

 United States Geological Survey, who have freely contributed from 

 unpublished manuscripts and notes. The most that can be claimed 

 for the grouping is that it represents possible relations. The implied 

 correlations should be tested rigorously by observations in the field 

 and modified as newly established relationships are determined. 

 Most of the sections have been grouped with reference to the top of 

 the Morrison as a datum plane. This plane was close to sea-level 

 and formed the floor on which the Dakota sandstone was deposited. 



COMPARISON OF SECTIONS 

 ARIZONA TO NORTHERN UTAH AND EASTERN WYOMING 



The La Plata sandstone described by Cross * in southwestern 

 Colorado has been traced by Gregory 2 southward into Arizona, 

 where the upper sandstone is called Navajo, the lower sandstone 

 Wingate, and the beds separating the two Todilto. These subdi- 

 visions have not been carried far to the north in the walls of the 

 canyon of the Colorado River, but it seems probable that the Gray 

 Cliff and Vermilion Cliff sandstones in the region of Henry Moun- 



1 Cross, Whitman, U. S. Geol. Survey Geol. Atlas. La Plata folio (No. 60), 



399. 

 "Gregory, H. E., U. S. Geol. Survey Prof. Paper 93, 1917. 



