26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 6g 



constitute what is here called the La Plata group {see pi. 2, fig. 2). 

 They occur principally in Colorado, eastern Utah, northern New 

 Mexico and Arizona. The group takes its name from southwestern 

 Colorado, where Cross ' first studied and described the deposits as 

 the La Plata sandstone. The original La Plata and its approximate 

 age equivalents cover some such areas as that shown in the accom- 

 panying figure 1, page 11. It includes the White Cliff and Vermilion 

 Cliff sandstones of Utah; the Navajo, Todilto, and Wingate of Ari- 

 zona ; the Wingate and other formations in western New Mexico ; 

 the Exeter sandstone of eastern New Mexico, and rocks in other 

 places which have been grouped by some geologists with the under- 

 lying Triassic and by others with the overlying Morrison, but which 

 are here regarded as being of essentially the same age. 



The La Plata sandstone and the formations believed to be its age 

 equivalents consist chiefly of massive cross-bedded, cliff-forming 

 sandstone (see pi. 1). They contain a subordinate amount of shale, 

 and in some places there are thin limestones which contain a few 

 shells of fresh-water invertebrates. In the southern part of the area 

 occupied by these deposits gypsum is abundant in the center of the 

 group. The typical La Plata is prevailingly light-colored, but in 

 northeastern Arizona and northwestern New Mexico its equivalent 

 formations are red, and in northeastern New Mexico they are pink 

 to buff-colored. In lithologic character and stratigraphic position the 

 White Cliff and Vermilion Cliff sandstones correspond closely with 

 lower La Plata, but there seems to be lack of general agreement as to 

 their exact correlation. Some facts seem to indicate that these cor- 

 respond to the two sandstones of the La Plata group; others, that 

 they represent only the lower sandstone, and are together equivalent 

 to the Wingate — a view which I am inclined to advocate after seeing 

 them in southern Utah. There is further disagreement as to the 

 relations in northern Colorado and Utah, some geologists correlating 

 the cliff-making sandstone (White Cliff of that region which some 

 call Nugget) with La Plata as a whole, others with lower La Plata 

 only. The correlation embodied in figure 2, page 13, harmonizes 

 with the known facts. 



Emery, 2 who is familiar with the formations in the Navajo country 

 in Arizona and who has recently (1917) examined the similar forma- 



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

 1899. 



2 Emery, Wilson B., Manuscript in preparation. 



