646 C. L. DAKE 



can with confidence be referred to the Navajo. The sandy beds 

 which he describes as occurring above the gypsum fit more closely 

 Gregory's description of the sandstones above typical Navajo, 

 being those considered by Gregory to constitute the McElmo. This 

 discrepancy could be brought about either by a mistake in the 

 identity of the sandstones called Wingate below the gypsum beds, 

 or by the absence of any true Navajo, which condition would allow 

 the gypsum beds at the base of the McElmo to rest directly on 

 Wingate or Todilto. 



From an extensive personal survey of these formations over a 

 region that includes the Water Pocket Canyon, Dirty Devil 

 Canyon, the Moab Valley, the Big Indian Uphft, the San Juan 

 Uplift, the Fort Defiance Uphft, and the Zuni Uplift, the writer 

 is reasonably familiar with the lithologic variations of these forma- 

 tions, and in view of the facts presented he is forced to the con- 

 clusion that these marine Jurassic beds are not the equivalent of 

 the Todilto, but that they are above the Navajo sandstone. 



Whether or not they are a part of the true McElmo is another 

 matter. The writer is quite in accord with Emery in believing that 

 a pronounced break occurs at the base of the Salt Wash con- 

 glomeratic sandstone. A similar break, with heavy conglomeratic 

 beds, was noted by the writer at the Sundance-Morrison contact 

 in the valley of the south fork of Shoshone River, Wyoming. The 

 following is quoted from the unpublished manuscript of a bulletin 

 on that region prepared by the writer for the Wyoming State 

 Geological Survey: "At several points, particularly near the south- 

 east corner of section 19, T. 51 N., R. 103 W., there lies between 

 typical Sundance and typical Morrison from 5 to 20 feet of coarse 

 pebbly sandstone. The pebbles are all small, mostly less than one- 

 fourth inch in diameter, and consist of gray shale, gray sandstone, 

 black chert, and quartz, mostly well worn and rounded." A break 

 at this horizon, of great importance, is considered probable by 

 Schuchert.^ 



This break, if sufficiently widespread, might well justify the 

 separation of the beds below the Salt Wash member as a distinct 

 formation, equivalent to the Sundance. 



' Charles Schuchert, "Age of the American Morrison and East African Tendaguru 

 Formations," Bull Geol. Soc. America, XXIX (1918), 245-80. 



