RECONNAISSANCE IN COLORADO AND UTAH 637 
While this reconnaissance was in progress Stanton and others were 
demonstrating that the beds hitherto referred to the Dakota to the 
east of the mountains, in the Arkansas Valley, were in part of the 
Comanche or Lower Cretaceous Series (36). Our observations 
were not directed to this point, but a re-examination in 1906 of the 
Dakota on the south flank of the San Juan failed to reveal ground for 
assigning any part of the formation called the Dakota in the San 
Juan folios to the Comanche Series. 
JURASSIC FORMATIONS 
All geologists who have examined the Mesozoic section of western 
Colorado have been impressed with the strong lithologic resemblance 
exhibited by several hundred feet of strata, occurring immediately 
below the Dakota to the fresh-water Jurassic beds found along the 
eastern base of the Front Range and characterized by the wonderful 
Dinosaurian fauna exploited by Marsh and others. With one 
exception, to be considered below, this lithologic similarity and corre- 
sponding stratigraphic position have been considered sufficient to 
warrant the assignment of the western slope beds to the Jurassic. 
The first to give a formation name to those strata was Eldridge 
(13), who called them the Gunnison formation. In the San Juan 
region it was found better to divide the Gunnison into the McElmo 
and La Plata formations, the former to include the alternating sand- 
stones and variously colored marls and shales of the upper part of 
the section, and the latter the heavy sandstones of the lower portion. 
THE McELMO FORMATION 
Before the McElmo beds were so named (3) they had been studied 
in the Telluride quadrangle at the head of San Miguel and Dolores 
Valley, and had been traced for some distance down each stream. 
They are continuously exposed down the canyon of the former to 
the Dolores River and have a wide distribution in the Uncompahgre 
Plateau, about the La Sal Mountains and in the lower Dolores and 
the Grand River valleys. This is clearly stated by Peale in the Report 
for 1875 (29). But the Hayden map covering the area just mentioned 
shows “‘ Lower Dakota”’ beds as present beneath the plateau-making 
Dakota proper, and in Peale’s Report for 1876 (30) he divides the 
