558 



INDEX TO THE STEATIGRAPHY OF NOETH AMERICA. 



Correlation of formations. 



AH geologists who have examined the Mesozoic section of western Colorado have been 

 impressed with the strong hthologic 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 

 the corresponding stratigraphic position have been considered sufficient to warrant the assign- 

 ment of the western slope beds to the Jurassic. 



The first to give a formation name to those strata was Eldridge, who called theiia the Gun- 

 nison formation. In the San Juan region it was found better to divide the Gumhson into the 

 McElmo and La Plata formations, the former to include the alternating sandstones and variously 

 colored marls and shales of the upper part of the section and the latter the heavy sandstones 

 of the lower portion. 



Before the McElmo beds were so named they had been studied in the TeUuride 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. * * * 



The McElmo beds in characteristic development were seen by us in Dry Valley and on the 

 eastern flanks of the La Sal Mountains, in Dolores Valley, and in many places on Uncompahgre 

 Plateau, as far north as Unaweep Canyon. To the north from that locality Peale refers to the 

 formation as maintaining the same general character. No representative of the marine Jurassic 

 reported by Powell and others from Utah was observed by us. 



Rumors of large bones, presumably in the McElmo beds, have come to my attention sev- 

 eral times in recent years, but never with the exact locality named, and no trace of such- 

 remains has been found in the San Juan region. 



