138 CARBONIFEROUS FORMATIONS AND FAUNAS OF COLORADO. 



more I'lH't. The Jurassic strata luue no iicaring upon th(> present discussion, though 

 possibly the Triassic ones, which may be partially equivalent to the Wyoming sand- 

 stone, are germane to it. It is to be noted in this connection that Riggs conceives 

 tlie granitit' roclc beneath tlie Triassic to be of intrusive origin (see pi. 39), but this 

 formation has been regai-ded by the Hayden survey, and usually, I believe, as 

 Archean. 



It seems that the Jurassic beds of this paper, which have a thickness of 500 feet, 

 are the same as the McElmo formation of the Telluride quadrangle, and thej- here 

 have about the thickness as well as the character of that formation as it appears in 

 the Rico region. It is also the Jurassic of Peale, though considerably thicker, and it 

 does not contain the basal white sandstone which Peale in his report for 1876 includes 

 in the Jurassic. 



The Triassic is comparable in character and thickness to the beds discriminated 

 by Peale under the same name in 1876. His Triassic of 1876 was much thicker. The 

 character of Riggs's Triassic is similar to that of the Dolores formation, but it is 

 thinner. It is difficult to understand the absence of the La Plata in Riggs's section. 



