RESUME OF LITERATURE. 125 



The Rico formation includes the lower portion of the Red Beds in this region, 

 the upper portion being described as the Dolores formation. Lithologicall}? the 

 two formations are quite similar, and their discrimination is based chiefly upon 

 the e^^dence of fossils. The fauna of the Rico formation is of Carboniferous age, 

 while that of the Dolores is Triassic. The boundary between the Rico and the 

 Hermosa formations is well marked, but that between the Rico and Dolores " is at 

 present entirely artificial, being based upon the highest known occurrence of the 

 Rico fossils. The former is made to include onlj- strata characterized by the Rico 

 fauna, while the latter comprises the apparentlj^ unfossiliferous medial portion of the 

 Red Beds, together with the upper part, of known Triassic aflinities. The actual age 

 of the unfossiliferous Red Beds is thus left in doubt; they may eventually prove to 

 be either Permo-Carboniferous, true Permian, or Trias. They correspond to what 

 has been called Trias throughout the Rocky Mountain province." The thickness of 

 the Rico formation thus determined is about 300 feet, and it is made up of " sand- 

 stone and conglomerates with intercalated shales and sandy fossiliferous limestones." 

 The general characteristics of the Rico formation are again described as being, first, 

 "its calcareous nature, in which it resembles the strata above and below; second, the 

 very arkose character and the coarseness of the sandstones, in which respect it differs 

 from the Hermosa and resembles the Dolores; and, third, its chocolate or dark- 

 maroon color, which contrasts sharply with the gray or green of the Hermosa and 

 which is more or less distinct from the bright vermilion of the Dolores." 



With the Rico formation the portion of the section falling within the scope of 

 nij work is exhausted, but I will briefly recapitulate the important characters of the 

 Juratrias groups. "All the Juratrias formations of the San Juan region," it is 

 reported, "are represented in the Rico Mountains. Beginning at the base, they are 

 the Dolores Red Beds, 1,600 feet in thickness; the La Plata sandstone, 250 feet or 

 more; and the McElmo shales and sandstones, exposed to a thickness of 300 feet 

 within the area represented on the map, but having a total thickness of nearly 900 

 feet in the region adjacent." 



The. La Plata sandstones and McElmo formations are said by Cross and Spencer 

 to I'epresent, respectively, the lower and upper portions of the Gunnison formation 

 of the Crested Butte region. The Oura}' limestone, as alreadj^ shown, can be corre- 

 lated with the Leadville limestone of that area, so that the Molas, Hermosa, Rico, 

 and Dolores formations in one area and the Weber and Maroon formations in the 

 other occupj^ similar positions in the section. The Weber formation appears to 

 have no especially discriminated lithologic equivalent in the San Juan region. 

 The Hermosa formation, in thickness, lithologic character, and position, is similar to 

 the lower of the two divisions recognized in the Maroon conglomerate. The Rico 

 formation in its color and lithology is similar to the upper division of the Maroon, 



