452 CROSS AND HOWE — RED BEDS OF SOUTHWESTERN COLORADO 



Plata folio (4), with provisional inclusion of the lower unfossiliferous 

 member of the section. Reviewing the conditions, it is remarked : 



11 It is evident that future discoveries may not only place the upper boundary of 

 the Rico formation at a higher horizon, but may also establish an intermediate 

 Permian formation between the Rico and the Dolores." 



The Rico formation is not exposed in the La Plata quadrangle. 

 In the report on the Geology of the Rico mountains the following 

 characterization of the Rico formation and its relations was given : 



" It is here proposed to apply the name Rico to a formation assumed to be about 

 300 feet in thickness, occurring between the Hermosa or characteristic Pennsylva- 

 nian Carboniferous and strata assigned at present to the Trias of the San Juan 

 region, the Dolores formation. It is made up of sandstones and conglomerates, 

 with intercalated shales and sandy fossil iferous limestones. In its lithological 

 features it resembles the strata immediately above it, but its fossils are distinctly 

 of Paleozoic age, and while many of its forms are common to the Hermosa forma- 

 tion, others are of Permian type, so that it seems proper to designate its age 

 Perm o- Carbon iferous [Permo-Pennsylvanian], to indicate that it is transitional 

 between these divisions of the Carboniferous system. In the Rico region the for- 

 mation is conformable upon the Hermosa and is followed by the Dolores with 

 seemingly perfect parallelism of stratification. The fauna as a whole has an aspect 

 quite different from that of the Hermosa, since it is largely composed of lamelli- 

 branchs as opposed to the brachiopod assemblage of the lower formation. The 

 boundary between the Rico and Dolores formations is at present entirely artificial, 

 being based upon the highest known occurrence of the Rico fossils. The former is 

 made to include only strata characterized by the Rico fauna, while the latter com 

 prises the apparently unfossiliferous medial portion of the Red beds, together with 

 the upper part of known Triassic affinities. The actual age of the unfossiliferous Red 

 beds is thus left in doubt. They may eventually prove to be either Permo-Carbon- 

 iferous, true Permian, or Trias. They correspond to [a part of] what has been 

 called Trias throughout the Rocky Mountain province" (5). 



The remainder of the Red Bed sections about Rico was referred to the 

 Dolores with the provision that the name should apply to Triassic strata 

 and expression of doubt as to the age of the unfossiliferous beds, in 

 harmony with the treatment of the subject in the Telluride folio. The 

 fossil-bearing and fossil-free parts were described separately. 



It was further pointed out that the unfossiliferous Red beds corre- 

 sponded in position and character to strata assigned by various authors 

 to the Permo-Carboniferous or the Permian, in the adjacent Plateau 

 province of Utah and Arizona and in other localities. 



The views expressed in the Rico report on the age and relations of the 

 Rico formation were based mainly on the opinion of G. H. Girty as to 

 the invertebrate fauna. That opinion was more completely stated by 

 Girty in his full discussion of the Carboniferous faunas of Colorado 

 (15). The more recent work in the San Juan region, and especially 



