THE BASIN PROVINCE 157 



San Juan region the Dolores formation. It is made up of sandstones and con- 

 glomerates with intercalated shales and sandy fossiliferous 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 formation, others are of Permian type, so that it seems proper to desig- 

 nate its age Permo-Carboniferous, to indicate that it is transitional between 

 these divisions of the Carboniferous system. In the Rico region the formation 

 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 lamellibranchs 

 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 

 comprises the apparently unfossiliferous medial portion of the Red Beds, together 

 with the upper part, of known Triassic affinities. The actual age of the un- 

 fossiliferous Red Beds is thus left in doubt; they may eventually prove to be 

 either Permo-Carboniferous, true Permian, or Trias. They correspond to [a 

 part of] what has been called Trias throughout the Rocky Mountain province." 



In their 1905 paper, Cross and Howe 1 say of the Rico formation: 



"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. The more recent work in the San Juan 

 region, and especially in the Animas Valley, has shown that the Rico formation 

 is not a persistent feature of the Red Bed section, nor its fauna so markedly dis- 

 tinguishable from that of the Hermosa beds, as was seemingly the case from 

 observations in the Rico mountains. At even a few miles distance to the east 

 or southeast from that district, the transition from the unfossiliferous Red Beds 

 to the Hermosa is no longer through a marked reddish zone 300 feet in thickness. 

 The Rico fossils are found in certain peculiar limestones, plainly to be correlated 

 with those so marked in the Rico formation, but these fossil-bearing strata are 

 not necessarily intercalated in a red section and are limited to a very narrow 

 band. Moreover, there is mingling of forms supposed originally to be char- 

 acteristic of the Rico with those of the Hermosa. These observations make the 

 Rico formation a local development, having less importance in the analysis of 

 either the Red Beds or the Carboniferous section than was at first assigned to it" 



Cross and Howe, in the paper just cited, make several observations worthy 

 of note on the correlation and interpretation of the red beds of southwestern 

 Colorado. 



(Page 466.) " If the Aubrey and Hermosa are practically equivalent, as the 

 stratigraphic relations suggest, the Cutler beds occupy a position corresponding 

 to that of the Permian of the Kanab Valley in Utah and the formation of the 

 Zufii Plateau referred to the Permian by Dutton * * *." 



1 Cross, Whitman, and Ernest Howe, Red Beds of Southwestern Colorado and Their Cor- 

 relation, Bull. Geol. Soc. Amer., vol. 16, p. 452, 1905. 



