LITERATURE 453 



in the Animas valley, has shown that the Rico formation is not a per- 

 sistent 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 unfos- 

 siliferous Red beds to the Hermosa is no longer through a marked red- 

 dish 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 inter- 

 calated in a red section and are limited to a very narrow band. More- 

 over, there is mingling of forms supposed originally to be characteristic 

 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. Further reference to this question will be made in 

 discussing the correlation of the San Juan formations. 



THE OURAY VNCONFORMITY 



As was long ago made plain by the Hayden maps of southwestern 

 Colorado, the western San Juan district is a center at which a domal 

 elevation of Paleozoic and Mesozoic sediments took place at the close of 

 the Cretaceous. Earlier movements of a similar character also took place 

 at this center, their effects being with difficulty distinguishable from those 

 of the post-Cretaceous uplift. The domal structure is well shown by the 

 attitudes of the strata in the deep valleys of the Animas, Dolores, San 

 Miguel, and Uncompahgre rivers, which radiate from a central area now 

 occupied mainly by volcanic mountains. There was practically a plana- 

 tion of the apex of this dome before the earliest volcanic tuffs were 

 deposited, so extensive that granites, schists, and Algonkian sediments 

 were exposed by that planation, and these rocks have now been once 

 more revealed, to some extent, on both the northern and southern sides 

 of the mountains, through the removal of the volcanics. A review of 

 San Juan structure may be found in the Telluride folio (3), and this 

 broad subject will not be further discussed in this place. 



The Uncompahgre valley exhibits the only extensive section of the 

 sedimentary rocks to be found on the northern slope of the San Juan. 

 That condition is mainly due to the fact that to the east of the valley the 

 Paleozoic and early Mesozoic beds were removed as a result of pre- 

 Triassic and pre-Jurassic uplifts and succeeding erosion, while in the 

 Tertiary period the Cretaceous beds were also to a great extent eroded, 

 so that in some places the volcanics rest on gneiss or schist. 



