126 CARBONIFEROUS FORMATIONS AND FAUNAS OF COLORADO. 



and both arc cliaractcri/.cd by their arkoso character, indicating that erosion Inniny 

 cut throuyli tht> overlying sediments was phiying upon the granitic basement roeU. 

 The Rieo is, however, very much thinn(>r, and its characteristic fauna has as yet not 

 been found in the Maroon conglomerate, the fauna of tlie upper part of which is 

 unknown. Tlic brigiit-red Dolores beds appear to have no equivalent in the Crested 

 Butte quadrangle, but they can be compared to the similar deposits described by 

 Spurr at Lenado Can3'on near Aspen. The time interval between the Rico fauna 

 and that characterizing the top of the Dolores is so great that 1 believe that a 

 pex'iod of erosion or of nondeposition must have intervened, even though it is not 

 now recognizable in the mutual attitude of the sediments. With such a theory the 

 great disparity in thickness between the Eico formation and the upper Maroon is 

 in accord, and also the greath^ reduced thickness of the upper Maroon in the 

 southern part of the Crested Butte quadrangle reported by Eldridge. On the other 

 hand, the Gunnison formation was preceded in the Crested Butte region by a 

 recognized unconformity, and it is bj' pre-Gunnison erosion that the apparent absence 

 of the Dolores formation in that region may be accounted for. 



The paper which furnished the foregoing brief summarj' was one of the last 

 dealing with the geology of this area, but because it establishes for the first time 

 a section and a nomenclature, its consideration has been, as a matter of convenience, 

 taken up before those of earlier date. 



In the geologic reports which accompanj' Ruffner's report upon a reconnais- 

 sance in the Ute country" are accounts bj' F. Hawn and L. Hawn, of observations 

 at Animas Park and Animas River in the San Juan Mountains. Most of the strata 

 described along the Animas seem to belong to the Hermosa formation, and Car- 

 boniferous faunas are cited from them in a number of instances. At two localities 

 in the vicinity of Engineei' Mountain fossils are cited from a clierty limestone, which 

 are said to be of Eocarboniferous age. The following species are reported (p. 68): 

 Ci/athqphylhim, Gorgonia, Crinoidea, Productus reticulahis, and Euomphalus latus. 

 Ks, the character of the rock, the localitj^, and, to a certain extent, the fauna agree 

 with our collections from the top of the Ouraj^ limestone, it seems probable that 

 Hawn was correct in the age determination of these beds. The red sandstone 

 mentioned on page 68, and reported as 1,000 feet thick, possibly represents the Rico 

 formation, unless the color is due to metamorphism as suggested by Hawn. 



Stevenson's report in the third volume of the Wheeler surve}^* touches briefly 

 upon the geologj^ of the San .Tuan region."^ The only beds which come "svithin the 

 purview of this discussion are referred to the Carbonifei'ous and the Triassic. Thej' 

 were seen chieflj^ in the Animas Vallej^ The Carboniferous, distinguished b_y its 



a Ru£Ener's Kept. Eecon. in the Ute Country, Forty-third Cong., 1st sess., House Ex.- Doc. No. 193, 1871. 

 ftU. S. Geog. Geol. Surr. W. 100th Mer., Kept., vol. 3, 1875, pp. 307-501. 

 clbid., p. 374 et seq. 



