122 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



"The Hermosa and Aubrey [Kaibab] faunas are both regarded as Pennsyl- 

 vanian, but Mr. Girty informs us that the Hermosa has no species in common 

 with the typical Aubrey [Kaibab] of the Grand Canyon section, as far as known. 

 Mr. Girty further states that the lower Aubrey fauna from beds at the junction 

 of the Grand and Green Rivers, comprising a large part of the Aubrey fauna 

 described by White in Powell's Unita report, is markedly different from the 

 Aubrey of the Grand Canyon, as it also is from the Hermosa fauna of Colorado. 

 These faunal differences must seemingly be explained in one of three ways: (i) 

 by a rapid gradation of forms within a comparatively narrow zone; (2) by the 

 assumption of an effective barrier between the Aubrey and Hermosa seas, extend- 

 ing for hundreds of miles from eastern New Mexico west and northwest across 

 New Mexico and Utah ; or (3) by assuming one of the formations to be younger 

 than the other, and that the Pennsylvanian section is incomplete both in Colorado 

 and in the Plateau country * * *." 



It is probable that the Hermosa is the equivalent of the Weber interval 

 to the north and either equivalent to, or a continuation of, the Aubrey. 

 If the latter, there is evidence of a transition into the conditions which 

 determined the Weber grits which cover such a large area to the north. 



The first appearance of the Weber formation in a locality where it has 

 been described is in the area of the Tenmile quadrangle in west central 

 Colorado. In this region, Emmons^ says of it: 



" Weber formation. — This formation constitutes the most siliceous member of 

 the Carboniferous system, and corresponds in a general way to the Weber 

 quartzite of the Wasatch Mountain section and to the Lower Aubrey [Kaibab] of 

 the Colorado Canyon section. It includes a lower calcareo-argillaceous member, 

 designated in the Leadville report the Weber shales, the main siliceous formation 

 being there called the Weber grits. 



"The Weber shales constitute a transition series between the massive lime- 

 stones below and the coarse sandstones of the Weber grits above. They consist 

 in the Mosquito Range of argillaceous and calcareous shales, alternating with 

 quartzitic sandstones. The former are generally very carbonaceous and often 

 contain seams of impure anthracite, up to several feet in thickness, but of no 

 commercial value. The calcareous members sometimes develop considerable 

 beds of impure limestones, generally fossiliferous, which are distinguished from 

 those of the lower formation by fossils that are exclusively of Coal Measure 

 aspect. The thickness of this series, which is very variable, is assumed in this 

 region to be about 300 feet. It occurs in the valley of Eagle River, just west of 

 the limits of the quadrangle. 



"To the Weber grits belong the lowest beds exposed in the Tenmile district 

 west of the Mosquito fault. Their average thickness here, as in the Mosquito 

 Range, is about 2,500 feet. They consist mainly of coarse sandstones or grits, 

 often very micaceous, with a subordinate development of shales and a few thin 

 and non-persistent beds of dolomitic limestone. The sandstones are generally 

 light gray in color, but near the base of the series are sometimes quite dark from 

 the presence of finely divided carbon, probably in the form of anthracite or 

 graphite. Their prominent constituents are quartz and feldspar, evidently 

 derived from the Archean; pink orthoclase is sometimes so abundant as to 



^ Emmons, S. P., Tenmile District Special Folio, No. 48, U. S. Geological Survey, p. i, 1898. 



