RESUME OF LITERATURE. 55 



locephalus fauna are said to have been found. The same is true of the Yule lime- 

 stone, which thoug-h from 260 to 400 feet thick at Aspen only reaches 120 to 160 feet 

 in the Tenmile district. Similarly also with the Parting quartzite, which measures 

 67 feet in a typical detailed section near Aspen, while here it is said to be from 15 to 

 60 feet. Emmons maps these beds as Silurian in the Tenmile folio. His position is 

 best defined in his own words, and I quote as follows: 



' ' Near Leadville evidence has been found of an unconf ormit}' by erosion between 

 thera and the overlying beds. While no fossils have been found which would con- 

 nect them with either the Silurian or the Carboniferous system, they have hitherto 

 been mapped under the Silurian color, mainh^ because of this unconformity, which 

 might account for the apparent absence of Devonian strata. 



"Within the last few j^ears, however, Devonian fossils, principally fish remains, 

 have been found in other parts of the Rocky Mountains in beds that occup}^ a strati- 

 graphic position so closely corresponding to that of these quartzites that it has become 

 probable that the latter are of Devonian age also. Nevertheless, in the absence of 

 direct evidence, it has not been judged wise to designate them by Devonian color on 

 the sheet showing structui'al sections." 



Like the earlier beds, the Leadville limestone is thinner in the Tenmile district 

 than at Aspen, varying from 300 feet at the latter to 200 feet at the former, where it 

 is sometimes much reduced. Emmons refers the Leadville limestone to the Missis- 

 sippian period, and in this he is with little doubt correct. 



The Weber shale and the Weber grits are grouped in the Tenmile folio under 

 the name of the Weber formation, which with the series of beds called here the 

 Maroon represents the combined Weber and Maroon formations of the Aspen mono- 

 gi'aph. It seems fair to infer that the Weber shale of the one is the equivalent of 

 the Weber formation of the other (Aspen). In this case also the decreased thickness 

 of the Weber in the Tenmile region is a peculiar feature. It has there a thickness 

 of 300 feet, while at Aspen it measures but little less than 1,000 feet. In the 

 Tenmile folio the limestones of this formation are said to contain fossils of Coal 

 Measure age, and our collections from both the Leadville and Crested Butte districts 

 confirm this statement. 



The upper portion of the Weber formation of the Tenmile folio, or the Weber 

 grits, as it is there called, must, along with the Maroon formation of the same 

 report, represent the Maroon formation of the Aspen monograph. In the case of 

 this formation the thickness, in marked contrast to the formations which preceded 

 it, is the same in both the Aspen and Tenmile districts, being measured at 4,000 feet 

 in both. The Maroon formation in the Tenmile district is divided from the Weber 

 grits below by the Robinson limestone, and its upper limit is taken at the Jacque 

 Mountain limestone. Both of the strata last mentioned are said to contain 

 invertebrate faunas of Upper Coal Measure type. 



