244 CARBONIFEROUS FORMATIONS AND FAUNAS OF COLORADO. 



The preceding- table shows thi' local distriliutioii of spcrics of the Upper Cai'bon- 

 it'erous strata of the Leadville royioii. The la.st live coluniiis show their raiif>-e in 

 the five stratigraphic divisions which have been recognized. They indicate two 

 somewhat different and contrasting, though both Upper Carboniferous, faunas. 

 One of these is exeniplitied by the fossils of the Weber shales. To the other those 

 of the upper division of the Weber belong. The fauna now known from the Kobin- 

 son limestone is a meager one. Thus far the evidence does not indicate that the 

 Robinson fauna differs essentially from that of the upper Weber. The middle 

 division of the Weber also has furnished a very limited fauna. All of its species 

 are common to the contiguous formations, both above and below, and there is no 

 evidence for uniting it with one rather than another, or for retaining it as coordinate 

 with either. The upper fauna is especially characterized b}' the number of its 

 pelecypod and gasteropod species, which were obtained, however, in large part at 

 a single locality. 



The difference in facies between the Weber shales ,and the middle and upper 

 Weber is as marked in this case as in the corresponding formations of the Crested 

 Butte region. While only 4 species of the true Mollusca have been found in the 

 Weber shales in this area, almost the entire brachiopod representation is centered 

 there, only 7 species being found in the four upper stratigraphic divisions. Even 

 among these the most numerous representation is in the Weber shale, the only 

 instance where the range is nearlj' uniform being in the species Seminula subtilita. 

 However, while the Weber shale has perhaps been sufficiently collected to warrant 

 the belief that its main features are known, this is far less true of the upper beds of 

 the section. 



Comparing the formational faunas of the Leadville area after the same method 

 that was employed with those of the Crested Butte region, the following facts 

 appear: In the case of the lower and upper Weber, these two formations have a 

 joint fauna of 35 species, of which onl}" 5 are contained in common. The relation of 

 these faunas will then be expressed by the fraction ^^ or by the percentage 14.3. 

 If, however, the lower Weber is contrasted with all the series above, the percentage 

 is raised to 16.2, for the total fauna is 37 and the common species 6. 



The Leadville area is related to the Crested Butte region geologically and 

 geographically, and a general faunal agreement can be traced. In both areas the 

 fauna characteristic of the Rico formation of the Animas region is wanting, as will 

 appear later. When the faunas of the different divisions of the Carboniferous in the 

 Leadville region are tabulated with those of the Crested Butte region a certain cor- 

 respondence in the abundance and range of species is disclosed. The evidence thus 

 obtained indicates that the Weber shale of the Leadville region is the representative 

 of the Weber formation of the Crested Butte region, and that the upper member of 



