FAUNAL EVIDENCE AND CORRELATION. 



MISSISSIPPIAN. 



In Colorado, and generally throughout the AVest, the Upper and Lower Car- 

 boniferous faunas are well differentiated. In Colorado the only Mississippian hori- 

 zon is that of the Leadville limestone and its equivalents, the Ouray and Millsap 

 limestones. This formation contains at many points a characteristic Upper Devonian 

 fauna in its lower part, and at the top a fauna equally distinctive of Carboniferous 

 time. This is trae of the Crested Butte region, the San Juan region, and the beds 

 at Salida. At some points only the Devonian fauna is known, and at others only 

 the Mississippian; but in A'iew of the scanty character of our collections from manj^ 

 of the areas, the expectation is reasonable that if careful search be made both faunas 

 will be found at some of the points at which onl\' one is now known. 



The Mississippian fauna of the Leadville limestone represents the earlv portion 

 of Mississippian time, and is related to the fauna which occurs with such wonderful 

 persistency throughout the West. It is closely similar to the fauna of the Cuyahoga 

 shale of Ohio and the Chouteau limestone of Missoui'i, and this relationship has been 

 generally recognized. Of this further proof need hardly be offei'ed than the.faunal 

 lists themselves. The evidence both of species and of genera is quite clear in 

 indicating the early Mississippian. 



In Colorado we have no upper Mississippian faunas. This is true of a large 

 part of the western country, e\'en where early Mississippian rocks are known. On 

 the other hand, at a few points, unmistakable faunas of the Genevieve period have 

 been found, so that there is no reason to believe that these rocks and faunas have 

 never been deposited in the West. In Colorado there is satisfactory evidence that 

 the Leadville limestone was followed, though probably not directly, by a period of 

 elevation and erosion; and I believe that the absence of Genevieve faunas, both in 

 Colorado and elsewhere in the West, is in large measure accounted for through the 

 same circumstance. In the Leadville limestone, therefore, so far as its fossils have 

 come into my hands, I believe that Genevieve time is not represented, nor probablj'' 

 Keokuk or upper Burlington. The Leadville fauna, according to my interpretation, 

 represents, in the Carboniferous, Kinderhook, and possibly lower Burlington time. 



I have spoken of the Leadville fauna as a unit. In fact, collections from different 

 disconnected areas show rather strong local groups of species. This is no doubt in 

 part due to local differences in the condition of environment. It is in part the result 

 of the scantiness of many of our collections. Mississippian faunas have been found 

 in Colorado, in the San Juan region, in the Crested Butte quadrangle, in the Aspen 

 district, in the Leadville district, at Salida, at Canyon, in the Pikes Peak quadrangle, 



217 



