FAUNAL EVIDENCE AND CORRELATION. 229 



segregations of nearly black color, while many of those from the San Juan region 

 are inclosed in cherts of a nearl}' white tint. Thus the collections having this rather 

 puzzling facies are characterized by their cherty matrix. While the fauna of the 

 San Juan, Crested Butte, and other areas occurs at a number of points both north, 

 east, south, and west of central Colorado, I do not recall the Front Range association 

 of species from any other point. It is the grouping of these species rather than the 

 species themselves that is novel. Considered individually, each member of this fauna 

 is found elsewhere or has close allies, especially in the early Mississippian. Ortho- 

 thetes ineqiMlis, Spirifer centronatus, Spirifervna solidirostris, Semimda humilis, and 

 Strehlopteria media are found in the Waverly faunas. The nearest allies of Sjpirifer 

 sp. h occur the one at the very base of the Kinderhook, the other at the base of the 

 Burlington in the Mississippi Valley. Oransena ■nibelliptica var. liar ding ensis is 

 allied to upper De\'onian and Waverly forms. Eumetria ivoostei'l might be either 

 lower or upper Mississippian. Straparolhis cf. sjyergenensis and Semimda sid}qii;ad- 

 rata? are Genevieve species, but have allied forms in the early Mississippian. On 

 the whole, I should estimate the horizon of this fauna as in the Eo- or Meso-Missis- 

 sippian, not far, in fact, from that of the other group. The geologic evidence tends 

 unmistakabh' to the same conclusion. The identitj- of the Leadville limestone of 

 Leadville with beds at Aspen and in the Crested Butte quadrangle. has been rather 

 generally asserted by geologists thari doubted by anyone, yet the fauna of the 

 typical Leadville limestone is distinctly unlike that of the Crested Butte quad- 

 rangle. Its fauna allies it closely with that of the Millsap limestone. The faunas 

 of the Millsap limestone along the Front Range are correlated with that of the Liead- 

 ville limestone at Leadville h\ paleontologic evidence. The latter is correlated with 

 those of the Leadville limestone at Aspen and in the Crested Butte quadrangle by 

 stratigraphic evidence, and these again with the Ouraj' limestone of Colorado by 

 paleontologic evidence. The two groups of Mississippian faunas of Colorado, both 

 by stratigraphy and paleontology^ are placed at about the same horizon. Their 

 difference in facies is partly due to the incompleteness of our collections, but partly 

 to the selection of local environment, which was possibl}' somehow connected with 

 cherty segregation. Most authors have considered central Colorado to have been the 

 site of a number of large islands occupjdng the position of the present Archean 

 mountain masses. One asks the question whether the peculiarities of the faunas at 

 Leadville and the Front Range are connected with these conditions. This is possibly 

 the case. But it should be borne in mind that this fauna is a marine one, so that its 

 habitat could hardlj^ have been a landlocked basin. Besides which, all the species 

 found occur elsewhere themselves or have closely allied forms. The physical con- 

 dition could not have been in the nature of a barrier, for the faunas of the Elk and 

 San Juan Mountains are found to the north, south, and east of this area. So far as 



