368 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



or semicrystalline limestone. The thickness is about 150 feet. The greater part of this forma- 

 tion appears to be of Devonian age, and it was only comparatively recently that a Mississippian 

 fauna was obtained from its upper portion. The latter is intimately related to the Mississippian 

 faunas at Leadville and A.spen, and also to the Waverly fauna of the Wasatch limestone of Utah, 

 the Madison limestone of Wyoming and Montana, the typical Waverly of Ohio, and the 

 Chouteau limestone of Missouri. The fauna of the Millsap limestone, as we know it from 

 Perry Park and also from Garden Park and Canyon, and that of the Leadville hmestone at 

 Leadville present a rather peculiar facies, but even this fauna seems to be related to those of 

 the early Mississippian, and it is somewhat singular that the Leadville fauna is more nearly 

 alhed to that of Perry and Garden parks, while lithologically and stratigraphically the Leadville 

 as a formation is especially to be compared with the Leadville of Aspen and of the Crested 

 Butte region. I think we can safely regard these faunas as varying facies of a single widespread 

 and contemporaneous fauna, the differences being due in part to varying environmental con- 

 ditions and in part to quite a different cause. Our collections from these beds are extremely 

 meager, and to this circumstance can be referred some of the differences at present existing. 



^ Sj* ^ SfC ^ 5}£ 5f* ^ ^ 



It is now fairly certain that the Carboniferous portion of the Leadville, Ouray, and Millsap 

 hmestones are of early Mississippian age. 



This formation, or, as it has received several names, this horizon, is widely distributed in 

 Colorado. It is known in the Grand River region (the Devonian portion has been recognized 

 on the White River and at Glenwood Springs and the Carboniferous probably on Eagle River), 

 in the Elk Mountain region, in the South Park region, in the northern part of the Sangre de 

 Cristo Range, in the San Juan region, and along the Front Range. It is probably lacking over 

 much of the Front Range and Sangre de Cristo areas, in the Uinta Mountains, and in the valleys 

 of the Dolores and the Grand, and it is concealed over extensive areas by deposits of later 

 geologic age; but it is perhaps the most widespread of all the Paleozoic formations except those 

 belonging to the Pennsylvanian. The remarkable persistency of this horizon as a hmestone 

 formation, not only in Colorado but throughout the West, and the widespread distribution of 

 essentially the same fauna would argue extended and uniform marine conditions during Missis- 

 sippian time. 



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The Lower Carboniferous period was followed by an epoch of elevation and erosion. At 

 all events none but the early portion of Missisippian time is apparently represented in the 

 Colorado sediments. The chronological point at which this elevation took place is, except in a 

 general way, unknown. It may have occurred soon after the formation. of the Leadville lime- 

 stone, the absence of the upper Mississippian horizons being due not so much to erosion as to 

 nondeposition; but it seems to me rather more probable that this episode was nearly contem- 

 poraneous with the elevation in eastern North America at the close of Mississippian time. That 

 the elevation is of wide extent is indicated by the absence over such large areas in our Western 

 States of upper Mississippian faunas. To the erosion which resulted from this elevation the 

 variation in thickness of the Leadville hmestone in different sections can often be ascribed. 

 When subsidence again permitted the formation of sediments, the earUest deposits were 

 frequently of a less purely marine character, and shortly a great thickness of sands and con- 

 glomerates intermingled with nonpersistent limestone bands began to form over great areas in 

 the State. 



The Paleozoic section of central Colorado, by which term I would incliide the Grand River, 

 Elk Mountain, and South Park areas, is singularly uniform, and this is scarcely less true of the 

 strata of Carboniferous age than of the earlier Paleozoics. Though these are fairly constant in 

 lithologic characters, the nomenclature employed for them has varied somewhat; Thus in the 

 Anthracite-Crested Butte folio the Carboniferous formations are called the Leadville hmestone, 

 Weber hmestone, and Maroon conglomerate; in the Aspen monograph the Leadville hmestone, 

 Weber formation. Maroon formation, and the Triassic; in the Tenmile foho the Leadville 

 limestone, Weber shale, Weber grits, Maroon formation, and Wyoming formation; and in the 

 Leadville monograph the Blue limestone, Weber shale, Weber grits, and Upper Coal Measures. 



