170 CARBONIFEROUS FORMATIONS AND KAUNAS OF COLORADO. 



C'.irlKniil'orous. 'Walcotl in his Canyon scclion, Cros.s in" tlie I'ikos I'eak I'olio, and 

 Gilbert in the Puoblo folio refer the Millsap liiiiestone to the Carboniferous inerel}', 

 without being sur(> to what portion of the system it ))elonged. Lee, in discussing 

 th(^ Pikes Peak section, uses my determination of the age of the Millsap. Emmons 

 in tile 'riMunile folio. Eldridge in the Anthracite-Crested Butte folio, and appar(>ntly 

 Spurr in the Aspen monograph, refer the Leadvilie limestone to tlie Lower 

 Carboniferous in its true sense as a division of the geologic time scale. Peale in liis 

 Eagle River section seems to be uncertain whether the corresponding beds were 

 Carboniferous or Devonian. Cross and Spencer refer the Oura}^ limestone to the 

 Devonian, because a Devonian and no Carboniferous fauna was known from it at 

 the time of their writing. It is now fairly certain that the Carboniferous portion 

 of the Leadvilie, Ouray, and Millsap limestones 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 Glen wood 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 limestone 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 Mississippian time. Whatever may 

 have been the conditions prior to the formation of the Leadvilie limestone, I believe 

 that there were no land areas in Colorado during the Leadvilie epoch. Aside from 

 the persistency of the fauna and the lack of an3rthing like shore deposits at this 

 horizon, the fact that the Leadvilie nowhere, so far as I have I'ead, lies upon the 

 Archean is evidence in point. As the Leadvilie period was one of subsidence, had 

 there been permanent Archean land areas deposits could hardly have failed to be 

 laid down upon some of the submerged land. The possibility that the waters 

 advanced only so as to fall short of at all points, or not to transgress, some former 

 level does not need serious consideration. On the other hand, I do not see that the 

 sedimentation of Leadvilie beds upon the Archean, even if it does occur, would 

 necessarily be evidence in favor of permanent Archean land masses in Colorado. 



PENNSYLVANIAN. 



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

 erosion. At all events none but the early portion of Mississippian time is 



