CAKBONIFEROUS UNDIVIDED. 



371 



that somewhere on the slopes of the San Juan Mountains notable remnants of these beds may 

 be found. 



From a comprehensive study by G. H. Girty of all Carboniferous invertebrate fossils thus 

 far collected in Colorado, and available to him, the following brief summary concerning the 

 Mississippian of the San Juan region may be made. Thirty-two determinable species have thus 

 far been obtained from the chert pebbles of the Molas formation, which were derived from 

 the eroded beds, and from two locahties in the upper zone of the Ouray limestone. This fauna 

 is closely similar to that of the Wasatch hmestone of Utah and the Madison limestone of the 

 Yellowstone National Park. It is also similar to the small faunas obtained in Colorado from 

 the Leadville limestone in the Mosquito Range, at Aspen, Crested Butte, and near Sahda, and 

 that from the MiUsap limestone of the Front Range near Canyon and in Garden and Perry 

 parks. Dr. Girty concludes that the Leadville limestone, which was originally assigned to 

 the Mississippian because of fossils found in its upper part, and the Ouray limestone, which 

 has been considered Devonian on account of the fauna in its lower and middle portions, are 

 equivalent. He thinks that careful search would in many cases show both faunas where only 

 one is now known. 



The Pennsylvanian of the San Juan district comprises, from the base up, the 

 Molas, Hermosa, and Rico formations. These are succeeded by the Cutler (Perm- 

 ian?) formation. The sequence is summarized as follows from the Needle Mountains 

 folio, ^^^* where full descriptions may be found : 



Generalized section for the Needle Mountains quadrangle. 



In southwestern Colorado and southeastern Utah the Carboniferous formations 

 distinguished in the San Juan district have recently been traced and correlated in 

 part with other occurrences in Utah, Arizona, and New Mexico. Fossils collected 

 by Newberry and Powell near the junction of Grand and Green rivers recently 

 compared by Girty with other collections made by Cross and Spencer at Moab, 



