Permo-Carboniferous Ammonoids of the Glass Mountains 23 



With these remarks of Dr. Beede should be compared his hst ot 

 fossils from the Gaptank formation published by Udden in his "Notes 

 on the Geology of the Glass Mountains," University of Texas Bulletin 

 No. 1753, 1917, p. 38 et. seq. 



Together with the ammonoids of our zone of Uddenites, we have 

 found some fossils of carboniferous aspect, but at the same time we 

 found intimately associated with the cephalopod-bearing beds several 

 Richthof enia and a little higher several Lyttonia and other brachiopods 

 of Permo-Carboniferous character, all far below the conglomerate of 

 the erosional unconformity at the beginning of the zone of Prothal- 

 assoceras. 



It seems therefore that we are justified in considering as the upper- 

 most Pennsylvanian the light gray limestones which follow the foot- 

 hills from Wolf Camp to Gap Tank, and to regard our cephalopod- 

 bearing dark shales and gray limestones as the lowest portion of the 

 Permo-Carboniferous. For the time being, this is certainly the best 

 solution of the problem, because it is easy to follow the upper limit of 

 the gray masses of limestone. If later detailed studies should demon- 

 strate that part of these limestones belongs to the Permo-Carboni- 

 ferous this would not have great influence on the main subdivision of 

 the beds. 



CORRELATION OF THE PERMO-CARBONIFEROUS OF THE GLASS MOUN- 

 TAINS WITH OTHER NORTH AMERICAN FORMATIONS 



The Permian of North America is still very little known. This 

 makes correlation difhcult, especially in those regions where no marine 

 fauna has been described. Most of the fossils cited are pelecypods, 

 which do not make it possible to distinguish well limited palaeontolog- 

 ical zones. Nevertheless, there are some regions which permit a more 

 exact correlation. 



The nearest place where a marine Permo-Carboniferous fauna has 

 been found is the Guadalupe Mountains in Culberson County, West 

 Texas. The detailed description of this fauna by G. H. Girty should 

 allow an exact correlation, but unfortunately the ammonoids de- 

 scribed are few and badly preserved. Nevertheless, the occurrence of 

 Waagenoceras (not "Waagenoceras" Cumminsi White) seems to 



