648 NEW YORK STATE MUSEUM 



Niagara, nor of any later Siluric age in east Tennessee, unless 

 the Niagara is represented in the extreme upper part of the 

 Rockwood formation, the shales and sandstones of this forma- 

 tion being as a rule succeeded in this area by the Devonic Chat- 

 tanooga shale. The existence of a land surface, extending west- 

 ward from the protaxis across east Tennessee to the western 

 slope of the middle Tennessee dome, therefore is assumed as 

 siiuric land filling the interval between the close of the Clinton or early 



in Tennessee 



and Kentucky Niagara to the middle Devonic. The Cincinnati dome also was 

 above sea level at the same time, and connected with the east 

 Tennessee land in such a manner that a broad bay was left 

 between the two domes. Neither of the latter was ever covered 

 entirely by Siluric strata, these being laid down only on their 

 gently sloping shores and in embayments produced by slight 

 warping of their surfaces. The succession of the deposits in 

 these embayments shows very clearly that the emergence at 

 the close of the Clinton was soon checked, and that gentle 

 subsidence prevailed in later Niagara time. 



Throughout Cayugan time, on the contrary, the Mississippian 

 sea was growing shallower, the floor of the sea having risen 

 almost gradually till, at the close of the Kondout, the whole 

 interior of the continent west of the Helderbergian barrier had 

 become land. This important emergence, for which we propose 



cayugan the name Cayugan. continued from Waterlime to Onondaga time. 



emergence ° a ' ° ' 



when the Mississippian sea again came in from the southwest, 

 spreading far and wide in the United States. In its eastward 

 progression this invasion (Onondaga) did not reach middle and ' 

 east Tennessee till near the close of the Black shale, which is 

 commonly correlated with the Genesee. The southern Black or 

 Chattanooga shale, however, may really represent late Devonic 

 time only, since in complete sections the shale in question seems 

 to pass very gradually into undoubted basal Mississippian (Car- 

 bonic) shales. 

 Helderbergian While both the Oswegan subsidence and the following Cayu- 



invasion 



gan emergence were affecting the area to the west of the Helder- 



