492 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



across northern Appalachia and brought with it the Schoharie fauna, rich 

 in cephalopods. The Saint Lawrence embayments had been continuous 

 from the earliest Devonic, and across the Taconic uplift, in free com- 

 munication with the Lower Devonic seas of the northern Appalachian 

 region. This intercommunication persisted into the Onondaga, when 

 decided secular changes took place in Acadia, which practically barred fur- 

 ther marine access to the Saint Lawrence waterways until Mississippic 

 time. This elevation is also recorded in Appalachia, but earlier, as the 

 siliceous materials of the Oriskany and Camden are clearly of this land. 

 The rejuvenation of Appalachia which began in the Oriskany dissipated 

 the southern portion of this sea, and the elevation became more decided in 

 Onondaga time, its deposits being absent throughout the Appalachian 

 area from central Pennsylvania southward. During the later Devonic 

 Appalachia continued to be elevated, more and more of the southern 

 Appalachian sea and the Mexico embayment being obliterated. 



The flood reached its maximum in the late Hamilton, and then another 

 great northern sea appeared — the Cordilleran sea, with its Euro- Asiatic 

 faunas, extending from the Arctic ocean to Iowa and from Montana to 

 northern Nevada. For a time the Kankakee axis separated this western 

 sea from the Mississippian sea, but at the close of the Hamilton the waters 

 spread across this narrow barrier, and to a limited extent the faunas of 

 the two areas mixed. This condition is best seen in the Devonic biotas 

 occurring about Milwaukee, Wisconsin. 



According to the maps, the transgression progressed as follows : 



North America. United States. 



New Scotland 6 per cent 13 per cent 



Becraft 6 " " 11 " " 



Decewville 8 " " 13 " " 



Middle Onondaga 16 " " 19 " " 



Late Hamilton 35 " " 32 " " 



This great transgression was first pointed out by Suess. He states : 



"A very considerable extension of the Devonian seas took place simulta- 

 neously from the Ural over the Russian plain toward the west and northwest, 

 and from the Rocky mountains across the valley of the Mackenzie to the east. 

 . . . The positive phase in the middle of the Devonian system thus mani- 

 fests itself on both sides of the Atlantic ocean at the same time" (II : 232, 233). 



Chemung emergence (see map, plate 77). — The Middle Devonic trans- 

 gression just described continued unbroken in the Cordilleran sea and in- 

 vaded further areas in the southwest; along the Atlantic, however, there 

 was marked unrest. As has been seen, Appalachia was in movement early 



