528 A. W. GRABAU PALEOZOIC DELTA DEPOSITS OF NORTH AMERICA 



northern Pennsylvania in Upper Silnric time. In Upper Devonic time 

 one or more of the centers became shifted still farther to the north, 

 though an additional center seems to have persisted in . Pennsylvania. 

 Finally, in Mississippic and Carbonic time, a southward migration of 

 the centers of deposition set in again. The discussion of these facts, 

 together with the details of the deposits involved, are reserved for a 

 subsequent paper. 



