148 DAVID WHITE 



genera, Lesleya, Megalopteris, and Whittleseya, apparently originated 

 in America whence they spread northeastward. The same is true of 

 the lower Pottsville Phyllotheca, which appeared a little later in 

 Heraclea, and finally, in a new group of forms, became somewhat 

 characteristic of the Gangamopteris flora of " Gondwana land." 

 On the whole, therefore, it is probable that the Westphalian flora 

 is the joint contribution of the lagoonal — i. e., coal-forming — re- 

 gions of western Europe and eastern America. Free inter-com- 

 munication was almost certainly by an Arctic land bridge, possibly 

 by way of a Greenland-Scandinavian shore connection. The general 

 regional distribution of the Pennsylvanian floras is shown in Fig. i. 



The Stephanian 



Conditions of deposition and probable equivalents. — The Stephanian 

 or Ouralian (including the Gschellian) of Europe dates from the 

 Hercynian uplift. Prior to this movement the sea had reached its 

 maximum extension in the coalfields of the northern hemisphere. 

 The Hercynian thrust caused its practical expulsion from the old 

 synclines of western Europe and the creation, especially to the south- 

 ward, of new basins, mostly of fresh or brackish water, to which were 

 transferred the scenes of coal-formation. In America the line between 

 the Westphalian and Stephanian is not yet accurately drawn, the 

 fossil floras being not studied in sufficient detail. In view, however, 

 of the paleobotanical evidence indicative of a point near the Allegheny- 

 Conemaugh boundary, I, personally, am inclined to regard the forma- 

 tion of the Mahoning sandstone (conglomeratic), the changed sedi- 

 mentation of the Conemaugh formation, the probable upwarp of the 

 southern Appalachian region which later resulted in the exclusion of 

 the sea from the northern area also, and the consequent climatic 

 changes, as due to the same great orogenic influence. Accordingly I 

 would provisionally place the greater part, if not all, of the Cone- 

 maugh together with the Monongahela in the Stephanian. 



The final exclusion of the sea from the Appalachian trough appears 

 to have occurred soon after the deposition of the Ames limestone, near 

 the middle of the Conemaugh, since, according to reports, only fresh, 

 or possibly brackish water mollusca occur in the higher terranes.' 



■ I. C. White, Report II A, Geol. Surv. W. Va., p. 622. 



