146 DAVID WHITE 



tion of additional as well as more complicated synclines. In these 

 at first greatly restricted basins the sea began a great readvance, 

 attended by the subsidence and deformation of the new basins under 

 loading. These changes were most active during the Pottsville, the 

 lowest of the American Pennsylvanian series. The new climatic and 

 terrestrial conditions were most favorable to the extraordinary growth 

 and differentiation of the plant life which spread across broad base- 

 level plains and over the marshes and lagoons that flanked its long 

 inland-reaching estuaries. 



Appalachian deposition. — In the Appalachian trough the basin in 

 earliest Pottsville time was confined to a very narrow estuary extending 

 from the anthracite region of northeast Pennsylvania southward to 

 Alabama where it became confluent with a small lobe of the sea 

 extending northward toward southern Illinois and Indiana. Sub- 

 sidence under loading was most rapid, with successive periods of sea 

 advance to the north and west during all Pottsville time, at the close 

 of which the water-level may have proximated its greatest Pennsyl- 

 vanian extent; at least it exceeded the present limits of the coalfields.' 

 The enormously expanded water-level of the Allegheny formation 

 attended a westward migration of the axis of deposition, probably 

 with the development of minor subordinate basins. Slight warping 

 occurred during the period, especially to the southward, so that the 

 connection with the Eastern Interior basin presumably migrated north- 

 ward into the Kentucky region, foreshadowing the probable exclusion 

 of the sea from the southern area of the present Appalachian coalfield 

 during later Conemaugh and Monongahela time. 



Rapid Evolution. Floral characters definite, with many short-lived 

 genera in early Pennsylvanian. — The most rapid evolution of the 

 Pennsylvanian flora occurred during Pottsville time, though the 

 generation of new forms continued at a slower rate into the Allegheny, 

 which, together with the Pottsville, approximately represents the 

 Westphalian or Muscovian in North America. 



The Westphalian is the period of Cheilanthites, Mariopteris, 

 Diplothmema, Crossotheca, Eremopteris, Palmatopteris, Lonchop- 

 teris, Megalopteris, Lesleya, Neriopteris, Bothrodendron, Ulodendron, 

 Lepidophloios, and Whittleseya, besides a number of genera repre- 



1 Bull. Geol. Soc. Amer., Vol. XV (1904), p. 267. 



