PALEOGEOGRAPHIC MAPS 

 DEVONIAN AND MISSISSIPPIAN 



BAILEY WILLIS 



U. S. Geological Survey 



5 AND 6. MIDDLE DEVONIAN AND MISSISSIPPIAN' 



The archipelagic condition of North America which began in 

 the Ordovician persisted through the two succeeding periods with 

 many changes of land and sea. Any refined study of these changes 

 involves somewhat precise correlations which have already been 

 carried far. The map here presented is of one passing phase only. 

 The time represented is that before and after the invasion of the 

 Hamilton fauna into the New York embayment, as is indicated by 

 the temporary land barrier shown in Illinois and Missouri. The great 

 thickness of sediments in the eastern Appalachian trough indicates 

 marked orogenic movement during the middle and upper Devonian 

 in the land lying toward the Atlantic. The southeastern expansion 

 of the sea over Appalachia began apparently in middle Devonian 

 and extended into upper Devonian time. 



The distribution and character of the Mississippian sediments 

 leads to the inference that the time was one of an extended epicon- 

 tinental sea with low and relatively limited lands. The archipelago 

 of the immediately preceding period gave way to a general sub- 

 mergence of all the southwestern portion of the continent. In the 

 far north conditions were favorable to the deposition of coal and other 

 continental deposits associated with marine beds. The Atlantic 

 and eastern portion of the interior sea and the wide sea covering all 

 the western states present differences of habitat which are empha- 

 sized by Dr. Weller in his discussion. Toward the close of the 

 Mississippian or early in Pennsylvanian time, an extensive land 

 area emerged in the Colorado-New Mexico region, as indicated by 

 erosion of the Mississippian sediments. 



I Prepared in collaboration with Dr. G. H. Girty. 



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