PALEOGEOGRAPHIC MAPS' 

 MIDDLE ORDOVICIAN AND SILURIAN 



BAILEY WILLIS 

 U. S. Geological Survey 



3 AND 4. MIDDLE ORDOVICI.AN' AND SILURIAN^ 



The passage from the upper Cambrian to the Ordovician appears 

 to be marked in many localities by inconspicuous but notable evi- 

 dences of non-deposition or erosion which may be attributed to 

 submarine scour or the actual subaerial denudation of low-lying 

 lands. The phenomena differ from those which commonly accom- 

 pany marked continental deformation. They are believed to have 

 resulted from the deepening of ocean basins which gave rise to a 

 widespread ebb of the epicontinental seas. Effects of continental 

 warping of a subordinate character may naturally have accompanied 

 the sub-oceanic movements. The conditions of oceanic circulation 

 which result from a consideration of the probable distribution of 

 seas and lands are those of general northward currents flowing from 

 the Gulf of Mexico through to the Arctic. They carried with them 

 the characteristic middle Ordovician fauna, which, however, developed 

 local diversities in the eddies of the North American archipelago. 

 In contrast to the central marine currents and their fauna we have 

 the polar southward-trending return currents which may have been 

 congenial to the graptolites. Their distribution would seem to ex- 

 plain the similarity of graptolite faunas in the eastern and western 

 troughs. A peculiar circumstance is suggested in the occurrence 

 of the graptolites in Arkansas. This is recognized on the map by 

 the crossing of the arrows indicating marine currents. It is a well- 

 established fact of oceanography that marine currents pass over or 

 under one another, and this fact affords a possible explanation of 

 the relations which appear to have existed in Tennessee and Arkansas. 

 As a supplemental hypothesis the student should consider Professor 



1 Based largely on data furnished by C. D. Walcott. 



2 Based on data furnished by Dr. E. M. Kindle. 



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