PALEOGEOGRAPHIC MAPS OF NORTH AMERICA! 
RAILEY WILLIS 
U. S. Geological Survey 
3. MIDDLE ORDOVICIAN NORTH AMERICA 
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 subaérial 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 
Chamberlin’s suggestion of an inversion of oceanic circulation which 
is based upon the possibility that saline equatorial waters may have 
been denser than, and have sunk beneath, relatively fresh and lighter 
« Published by permission of the Director of the U. S. Geological Survey. 
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