670 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



With the close of the Utica age of the Eden epoch the Atlantic connec- 

 tions were either partly or wholly closed or the marine currents which 

 had previously swept through them were diverted. At any rate, the suc- 

 ceeding middle Eden fauna which lived in the now greatly expanded sea 

 is almost entirely southern in origin. That limited communication with 

 the Atlantic was maintained for some time after the Utica proper, 

 or was reestablished in a brief subsequent time, is suggested by the mid- 

 dle Eden invasion of Triarthrus, mentioned on page 296. Before the 

 close of the Eden, however, the fauna became so purely southern in type 

 that direct connection between the Appalachian troughs and the Atlantic 

 seems out of the question. The tilt of southeastern North America 

 toward the northern Atlantic which prevailed during the Utica was thus 

 reversed in direction, the resulting late Eden attitude of the affected 

 areas with respect to sealevel being, furthermore, in essential respects 

 much like that which prevailed in the late Trenton ages. 



The differential oscillations which occurred during the course of the 

 Clinton epoch make a case having much in common with the preceding 

 Eden example. The essential features of the Clinton movements are 

 suggested in the discussion of the Silurian sequence on page 558. Briefly 

 restated in a manner suited to the present purpose, they are as follows : 

 At the beginning of Clinton time the Brassfield sea invaded the Ohioan 

 province by way of the Mississippi embayment to the vicinity of Hamil- 

 ton, Ontario. After an interval, however, the succeeding pre-Rochester 

 Clinton faunas invaded the Appalachian and Allegheny basins from the 

 east. Reversal of tilt again took place during the Rochester, but on this 

 occasion it seems to have been accomplished without complete interrup- 

 tion of sedimentation in the Appalachian Valley. In fact, there is an in- 

 termingling of Atlantic and Gulf migrants in the Rochester part of the 

 Clinton in Pennsylvania and Maryland that is quite absent in the fauna 

 of the Osgood limestone, the southern representative of the Rochester. 

 Whether the indicated Atlantic connection was maintained to the close 

 of the Rochester age has not been determined. However, it is not to be 

 doubted that complete emergence of the Atlantic border set in directly 

 after. 



Similarly alternating Gulf of Mexico and Atlantic invasions are sug- 

 gested by the distribution of the Onondaga, Marcellus, and Hamilton 

 faunas and deposits in southeastern North America. Perhaps these are 

 no less good illustrations of this principle, but in the absence of definite 

 knowledge respecting the structural relations of the beds I hesitate to 

 say that the movements in these eases were consummated without inter- 

 vening emergence. 



