424 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



Maqnoketa are not easily determined. I can not take the space to argue 

 the question here but must content myself with the statement that after 

 careful study it has been decided to place them above the Maquoketa. If 

 this assignment is correct, then continental tilting had again intervened 

 and reversed the direction of marine invasion, for the greater part of the 

 Whitewater and Saluda faunas are undoubtedly sou*thern in origin. It 

 is these faunas, too, that I correlate with the typical Medina fauna of 

 New York, the latter comprising such of the elements of the former as 

 were suited to existence on a sandy beach. The geographic arrangement 

 of this time then would be to the east of the Mississippi much like that 

 marking the succeeding early Clinton or Brassfield stage. 



At Cape Girardeau, Missouri, the Thebes facies of the Maquoketa 

 shale is followed first by the Girardeau limestone, next by a limestone 

 correlated with the Noix oolite of Lincoln County, Missouri. This is 

 succeeded by another thin limestone representing the earliest Clinton or 

 Brassfield of Kentucky. Of these three zones, each of which is uncon- 

 formable to the next, the first or Girardeau is a southern invasion that 

 did not extend far north in the valley, but the second evidently came in 

 from the north, being entirely unknown in any more southern locality, 

 while its distinctive coral fauna is very widely recognized in northern 

 areas. The third, with its distinctive early Clinton fauna, is known 

 from Oklahoma on the southwest to Hamilton, Ontario, on the north- 

 east. It undoubtedly invaded the continent from the south. Consider- 

 ing the wide differences in distribution of the faunas and deposits of 

 these three zones it is evident that each was introduced and terminated 

 by a diastrophic movement. 



Magaran oscillations. — Concerning the succeeding Silurian ages, the 

 evidence afforded by the fossils establishes beyond reasonable doubt 

 that the dolomitic rocks of these ages in the north were deposited in 

 waters connecting with the Arctic sea, by way of Hudson Bay, while the 

 purer Silurian limestones of the south were in direct communication 

 with the Gulf and middle Atlantic. To what extent the respective parts 

 of these two series of deposits are contemporaneous has not been finally 

 determined. The problem is exceedingly difficult, but far from hopeless. 

 In this connection, the point of interest is to determine whether the 

 lithologically and faunally distinct deposits were laid down contempor- 

 aneously in somewhat precariously separated seas or whether alternate 

 tilting of the continent to the south and north, as in middle Ordovician 

 ages (see pages 367 to 371), is a more important cause of the observed 

 differences. That the latter condition did occur at times in the Silurian 



