370 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



fauna. Further, with the exception of a small percentage of species, 

 most of them of wide, possibly cosmopolitan range, the first three faunas, 

 which invaded the continent from the north and west, are entirely dif- 

 ferent from those of the fourth fauna, which invaded from either the 

 Atlantic or the Gulf of Mexico. 



In view of the fact that the first or Decorah shale fauna maintains its 

 essential characteristics from Missouri to Baffinland it does not seem 

 reasonable to suppose that it could have been so changed as to be unrec- 

 ognizable in the Appalachian Valley, in Colorado, and generally in the 

 western part of the continent where Black Eiver-Trenton faunas are 

 known. Nor can we believe this of the third or Prosser limestone fauna, 

 which likewise is recognized in Missouri and Baffinland, and is typically 

 developed in New York, New Jersey and Canada, but which has not 

 been observed west of the Mississippi Valley nor south of the Ohio. 

 Neither has it been recognized in the Appalachian Valley south of New 

 Jersey. It would be even less reasonable to assume that the two southern 

 faunas, that is, the second and fourth of these really very different 

 faunas on which the oft-asserted great early Trenton submergence is 

 so insecurely based, suddenly lost their identities beyond the areas in 

 which they are easily recognized. On the other hand, if we admit for 

 a moment the possibility of their contemporaneity and the continuity 

 of the waters in which they lived what possible reason might be advanced 

 to explain why these northern and southern faunas failed to mix freely 

 in the middle ground ? 



This being a time of great sea transgression when, according to 

 Willis's theory, the continental seas were swept by strong currents, we 

 ask ourselves, is it possible that these cuiTents were so capricious that 

 when the first (Decorah) and third (Prosser) faunas prevailed in the 

 middle and northern areas where we now find them the currents pre- 

 vented deposition in the south? Or that when the presence of the 

 second (Kimmswick) and fourth (Praspora simulatrix) faunas were 

 being recorded in the south the currents prohibited sedimentary records 

 in the north? What possible reason could have occasioned such extra- 

 ordinary vacillation of current efficiency? Or, to go further, on what 

 grounds might we conceive of marine currents that could sweep cleanly 

 the basins of half a continent and permit unimpeded deposition in the 

 altogether similar shallow seas on the other half? 



If we did not know from actual superposition of the beds in eastern 

 Missouri that the Kimmswick fauna intervenes between the first and 

 third and that in New York the fourth succeeds the third, the anomalies 



