516 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



(11) Provincial differences in introductory faunal facies. — A given 

 time may begin in one area with a fauna differing greatly from that 

 which introduces the same period elsewhere. In one case the general 

 facies is obviously like that of faunas preceding it in the same area; in 

 the other it is more like the locally succeeding faunas. In the former it 

 is usually found that the same basin contained a large fauna during the 

 age immediately preceding and that this older fauna simply continued 

 into the next, or was replaced by a derivative of the preceding fauna 

 evolved in the same oceanic basin during an intervening stage of sea 

 withdrawal. In the latter case the new sea submerged, ur the new fauna 

 entered, areas that for a long time previously had been either land or 

 were merely inhospitable to types of marine life characterizing the in- 

 vading new fauna; or the new fauna invaded from a different oceanic 

 basin than the one which supplied the preceding local facies. Examples 

 of the former condition are the Ohio-Indiana Kichmond and the Brad- 

 fordian of Pennsylvania and New York, in both of which the new faunas 

 found the ground already occupied by large and as yet but little changed 

 descendants of preceding vigorous and in a way indigenous faunas. The 

 latter condition is exemplified by the western Eichmond and better by 

 the Medina Eichmond of Kew York and the Appalachian area generally ; 

 also usually by the middle western Kinderhook faunas. In each of these 

 instances the fauna is markedly different from the one next beneath it. 



The new facies in these cases invaded areas that for a long time pre- 

 viously had not been submerged. As they were not obliged to contest the 

 ground with surviving older faunas that elsewhere (as in the vicinity of 

 Cincinnati) still had access to certain continental basins, the local faunal 

 break under these circumstances may be very sharp. It is especially so 

 when, as in the case of the Eichmond fauna following the Trinucleus 

 fauna in the Viola limestone in central Oklahoma,^^ the locally superposed 

 faunas invaded from different oceanic provinces. If, however, they 

 invaded from the same oceanic basin, even if a long period intervened, the 

 value of the hiatus may be obscured by inherited similarities. This con- 

 dition is illustrated by the Black Eiver and Eichmond phases of the slowly 

 modifying Arctic fauna, which so often succeed each other without inter- 

 vention of Trenton and Cincinnati faunas. 



This principle may be expressed somewhat differently as follows: 

 Though the general facies of a fauna may be greatly like that of a certain 

 period, it may nevertheless belong to the next succeeding period. Thq 

 true age of such faunas is usually indicated by one or more types else- 



Tlshomlngo Folio, Geol. Atlas U. S., No. 98, 1903. 



