592 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



formation, differing from the preceding case in that the age of the bed 

 varies from place to place according to the stage of the transgression. 

 The relation of the Tellico sandstone to the Ottosee shale is more like 

 that of the Kiefer to the McKenzie, only in this case the lower introduc- 

 tory member is of sufficient thickness and areal extent to rank as a sepa- 

 rate formation. 



In applying the principle great caution is always to be observed in 

 determining the presence or absence. of a stratigraphic break at the top 

 of the clastic bed. Thus the contact of the Saint Peter sandstone and the, 

 succeeding limestone in the upper Mississippi Valley often suggests gra- 

 dation from the sandstone through shaly sandstone and calcareous shale 

 to the limestone. In fact, however, there is an important hiatus at the 

 top of the Saint Peter, and the apparent transition suggested by the over- 

 lying mixed material is occasioned solely by the sandy character of the 

 redeposited regolith of preceding Saint Peter land areas. In this case, 

 then, only the variable and mixed intermediate bed, and not the Saint 

 Peter itself, is to be regarded as the introductory deposit of the overlap- 

 ping liihestone formation. The Oriskany-middle Devonian sequence in 

 the Appalachian Valley and in 'New York exhibits precisely analogous 

 conditions. 



The first appearance of fossil species and faunas. — As a general con- 

 ception the first introduction of organic types is regarded as marking the 

 beginning and not the closing term of geologic ages. It is deemed a 

 valid interpretation even when the general composition of a fauna which 

 contains a considerable number of such types is greatly like some older 

 facies. This opinion and consequent practice is based (1) on experience, 

 which teaches that reappearances (survivals) of faunal facies are more 

 common in stratigraphic history than are preexistences, and (2) on the 

 fact that the sedimentary record was frequently broken by inaccessibly or 

 poorly recorded intervals in which new things were evolved without 

 complete extinction of the old. The essential truth of the conception is 

 clearly proved by prevailing views respecting reappearances of faunas, 

 whose discrimination depends on a relatively small number of diagnostic 

 new forms by which each is distinguished from all the others and not on 

 comparisons involving their more numerous associates. 



Time was required to evolve these new forms, so that their first ap- 

 pearance in continental seas may as a rule be supposed to have followed 

 an inaccessibly recorded period during which they came into being. Or 

 it may be that they are migrants from some other province whose previ- 

 ous exclusion was due to some physical barrier ; or. if they happen to be 



