480 E. O. ULRICH CORRELATION OF THE STRAND-LINE 



vaded southwestern Virginia already in late Helderbergian — evidently 

 Becraft — time. 



FAUNAS OF DISTINCT MARINE PROVINCES DIFFER IN COMPOSITION AND 



SEQUENCE 



Experience has definitely shown that the foregoing generalizations are 

 well founded. Incontestable evidence has forced the admission that in 

 both the composition and the sequence of fossil faunas, and to a consid- 

 erable extent also in their vertical or time ranges, the records in the dif- 

 ferent provinces vary more or less decidedly. To meet this condition it 

 has become necessary to compile faunal standards for each province. 

 Within the borders of each province the percentage method of estimating 

 faunal, consequently time relations, is still useful ; but even in these cases 

 the result of such direct matching of faunas should not be accepted with- 

 out verifications by other criteria. That would be allowable only wben 

 the comparison shows detailed specific agreement. 



To illustrate : We know of contiguous formations containing closely 

 simulating faunas that on investigation proved to be separated by an 

 unconformity. This structural relation at once suggested a much greater 

 difference in age than their contiguity and faunal similarity indicated. 

 Subsequently it became evident that the likeness in fossil contents meant 

 only that the two formations derived their faunas from the same oceanic 

 realm. 



Fortunately, in one of the cases referred to, the true relations of the 

 formations are shown in a neighboring area, where a wedge, reaching 

 thousands of feet in thickness, is introduced between them. Moreover, 

 in the latter area the intercalated formation contains a large fauna so 

 radically different as to immediately suggest a widely distinct age. How- 

 ever, this is not the case, the striking difference having no further sig- 

 nificance than that the simulating lower and upper faunas invaded the 

 area from the Gulf of Mexico, whereas the altogether different interven- 

 ing fauna was developed in the middle and north Atlantic. 



When it comes to correlating formations in different provinces, all 

 strictly paleontological criteria depending on comparisons of general 

 faunal aspects fail as a rule to give lasting results. The best exceptions 

 to the rule are those furnished by formations and faunas which transgress 

 from one province to another. The exceptions include the widely rang- 

 ing, current-borne, pelagic faunas which, however, because of the usual 

 absence of transcontinental currents, seldom invaded the interior conti- 

 nental basins. But they do not include the so-called "cosmopolitan" 

 bottom faunas, for none really deserving this designation ever existed. 



