REVISION OF STRATIGRAPHIC CLASSIFICATION 297 



the Pulton, or true Utica. The limy shales of the Economy member of 

 the Eden between the two occurrences contain a prolific fauna differing 

 radically from the preceding Utica life. A few species of the earlier 

 fauna, notably two of its graptolites (CUmacograptus typicalis and 3Ias- 

 tigograptus tenuiramosus) continue into the succeeding Eden faunas; 

 but the general aspect of the latter, with their wealth of bryozoa, calca- 

 reous brachiopods, crinoidea, pelecypods, and gastropods, all indicative of 

 warm water, shows that very different marine conditions had abruptly set 

 in. The character of these changes might be discussed with advantage, 

 but as they are not immediately essential, we will pass to the point it is 

 wished to make, namely, had the true Utica failed to reach Cincinnati, 

 or if the bed did not come to the surface at that point, in which case 

 Triarthrus hecH would have been known here only from its later South- 

 gate Eden occurrence, either of two inferences might have been drawn. 

 According to the first, the Southgate Eden would be correlated with the 

 New York typical Utica and the marked differences in their faunas 

 ascribed to local variation. If, on the other hand, the newer age of the 

 Southgate had been recognized, then it might well be cited as an in- 

 stance of shifting faunas. That neither of these possible inferences is 

 correct is indicated by the earlier occurrence at Cincinnati, not only of 

 the trilobite, but also a number of its constant associates in the New 

 York Utica. This interpretation is finally established by a similar reap- 

 pearance of T. lecki in New York, where Walcott and others before me 

 found this species in the lower part of the Lorraine shales, in beds ap- 

 parently exactly correlatable with the Southgate shale at Cincinnati. 



As interpreted by me, the known facts concerning the distribution of 

 the T. beclci fauna in the Utica and Southgate members of the Eden 

 indicate, not a slow, tangential shifting of the fauna, but a brief revival 

 of conditions — presumably subsidence in the region of the Saint Law- 

 rence or of the Chesapeake Bay region — that permitted a second rapid 

 invasion of the interior basins as far west as Cincinnati. 



Rate of progress in migration of faunas. — In my opinion, most in- 

 stances of supposed "shifting^^ rest on incomplete observations of reap- 

 pearing faunal elements. These successive occurrences were not accom- 

 plished by slow geologically measurable stages, but they took place at 

 rates of progress so rapid that it is impractical, if not impossible, to ex- 

 press them by the smallest formational unit now recognized. 



That marine faunas usually migrated as rapidly as the invading waters 



themselves is shown also by the fact that where a widely transgressing 



sea is represented locally by perhaps only a few inches of deposits these 



may be, and usually are, filled with characteristic fossils. For instance, 



XXI— Bull. Geol. Soc. Am., Vol. 22, 1910 



