PROPER USE OF FOSSILS IN CORRELATION 487 



many species of Atlantic origin that are absent in western New York 

 and lacks even a greater number of the species of the southern region 

 that are abundant at the type locality of the Bochester and in correspond- 

 ing deposits in the Ohio Valley. 



Because of its bearing on correlation, it is highly important to note 

 that practically all of the fossils of the McKenzie that have "Niagaran 

 affinities" are allied to members of the Atlantic fauna that invaded the 

 north Middle Appalachian region during the Eochester age. Those that 

 have relatives also in the more western Niagaran faunas are types that 

 were then common to both the southern and the northern Atlantic realms. 

 The presence of these Atlantic-Niagaran types in the McKenzie, there- 

 fore, is not particularly diagnostic. Moreover, as these types are known 

 to have maintained their existence in the north Atlantic through the 

 Cayugan into the Devonian, invading the Appalachian troughs when 

 opportunity offered, and as their McKenzie representatives are at least as 

 different from their positively determined Niagaran facies as from their 

 late Cayugan and Helderbergian descendants, it seems reasonable to con- 

 clude that they are post-Niagaran in age. 



But there are two other facts that must be taken into account in 

 deciding the age of the McKenzie formation, and both are in harmony 

 with the trend of the evidence already presented. First, the Lower 

 Cayugan formations in New York are almost barren of normal marine 

 faunas ; second, the Upper Niagaran deposits which are well developed in 

 western New York pinch out rapidly in going eastward, being entirely 

 absent in the eastern third of the State. The Silurian sequence in cen- 

 tral to eastern New York, therefore, is (1) Medinan, (2) Clinton group 

 of the Niagaran, (3) hiatus corresponding to the Upper Niagaran or 

 Lockport group, (4) Cayugan deposits resting on the Clinton, and (5) 

 one or another of the Helderbergian formations following the "Tentacu- 

 lite" or typical Manlius limestone. The oscillating movements which 

 caused the differences observed in comparing the stratigraphic sequence 

 in western and eastern New Y r ork doubtless similarly affected the areas 

 to the south in Pennsylvania and Maryland. 



The diastrophic evidence thus appears to support the testimony of the 

 fossils as indicated by minute comparisons involving the whole fauna 

 and not merely the small part that is loosely identifiable with described 

 species. Taking into consideration other factors that limit the possi- 

 bilities upward, but which it is unnecessary to discuss here, there is but 

 one possible conclusion, namely, that the McKenzie formation of Mary- 

 land and southern Pennsylvania is early Cayugan and not late Niagaran 

 in age. 



