REVISED PRINCIPLES OP CORRELATION 485 



cently read by me, in which the writer endeavors to show that the Mc- 

 Kenzie, a post-Eochester Silurian formation that is widely distributed in 

 the middle third of the Appalachian Valley region, is of late Niagaran 

 and not of early Cayuga age, as some have claimed. 



He begins his argument with the statement that the fauna of the for- 

 mation, so far as known to him, comprises a total of 53 species. Of this 

 number 37 are said to be new and confined to the formation in question. 

 The remaining 16 species are referred more or less definitely to previ- 

 ously described species. The 37 new species, being unknown elsewhere, 

 he dismisses as having no bearing on the age of the formation. Pro- 

 ceeding, then, with an analysis of the identified old species, he finds that 

 9 have "distinctly Niagaran affinities," 3 have Cayugan and Helderber- 

 gian affinities, 3 are widely ranging Silurian species, and, finally, 1 that 

 has so wide a range that it may be set aside as "not significant." On 

 account of the preponderance of the Niagaran affinities thus indicated by 

 the previously described species, the formation is determined to be of 

 Niagaran age. 



Carried out in the usual manner, wherein the fossils are identified with 

 rather loosely conceived specific units, this method of correlation can not 

 be too strongly condemned. It would be admissible and defensible only 

 when the identifications are of minutely discriminated mutations. And 

 in that case the same bed would not contain both distinctly Helderber- 

 gian and unqualifiedly Niagaran fossils. In the instance referred to, I 

 know that the 9 fossils cited as indicating distinctly Niagaran affinities 

 were not identified by those biologically unimportant peculiarities of sur- 

 face marking which alone are reliably indicative of contemporaneity. In 

 fact, most of them can not be so accurately identified because their pres- 

 ervation is inadequate for the purpose. Moreover, even in the state of 

 preservation in which they are available, these so-called Niagaran fossils 

 suggest enough of difference from their Niagaran allies to cause other 

 students of the fauna not only to question the asserted identities but in 

 most instances to deny them. 



Further, it is important to note that all of these Niagaran species are 

 represented by unquestionable descendants in post-Niagaran — especially 

 Helderbergian and Oriskany — rocks. They must, therefore, have existed 

 somewhere during the intervening Cayugan epoch. In view of the fact 

 that their respective stocks continued to exist into the Devonian — modify- 

 ing, of course, slowly in the meantime — is there any warrantable objec- 

 tion to viewing these 9 "Niagaran" species of the McKenzie as represent- 

 ing Cayugan stages in their development? This suggestion is further 



