REVISED PRINCIPLES OF CORRELATION 483 



vestigations in east Tennessee showed that the contact between the Low- 

 ville and the Stones Eiver group, of which the Murfreesboro limestone 

 constitutes the lowest of four formations, represents a hiatus that opens 

 eastwardly to make room for another thick group of sediments. The 

 intercalated group — the Blount group of my classification — reaches an 

 aggregate thickness of more than 3,000 feet of limestone and shale, a 

 part of which corresponds in age to the Upper Chazy of New York. 



In this manner the range of Gonioceras, formerly believed to be repre- 

 sented by specifically indistinguishable occurrences in widely separated 

 places and to indicate a Black Eiver zone whose maximum thickness in 

 New York is less than 10 feet, has been expanded to cover a thickness of 

 chiefly limestone deposits aggregating approximately 4,700 feet. This 

 conclusion, I am glad to say, has since been largely verified by the dis- 

 covery in New York of both generic types in two formations — the Chazy 

 and Pamelia limestones — that are much older than the "7-foot tier" of 

 the Watertown limestone in which these cephalopods were originally 

 found. 



It should be noted that these extensions of vertical range are not based 

 so much on new discoveries in beds previously known to be younger or 

 older as on corrections of previous opinions regarding the age of some of 

 the deposits containing them. It is this fact that illustrates the folly 

 and danger to successful correlation of the common practice of assuming 

 knowledge respecting the range of fossil genera that actually we do not 

 possess. Experience clearly discourages the assumption that genera, or 

 even subgenera and broadly conceived species, will stay in the strati- 

 graphic limits to which they have been assigned. 



And yet, up to very recently and I fear even to the present time, pale- 

 ontologists have accorded a greater value in correlation to affinities of 

 generic rank than to those of a specific grade. The idea is used in a 

 recent paper by Dr. George H. Girty, who, in advocating the Devonian 

 age of the Bedford shale of Ohio, emphasizes the fact that the "Carbon- 

 iferous affinities of certain Bedford fossils are specific, while the De- 

 vonian ones are generic." 



If there is any logical reasoning back of this proposition, I fail to see 

 it. There might be some roundabout defense of the principle in the case 

 of first appearances, but certainly none where the further range and final 

 extinction of genera and species are concerned. Its most obvious fallacy 

 lies in disregarding the large element of chance and probability. Thus 

 we may readily conceive of the possibility of a variety or even of a species 

 becoming extinct at times of stress not only locally but generally. But a 



XXXV — Bull. GEOii. Soc. Am., Vol. 27, 1915 



