298 ANNALS NEW YORK ACADEMY OF SCIENCES 



Some diversity of opinion exists as to the principles which should 

 govern the interpretation of paleontological evidence in determining the 

 boundaries of geologic systems. I propose to consider these different 

 views and to decide which is to be employed in the present investigation. 

 Thus, to take a put case, it is sometimes said that the line between the 

 Devonian and the Carboniferous should be placed at the first introduc- 

 tion of Carboniferous species. This principle seems by implication or 

 otherwise to be adopted by Glenn and Butts and Clarke in their discus- 

 sion of the geology of the Olean quadrangle and other areas in western 

 ISTew York. 6 It is true that, in adopting this principle in the Olean rock 

 section, the authors were influenced by the belief that the interval below 

 the Olean conglomerate member of the Pottsville formation in the Olean 

 quadrangle corresponded to the interval similarly underlying the Olean 

 conglomerate in northwestern Pennsylvania and northeastern Ohio, so 

 that several hundred feet in the one section would have to correspond to 

 the authentic Waverly group in the other. 7 How that it is known, or at 

 least seems highly probable, that, owing to erosion which preceded the 

 Olean conglomerate, that member rests on lower and lower strata as it is 

 followed eastward, so that at Warren, Pa., most of the Waverly rocks 

 are missing and they are not known to appear -in the sections farther 

 east, — with this condition of affairs granted, it is possible that the au- 

 thors would not have adopted the principle of first appearance. It is 

 necessary, however, to examine this principle to see whether it is gener- 

 ally applicable or applicable to the present case. 



The meaning of the principle as stated above is clearer at first sight 

 than when it is examined more closely. The implication largely turns 

 on the meaning which is given to the terms "Devonian" and "Carbon- 

 iferous" species, and, at the risk of drifting into something like the 

 Greek dialectic, it will not be unprofitable to consider this question. By 

 Carboniferous species may be meant (a) any species which has been 

 found in rocks of Carboniferous age; (&) a species especially abundant 

 or widely spread and persistent in rocks of that age though occurring 

 elsewhere, or (c) a species known only from the Carboniferous, of at 

 least known there but not in the other geologic system with which com- 

 parison is made. 



It is evident that, in the present instance, the words "Carboniferous 



6 New York State Mus. Rept, vol. 56, pt. 2, 1903, pp. 985, 991 and 999. Clarke gave 

 a somewhat more extended discussion the year previous (idem, vol. 55, 1902, p. 524) in 

 which he showed that a marked faunal change took place at the hase of the "Brad- 

 fordian," but not that the "Bradfordian" fauna had a preponderating Carboniferous 

 aspect. 



' Loc. cit., p. 991. 



