Williams.] CLAYPOLE. 127 



The principle underlying the new interpretation of this problem was 

 suggested several years earlier in a paper read before the American 

 Association in 1881. x 



In this paper announcement was made of the discovery of a distinctly 

 Hamilton fauna, all the species of which had been heretofore consid- 

 ered as strictly Hamilton species, in arenaceous shales several hundred 

 feet above the Genesee shales, at Ithaca, New York. It is separated 

 from the typical Hamilton fauna by four distinct faunas ; those of the 

 Tully limestone, the Genesee shale, the Spirifera Icevis fauna, called 

 Portage in the State reports, and a fauna described in this paper from 

 shales overlying the last, called " Ithaca shales," resembliug the Gene- 

 see shale fauna, but evidently a later stage of it. This black shale 

 was regarded by the author as u a single continuous fauna.' 7 He says : 



Its appearance in the rocks of central New York in three separate zones, called the 

 Marcellus shales, the Genesee slate, and the Ithaca shales is regarded as evidence of 

 interrupted incursion, eastward of the conditions which were continuous over some 

 portions of the interior of the Devonian intercontinental sea, where the three New 

 York zones were represented by one continuous series of Black shales. 



The hypothesis is also advanced that (a) the Hamilton and Chemung faunas were 

 probably coexistent with this Black-shale fauna; and (6) were respectively the 

 northern and southern faunas of a western coast line of the open ocean on the east- 

 ward of this continent ; and (c) the appearauce of the Chemung fauna, displacing the 

 Hamilton faunas, in the latitude of New York and Pennsylvania, was the resultant 

 of some grand changes in the relations of the ocean and continental borders, by 

 which tropical conditions of the ocean were advanced northward, occasioning the 

 shifting of the Hamilton faunas toward the North pole ; so that (we may suppose) 

 at the time when the Chemung fauna was dominant over the northeastern United 

 States, rocks being deposited in the arctic latitudes received a Hamilton fauna ; and 

 (d) finally, these changes were'gradual, the shifting of the faunas northward beginning 

 as early as the beginning of the Portage epoch, and continuing far into, and perhaps 

 after the close of the Chemung epoch, with some oscillation of the couditious, causing 

 traces of the Hamilton to recur at the base, and possibly a second time higher up in 

 the midst of Chemung rocks and faunas. 



The fundamental idea inspiring the paper was an application of con- 

 ditions of modern biology to the interpretation of the fossil faunas. 

 As in the present seas many faunas are known to coexist in the same 

 ocean basin, their particular constitution and characteristics being de- 

 termined in great measure by differences of environment, bathymetri- 

 cal conditions, temperature, purity of water, etc., so in the past, it is 

 supposed, similar differences in the fauuas will be found to mark deposits 

 which were made at the same time, but under different conditions. 

 And in the second place since oscillations are known to have occurred 

 and currents are supposed to have existed in the ancient as in the 

 modern oceans, according to the theory it is reasonable to expect a 

 more or less constant chauge of the conditions of environment at any 

 particular geographical position, and consequently a shifting backward 

 and forward over it of the faunas during the accumulation of the sedi- 



1 The Recurrence of Faunas in the Devonian Rocks of New York, by H. S. Williams. Proc. Am. Ass. 

 Adv. Sci., vol. 30, pp. 186-131. 



