PRINCIPLES OF STRATIGRAPHIC CORRELATIONS 517 



where characteristic of the later period. Examples : The Eichmond and 

 the Helderbergian faunas, the Glen Park Kinderhook fauna, and the 

 Bradfordian Kinderhook fauna. The Eichmondian faunas are decidedly 

 Ordovician in general aspect, and the Helderbergian series contains a 

 large percentage of species reminding more of middle and late Silurian 

 than of Devonian forms. Yet, relying on diastrophic criteria and the 

 introduction of new organic types, the former are shown to be early 

 Silurian in age and the latter early Devonian. Similarly the Glen Park 

 fauna recently described by Weller, though reminding strongly of the 

 Hamilton, is of Kinderhook age. The Bradfordian also is of Kinderhook 

 age despite the greatly predominating Chemung element in its fauna. On 

 precisely the same grounds, namely, diastrophism and introduction of 

 new types, I refer the Sainte Genevieve to the base of the Chesterian and 

 the Warsaw to the base of the Meramecian. 



(12) Value of pelagic species in correlation. — Pelagic species and 

 faunas, notably the graptolites and the later thin-shelled coiled cephalo- 

 pods, are of great value in intercontinental correlation. Their use in cor- 

 relating the deposits of the more shallow and limited Paleozoic continental 

 seas, however, is reduced to the infrequent occasions when these seas and 

 troughs became thoroughfares for oceanic currents. As a rule such oc- 

 currences are marked by black shale deposits. The Levis and Normanskill 

 graptolites, found in Sweden, Great Britain, Newfoundland, Quebec, New 

 York, Arkansas, and Australia, are excellent examples. 



(13) Eelation of marine currents to faunal distribution. — Absence of 

 certain classes of invertebrates in the whole or parts of continental seas is 

 more often due to discontinuance of currents than to increasingly unfa- 

 vorable environment. Such progressive changes in the composition of 

 fossil faunas, therefore, are not indicative of corresponding differences in 

 age ; neither do they indicate provincial differences. They mean only that 

 the marine currents to which the organisms in question owe their presence 

 in the basin either lost their efficiency or that they were diverted to off- 

 shore areas that were more or less unfavorable to the development of 

 shallow-water life. 



Striking differences are often found in comparing lists of fossils gath- 

 ered at distant outcrops that are referred on perfectly competent evidence 

 to the same age and same continental sea. These differences are espe- 

 cially noteworthy in the case of widely recognized formations, which 

 maintain essentially similar lithologic characters and in which, therefore, 

 the observed changes are not readily explained on the ground of unfavor- 

 able environment. On close comparison of such faunal lists, it is usually 



