286 H. S. WILLIAMS SHIFTING OF DEVONIAN FAUNAS 



by the continuous fauna of the black shales of Ohio, Indiana, Kentucky, 

 and Tennessee ; and also, in the same paper, the hypothesis of shifting of 

 faunas was applied to the Hamilton and Chemung faunas of central New 

 York.2 Since that time a large amount of evidence has been accumulated 

 confirming these hypotheses. These hypotheses are intimately correlated. 

 Eecurrence or the departure of a fauna, its replacement by another, and 

 its final reappearance in the same section at a higher level become the 

 facts on which the hypothesis of shifting of the faunas is based; and the 

 assumption of continuance and shifting of a fauna without losing its 

 characteristics appears to be the only satisfactor}^ explanation of its 

 recurrence. 



Facts on v^hich the Hypotheses rest 



The following facts explained by these hypotheses are among the more 

 important which have come to light in the course of my studies. 



0AT8KILL SEDIMENTATION 



This was shown to be thicker and to start lower down in the geological 

 column in eastern 'New York than in middle and western New York. In 

 eastern New York it began while the Hamilton marine fauna was still 

 present and cut it off, bringing in estuarian conditions with a brackish 

 water and land fauna and flora. In central New York no Catskill sedi- 

 mentation is present until after the arrival of the Chemung fauna, and in 

 western New York no trace of the Catskill type of sediments appears till 

 after the close of the Devonian. These facts are direct evidence of shift- 

 ing of the environmental conditions of the edge of the continent west- 

 ward as the deposits of the middle and upper Devonian vfere being laid 

 down. With this shifting westward of the off-shore conditions of the sea 

 there went on a corresponding shifting of several faunas that were ad- 

 justed to each phase of those conditions.^ 



REVERSAL OF ORDER IN SUCCESSION OF FAUNAS 



The appearance of the dominant species of the general fauna in re- 

 versed order of succession at the close of a fossiliferous zone. 



The cases of Spirifer IcFvis in the Ithaca zone and of the frequent 

 appearance of Leiorhynchus at the opening and close of a fossiliferous 

 zone were among the earliest observed facts suggesting an actual shifting 



2 Proceedings of the American Association for the Advancement of Science, voL xxx, 

 p.186, etc. 



' "On the classification of the upper Devonian." Proceedings of the American Asso- 

 ciation for the Advancement of Science, vol. xxxiv, 1885, p. 222. 



