82 NEW YORK STATE MUSEtJM 



at Dalhousie is locally known as the "Bon Ami rocks." It is more- 

 over, a mass of eruptive, the extremity of one of the parallel apophyses 

 extending eastward from the great massive of Dalhousie mountain. The 

 fossiliferous rocks are exposed for the most part in Stewart's cove, some 

 distance south of this point. 



Chapter 4 



THE DEVONIC AGE OF THE HELDERBERGIAN FAUNA AND 

 THE BASE OF THE DEVONIC SYSTEM IN NEW YORK 



The fact of the presence of numerous Helderbergian species in the 

 fauna of the Oriskany of Becraft mountain, as an integral part of that 

 fauna, not a casual intermixture, is sufficient demonstration that the 

 fauna of the Helderbergian became modified in its continued existence 

 by the departure or extinction of certain of its species only. A fair 

 percentage kept the field up to the time of and pending the incursion 

 of species of the early Oriskany. In this way the former became a 

 true and proper part of this new fauna with whose indicial species it 

 coexisted throughout the remainder of its duration. A modification 

 so gradual as to permit such an uninterrupted existence can not sever 

 the close relation of the one fauna in its entirety to the other. It is 

 therefore a natural corollary from the account given of the Oriskany 

 fauna, to consider briefly the relation of the organic assemblage con- 

 stituting the typical and normal Helderbergian to the Devonic type of 

 organic life, and that formation in its relation to the Devonic system. 



We may postulate this proposition: the sections on which the term. 

 Lower Helderberg group, was based are in Albany and Schoharie coun- 

 ties N. Y. ; it is here that the formation attains its greatest and most 

 differentiated sedimentation as well as its highest profusion of organ- 

 isms ; it is therefore to these sections and their organic contents that 

 every argument in analysis and correlation of the fauna and its con- 

 taining sediments must be eventually referred. 



