628 NORTH AMERICAN INDEX FOSSILS. 



IV. The Devonic System. 



General Subdivision. — In eastern North America, especially New 

 York, the Devonic admits of the following subdivisions :-^ 



Upper Devonic. 



Chautauquan — Chemung and Catskill. 

 Senecan. 



Portage beds. 



Genesee shale. 



TuUy limestone. 



Middle Devonic. 

 Erian. 



Hamilton beds. 



Marcellus shales. 

 Ulsterian. 



Onondaga (Corniferous) Hmestone. 



Schoharie beds. 



Lower Devonic. 

 Oriskanian. 



Esopus (Caudagalli) beds. 



Oriskany beds. 

 Helderbergian. 



Port Ewen beds.-^ 



Becraft limestone. 



New Scotland beds. 



Coeymans limestone. 



The typical Helderbergian^^ is restricted to the Appalachian area 

 from New York to Maryland. Southward, it is represented by 

 some of the beds referred in Virginia to the Hancock limestone; in 

 western Tennessee, by the Linden beds, in Illinois by the Clear 

 Creek limestones (including the Oriskany) and by " Lower Helder- 

 berg" limestones (Hunton limestone in part) in Oklahoma. In 

 Maryland, the Oriskany is known as the Monterey formation, and 

 in western Tennessee it is represented by the Camden chert. In 



^^ Clarke and Schuchert, Science, Vol. X., 1899. 



^Included with the Oriskany by Chadwick (loc. cit.). 



^ Schuchert, Charles, Btill. Geol. Soc. Am., XI., pp. 24-332, 1900. 



