624 NORTH AMERICAN INDEX FOSSILS. 



Bossardvllle limestone. 

 Poxino Island shale. 



{Probably a hiatus and dis conformity.) 



Middle Siluric. 



Longwood shales and sandstones. 

 Shawangunk conglomerate. 



{Great hiatus and generally an unconformity.) 



SUBFORMATION. 



Upper Ordovicic — Hudson shales. 



Southwestward, in the Appalachians of south central Pennsyl- 

 vania, and Maryland, the base of the Siluric is formed by the 

 Tuscarora sandstone and conglomerate followed by the "" Clinton " 

 {Niagaran) shales and sands, and by red shales of Salina age 

 {Longwood, Bloomsburg, etc.). This is succeeded by the Lewis- 

 town limestone series. This last division varies somewhat in value 

 — but is of Monroan age, representing in most regions, the upper 

 Monroe. In Maryland, the lower Monroe is represented by shales 

 and limestones commonly called '' Salina " ; the true SaHna being 

 absent. A pronounced disconformity separates the lower Monroe 

 from the Middle Niagaran (Rochester) which in turn is underlain 

 by Clinton and Tuscarora. The Upper Monroe beds have recently 

 been named the Corrigan limestone.^^ 



In Virginia and Tennessee, the Tuscarora is represented by the 

 Clinch sandstone, and this is followed by the Rockwood formation 

 with its iron ores, the age of which is Niagaran. Higher Siluric 

 beds are generally absent, the succeeding beds being either Lower 

 Devonic (Helderbergian) or the Black shale of later age. 



West of New York, the Medina gradually dies out, the Clinton 

 being mostly calcareous and resting often with a hiatus of greater 

 or less extent, upon the Ordovicic. The section is most complete 

 in Canada, Michigan and northern Ohio, where the following suc- 

 cession occurs :^^ 



SUPERFORMATION. 



Middle Devonic — Onondagan. 



{Great hiatus and disconformity.) 



" T. Poole Maynard, Geol. Soc. Am. Bull. 



^* Lane, Prosser, Sherzer and Grabau, Bull. Geological Soc. Am., XIX., 553-556. 



