THE SILURIAN PERIOD. 371 



The principal subdivisions of this classification, modified as below, 

 will be followed: 



Silurian 



Cayugan 



(Neo- or Upper Silurian) 

 Niagaran 



(Meso- or Middle Silurian) 

 Oswegan 



(Paleo- or Lower Silurian) 



In this case there is some objection to the use of the terms Lower, 

 Middle, and Upper Silurian, since Lower Silurian has been used as a 

 synonym for Ordovician, and Upper Silurian for Silurian, as that term 

 is here employed. The subdivisions in the third column of the above 

 table (p. 370) are to be accepted for New York, but are not all 

 applicable to other regions. 



A brief sketch of the nature and distribution of these principal 

 subdivisions of the system will throw much light on the history of 

 the continent during the Silurian period. 



The Oswegan Series. 



The Oneida formation. — The lowest and therefore the oldest for- 

 mation of the Silurian system is the Oneida. It appears at the surface 

 at various points in the Appalachian belt south of the Shawangunk 

 (pronounced Shongum) mountains (N. Y.), though different names 

 are applied to it in different regions, and it is not everywhere separated 

 from the succeeding Medina. To the south, the formation seems 

 to be restricted to the western part of the Appalachian system. Since 

 the formation appears to rest conformably on the Ordovician, it is 

 inferred that sedimentation was not interrupted, in the region where 

 the formation occurs, during the transition from the Ordovician period 

 to the Silurian. 



The Oneida formation of the Appalachian belt is not known to 

 extend west of the mountains, and it is now believed to have been 

 accumulated in a narrow basin within the present Appalachian belt, 

 completely shut off from the great interior (see Fig. 182, p. 390). It 

 is not known whether the area where it occurs was submerged, or 

 whether it was the site of subaerial deposition. The fact that the 

 formation is without fossils lends color to the view that its sediments 



pp. 141-152), in 1846. It had been used earlier in a more restricted sense (Mather, Nat. 

 Hist, of N. Y., Pt. IV, pp. 353-4. 



