THE SILURIAN PERIOD 537 



Like the preceding system, the Silurian may be divided into 

 three principal series, as follows: 1 



Manlius limestone 



Cayugan 



(Upper Silurian) 



Silurian <( Niagaran 





(Middle Silurian 



Oswegan 



(Lower Silurian) 



Rondout waterlime 



Cobleskill limestone 



Salina beds 



Guelph dolomite 



Lockport limestone 



Rochester shale 



Clinton beds 



Medina sandstone 



Oneida conglomerate (and perhaps the 



Richmond beds) 



go 

 d b 



Each of these series is subdivided into several formations, but 

 the subdivisions of one place do not fit another. A brief sketch of 

 the nature and distribution of these principal subdivisions of the 

 system affords an outline of the history of the continent during the 

 period. 



Silurian of the East 



The Oswegan series. The principal subdivisions of this series in 

 the east are the Oneida and the Medina. Both appear at the sur- 

 face south of Lake Ontario, and perhaps in the western part of the 

 Appalachians farther south. 2 The Oneida consists of conglom- 

 erates and sandstone, and the Medina of sandstone and shale. 

 The sediments of these formations appear to have been deposited 

 in a shallow interior sea, as shown by their fossils, and by the cross- 

 bedding, the ripple-marks, etc., which affect its layers. The Medina 

 formation extends farther west than the Oneida, reaching eastern 

 Ohio and Ontario. Its distribution, as compared with that of the 

 Oneida, points to a subsidence of the eastern interior during the 

 early part of the Silurian period. Both formations are probably 

 continuous beneath younger strata over considerable areas south 

 of Lake Ontario and the Mohawk valley, and west of the Appala- 

 chians. 3 Nothing is known of the sediments which were accumu- 



1 In this case, there is some infelicity in the use of the terms Lower Silurian, 

 Middle Silurian, and Upper Silurian, for the subdivisions of the system, since 

 Lower Silurian was long used as a synonym for Ordovician, and Upper 

 Silurian for Silurian, as that term is here employed. 



2 Some of the formations formerly classed as Oneida in New York and New 

 Jersey are now regarded as of Salina age (see p. 543). 



3 Perhaps the Richmond beds, the Maquoketa shales, etc. See foot-notes, 

 pp. 509 and 510. 



