CHAPTER XVII 



THE SILURIAN (UPPER SILURIAN) PERIOD 

 FORMATIONS AND PHYSICAL HISTORY 



The changes which brought the Ordovician period to a close 

 marked also the inauguration of the Silurian. These changes 

 included movements which affected (i) small areas intensely, and 

 (2) broad areas slightly. From the standpoint of continental his- 

 tory, the latter were the more important. These changes were 

 accomplished slowly, and after they had taken place, the area of 

 land in North America was greater than at any time since the early 

 Cambrian. The increase in land meant lengthened streams, and 

 presumably increased erosion. 



It is safe to assume that at the opening of the period clastic 

 sediments were accumulating about the immediate borders of the 

 lands, and as far out as waves and currents were able to transport 

 detritus, and that elsewhere sediments of organic origin were rela- 

 tively more important. Though sedimentation was interrupted in 

 regions which emerged from the sea at the close of the Ordovician 

 period, the interruption was not universal, and Silurian strata are 

 locally conformable on the Ordovician in the continents, and gen- 

 erally, it is presumed, in the ocean basins. 



In New York, the Silurian is subdivided as follows 1 : 



Manlius limestone 



Cayugan J Rondout waterlime 



(Upper Silurian) Cobleskill limestone 

 I Salina beds 

 \ Guelph dolomite 



Silurian ~\ Niagaran j Lockport limestone 



(Middle Silurian) 1 Rochester shale 



(_ Clinton beds 

 Oswegan ( Medina sandstone 



(Lower Silurian) -s Oneida conglomerate (and perhaps the 

 I Richmond beds) 



1 There is infelicity in the use of the terms Lower, Middle, 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. 



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