370 



GEOLOGY. 



period beds of clastic sediments were accumulating about the imme- 

 diate borders of the lands, and as far out as waves and currents were 

 able to transport abundant detritus from the land, and that elsewhere 

 sediments of organic origin were relatively more important. Though 

 sedimentation was interrupted in the regions which emerged from 

 the sea during the transition from the Ordovician period to the Silurian, 

 it must be assumed that such interruption was not universal, and that 

 where it did not take place, the Silurian strata are conformable on 

 the Ordovician. It may be assumed further that at each stage of 

 the Silurian period, the exposed portions of each older formation were 

 contributing to the system then in process of formation. The areas 

 of sedimentation at any particular stage of the Silurian cannot now 

 be determined by the geography of that stage; rather is the geography 

 of the successive stages of the period determinable in some measure 

 by the successive areas of sedimentation. 



Subdivisions of the Silurian System. 



The Silurian system of rocks is divided into a number of formations 

 or series of formations. The principal subdivisions which have been 

 commonly recognized in America are given below. 



The Helderberg series, though sometimes classed as Silurian, is 

 here classed with the next later (Devonian) system. 



[(Lower) Helderberg?] 

 5. Salina 

 4. Niagara 

 3. Clinton 

 2. Medina 

 1. Oneida 



In place of the above classification, the following has recently been 

 proposed for New York : l 



Ontaric 2 



or 

 Siluric 



Cayugan 

 Neontaric 



Niagaran 



(Mesontaric) 



Oswegan 



(Paleontaric 



Manlius limestone 

 Rondout waterlime 

 Cobleskill limestone 

 [ Salina beds 

 f Guelph dolomite 

 j Lockport limestone 

 J Rochester shale 

 [ Clinton beds 



(Medina sandstone 

 Oneida conglomerate 

 Shawangunk grit 



1 Clark and Schuchert, Science, Vol. X, 1898, p. 876, and Am. Geol., Vol. XXV, 

 1900. Slightly modified by Hartnagel, N. Y. State Mus. Bull. 69, 1903, p. 1160. 



2 The term Ontaric was first used in this sense by Emmons (Agr. of N. Y., Vol. I, 



