CHAPTER XXIII 



ORDOVICIAN (OR LOWER SILURIAN) PERIOD 



Murchison divided his great Silurian system primarily into 

 two parts, Upper and Lower. This method of classification is 

 generally followed even at the present day, although it is widely 

 recognized that the most decided break in the entire Palaeozoic 

 group is the one between these divisions. In 1879 Lapworth 

 proposed to give due emphasis to this distinction by erecting 

 the Lower Silurian into a separate system, the Ordovician. The 

 name is taken from the Ordovici, an ancient British tribe which 

 dwelt in Wales during Roman times. Lapworth's example is 

 now largely followed in England and the United States, but on 

 the continent of Europe the name Silurian is still retained for 

 both systems. 



The classification and subdivision of the American Ordovician 

 were first worked out in the state of New York, and consequently 

 the New York scale serves as a standard of reference for the rest 

 of the continent. It is given in the following table from Dana : — 







'3- 



Hudson or Cincinnati Stage 



( 2. 



Ordovician 

 System. ] 



I,. 



Trenton Series. < 

 Canadian. Series. 



2. 

 1. 



2. 



Utica Stage. 

 Trenton Stage. 

 Chazy Stage. 

 Calciferous Stage. 



The passage from the Cambrian to the Ordovician was a grad- 

 ual one, not marked either in North America or in Europe by any 

 decided physical break, but by a change in the character of the 

 fossils. By the end of the Cambrian period a vast interior sea 

 had been established over what is now the Mississippi valley. 

 This great sea was separated from the Atlantic by the Appalachian 



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