CHAPTER VIII. 



THE DEVONIAN PERIOD. 



Formations and Physical History. 



The division plane between the Silurian and Devonian systems is 

 less clearly marked than that between the Silurian and the Ordovician. 

 As already indicated, there is difference of opinion as to whether the 

 Helderberg 1 formation belongs to the one system or to the other. 



Whatever the final reference of the Helderberg formation, its rela- 

 tions to the immediately preceding and succeeding formations serve 

 to show that the separation of the Devonian from the Silurian is less 

 distinct than that of the Silurian from the Ordovician. Under either 

 of these points of view, the sea again invaded the land early in the 

 Devonian period, perhaps establishing some such relations as shown 

 in Fig. 191. During the period, there were changes in the relations 

 of land and water, some of which were of great importance in their 

 effects on the life of the period. These will be noted later. 



The subdivisions of the Devonian which have been commonly 

 recognized in America (the Helderberg being included) are as follows, 

 the several formations being numbered in chronological (and strati- 

 graphical) order: 



5. Chemung-Catskill: 



Chemung. 



Portage. 

 4. Hamilton: 



Hamilton. 



Marcellus. 

 3. Corniferous: 



Corniferous. 



Schoharie and Cauda Galli. 

 2. Oriskany. 

 1. Helderberg. 



1 As originally classified, there was a Loioer and an Upper Helderberg formation. 

 The latter is now more commonly known as the Onondagan (Corniferous). In this 

 volume the term Helderberg is applied to the former only. The adjective "lower" 

 therefore becomes superfluous. 



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