CHAPTER XVIII 



THE DEVONIAN PERIOD 



FORMATIONS AND PHYSICAL HISTORY 



The most important physical events of the Devonian period in 

 North America were the invasions of large areas of the continent by 

 the sea, which came in from different directions in successive epochs. 

 The invasions appear to have been from the east, perhaps the north, 

 the south, and the northwest, in turn. 



Early in the Devonian period, the sea covered the present area 

 of land to. some such extent as shown in Fig. 356. During the 

 period, there were notable changes in the relations of land and 

 water, with corresponding changes in the life of the period. 



The subdivisions of the Devonian system now recognized in 

 New York (where the Devonian is best known) are as follows: 



{Chautauquan Chemung and -Catskill 



f Portage beds 

 Senecan 



Devonian 



Middle Devonian 



Erian 



Ulsterian 



-? Genesee shale 

 I Tully limestone 

 J Hamilton shale 

 1 Marcellus shale 



C Onondaga limestone 

 -I Schoharie grit 

 I Esopus grit 



f Oriskanian Oriskany beds 



f Kingston beds 



Lower Devonian \ , , J Becraft limestone 



I Helderbergian -j New ^^ beds 



[ Coeymans limestone 

 The subdivisions of the last two columns do not fit regions 

 remote from New York, though the names, Helderberg (or Helder- 

 bergian), Oriskany, Onondaga (Corniferous), Hamilton, Portage, 

 and Chemung, have rather wide application. 



Devonian of the East 



The Lower Devonian. The known Helderbergian series, largely 

 limestone, is known in (i) the northeast, (2) the Appalachian belt, 



402 



