THE DEVONIAN PERIOD 561 



outside the state, but other local names are used instead of these 



in many places. 



Devonian of the East 



The Lower Devonian. The known Helderbergian series is con- 

 fined largely to the eastern part of the continent. It is known (1) 

 in Maine and farther northeast, (2) in the Appalachian belt, and 

 (3) in the lower Mississippi basin (Fig. 407) . It is largely limestone, 

 300 to 600 feet thick in eastern New York and Pennsylvania, but 

 thinner to the west. 



The Oriskany formation is best known in the northern Appala- 

 chian region, but beds of the same age have a wide though not well 

 determined distribution farther west. They are also present in the 

 northeast (Gaspe, New Brunswick, and Nova Scotia). Where best 

 known, the Oriskany is made up chiefly of coarse sandstone. From 

 the vicinity of Cumberland, Md., where the formation has a thickness 

 of a few hundred feet, it thins to the northeast and the southwest, 

 and loses its most distinctive faunal characteristics. 



The Middle Devonian. The most important formations of the 

 Middle Devonian are the Onondaga and Hamilton, which are more 

 wide-spread than the Lower Devonian formations. In the east, the 

 Onondaga formation is underlain by clastic beds (Esopus and Scho- 

 harie) the equivalents of which* have not been differentiated with 

 certainty west of the Appalachian region. They appear to be shore 

 formations, and the lower beds of the Onondaga limestone may 

 have been in process of deposition over the eastern interior while 

 the Esopus and Schoharie beds were accumulating in the Appala- 

 chian belt. If this is the case, the conditions for limestone accumu- 

 lation were presently extended eastward. 



The Onondaga limestone is found from New York to the Missis- 

 sippi (Fig. 408). It rests on Silurian beds, 1 often with little 

 evidence of unconformity. The epicontinental sea in which the 

 limestone was formed was relatively clear and shallow, as shown 

 by the composition of the rock and the character of the fossils it con- 

 tains. In many places the limestone is rich in coral, and locally 

 the coral-reef structure is as perfectly shown as in the reefs of 

 modern times. This is true, for example, at the rapid's of the Ohio 



1 See Reports of Iowa, Missouri, Arkansas, Indiana, and Michigan. 



