430 GEOLOGY. 



northwestern sea which stretched from Missouri on the southeast, 

 through the Mackenzie basin, to the Arctic Ocean on the north. 

 Toward the end of the Hamilton epoch, this barrier seems to have 

 been removed sufficiently to allow the waters and the life on opposite 

 sides to mingle freely. The removal may not have affected the whole 

 of the barrier, but it seems to have been sufficient to accomplish the 

 results indicated. 



Belief in the barrier and in its removal is based primarily on pale- 

 ontological evidence to be sketched later, but it may here be stated 

 that up to the end of the Hamilton, the fauna to the west of the sup- 

 posed barrier was unlike that to the east, while during the next 

 succeeding epoch the fauna on the northwest seems to have invaded 

 the eastern interior sea. 1 



The northeastern region. — Rocks (sandstone) of Middle Devonian 

 age, presumably Hamilton, are known to occur in the eastern region 

 (Gaspe), where some of them appear to be of fresh- water origin. The 

 evidence of this origin is found in the abundant land-plant fossils. 

 Some portions of these beds are probably the time-equivalents of the 

 Hamilton of the interior, but their exact equivalency has not been 

 established. 



Geographic changes during the Middle Devonian. — The distribu- 

 tion of the Hamilton beds indicates several changes in the geography 

 of the continent, as compared with that of early Devonian time. These 

 were (1) a slight elevation (relative) to the east, interrupting for a 

 time the connections between the sea of the Appalachian area and 

 the Atlantic; (2) a subsidence to the southward in the interior, sub- 

 merging areas which had long been land, and perhaps connecting 

 the interior sea with more southerly waters; and toward the close of 

 the epoch, (3) the removal of the barrier which separated the north- 

 western interior sea from the eastern. This last change may perhaps 

 be looked upon as marking the closing stage of the Hamilton epoch. 



The Upper Devonian. 



The Upper Devonian series has a distribution (Fig. 196) similar 

 to that of the Middle Devonian, though it is perhaps less widespread 



1 The Hamilton fish fauna of Wisconsin, Illinois, and Iowa is thought by Eastman 

 to have been related to the Onondagan fish fauna of New York. Bull. Geol. Soc. 

 Am., Vol. XIII, p. 537. 



