GONDITIOI^rS OF DEFOSITIOK OF BEDS. 15 



been subjected to recent and important alterations of level, there are many 

 considerable basins along its shores which appear to be, in their structure 

 and history, much like those which, according to the hypothesis we are 

 discussing, were developed along this coast line during the Carboniferous 

 period. Albemarle and Pamlico sounds, and the bays of the Chesapeake 

 and Delaware, need only a continuance through a considerable extent of 

 geological time of the conditions which now exist to bring about the 

 formation of accumulations essentially like those under consideration. So, 

 too, certain basins along the Grulf of Mexico, particularly Mobile Bay and 

 the trough of the Mississippi, are the seats of extensive estuarine accumula- 

 tions which in the ages to come may take on much the same aspect as the 

 Narragansett Basin. 



A little consideration will show the reader that a river valley in its 

 lower parts naturally becomes the seat of sedimentation. An inspection of 

 the maps of shore lines will make it plain that more than half the great 

 rivers of the world have the lower parts of their valleys flooded in a way 

 which clearly indicates that these estuarine regions have recently been 

 brought beneath the level of the sea and thus converted from fields of 

 erosion to those of deposition. The generality of the fact that the great 

 rivers, notwithstanding the evident tendency to accumulate delta deposits 

 about their mouths, enter the sea through their own submerged valleys, is 

 probably in many cases to be accounted for by the fact that, while the con- 

 tinental masses as a whole tend rather constantly upward, their shores, 

 being near the seat of maximum sedimentation, naturally tend downward, 

 in the manner now recognized as resulting from the imposition of a great 

 load of sediments on any part of the earth's surface. We may therefore 

 regard the occlusion of river valleys by excessive sedimentation, which 

 takes place coincidently with the subsidence of the trough below the 

 level of the sea, as a normal feature in the history of any shore which h 

 intersected by river valleys. 



If the hypothesis which is here adduced to explain the main peculiar- 

 ities of the East Appalachians be established, it is clear that, considered 

 from the point of view of their origin, we must accept a new specific group 

 of mountains, one characterized by features in the main determined by the 

 fact that the beds of which they are composed have been laid down in a 

 formerly existing erosion basin, originally due to stream work, though it may 



