484 ROLLIN D. SALISBURY 



the level of the sea is concerned, for sinking of the bottom increases 

 the capacity of the basin, just as the deposition of sediment in it 

 decreases it. At those periods when sedimentation in the oceans 

 exceeded subsidence of the ocean beds, the sea-level must have risen, 

 and the lower parts of the continents must have been submerged. 

 At those periods when subsidence of the ocean bed increased the 

 capacity of the basin more rapidly than sedimentation decreased it, 

 the water would have been drawn off the continental shelves, and 

 lands would have emerged. It seems not improbable that this has 

 been a main factor in bringing about repeatedly the submergences of 

 great parts of the continental areas, in the course of geological his- 

 tory. 1 The periodic sinking of the continents bodily has doubtless 

 also been a factor leading to the same result. 



It is to be understood that all changes of sea-level, due to sedi- 

 mentation and warping, have probably taken place very slowly — 

 so slowly that their effects, in all probability, would not have been 

 conspicuous to observers, had there been observers to witness them. 



1 The effectiveness of gradation (degradation and aggradation) in bringing about 

 considerable submergences of land areas was first urged, so far as I am aware, by 

 Professor Chamberlin. 



