6io FRANCIS PARKER SEEPARD 



to cause the large variations in the sea-level in a relatively short 

 period of time? If the evidence for the greater density of sub- 

 oceanic material is correct, there is every reason to believe that the 

 ocean basins owe their depth to this extra density. They should 

 have become adjusted to this difference early in geological history, 

 just as the earth has adjusted its oblateness to its rate of rotation. 

 As there has been presumably little addition or subtraction from the 

 basins, there is Httle reason to believe that they should have to renew 

 this adjustment periodically. If the zones along shore sink, due to 

 loading, the compressive effect of such shrinking would be neghgible 

 because of the narrowness of the zone. 



As to the second point, let us see what the effect of erosion would 

 be as to raising the sea-level. It must be remembered that sedi- 

 ments are not all carried to the ocean directly, but that there is a 

 most important amount of deposition in the interior in river basins. 

 Let us suppose there was a sea incursion up the Mississippi due to 

 some sUght raising of the sea-level. The Mississippi and its lower 

 tributaries would begin to deposit in this sea and would build out 

 the land much more rapidly than they would raise the sea-level by 

 displacing the sea. Of course, if erosion continued long enough 

 there would be a tendency to submerge parts of the lands, but for 

 long periods after a great upheaval of the continents or a sinking 

 of the sea-level the continental boundaries might be expected to be 

 spread out rather than to retreat so far as erosion and deposition are 

 concerned. The oscillations accompanying the advance and the 

 retreat of the seas could hardly have been produced by the displace- 

 ment of the water by sedimentation. Orogenic disturbances could 

 easily have produced such an effect. 



If diastrophism is periodic and world-wide in its effects, it is only 

 reasonable to infer that during a disturbance there would be folding 

 in parts of the ocean basins as well as in parts of the continents. If 

 this were the case, the sea-level would rise and the lower parts of the 

 continents, which were not affected by the diastrophic movements, 

 would be submerged. 



If on the other hand we consider that diastrophism has been 

 continuous, but at some times more concentrated under the oceans 

 and at others more concentrated within the continents, we can 



