MOVEMENTS AND DEFORMATIONS 353 



latter. Whatever change affects the capacity of the abysmal 

 basins' affects the sea-level. If the basins are increased, the sea 

 settles deeper into them; if they are decreased, the sea spreads 

 out more widely on their borders. The one thing that gives a 

 measure of stability to the sea-level is the fact that all the great 

 basins are connected, and so an average is maintained. For this 

 reason the sea-level is the most convenient basis of reference, and 

 has become the accepted datum-plane, notwithstanding its in- 

 stability and its complete subordination to the lithosphere. If 

 there were some available mode of measuring the distance of sur- 

 face points from the center of the earth, it would give absolute data 

 and would reveal much that is now uncertain respecting the real 

 movements of the surface. 



The classes of movements. The existence of land depends on 

 protuberances on the surface of the lithosphere. If the lithosphere 

 were perfectly spheroidal, water would cover it everywhere to a 

 depth of nearly two miles. To maintain the existence of land the 

 protuberances must be renewed from time to time; otherwise the land 

 would in time be degraded to the lowest depths of wave action. 

 The renewal has been brought about again and again in geologic 

 history by movements that have restored the protuberances. With 

 every such movement, the oceans seem to have withdrawn more 

 completely within the basins, while the continents have stood forth 

 more prominently until again worn down. This renewal of protu- 

 berances appears to have been periodic in its great features, with long 

 intervals between. In these intervals, the land was worn down by 

 rivers, waves, etc., and the sea crept out upon the lower parts of 

 the continents, forming continental shelves. Before complete sub- 

 mergence was effected, however, renewed deformations checked 

 the progress of submergence and rejuvenated the continents. 



Beside the great movements, numerous minor warpings or 

 oscillations of the surface have been in almost constant progress. 

 Some of these are probably only incidental features of the larger 

 movements made necessary to complete the adjustment to the 

 smaller stresses left after the great stresses are relieved, but others 

 are probably due to local and special causes of the lesser sort. 



