SECULAR MOVEMENTS 219 



withstanding its instability and its complete subordination to the 

 lithosphere. If there were some available mode of measuring the 

 distance of surface points from the center of the earth, it would 

 reveal much that is now uncertain respecting the real movements 

 of the surface. 



Periodic and aperiodic movements. The existence of land de- 

 pends on protuberances of the surface of the lithosphere. If the 

 lithosphere were perfectly spheroidal, water would cover it every- 

 where 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. Such renewal has been brought about again and again in 

 gi-ologic history. With every movement which restored the pro- 

 tuberances, the oceans seem to have withdrawn more completely 

 within the basins, while the continents have stood forth more 

 prominently until worn down again. This renewal of protuberances 

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

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

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

 the continents, the continental shelves. Before complete sub- 

 mergence was effected, renewed deformations checked the progress 

 of submergence and rejuvenated the continents. 



Beside the great periodic movements, minor warpings or oscil- 

 lations of the surface have been in almost constant progress. Some 

 of these are probably incidental to the larger movements, but others 

 probably are due to local causes. 



Minor Movements 



Gentle movements seem to have affected nearly every portion 

 of the surface of the lithosphere at nearly all stages of its history. 

 They have had much to do with the particular places and forms 

 of deposits. Slow sinkings of sea-borders have permitted deposition 

 to go on in shallow water for long periods without building the bot- 

 tom up into land, and slow relative swellings of land tracts have 

 renewed the sources of sediments for such deposits. Such move- 

 ments shift shore lines, and with them the areas of erosion and 

 deposition. These movements may have amounted to a few inches, 

 or a few feet per century. In some cases they appear to have been 

 reciprocal, one area being bowed up while another near by is bowed 

 down. How far these are merely local or regional, due to loading, 



