326 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



Maryland, which I take to have lain at that time within the belt of 

 peripheral isostatic elevation, the land was pushed up with resultant 

 apparent or relative sinking of sea level. Farther south, beyond the 

 belt of peripheral bulging, the Atlantic shore probably shared in the 

 eustatic rise of sea level that prevailed generally, because of the tem- 

 porarily decreased capacity of ocean basins except in the areas 

 affected immediately and differentially by the ice sheets. 



Correlation of Pleistocene sea beaches in Maryland and Maine 

 therefore suggests and perhaps requires comparison of the low 

 beaches in Maryland with high beaches in New England. 



Because of this dissimilarity in manifestation, it seems to me that 

 it is only in the warm temperate and tropical zones lying well be- 

 yond the areas in which isostatic balance would be materially dis- 

 turbed by known ice loading of lands, that the sequence and amount 

 of the several glacially controlled Pleistocene changes of sea level 

 are recorded in their proper relations to the actual fluctuations of the 

 volume of sea water and to the capacity variations of the basins 

 holding it. But even in tropical areas the complete sequence of the 

 oscillations and the immediate cause of each can not be worked 

 out without taking strict account of what was happening at the same 

 times in higher latitudes. 



In thinking of the progressive and regressive sequences of move- 

 ments it is well to remember that ice loading and sediment (rock) 

 loading of epicontinental areas are comparable in their deforma- 

 tional effects on the lithosphere only in one respect— that is, in both 

 cases the loaded area sinks. They differ, primarily, in that the ice 

 cap originates on, and spreads outwardly from, normally positive 

 areas, whereas the rock sediments are laid only in areas of relatively 

 negative tendencies. Subsidence because of ice loading, therefore, 

 is an abnormal process in that it is carried on under unusual condi- 

 tions, so that normal gravitational tendencies are reversed; in the 

 other case not only the process but the results also are perfectly in 

 accord with the normal gravitational tendencies of the affected areas. 

 Next, they differ in that the ice sheets presently melt away, whereas 

 the water-laid rock deposits commonly remain as a permanent asset 

 of the area covered by them. A third difference is that in the first 

 cases the removal of the ice load tends to reestablish the normally 

 positive tendencies of the deglaciated areas, whereas in the areas 

 loaded with rock deposits their normal negative tendency is not 

 reversed. 



Finally, there is the rather generally accepted belief among stra- 

 tigraphers and students of paleogeography that in the past the ad- 

 vances of the sea usually were slow and gradual, whereas the re- 

 treats were more rapid and relatively impulsive. Many facts in 



