OSCILLATORY CHARACTER OF CONTINENTAL SEAS 353 



prevailed, and warmer climates always returned when the continents had 

 been reduced by erosion or other processes to approximate baselevel, and 

 when they were more or less widely submerged beneath the oceanic waters. 

 Under this supposition relatively frigid conditions may have occurred 

 during any of the highly emergent phases. But the subject is exceedingly 

 complex and many other factors, such as the depletion of the moisture 

 and carbon dioxide content of the atmosphere, clouds and change in pre- 

 vailing direction of winds, and the absorption and radiation of heat 

 under varying climatic conditions, some or all of which doubtless con- 

 tributed and possibly were more important in bringing about climatic 

 changes than were elevation and size of land areas. Eegarding most of 

 these other causes all I care to say at present is that their operation prob- 

 ably was very effective only at times of exceptional continental expansion 

 and elevation. 



Another suggestion relating especially to the several advances and re- 

 treats of the Pleistocene ice-sheets : Assuming that elevation is compe- 

 tent to bring about glacial conditions in areas of abundant precipitation, 

 it seems to me that the subsequent melting and retreat of the ice-cap may 

 be due chiefly to subsidence of the areas, and that the subsidence resulted 

 from overloading. In other words, that the isotatic equilibrium had been 

 disturbed by loading, and that subsidence set in when the ice attained 

 a certain limit of thickness. As the loaded areas sank, more widely 

 distributed compensatory upward movement probably occurred in the 

 more interior unglaciated regions. Eeaching the level of melting, the ice- 

 cap was gradually removed, only to be rebuilt when the direction of move- 

 ment was first stopped and then reversed. By such alternate loading and 

 unloading we may perhaps explain the unquestionably established periodic 

 advance and retreat of the ice-sheet. That the process here briefly out- 

 lined, which is in essential accord with the views of Dutton, de Geer, and 

 others, is of itself competent to cause the alternations of growth and re- 

 striction of the ice-sheet I am not prepared to say, but that the effect of 

 overloading comparatively local areas with ice, in the way of unsettling 

 isostatic conditions, has not received adequate consideration from most of 

 those who have sought to account for the observed phenomena, seems un- 

 deniable. Perhaps, because of my high regard for the principles of isos- 

 tacy, I am overestimating their bearing on this Pleistocene problem. 

 However, since these physical problems are somewhat out of my special 

 lines, it is with much diffidence that I venture an opinion. 



The hypothesis of reversed oceanic circulation. — In passing it seems 

 desirable to notice an important suggestion made a few years ago by 



