246 ANNUAL, EEPORT SMITHSONIAN INSTITUTION, 1924 



ice ages, and, further, thought that subsidence of one region caused 

 uplift of other portions of the earth's crust; so that glacial condi- 

 tions may have prevailed alternately in the Northern and South- 

 ern Hemispheres or in North America and Europe. But there 

 are weighty reasons enough for assuming a simultaneous appearance, 

 not only of the glaciations on the Northern and Southern Hemis- 

 pheres, but also of the successive Quaternary ice ages in different 

 quarters of the world. 



The supposition that the dissolution of the ice caps resulted from 

 the depression of the regions under the load of the ice, has, however, 

 been considered fallacious." The depression being, namely, at 

 isostatic equilibrium only about one-third of the thickness of the 

 ice, the surface of an ice-covered region is always more elevated than 

 the region was before its glaciation. Consequently, the ice cap does 

 not disappear. Such a reasoning is right, if one has in view only 

 the isostatic depression. But here come the changes of sea level, 

 too, as we shall see. 



At the beginning of the Quaternary period the continents had 

 orocratic relief, as at the beginning of the earlier miothermic ages. 

 Their higher parts became glaciated, the ice caps grew bigger, as 

 described above, the sea was chilled, and the climate deteriorated. 

 Enormous quantities of water being confined in the land ice, the sea 

 level, and with it the snowline, sank. The ice fields grew still larger, 

 until they extended to so low levels or latitudes that the heat of the 

 summer stopped them at a certain maximum. However, an equilib- 

 rium between glaciation and melting did not yet come about. For 

 as the ice caps got bigger and thicker, they loaded more and more 

 the areas occupied by them. The crust of the earth gave way and 

 began to sink, but not in the same proportion as the ice load in- 

 creased. The depression certainly reached its greatest depth only 

 after, and long after, the maximum of glaciation, just as in the 

 contrary case, the upheaval of Fenno-Scandia and other formerly 

 glaciated regions has continued long after their release from the 

 weight of the ice, and still continues. 



As the depression proceeded, the ice-covered regions came gradu- 

 ally into lower positions in relation to the levels of the snow and 

 glacier lines for maximum glaciation. The ablation now gained 

 upon the advance of the glaciers; the ice masses began to diminish. 

 The addition of water to the sea raised its level, and thus also the 

 snow line. This accelerated the melting of the ice, the sea level rose 

 steadily, and so on. The more the sea level rose, the better the warm 

 sea currents could pass over submarine thresholds and penetrate 

 to higher latitudes. All this worked together to lessen glaciation 

 and hastened the amelioration of the climate. 



"R. D. Salisbury, Editorial note: Journal of Geology, li, Chicago, 1894, pp. 222-224. 



