THE PLEISTOCENE OR GLACIAL PERIOD. 425 



geographic changes would be a natural consequence of such a move- 

 ment, and the effects of direct elevation and of attendant geographic 

 changes have been variously combined in the different phases the 

 hypothesis has assumed. As chief evidence of the elevation postu- 

 lated, the buried valleys of the sea coasts, especially those of the north- 

 ern latitudes, have been cited, and it is held by many advocates of 

 this hypothesis that the 4000 feet or more of elevation thought to 

 be indicated by the northern fiords, together with abetting geographic 

 changes, were competent to produce the Pleistocene glaciation. Those 

 who question this view doubt whether this elevation was contem- 

 poraneous with the ice development, and cite, as grounds for believing 

 that it was earlier, the magnitude of the erosion indicated by the fiords 

 compared with that which the glacial formations have suffered. They 

 cite also the direct evidences that the valleys formed during this period 

 of elevation were already present when the ice invasion took place. 

 On the other hand, they offer evidences that the land was often lower 

 than at present at certain important stages of the glacial period. It 

 is explained by the advocates of the hypothesis of elevation that the 

 glaciating effects must have lagged behind the elevation itself, and 

 that the accumulation of ice might well have produced depression, 

 and led to its own destruction. 1 It is not, however, clear to those 

 who doubt the hypothesis that the glaciation should have lagged so 

 far behind the elevation as to result in the great discrepancy observed 

 between the erosion of the period of elevation, and the erosion of the 

 earliest drift-sheets. The hypothesis of elevation also encounters 

 difficulty in satisfactorily explaining the interglacial intervals which 

 are now well established by abimdant evidence, and also in accounting 

 for the markedly mild climate of one or more of these intervals, which 

 seems to imply a disappearance of the ice at least as complete as that 

 of today. Unless some other agency than elevation be called into 

 play, it seems necessary to postulate that a great elevation of a large 

 part of two continents, followed by depression, was repeated as often 

 as there were great oscillations in the ice development. The advo- 

 cates of elevation have naturally questioned the adequacy of the evi- 

 dence that the oscillations of the ice-sheets were really great, and 

 they have usually held that the ice period was relatively short and 



1 On this point see Jour. Geol., Vol. II, 1894, p. 222. 



