THE CAUSE OF THE GLACIAL PERIOD. 499 



mous pressure and high temperature of the earth's interior, and 

 wide diversity in speculations on this subject will probably long 

 continue. Professor Shaler, while holding that the earth is 

 mainly solid throughout, perhaps having in its most mobile 

 layer beneath the crust " a rigidity such as belongs to the 

 metals of average resistance to compression," yet is one of the 

 earliest and most decided advocates of the opinion that the 

 weight of an ice-sheet may depress the area on which it lies, 

 and that the departure of the ice would be attended by re- 

 elevation. In comparison, however, with the physical condi- 

 tions and laws familiar to us upon the earth's surface, the 

 subsidence and elevation of extensive areas, as of nearly all 

 glaciated regions, seem to demonstrate a mobility of the earth's 

 interior as if it were fused rock. The same conclusion is in- 

 dicated by volcanoes, which are probably the openings of molt- 

 en passages that communicate downward through the crust to 

 the heavier melted interior, thence deriving their supply of 

 heat, while their outpoured lavas consist largely or wholly of 

 fused portions of the crust, the phenomena of eruption being 

 caused by the access of water to the upper part of the molten 

 rock near the volcanic vent. But the great plications of the 

 strata in the formation of mountain -chains evidently involve 

 only the upper part of the earth's crust, crumpled into smaller 

 area in adapting itself to the diminishing volume of the lower 

 portion of the same crust, which, with the nucleus, is under- 

 going contraction on account of the gradual loss of its heat, 

 and perhaps also on account of progressing solidification and 

 compression. There is in this process no dependence on the 

 molten condition of the interior, except as that seems to be 

 necessary for distortion of the earth, both of the crust and 

 nucleus or mobile layer enveloping the nucleus, whereby con- 

 siderable shrinkage of volume can take place before the ac- 

 cumulated strain becomes sufficient for the formation of a 

 mountain-chain. At the present time depressions and eleva- 

 tions, probably caused by accumulating strains, are slowly 

 changing the relations of land and sea upon many parts of the 

 earth's surface. In the same way the downward and upward 

 movements which would be caused by the burden of the ice- 

 sheet and its removal are doubtless in many places complicated 



