496 THE GLACIAL LAKE AGASSIZ. 



resistance to compression," yet is one of the earliest and most decided advo- 

 cates of the opinion that the weight of an ice-sheet may depress the area on 

 which it Hes, and that the departure of the ice would be attended by reele- 

 vation. In comparison, however, with the physical conditions 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 conclu- 

 sion is indicated by volcanoes, which are probably the openings of molten 

 passages that conuuunicate downward through the crust to a heavier melted 

 portion of the interior, thence deri^dng their supply of heat, while theu- 

 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 onh^ the 

 upper part of the earth's crust, crumpled into smaller area in adapting itself 

 to tlie diminishing volume of the lower portion of the same crust, which, 

 with the nucleus, is undergoing 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 plastic or 

 perhaps 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 considerable shrinkage of volume can 

 take place before the accumulated stress becomes sufficient for the forma- 

 tion of a mountain chain. At the present time depressions and elevations, 

 probably caused by accumulating stresses, 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 com- 

 plicated by concomitant or subsequent movements thus due to deformation 

 under stresses, by which the ele\ation attributable to the departure of the 

 ice-sheet may be augmented or partly or wholly counteracted, giving much 

 irregularity to the glacial and postglacial oscillations of the land. 



The area of Lake Bonneville has experienced changes of level since 

 the formation of its highest shore-line, Avhich Mr. Gilbert finds to be in har- 



