THE STRENGTH OF THE EARTH'S CRUST 39 



between 9,000 and 10,000 feet, the Antarctic attaining to about 

 11,000 feet. The average thickness of the ice must be thousands 

 of feet in each case. The development of these ice caps during 

 the refrigeration of climate which marked the later Tertiary must 

 have imposed upon the crust great loads of wide span. If isostatic 

 equilibrium was previously complete to a large degree, the ice 

 mantles should give valuable measures of crustal strength. For 

 this purpose, however, a set of gravity measurements should be 

 carried inland and reduced by the Hayfordian method. The facts 

 that these two ice-mantled areas are both high plateaus, and 

 that no other adjacent unglaciated land is of similar topographic 

 character, suggest that these regions may be competent to carry 

 great thicknesses of ice without isostatic yielding. There is no 

 present basis, however, for making a quantitative estimate. It 

 must be borne in mind, furthermore, that the ice mantle is only 

 about one-third of the density of rock and that lofty mountains 

 exist in both regions, showing that these lands would possess con- 

 siderable mean elevation even without the presence of the ice. 

 The effect of the difference of density between ice and rock may 

 be appreciated by considering that an ice sheet 3,000 feet in thick- 

 ness would possess the same mass as a layer of rock 1,000 feet thick. 

 For isostasy to remain perfect after the development of this ice 

 sheet, the crust would have to sink 1,000 feet, but the surface of 

 the ice would still be 2,000 feet above the former level and give 

 an appearance of load which would not in reality exist. 



This is a problem meriting research for several reasons. A 

 knowledge of the load which is sustained by these regions would 

 show to what degree the warpings connected with the extinct 

 Pleistocene ice sheets were mere elastic responses to load, to 

 what degree they marked subcrustal plastic flow working toward 

 isostasy. The results could be appHed also to the problem as to 

 how far from isostatic equilibrium a continent might come to lie 

 as a result of continent-wide base-leveling in a period of geologic 

 quiet. It seems not impossible that the stress-curve due to the 

 portion of the glacial load which is elastically sustained would give 

 stress-differences greater at a depth of 300-500 km. than those 

 shown by curve C, Fig. 18. Such an investigation may then 



