330 Status of the Theory of Isostasy. 



The broad areas recently subjected to continental 

 glaciation are observed to be in a depressed condition, 

 greatest where the ice was thickest. The present depar- 

 ture from equilibrium must presumably give, in those 

 regions, an upward strain resisted by the strength of 

 the crust and subcrust. The stability of a continent 

 during a partial cycle of erosion implies the same resist- 

 ance to upwarping forces as does the persistence of 

 a depressed condition after glaciation. The Nile and 

 Niger deltas, on the other hand, show the capacity of 

 the crust to resist downward stresses. Irregularities in 

 density not related to the topography give indications 

 of being comparable in magnitude of loads to mountains, 

 and these to be supported through geologic ages by the 

 strength of the crust. 



Mathematical solutions of isostatic compensation de- 

 pending on differences of density of the order of 0-01 

 therefore have no geologic meaning. In D, the curve 

 of compensation extending to 178-6 miles, everything 

 below 100 miles may therefore be dropped so far as the 

 geodetic evidence and the geological argument are con- 

 cerned. If such slight uniform differences of density 

 exist at those depths between continental and oceanic 

 sectors, they exist irrespective of the geodetic and geo- 

 logic evidence. It is much more probable that the dif- 

 ferences of density at such depths are larger, more 

 irregular than the curve, positive and negative, and unre- 

 lated to isostatic compensation. The slender portion of 

 curve D below 100 miles is consequently nothing more 

 .than an abstraction, serving as a tail to which to tie a 

 mathematical argument. 



Having eliminated for good geological reasons curve 

 A and the lower part of I), it may be seen how poorly 

 taken is MacMillan's argument, based on curves A, B, 

 C, and D, that with a slight modification of the hypothesis 

 the depth of compensation could be made to retreat to the 

 center of the earth or vanish altogether. 



It may be stated in conclusion that the isostatic com- 

 pensation balancing continents against oceans, if cor- 

 responding to the depth of 70-7 miles for uniform 

 distribution, must have its center of gravity between 

 34 and 37 miles deep, but may be of any mode of dis- 

 tribution, so long as the density differences do not ex- 

 ceed a reasonable amount at any depth and the whole 



