6 W. Boivie — Relation of Isostasy to 



of gTavity consider, in some detail, the question of the 

 accuracy of the data. 



As to the question of the reliability of the computations 

 of the effect of isostatic compensation we have a different 

 proposition. In the first place, we may say that as far as 

 the mathematical work is concerned the results are 

 reliable and accurate, but the effect of the compensation 

 will vary somewhat depending on the assumptions made. 

 It should be pointed out that the effect of the compensa- 

 tion will be approximately the same for all methods of 

 distribution, vertically, which have the center of gravity 

 of the compensation approximately 50 kilometers below 

 the surface of the earth. Methods of distribution which 

 deviate materially from this condition will not give as 

 accordant results as those based upon this requirement. 



Uniform distribution of compensation vertically. 



We should consider here the question of distribution 

 of the isostatic compensation with respect to depth. 

 Uniform distribution appears to the geodesist to be a 

 logical or reasonable assumption, for, in the first place, 

 the geodesist is not supposed to know anything about 

 the geology of the isostatic shell, except in so far as 

 information may develop from geodetic investigations. 

 It seems probable that, if one had never heard of isostatic 

 compensation and he was told that a mountain mass or 

 a plateau is balanced by a deficiency of mass in a column 

 directly under the topographic feature and that this 

 compensating deficiency extends to a certain definite 

 depth, he would conclude that the deficiency of mass is 

 distributed uniformly throughout the column. He would 

 see no reason why compensation should be greater in one 

 part of the column than in another, although one would 

 be apt to believe that the compensation ends gradually 

 rather than abruptly at the lower end of the column. 



We could distribute the compensation in any one of a 

 number of ways, with respect to depth, and still have the 

 center of gravity of the compensation approximately at 

 50 kilometers. But who is to decide and upon what data 

 is the decision to be made as to what the actual distri- 

 bution is, that is, the one that is in accordance with the 

 truth f 



^ As a matter of fact, the writer believes that compensa- 

 tion varies in its distribution, vertically, from place to 



