Status of the Theory of Isostasy. 293 



sun in space, namely, by the bringing into harmony, except 

 for minor irregular discrepancies, of calculation and ob- 

 servation on large assemblages of precisely measured 

 data. Beyond this broader demonstration of the exist- 

 ence of isostasy lies, however, the field of competing 

 hypotheses. Such questions arise as: How closely re- 

 lated in horizontal area are the elevations and corre- 

 sponding defects in density? How closely balanced are 

 the elevations and densities! How near the surface are 

 the corresponding variations and in what manner do 

 they disappear with depth! "What are the causes of 

 variations of density and to what degree have they re- 

 mained constant for each area through geologic time! 

 To what degree are these questions determinate and to 

 what degree indeterminate? 



The quantitative data are mostly astronomic and geo- 

 detic, depending upon very precise measurements of the 

 direction of the vertical and the intensity of gravity; 

 the interpretation, however, is mostly geologic, having 

 to harmonize the geodetic indications with other aspects 

 of earth structure and history. Gfeodesists have been 

 hampered in interpretation from the fact that they 

 were not geologists; geologists have been hampered in 

 weighing the force of the geodetic evidence because of 

 lack of familiarity with the mathematics of the geodesist. 



In order to subject a large mass of complex data to 

 mathematical analysis, simple mathematical hypotheses 

 must be introduced and tested to see if they reduce the 

 data to some degree of order. Thus, in the determina- 

 tion of star drift and the sun's way through space rela- 

 tive to the stars, the average motions of groups of stars 

 are determined. A certain proportion of the drift, in 

 so far as it is common to all the stars, is regarded as 

 due to the sun's motion with respect to the stars. The 

 outstanding differences represent the errors of observa- 

 tion combined with the random motion of the individual 

 stars and groups of stars with respect to each other. 



In the analysis of the geodetic data bearing on isostasy, 

 a simple mathematical picture is consequently introduced, 

 but no one should imagine that nature is really so simple 

 as the mathematical picture of hypothesis. The geode- 

 sist, while freely admitting this distinction, tends to 

 minimize it; the geologist, familiar with the complexi- 

 ties of earth structure and seeing that topographic relief 



