306 Status of the Theory of Isostasy. 



Bowie, in 1917, has discussed the effect of the eleva- 

 tion of the station npon the intensity of gravity. 15 Four- 

 teen pairs of adjacent stations were selected which 

 showed a large difference in elevation between the mem- 

 bers of each pair. Three of these pairs were in the United 

 States. The algebraic differences of the gravity anomalies 

 under the hypothesis of extreme local compensation are 

 systematic, amounting on the average to 0-0013 dyne for 

 each 100 meters. Bowie shows that by making a more ac- 

 curate computation of the effect of topography, the sys- 

 tematic difference is slightly reduced; by taking the 

 rock as locally denser beneath the high station than 

 beneath the low one, it is further reduced; by assuming 

 that the compensation does not reach up to the surface, 

 it is still further reduced. By adjusting these factors 

 the differences in anomalies between pairs of stations 

 may be made to disappear. Bowie's argument is sound 

 and valuable ; it should be replied, however, that since 

 extreme local compensation cannot be true, the particular 

 algebraic difference of the anomalies which gives a geo- 

 detic measure of the error of local compensation should 

 not be made to disappear. Numerous pairs of stations 

 should be measured at different elevations and the cor- 

 rections for local variations of density should be derived 

 by geologic study. Such pairs would then be better 

 adapted to serve as a geodetic test. Regions of strong 

 local disturbances such as those which prevail near 

 Seattle should, however, be avoided. The problem illus- 

 trates the difficulty of purely geodetic tests of various 

 subordinate hypotheses entering into the theory of 

 isostasy. 



Bowie, discussing in 1917 the average anomaly given by 

 stations in mountainous regions, finds that regional com- 

 pensation with a radius of 59 kilometers satisfies the 

 data as well as local compensation, but regional com- 

 pensation to a distance of 167 kilometers not quite so 

 well. Bowie considers that local compensation is much 

 nearer the truth than this degree of regional compensa- 

 tion. To the writer the adverb "much" appears to be 

 open to question. Mountain regions should, however, 

 show narrower limits for compensation than the areas 

 of low relief. Over the great plains of the continent 

 the anomalies are equally small for local compensation 



15 William Bowie, op. cit., pp. 93-96. 



