314 Status of the Theory of Isostasy. 



corresponded to a mean departure of the surface of the 

 United States of about 500 feet from the level-giving 

 equilibrium, as many stations being positive as negative. 

 Later Bowie raised this estimate to 630 feet. They recog- 

 nize that it is difficult to give an accurate estimate. 

 Under assumptions equally or perhaps more probable, 

 however, the interpretation could be made 1,000 to 1,400 

 feet. The maximum anomalies indicate loads several 

 times as great. These departures from isostasy are 

 regional in extent, the larger loads being restricted to 

 smaller areas than are the lesser loads. 



Notwithstanding this lack of perfection in isostasy, 

 the gravity anomalies prove with great conclusiveness 

 the existence of regional isostasy to a notable degree. 

 This is best brought out by the map of gravity anomalies 

 for the United States as given by the Bouguer reduction, 

 which postulates no isostasy. The map shows large and 

 systematic errors introduced by this hypothesis, the 

 errors increasing with the elevation above the datum 

 plane. On comparing two stations, the influence of the 

 datum plane is eliminated and the discrepancy is seen to 

 depend largely upon the difference in regional elevation. 



Interpretations Adverse to Isostasy. 



Hobbs in 1916 contributed an article to the subject 

 of isostasy. 21 In criticism of Hayford's attitude toward 

 the geological interpretation of the geodetic data, he 

 cites examples which teach that although the methods 

 of exact science may not be lacking in precision, the 

 assumptions possess the same measure of fallibility as 

 those employed in other fields of science. Hobbs then 

 shows that along the Atlantic and Pacific mountain belts 

 and near the coasts the observed deflections of the ver- 

 tical due to topography are large, and are in the general 

 regions of strong seismicity. The arrangement of the 

 belts of seismicity and of high deflections along distinct 

 lines is no doubt in part due to the direction of rock 

 structures, but to the writer it appears to be due in part 

 also to the fact that observations are more numerous 

 in or near the large centers of population. These last 

 in turn are associated with certain lines, such as the fall 



21 W. H. Hobbs, Assumptions involved in the doctrine of isostatic com- 

 pensation, with a note on Hecker's determination of gravity at sea. Jour. 

 Geology, 24, 690-717, 1916. 



