Status of the Theory of Isostasy. 307 



or for regional compensation to a radial distance of 167 

 kilometers, the greatest distance for which the test was 

 made. It is seen that for such level tracts there should 

 be no distinction between local and regional com- 

 pensation, since the isostatic adjustment is there due to 

 the elevation of the continental platform above the ocean 

 basins, with a minimum of influence from local relief. 



"Where marked breaks in topography and geologic 

 structure coincide, it is probable that a rather close rela- 

 tion is to be found to sub-surface density changes. The 

 hypothesis of local compensation takes recognition of 

 such boundaries to geologic provinces, as illustrated by 

 slopes of the oceanic basins and the fault face of the 

 Sierra Nevada. The hypothesis of regional compensa- 

 tion, although truer in general, will, if applied with- 

 out geologic guidance, result in obscuring such real 

 boundaries. 



In Part III of "The strength of the earth's crust,' ' 

 the "influence of variable rate of isostatic compensa- 

 tion" is considered, and evidence given that the anoma- 

 lies are mostly the measure of real loads, due to irregu- 

 larities in density in the crust, with little or no relation 

 to the topography formed by erosive processes. 



In Part IV, on "Heterogeneity and rigidity of the 

 crust as measured by departures from isostasy," an 

 estimate is made of the magnitudes of the loads due to 

 irregularities of density not in harmony with topography. 

 The largest anomalies thus far found in the United 

 States are those at Seattle. The mean of the two com- 

 puted values of gravity on the hypothesis of local and 

 Uniform distribution of compensation to 113-7 kilometers 

 is 980-830 dynes. The mean of the observed values of 

 gravity is 980-728 dynes. The anomaly is — 0-102 dynes. 

 Considering this in connection with the other measure- 

 ments in the state of Washington leads to an interpreta- 

 tion in terms of mass. It seems that the Seattle anomaly 

 corresponds to a load measured by at least 5,000 feet of 

 rock and may reach a considerably higher figure, per- 

 haps 10,000 feet. 



The hypothesis of compensation complete at a depth 

 of 60 kilometers makes these anomalies — 0-100 dyne; 

 that hypothesis which assumes compensation complete at 

 the surface, the "Free Air" reduction, gives 0-104, 

 while the hypothesis of no isostasy, the "Bouguer" 



