THE STRENGTH OF THE EARTH'S CRUST 301 



sated by the corresponding variations in crustal density, not the 

 760 m. which is the average elevation of the United States above 

 sea-level. 



It is the belt of Pacific coast stations which measures more 

 closely than other groups the degree of compensation accompany- 

 ing the continental relief above the ocean bottoms. These stations 

 lie in groups 10, 9, and 14, for which the mean residuals are but 

 0.06, 0.05, and 0.06 of the mean topographic deflections respec- 

 tively. These residual deflections indicate that for this coastal zone 

 the departures from complete compensation amount to but 5 or 

 6 per cent. If the mean effective relief which controls this be 

 assumed as 4,000 m., then the mean departure from equilibrium is 

 represented by a mass 200 to 240 m. thick, approximately between 

 650 and 800 ft. On the other hand, groups 5 and 12 are those 

 farthest removed from the ocean basins and their deflections are 

 controlled most largely by the internal continental relations. For 

 them the departures from complete isostatic compensation as 

 measured by the ratio of the mean residuals to the computed 

 topographic deflections amount to 42 and 26 per cent. The mass 

 to which this is equivalent may be no greater than the 5 per cent 

 departure on the Pacific coast. These estimates fall into the same 

 order of magnitude as that of the masses represented by the 

 gravity anomalies. 



This reconnaissance of the problem is sufficient for present pur- 

 poses. It is readily seen that even greater difficulties stand in the 

 way of a precise statement regarding the equivalence of mass 

 corresponding to deflections of the vertical than arose in the inter- 

 pretation of the gravity anomalies. The residual for each observed 

 deflection is the sum of the influences of all the excesses and defi- 

 ciencies of mass as compared to solution H on all sides of a station. 

 The effect of each unit varies inversely with the square of the 

 distance and directly with the sine of the angle which the line of 

 force makes with the horizontal passing through each station. A 

 combination of the data from the measurements of the intensity of 

 gravity with those of the deflections of the vertical would apparently 

 be necessary to state for each region the equivalence in terms of 

 mass which is implied by the residual at each station. 



