152 JOSEPH BAKRELL 



These conclusions imply a weakness of the crust surprising to the 

 geologist and stand in marked contrast to those figures derived from 

 the study of the deltas of the Nile and Niger. This subject also will 

 be discussed later, as here it is desired to give only a summary state- 

 ment of the methods and conclusions. 



Hayford and Bowie on variations of gravity. — Regarding the rela- 

 tions of variations in gravity to isostasy, Hayford and Bowie state: 



As soon as it was evident that the proper recognition of isostasy in connec- 

 tion with computations of the figure and size of the earth from observed 

 deflections of the vertical would produce a great increase in accuracy, it 

 appeared to be very probable that a similar recognition of isostasy in connection 

 with computations of the shape of the earth from observations of the intensity 

 of gravity would produce a similar increase of accuracy. Logically the next 

 step to be taken was therefore to introduce such a definite recognition of isostasy 

 into gravity computations. Moreover, it appeared that if this step were taken 

 it would furnish a proof of the existence of isostasy independent of the proof 

 furnished by observed deflections of the vertical, and would therefore be of 

 great value in supplementing the deflection investigations and in testing the 

 conclusions drawn from them. In other words, the effects of isostasy upon the 

 direction of gravity at various stations on the earth's surface having been 

 studied, it then appeared to be almost equally important to investigate the 

 eft'ects of isostasy upon the intensity of gravity.^ 



In order to make the computations, the isostatic compensation 

 was assumed to be complete under every topographic feature and 

 uniformly distributed to a depth of 114 km. below sea-level, pro- 

 ducing hydrostatic equilibrium at this depth. The mean density of 

 2.67 was taken as applying to the whole zone to this depth. Under 

 land 3 km. high this gives a density of 2 . 60 from sea-level to a depth 

 of 114 km.; under ocean 5 km. deep a density of 2.74 from ocean 

 bottom to 114 km. below the bottom.^ 



The authors show that the topography and its compensation for 

 the whole earth must be taken into consideration. On these 

 assumptions the theoretic value of gravity was computed for every 

 station, 124 in the final publication. This computed value is sub- 

 tracted from the observed value and gives the "new-method" 

 anomaly for each station. The results are shown in Fig. 5. 



' Hayford and Bowie, 19 12, p. 5. 



- Hayford and Bowie, 1912, pp. 9, 10. 



