442 WILLIAM BOWIE 



surface material was light in density. Dr. David White, chief 

 geologist of the United States Geological Survey, suggested to the 

 writer that it may be possible to predict with considerable accuracy 

 whether or not crystalline rocks are close to the surface under a 

 station located on surface material of light density by considering 

 the size and sign of the anomaly. 



There has been considerable confusion in regard to the opinion 

 of Professor Hayford and the writer as to the density of material 

 between sea-level and the depth of compensation. Some assert 

 that we hold that the density is 2.67 for all of this material except 

 in so far as it is modified by the compensation. As a matter of 

 fact, neither of us has made any assumption as to the absolute 

 density of the material between sea-level and the depth of compen- 

 sation. In the investigations of isostasy it is not necessary to know 

 the absolute density in making computations of the effect of the 

 compensation of the topography. It is the deviations from normal 

 densities that are given sole consideration. 



A great deal of the gravity anomaly is eliminated by the applica- 

 tion of the effect of topography and compensation, but it is certain 

 that the remainder of the anomaly cannot be eliminated by applying 

 the actual density of the topography in the computations. The 

 density of 2.67 was used for all the land topography, while there 

 are local variations in the density of material amounting to 10 per 

 cent or more. It is, however, impossible that the true densities, 

 if applied, could have reduced materially the average gravity 

 anomaly. There is not enough topography to account for the 

 gravity anomaly. It is of course probable that in many cases the 

 anomaly would be slightly changed if a true density were used. 

 What applies to the density of the topography will of course apply 

 also to the density of the compensation. There is not enough 

 compensation, however distributed, to account for most of the 

 anomalies. As the effect of the compensation is the opposite of 

 that of the topography, the resultant effect is smaller than the 

 effect of the topography alone. We must conclude that no method 

 of distributing compensation applied generally to the country or 

 to the world will eliminate the gravity anomalies which we 

 now have. 



