THE STRENGTH OF THE EARTH'S CRUST 155 



the station or a large excess at great depth or to one side may have 

 the same effect. Therefore it is necessary to speak of the net 

 effective excess or deficiency of mass.^ A table is given showing 

 these relations, and as a mean working hypothesis it is assumed that 

 ordinarily each 0.0030 dyne of anomaly is due to an excess or 

 deficiency of mass equivalent to a stratum 100 ft. thick. In the 

 final paper it is concluded : 



From the evidence given by deflections of the vertical the conclusion has 

 been drawn that in the United States the average departure from complete 

 compensation corresponds to excesses or deficiencies of mass represented by a 

 stratum only 250 feet thick on an average. The gravity determinations 

 indicate this average to be 630 feet instead of 250 feet. In neither case is the 

 average value determined or defined with a high grade of accuracy. The 

 difference between the two determinations of the average value is therefore of 

 little importance. The determination given by the gravity observations is 

 probably the more reliable of the two. Each determination is significant 

 mainly as showing that the isostatic compensation is nearly perfect. 



The average elevation in the United States above mean sea level is about 

 2,500 feet. Therefore, from gravity observations alone the compensation may 

 be considered to be about 75 per cent complete on an average for stations in the 

 United States.^ 



This conclusion implies a somewhat greater rigidity to the crust 

 than that which is stated for the deflections of the vertical, but in 

 regard to the maximum horizontal extent which a topographic 

 feature may have and still escape compensation the authors still 

 express the belief that the limit is between one square mile and one 

 square degree. "It appears from the inconclusive evidence fur- 

 nished by the gravity observations that the radius of this area is 

 probably less than 18.8 kilometers."^ 



This review of the work of Hayford on deflections of the vertical, 

 and of Hayford and Bowie on the gravity anomalies has been given 

 in order that the methods of the work, its bearings on the strength 

 of the crust, and the conclusions which were reached, may be per- 

 ceived. It is seen that a large difference of view as to the strength 

 of the crust exists between this interpretation from the geodetic 

 evidence and that from the geologic. In the following pages will be 



' Hayford and Bowie, 1912, pp. 108-12; Bowie, 1912, p. 22. 



^ Bowie, 1912, pp. 22, 23. 3 Hayford and Bowie, 1912, p. 102. 



