THE STRENGTH OF THE EARTH'S CRUST 



217 



last part of Part II. It leads to a consideration of the relations 

 between mass, distance, and anomaly. 



Under the title of "Interpretation of Anomalies in Terms of 

 Masses "^ Hayford and Bowie show that the excesses and deficiencies 

 of mass to a great distance have an effect upon the gravity anomalies 

 and that therefore the guarded expression "net effective excess 

 (or deficiency) of mass" is necessary for correctness. They give 

 the following tabulation to show the influence of uncompensated 

 masses in the crust in giving gravity anomalies when the gravity 

 is computed on the assumption of isostasy f 



TABLE X 

 Each tabular value is the vertical attraction in dynes produced at a station by a 

 mass equivalent to a stratum 100 ft. thick, of density 2.67, and of the horizontal extent 

 indicated in the left-hand argument, if that mass is uniformly distributed from the level 

 of the station down to the depth indicated in the top argument and from the station in 

 all directions horizontally to the distance indicated in the left-hand argument. 



Radius of Mass 



1,280 m. (the outer radius of 

 zone E) 



166. 7 km. (the outer radius of 

 zone O) 



1,190 km. (or io°4o', the outer 

 radius of zone 10) 



Depth 



1,000 Ft. 



0.0029 

 0.0037 

 o . 0040 



S,ooo Ft. 



0.0018 

 0.0034 

 0.0037 



10,000 Ft. 



0,0011 

 o . 0034 

 0.0037 



15,000 Ft. 



o . 0008 

 0.0034 

 0.0037 



113. 7 Km. 



O , 0000 

 0.0024 

 0.0034 



On p. Ill it is concluded by these authors that the best working 

 hypothesis is to take 



each o . 0030 dyne of anomaly as due to an excess (or deficiency) of mass equiva- 

 lent to a stratum 100 ft. thick. This working hypothesis is equivalent, as 

 may be seen by inspection of the table just given, either to the assumption 

 that the excess (or deficiency) of mass is uniformly distributed to a depth of 

 113. 7 kilometers and extends to a distance of more than 166. 7 kilometers and 

 less than 1,190 kilometers from the station, or that it extends to a distance of 

 166. 7 kilometers from the station and is distributed to an effective mean depth 

 of more than 1 5,000 feet and less than 1 13 . 7 kilometers, or the working hypothe- 

 sis may be considered to be a combination of these two assumptions. 



The mean anomaly of 0.018 dyne, interpreted on this basis of 

 0.030 dyne being taken as equivalent to 100 ft. of mass, gives a 



Hayford and Bowie, p. 108. 



^ Ibid., 1912, p. 109. 



