OUR PRESENT KNOWLEDGE OF ISOSTASY 435 



stations in mountainous regions, above the general level. The 

 anomaly is +0.021 for a depth of 42.6 km. and it is —0.006 for a 

 depth of 184.6 km. 



Computations were made to obtain the most probable depth 

 from all the gravity data for the 219 stations in the United States. 

 Where all stations were used, the depth was found to be 67. 1 km. 

 It was realized that the stations on topography which was not in 

 mountainous regions were not well adapted for the determination 

 of a depth of compensation. Owing to the low character of the 

 topography, the effect of the compensation was nearly the same 

 regardless of the distance from the station to which it was extended. 

 This is due to the fact that the attraction of an indefinitely extended 

 disk containing a certain mass but of indefinite thickness will exert 

 the same attractive force on a given mass regardless of how far 

 from the disk the mass is placed. It is assumed, of course, that the 

 attracted mass is over the center of the disk. We can see, therefore, 

 that where the compensation is nearly the same in amount under a 

 unit area for an indefinite distance around the station it would, 

 although a number of kilometers in thickness, attract the pendulums 

 at a station by the same amount regardless of the thickness of the 

 disk or column of compensation. 



The condition is different in the mountain regions for those 

 stations which are above the general level, for there the topography 

 near the station is comparatively limited in horizontal extent and 

 the attraction of the compensation will depend upon the depth to 

 which the compensation is extended. The closer to the station the 

 greater of course will be the effect of the compensation and the 

 larger will be the plus value of the anomaly observed, minus com- 

 puted gravity. Where the compensation is extended to a great 

 depth, the effect of the compensation is decreased, the computed 

 value of gravity is necessarily larger, and the anomaly tends to be 

 negative. 



After consideration of all the facts, it was decided to determine 

 the most probable value of the depth of compensation from gravity 

 data by using only the stations in the mountainous regions below 

 and above the general level. This was done and the depth resulting 

 was 95 km. 



