Uplift and Subsidence. 7 



place, but that the average distribution for any large 

 area such as that of the United States approximates 

 uniform distribution. One can see that, if we took the 

 mean of all possible distributions, approximately uniform 

 distribution would result. 



Distribution of compensation horizontally. 



An attempt was made by the writer to show whether 

 it was better to have the distribution of the compensation 

 directly under the topographic feature or to have it distri- 

 buted regionally within certain limits of distance from the 

 station. The evidence seems to be strong that the distri- 

 bution could not be extended to as great a distance as 

 100 miles in all directions from the station, but that 

 results as accordant, or nearly so, were obtained when 

 the distribution was extended about 40 miles in all direc- 

 tions from the topographic feature as when the compen- 

 sation was assumed to be local. 



Effect of ignoring compensation for small areas. 



An attempt was made to show whether gravity anoma- 

 lies could be more nearly eliminated if we assumed that 

 local areas near gravity stations did not have their topo- 

 graphic features compensated. 



Computations of the effect of the isostatic compensation 

 out to distances of 18 miles and 36 miles in all directions 

 from gravity stations were made. These effects were 

 subtracted from the total effect of compensation of all 

 topography. The results indicated most clearly that the 

 gravity anomalies were largely increased by this method, 

 showing conclusively that we cannot ignore the compen- 

 sation of the topography for even small areas. Of course, 

 we may ignore the isostatic compensation of the topog- 

 raphy near a station where the relief is very low. But 

 the tests were made for stations having elevations greater 

 than 1000 meters (about 3300 feet). 



Relation between isostatic anomalies and topography. 



Tests have been made to show^ whether there is any 

 relation between the sign and size of the gravity anomalies 

 and the character of the topography. No such relation 

 has been found for the isostatic anomalies but decided 

 relations have been noticed for the anomalies obtained 



