314 JOSEPH BARRELL 



the earth should be sought out and measured in detail. For this 

 work it would seem that many new stations would have to be 

 established; in groups so as to reduce the errors of each locality; 

 in sets so as to attack particular phases of the problems. For 

 example, it would appear that gravity stations should be located 

 in pairs close together and of as great a difference in elevation as 

 possible. Certain stations should be located within areas of 

 plateaus spared by circimidenudation, such as the Cumberland and 

 Allegheny plateaus; others should be located in the broad erosion 

 basins. Deflection stations should be located on the lines separat- 

 ing regions of erosion from those of circumdenudation, and also on 

 the Hues separating areas of up warp from those of downwarp. A 

 network should inclose, finally, all centers of marked gravity 

 anomaly or topographic deflection. Such an increase in the number 

 of stations would permit the introduction of simple hypotheses of 

 variable depth and rate and regional limits of compensation. But 

 such an extensive program is within the reach only of some research 

 institution. It needs the co-operation of geologists and geodesists. 

 The location of stations with respect to surface features and their 

 geologic history should be controlled by the geologist. The density 

 of the rocks to the limits exposed by the structure should also be 

 determined by him. The geodesist, on the other hand, should 

 seek out the hidden heterogeneities in the crust and guide the 

 details of the work. 



[To be continued] 



