OUR PRESENT KNOWLEDGE OF ISOSTASY 443 



We must go below sea-level and below the beds of the oceans 

 to find the cause of the anomalies. This necessarily takes the 

 geodesist into the realm of geology, and it is there that he needs 

 the assistance of geologists who are familiar with the geological 

 history of the outer portions of the earth's lithosphere and of the 

 existence of materials that deviate from normal density. 



As to the process by which isostatic adjustment occurs, we must 

 consider this largely a matter of speculation. There is no geodetic 

 evidence on the subject. No one can say that he knows. Of many 

 theories or opinions one is inclined to accept that which appears 

 to be most reasonable to him. 



We may summarize the contents of this paper as follows: We 

 have sufficient geodetic data to prove that, for large areas, such as 

 that of the United States, considered as a whole, the condition of 

 isostasy is nearly perfect. The data also prove that the local devia- 

 tions from perfect isostasy are not more than about 25 per cent 

 an an average. If, however, we consider that the abnormally 

 heavy or light material which is found under a number of gravity 

 stations is compensated for by deficiencies or excesses of density 

 lower down in the lithosphere, we may assume that the deviation 

 locally from perfect isostasy is of the order of 10 or 15 per cent 

 rather than of 25 per cent. The writer believes that this assump- 

 tion is justified. 



There is no geodetic evidence to show whether or not regional 

 distribution out to a distance of 58.8 km. from a station is more 

 probable than the local distribution immediately under each topo- 

 graphic feature. There is geodetic evidence which makes the 

 local distribution of compensation or the distribution regionally 

 within 58 . 8 km. more probable than the regional distribution out 

 to a distance of 166.7 km. from the station. It may be possible, 

 though the writer believes it improbable, that there is a distance 

 between 58.8 km. and 166 km., which would give a more probable 

 regional distribution than the distances tested. 



The geodetic evidence favors about 96 km. as the depth of 

 compensation if the compensation is assumed to be distributed 

 uniformly between the earth's surface or sea-level and the depth 

 of compensation. There is no geodetic evidence to show that any 



