312 proceedings: geological society 



isostatic compensation locally or whether they are due to abnormally 

 heavy or light materials near the station but in the region below sea 

 level. This local deviation from normal in the gravity may be due 

 in part to an actual deviation from perfect isostasy and in part to the 

 abnormal densities near the surface. 



It is reasonably certain that the anomalies cannot be due entirely 

 to the use of erroneous densities for the topography, that is, the mate- 

 rial which is above sea level, nor to the method of distributing the com- 

 pensation with respect to depth. The largest anomaly, which is 

 0.093 dyne, is at Seattle and this is the equivalent of 3000 feet of ma- 

 terial of normal density. As a matter of fact, Seattle is located near 

 the coast and that station has an elevation of only 74 meters. We 

 must, therefore, conclude that the anomaly at this station is caused by 

 conditions which exist below sea level. 



It has been found, as a result of the investigations, that a regional 

 distribution of compensation out to a distance of 58.8 kilometers from 

 a station is just as probable as the local distribution of the compensa- 

 tion. It has also been found that a regional distribution out to a 

 distance of about 167 kilometers from the station is not so probable 

 as the regional distribution to the smaller distance mentioned above 

 or as a local distribution. There is no relation between the gravity 

 anomalies and the character of the topography. There is, however, 

 a decided relation between the gravity anomalies and certain geological 

 formations. There is a very strong tendency for gravity to be light 

 and the anomalies negative for stations located on the Cenozoic for- 

 mation and the opposite is true for stations on the pre-Cambrian 

 formations. 



If the compensation were assumed to be distributed uniformly with 

 respect to depth, then the depth of compensation, as derived from 

 gravity data alone, is 95 kilometers. This agrees fairly well with the 

 depth obtained from deflections of the vertical, but there is no geodetic 

 evidence that the compensation is distributed uniformly with respect 

 to depth. It is possible that some other method of distribution is 

 the true one but it is difficult or practically impossible ever to discover, 

 at least from geodetic evidence, what the true method of distribution is. 



In the other countries data as detailed as for the United States were 

 not available, but such tests as could be made indicated that isostasy 

 in them was about as perfect as in this country. It is hoped that 

 geodesists in the other countries will reduce their gravity stations for 

 topography and isostasy by the same method as that used by the United 

 States Coast and Geodetic Survey or by a similar one in order that more 

 exhaustive tests may be made of the isostatic condition throughout 

 the world. 



Discussed by White. 



David White: Discussion of gravity anomalies from the stratigraphic 

 standpoint. (No abstract.) Discussed by William Bowie. 



H. E. Merwin, Seer dor i). 



