ISOSTATIC ADJUSTMENT OF DELTA FORMATIONS 277 



the gravity anomalies tend to be negative in sign and are of about the 

 average size of the anomalies on other Cenozoic material. 



If we assume that the density of the delta material is about 2.2, it is 

 evident that the material now occupying the volume once occupied by 

 the water is 2.2 times as heavy as the water which formerly occupied the 

 space. If this added mass were an overload on the earth's crust, then we 

 should certainly expect the gravity anomalies to be positive, owing to the 

 increased attraction of the added material. The fact that the gravity 

 anomalies tend to be negative and often large would indicate that there 

 is considerable depth to the delta deposits, and that probably the column 

 under the delta formation has normal mass and is in isostatic equilib- 

 rium. This question of the delta material and the strength of the earth's 

 crust is treated in a paper 5 by the speaker which has recently appeared. 



Of eight gravity stations recently established by the Coast and Geo- 

 detic Survey on or very near to the delta of the Mississippi Eiver, four 

 have negative and four positive anomalies. The average of these anom- 

 alies with regard to sign is — 0.006. If we include the station at New 

 Orleans, previously established, the average is — 0.008. This is evidence 

 against the Mississippi Delta's being an uncompensated load. 



Completeness oe isostatic Adjustment 



If we find that there is no relation between the topography and the 

 deflections of the vertical or gravity anomalies, and if we can give a 

 rational interpretation of the sizes and signs of the anomalies found on 

 the Cenozoic and Precambrian formations, and if there is no definite rela- 

 tion between the signs and sizes of the anomalies and other geological 

 formations, are we not justified in assuming that, as far as the areas 

 covered by the investigations are concerned, they are in a state of equilib- 

 rium, and that isostasy within those areas is nearly perfect? It would 

 also seem to be reasonable for us to assume that other areas which have 

 not been included in our investigations, but which in their topographic 

 features and geologic formations are similar to the United States, Canada, 

 and India, are also in isostatic equilibrium. It may be that we would also 

 be justified in assuming that the crust under the ocean area is in equilib- 

 rium. It is possible that we are assuming too much in this, but it is the 

 speaker's belief that when we shall have perfected an apparatus for the 

 accurate determination of gravity at sea, we shall find the ocean areas in 

 about the same degree of isostatic equilibrium as is found for the land 

 areas. 



5 William Bowie : The relation of isostasy to uplift and subsidence. American Journal 

 of Science, vol. ii. July, 1921. 



