14 W. Botvie — Relation of Isostasy to 



gravity anomaly is -0.013 dyne. This is certainly an 

 indication that the Mississippi delta is not an overload, 

 for if it were the anomaly would nnquestionably be 

 positive. 



The writer predicts that there will be a decided ten- 

 dency for the isostatic gravity anomalies to be negative 

 at delta stations. 



Contraction of column under sediments. 



In discussing the question of the delta formations, we 

 assume that as the sediments are deposited, the surface 

 of the sedimentary^ material gradually increases in eleva- 

 tion. There are cases where this is not true, for we find 

 evidences of very thick sedimentary material, all of which 

 was deposited in very shoal water. In such a case we 

 have to assume that there must have taken place an 

 increase in the density of the material of the column as 

 sedimentation progressed. It is only by doing this that 

 we can have a column under sedimentary material in 

 equilibrium, because the material deposited is lighter than 

 the material whose space it occupied. This change in 

 density in the column is entirely apart from the isostatic 

 adjustment. It must be due to some chemical or physical 

 action of which we have no knowledge. It is possible that 

 the decrease in density in the material in the column mth 

 resulting subsidence of the surface began before sedimen- 

 tation started and really decided the region in which 

 sedimentation should occur. 



If we should have sediments to the depth of 40,000 

 feet, all deposited at about sea-level, and if the column 

 were in equilibrium before sedimentation began, then we 

 must have had an increase in the density of the material 

 of the column and a consequent shrinking of the original 

 material which is equivalent to about 8000 feet of sedi- 

 mentary material. This sedimentary material is not 

 above the sea-level surface and, as the sediments are not 

 able to lower the former surface of the column except by 

 their weight, we must assume that the space occupied by 

 the 8000 feet of sedimentary material must have resulted 

 from the contraction of the column. There undoubtedly 

 is a small amount of contraction in the original material 

 of the column, due to the pressure of the sedimentary 

 material on the column, but this contraction would be a 

 small percentage of the total contraction necessarv to have 



