202 



SCIENCE 



[N. S. Vol. XXXIII. No. 841 



are equal, and at any less depth, X, the 

 pressure is greater in A than in B. Now 

 assume that in the normal course of 

 events a large amount of material is being 

 eroded from the high surface of column A 

 and deposited on the low surface of col- 

 umn B. After this erosion has been in 



of the two columns were at the same level. 

 During the process of erosion and deposi- 

 tion the excess of pressure in A at any 

 level above the neutral level will continu- 

 ally decrease. Similarly, at any level be- 

 low the neutral level the excess of pres- 

 sure in B will continually increase as the 



5£/\ LETVEL 



OOEAN BOTTOM 



progress for some time the isostatic com- 

 pensation will no longer be perfect. The 

 pressure at the base of B will be greater 

 than at the base of A. The pressure very 

 near the top of B will still be less than at 

 the same level in A so long as the top of A 

 remains higher than the top of B. There 

 will be some intermediate level at which 

 the pressure in the two columns is the 

 same. Call this level of temporary equal- 

 ity of pressure in the two columns the 

 neutral level. As the process of erosion 

 and deposition progresses the neutral level 

 will gradually progress upward from its 

 original position at the base of the col- 

 umns. Eventually if no interchange of 

 mass took place between the columns ex- 

 cept at the surface, and no vertical dis- 

 placement occurred in either column, the 

 neutral level would reach the surface when 

 the process of erosion and deposition 

 became complete and the upper surfaces 



DEPTH or OOMPENSAriOlM 



erosion progresses and the neutral level 

 will rise. Thus there will be established a 

 continually increasing tendency for the 

 material below the neutral level in B to be 

 squeezed over into A. If the stresses tend- 

 ing to produce this undertow from the 

 lower part of 5 to A become greater than 

 the material can stand, the flow will take 

 place as indicated by the arrow in the fig- 

 ure. If the material flows without change 

 of volume, as if it were incompressible, the 

 upper part of A and its surface will be 

 raised, the upper part of B and its surface 

 will be lowered, the neutral level will sink 

 and an approximation to the original con- 

 ditions with complete isostatic compensa- 

 tion will be reestablished. 



This is the general case of isostatic re- 

 adjustment by the action of gravitation 

 alone. Gravitation tends to produce a 

 deep undertow from the regions where de- 

 position is taking place to the regions 



