THE STRENGTH OF THE EARTH'S CRUST 667 



crust at the bottom than at the top. Such a lesser rigidity may be 

 granted, but it is seen then that the landward undertow would be 

 greatest at the bottom and could not advance above a depth 

 indicated on the diagrams by T. At this point the stress is of the 

 opposite sign but of the same value as for the state of isostatic 

 balance in case A. If seaward flow did not take place at this level 

 in the first case, landward flow could not take place in the second. 



For the extreme case where isostasy is completely destroyed by 

 surface leveling, no water body remaining, T will rise upward to a 

 depth equal to one-half the depth of the zone of compensation. 

 If the surface of complete compensation be 76 miles deep, this 

 gives a minimum depth of 36 miles. For the undertow to reach 

 this height implies, however, not only the limiting case of complete 

 destruction of isostasy, but a crust only one-half as rigid at depth 

 T as at the surface and a previous state of expansive surface 

 stress as great as the outer crust could bear. On the other hand, 

 if tangential pressures due to centrospheric shrinkage should 

 co-operate with the stresses tending to restore isostatic equilibrium, 

 underthrust would become more effective below, but overthrust 

 would also become effective above. 



The disappearance of isostatic compensation at a certain level 

 means the disappearance of notable heterogeneity in the earth- 

 shell below, as argued in Part V (pp. 446-48). One of the possible 

 suppositions to explain this is to suppose that this shell is weaker 

 than the crust above and therefore the lateral thrust due to an 

 assumed initial heterogeneity would cause a lateral flow, a density 

 stratification, and a resulting disappearance of the postulated 

 heterogeneity. This supposition of a weaker zone finds support 

 in other lines of evidence. Therefore, although some lateral flow 

 at the base of the lithosphere may occur during the restoration of 

 isostatic equilibrium, it is to be expected that the bulk of such 

 flow will be below, for there the substance is more plastic and the 

 lateral stress is throughout at a maximum. 



Let attention be given next to the vertical as contrasted to the 

 lateral unbalancing brought in by the destruction of isostatic 

 equihbrium. The land-column becomes lighter, the sea-column 

 heavier, by amounts which are shown in the vertically lined stress 



