678 JOSEPH BARRELL 



if the restoration of isostasy was deferred until assisted by strong 

 tangential pressures due to centrospheric shrinkage, it might be 

 held that isostatic undertow could take place within the zone of 

 compensation and between 5 and T of Fig. 13B. If, furthermore, 

 compensation should be not uniformly distributed but taken as 

 largely concentrated in the upper part of the zone of compensation, 

 which however is contrary to the Hayfordian hypothesis, then the 

 forces making for undertow may correspondingly rise in the crust. 

 For these reasons it is seen that the previous argument from the 

 distribution of pressures is not final and that the physical conditions 

 involved in lateral flowage must also be considered. 



The only positive reason which has been advanced for seeking 

 to place the undertow within the zone of compensation is in order 

 to utiHze its viscous drag as a cause of folding. To become effective 

 the drag must be strong, the crust above by contrast weak and 

 therefore thin. The crumpling pressure on the surface of the 

 crust cannot be transmitted directly from the sinking area, as is 

 shown in Fig. 13, since the thrusting force is greatest at the bottom. 

 It must be supposed to arise from the viscous drag of the undertow. 

 But viscosity decreases the hydrostatic head with increasing dis- 

 tance from the source. Therefore, to permit a viscous flow at 

 a distance from the source of pressure implies a mobility within 

 that level of the crust which would make it wholly incapable of 

 carrying the stresses necessary to maintain its own isostatic 

 equihbrium. Therefore this level, by the very terms of the general 

 conception of isostasy, would become the bottom of the zone of 

 compensation. 



As another mechanical defect of the theory under review, it 

 is to be noted that the section of undertow taken by Hayford as 

 in the middle of the zone of compensation is not given as involving 

 more than half of that zone. This is as if a viscous fluid were 

 transmitted through a pipe in which the cross-section of pipe and 

 fluid were equal. To assume that the fluid is free to escape into 

 a region of less pressure at the far end and yet gives such a fric- 

 tional resistance against its walls as to be able to crumple up the 

 pipe is to assume that the two are of the same order of strength. 

 The materials of pipe and fluid might almost be interchanged. 



