UNDERTHRUSTING 



349 



Compensatory Flow and TTnderthhusting 



Let ABC, figure 9, be an elevated land area in isostatic equilibrium 

 with the area CD, the surface of which is at sealevel. Let the prism 

 ABCE be transferred to CD by erosion. As a result of the disturbance 

 of equilibrium, CD is depressed to CFD, and the mass of the prism CDF 

 is equal to that of the prism ABCE, though the volumes may differ, 

 owing to the less density of the newly laid sediments. The profile ABC 

 is reduced and tends to assume the form EC. The profile EC is, however, 

 never attained, owing to continuous compensation as the transfer of load 

 proceeds. For purposes of discussion, we may assume that compensation, 

 instead of proceeding pari passu with the transfer, begins after the latter 



Figure 9. — Diagram illustrating the Development of the curved Thrust, PQ, as a Result 



of isostatic Compensation 



has been completed, and that finally balance is restored, with no more 

 residual than initial strain. 



Compensation is effected by a deep transfer of a mass from the region 

 under CD to the region under CE, and this mass is the same as that 

 which has been moved from the left to the right of E at the surface. The 

 deep transfer is effected by viscous flow of solid rock. The depth at which 

 this transfer takes place is unknown, except that it probably lies within 

 the limits of the zone of compensation, which may be taken at 75 miles. 

 The viscosity doubtless diminishes with increasing temperature, and at 

 some particular unknown temperature the curve of decreasing viscosity 

 probably bends abruptly. Below the level marked by this temperature we 



