THE STRENGTH OF THE EARTH'S CRUST 671 



of weakness below, still only the basal part below the point T 

 would be competent to give a landward movement during the 

 restoration of isostatic equilibrium. But now it is seen that 

 in the asthenosphere the lateral pressures are transmitted 

 with greater amount, from a greater distance, and with a greater 

 cross-section. The zone is one without notable isostatic compen- 

 sation within it and is presumably more plastic- than the 

 basal part of the lithosphere. Therefore there is good reason 

 to believe that the subcrustal undertow is restricted to the 

 asthenosphere. 



The forces actually needed to produce flowage would be in 

 reality but a fraction of those indicated in Fig. 13B as existing in 

 the asthenosphere. The reason is that the greater part of the 

 vertical forces is consumed in producing flexure and shear in the 

 lithosphere. Only a residuum is needed to produce a slow plastic 

 flow in the shell below. For that reason broken lines are used in 

 that part of the stress diagram. The energy consumed within 

 the lithosphere by its deformation will be nearly independent of 

 the breadth of the columns; it will actually tend to become some- 

 what less with breadth because flexure on large radii will be favored. 

 The energy consumed in the asthenosphere will, on the other hand, 

 increase with the breadth of the columns, but will be spread over 

 a greater area. The temperature effect due to the absorption of 

 energy would appear to be a minor factor, for it cannot exceed that 

 energy which is supplied by the average vertical stress-difference 

 multiplied by the vertical distance moved. The average vertical 

 stress-difference will be the mean between that at the beginning 

 of movement and that residual stress remaining after the movement 

 is completed. 



In determining the scale of the diagrams of Fig. 13 the following 

 data were chosen. The land-column was taken in A as having a 

 surface elevation of 1,000 m. and a density of 2.70; the sea as 

 3,000 m. deep, and the rock below as possessing a density of 2.77. 

 The sea- water has a density of 1.03. These relations give an 

 isostatic balance at a depth of 122 km. In B, erosion of the land 

 to sea-level is supposed to have taken place and the sediment 

 spread with same unit weight over the sea-column that it had as 



