THE STRENGTH OF THE EARTH'S CRUST 739 



small in comparison with the depth of the loaded surface. Then 

 the upper half of the stress-diagram will lie within the lithosphere. 

 The lower half of the curve would also have to lie within the elastic 

 competence of the crust for corresponding depths. Suppose the 

 plane to lie near to either boundary of the lithosphere. The case 

 now approaches that of a surface load, one side of the stress- 

 diagram becomes largely cut off and the other increases. The 

 exact analysis is of course difficult and will not be attempted. 



Assume the loaded plane to be at a depth of 61 km., half the 

 depth of the lithosphere. The strength of the middle would not 

 then be utilized for support of the load. For a wave-length up to 

 100 km. Fig. 17 shows fairly well the distribution of stress and it 

 would be contained mostly within the middle of the lithosphere. 

 For wave-lengths of 200 km., however, the margins of the litho- 

 sphere would be subjected to greater strain than the interior, the 

 stress-diagram would be modified toward that existing in a loaded 

 beam, and, if the margins are weak, the structure is poorly adapted 

 to support the load. If the loaded plane is at greater depth, the 

 same wave-length will throw a greater proportion of the strain 

 upon the asthenosphere and the deeper parts of the lithosphere. 

 If these are incapable of supporting the resulting stresses, again 

 a modification of the diagram would occur, involving bending 

 moments in the stronger part of the crust. 



If, now, it be assumed that the upper half of the lithosphere is 

 decidedly stronger than the lower half and that the maximum 

 strength is at some depth below the surface, it is seen that the 

 maximum outstanding masses which the crust could carry would 

 be disposed in the outer quarter or third of the lithosphere. But 

 this is just what was found to be the case as the result of the studies 

 made in Part V. Therefore the accordance between the geodetic 

 evidence and the consequences of the assumption raise it to the 

 dignity of a presumption. It may be taken as a working hypothe- 

 sis that the greater outstanding masses are limited in their positions 

 by the limitations of crustal strength. Mental reservation must 

 be made, however, as to the possibility of other more important 

 determining factors, such for example as the nature of igneous 

 activity, in limiting the zone of large outstanding masses. An 



