THE STRENGTH OF THE EARTH'S CRUST 665 



small at and below the base of the lithosphere. On account of the 

 incompleteness of local compensation, the irregularities and uncer- 

 tainties of the actual facts of nature, the Gordian knot of a solution 

 may be cut by simply assuming for present purposes the form 

 of diagram given by hydrostatic pressures due to a compensation 

 uniformly distributed. The approximate stress-differences will be 

 given by taking one-fourth of the values given by the hydrostatic 

 pressures. This transfers the problem from the difficult field of 

 zonal harmonics to the simple one of hydrostatics, and perhaps does 

 not introduce errors greater than those involved in the differences 

 between nature and the postulates which form the foundation of 

 the solution by zonal harmonics. This hydrostatic diagram is 

 shown accordingly in Fig. 13. 



It is held by the advocates of extreme isostasy, however, that 

 for long-continued stresses the crust is very weak; in other words, 

 the elastic limit is low, and slow plastic deformation readily 

 occurs which tends to dissipate the stress-differences and re- 

 establish isostatic equilibrium. To the extent to which this 

 is true, the real diagram of lateral stresses' would approach the 

 hydrostatic diagram here given and measure the forces producing 

 plastic flow. 



It has remained, however, for the opponents of the hypothesis 

 of local and nearly perfect isostasy to point out, what is here 

 illustrated graphically, that the extreme theory requires a belief 

 in vertical weakness but lateral strength. If it were not for lateral 

 strength the land-column would crowd against the sea-column, 

 more at the top than at the bottom. Flowing out with a glacier- 

 like motion over the upper part of the sea-column, the land-column 

 would settle at the top and become shorter. This in turn would 

 bring about a vertical elevatory pressure against its bottom, the 

 column would rise, lateral creep would continue with equal pace, 

 and the end result would be a density stratification in which the 

 continental crust would come to overlie the oceanic crust. The 

 limit of such an action would be given by the decreasing surface 

 gradient, this finally becoming so gentle as to stop the glacier-like 

 flow. The lack of such an effect implies of course that the lateral 

 stresses of the outer part of the lithosphere lie within the elastic 



