THE STRENGTH OF THE EARTH'S CRUST 31 



intensity. Deltas built into deep seas seem best adapted to give 

 quantitative measurements. Those of the Nile and the Niger 

 therefore are subjected to detailed study. They indicate that the 

 earth is competent over those regions to sustain stresses due to 

 sedimentation which are measured by the weight of several thou- 

 sand feet of rock, even where the load is continuous over tens of 

 thousands of square miles. Whatever response there may be is so 

 slow that the deposition is able to keep pace with subsidence and 

 maintain the load as a permanent stress of this magnitude upon the 

 crust. By analogy the conclusion may be applied to other parts 

 of the earth, and to those negative loads created by the erosion 

 to base-level of regions previously unwarped to an elevation pre- 

 sumably near to that which would give isostatic equilibrium. 

 Consequently, also, the crust should be able to bear in consider- 

 able degree the folded and overthrusted structures piled up by the 

 tangentially compressive forces which periodically operate to such 

 large degree within its outer shell. Deeper changes, involving 

 changes of density, are involved, however, in orogenic processes 

 and express themselves in vertical warpings associated with, and 

 following after, folding. This association of vertical and tangential 

 forces complicates the problem of the crustal strength needed to 

 support mountain range?. 



The measures derived from the study of deltas are more in 

 accord with those larger estimates of the strength of the crust 

 obtained by Putnam and Gilbert in 1895 from a transcontinental 

 series of gravity measurements in which was developed and em- 

 ployed for the first time the conception of local rigidity but regional 

 isostasy.' Their conclusions have been thought to be superseded 

 and controverted, however, by much more elaborate and complete 

 geodetic studies, first by Hayford, and later by Hayford and Bowie, 

 which went to show that the crust was very much weaker and in 

 much more perfect static equilibrium. 



The calculations of Hoskins tended to show also that the crust 

 within the zone of isostatic compensation could not bear perma- 

 nently loads as great as those apparently imposed by these deltas. 

 If, however, the great hydrostatic pressures within the deeper crust 



' Bull. Phil. Soc. Wash., XIII (1895), 3i-75; Jour- GeoL, III (1895), 33i-34- 



