THE STRENGTH OF THE EARTH'S CRUST 511 



widespread weakening of the crust, both through the physico- 

 chemical effects of the exalted temperatures and the structural 

 effects of the intruded viscous fluids. 



The rapidity of the application of stress is a variable in itself 

 and furthermore has variable effects, but would seem, however, 

 to be the least important of these several factors. The movements 

 of horizontal compression and vertical warping are slow and give 

 time for recrystalhzation in the deeper crust. In this way they 

 meet a lesser resistance than would rapid stresses. Where the 

 temperatures are close to those of fusion it would seem in fact that 

 rock flowage by recrystallization, developing the gneissoid structure, 

 should demand markedly less shearing stress than the process of 

 granulation. The gnarled and twisted rocks of the Archean speak 

 of the presence beneath them of molten magmas rather than of an 

 enormous degree of compressive forces upon them. But ready 

 yielding by recrystallization in one place would permit the con- 

 centration of mashing stresses upon other localities and raise the 

 strain to that intensity needed for granulation. An enormous 

 depth of cover, such as Adams' experiments have been thought to 

 show, is not suggested by the geologic evidence, nor apparently 

 is it demanded by a completer theory. 



In fault movements and in dike or sheet intrusion accompanied 

 by the expansion of gases are two sources of rapid application of 

 forces. It is probable, however, that their deformative action is 

 confined to the outer ten miles of the crust, and their consider- 

 ation need not detain us in the evaluation of those factors of 

 strength which concern the crust as a whole. 



Summing up the conclusions from these various lines of evidence, 

 physical and geological, it is seen that they suggest a rapid increase 

 of strength with depth, then the gradual passage into a deep zone 

 of lowered strength. The limits and values, however, are variable 

 with time and place. Such a distribution of strength as is indi- 

 cated by these independent lines is in accord with the interpretation 

 of the geodetic evidence showing the existence of crustal compe- 

 tence to support heavy loads over certain limits of area, coexist- 

 ing with flotational equilibrium over much broader regions. 



