654 LOUIS VESSOT KING 



curve and the axis xx. Thus for complete stability the entire series 

 of points representing stress-differences beneath a continental eleva- 

 tion must fall in this latter region. It is thus evident that the 

 existing theories of isostasy should, in considering the equilibrium 

 of stresses called into existence by continental elevations and moun- 

 tain ranges, take account of a " compensation of plasticity''' — i.e., of 

 the increased stiffness or resistance to deformation — of the under- 

 lying rock when submitted to greater hydrostatic pressure. With 

 the reservation already made as to the possible influence of tem- 

 perature, we have a considerable basis of evidence in favor of the 

 conclusion that at any time in the past history of the earth continen- 

 tal elevations might have attained much greater altitudes above sea- 

 level than any at present existing, without giving rise to stress- 

 differences in the earth's interior sufficiently great to have caused 

 rupture or breakdown, owing to the much increased "resistance to 

 flow" set up in the rock material by the great pressure of the over- 

 lying crust. We should conclude also that, in the event of flow 

 occurring, the region of flow would be confined to a region of the 

 earth's crust comparatively near the earth's surface. The increas- 

 ing limiting stress, with pressure characteristic of rock material 

 made to flow as in Adams' and Bancroft's experiments, leads one 

 to the conclusion that great movements of the earth's crust have 

 for the most part always proceeded by extremely slow and con- 

 tinuous adjustments to pressure conditions, and not, as supposed 

 by some geologists, 1 by a series of cataclysmal collapses of the type 

 which would occur if the material of the earth's crust possessed in 

 all circumstances a unique and definite limiting strength analogous 

 to that obtained by crushing a specimen, unsupported laterally, in 

 a testing machine. The further consideration of these problems 

 must, however, be left over for further discussion. Enough has 

 been said to make it evident that the results of Adams and Bancroft 

 have provided much needed data in the light of which many of the 

 existing theories of geodynamics may require considerable modi- 

 fication. 



1 G. A. J. Cole, Presidential address delivered before the geological section of the 

 British Association, Manchester meeting, 1915, B. A. Report, pp. 403-20. 



