May, 1922 dynamics of the lithosphere 207 



would be set up in surrounding rocks by elevation or depression 

 at a distance without local diastrophism. The amount of 

 accumulated stress resulting in sudden adjustment by fracture 

 and displacement will depend upon (1) the intensity of the 

 lateral stresses, and (2) the plasticity of the rocks by which 

 strain and stress can be adjusted without fracture. 



Continental Creep has not entered into our studies. It is 

 not, however, too much to suppose, since we know the rocks to 

 be highly plastic, that the difference of pressure on the land side 

 and water side of the continental slope will result in creep 

 toward the ocean; the same sort of phenomena that give rise 

 to mountains. Yet if this is true, the whole process maintains 

 isostatic adjustment throughout, for anomalies of gravity 

 nowhere indicate a general excess of gravity along the con- 

 tinental platform, which lack of adjustment would require. 



Some Immediate Causes of Adjustments. — While much of the 

 evidence presented in this section on "The Crust" indicates 

 rather rapid adjustment to anomalies, it is true that there would 

 be no anomalies if the adjustment did not lag more or less 

 behind the causes which produce maladjustments. This is 

 only another way of saying that stresses and strains are cumu- 

 lative, that they may be held by the structures for some time, 

 but that ultimately the earth structure has failed to hold them, 

 as shown by Leith.^* If the strains are held a longer time and 

 become great, their release causes large and widespread adjust- 

 ments. If the stresses are frequently released, only smaller and 

 often local adjustments are necessary. When the balance of 

 stress and structural strength is almost struck, but little is 

 necessary to start adjustments. Tides, distant earthquakes, or 

 even changes in atmospheric pressure are believed to be com- 

 petent to start potential movements. 



Deltas furnish a good line of evidence of almost continuous 

 adjustment, for if adjustment did not take place rather rapidly, 

 deltas and other areas of loading would indicate a positive 

 anomaly. A positive anomaly is found at the mouth of the 

 Hudson and through the Chesapeake Bay region. But we know 

 that these areas are in a stage of subsidence, so that adjustment, 

 if not complete, is taking place. On the other hand the delta of 

 the Mississippi shows negative anomaly, due perhaps to the 



2'C. K. Leith, Structural Failure of the Lithosphere, Science, 1921, pp. 195 flf. 



