372 B. WILLIS ROLE OF ISOSTATIC STRESS 



111 spite of this disagreement with my colleagues as to the degree ot 

 isostatic compensation which may actually exist, I recognize that there 

 is an isostatic stress which is set up by erosion and deposition or by erup- 

 tion of basic lavas. It will suffice in this brief statement to consider only 

 erosion and deposition. The question which I propose to discuss relates 

 to the competency of that stress to cause movement in the lithosphere, or 

 possibly only to direct movements which arise from other causes. 



Curve of isostatic Stress 



It may be agreed that the lithosphere, at least that part of it which lies 

 within the zone of compensation, is highly elastic, like a musical string, 

 and responds readily to changes of pressure or temperature, but within 

 elastic limits. If so, the effect of erosion in unloading a mountain region 

 must be similar to that produced by taking weights off a spring balance ; 

 and, similarly, the effect of loading by the deposition of sediment in a 

 depressed basin must be like that produced by placing weights on a 

 spring. When these two effects occur in adjacent parts of a continuous 

 mass, the downward stress in the one will be linked with the upward 

 stress in the other in such a way that there will be a tendency for move- 

 ment to take place from the loaded toward the unloaded section. The 

 lines of continuous stress will thus form a curve, which will extend down- 

 ward to a certain depth in the loaded section and thence proceed hori- 

 zontally or diagonally upward toward the unloaded section, approaching 

 the surface in a steeply inclined or vertical direction. The curve thus 

 formed may be called the curve of isostatic elastic stress. 



Xow if the isostatic elastic stress be adequate to cause movement, there 

 will be a flow of material from the loaded to the unloaded section, and 

 inasmuch as the stress originates in the unloaded section, the flow would 

 be in the nature of an undertow. This is the movement postulated by 

 Hayford, who, in that concept, follows the original idea of Button. On 

 the other hand, if the isostatic stress remain within the elastic limit, it 

 will not cause movement. Nevertheless, it may direct molecular dis- 

 placements due to other causes. 



Incompetence of isostatic Stress to produce Motion in the 



Lithosphere 



Since Adams has shown that the, strength of rocks increases notablv 

 under pressure, and that the lithosphere is therefore much stronger than 

 we were wont to consider it, the comparative weakness of isostatic stress 

 has become very evident. According to Adams's experiments, the rocks 



