LOCALIZED MODIFICATION OF STRENGTH IN SURFACE SHELL 313 



streaming which this has come to carry to some students. Thus changes 

 in the average relative densities of the two parts of the region of activity 

 are brought about. The mountain column will not rise quite so much as 

 it is lowered by denudation during a given interval, because relatively 

 heavy matter moves in beneath it in establishing readjustment ; and the 

 sea column will not sink quite so much as it is upbuilt, because the addi- 

 tion of a comparatively thick layer of unconsolidated, and therefore light, 

 sedimentary matter causes the column to press out from under it a rela- 

 tively thin layer of heavy yielding matter; and there will be lag in this 

 process of readjustment at every stage. The mountains will not rise nor 

 the sea-floor sink so fast nor so far as they would if the process encoun- 

 tered less resistance of a viscous nature. 



Defokmation during the Erosion-Deposition Cycle 



The process by itself will necessarily involve deformation. There will 

 be subcrustal tractions; shearing stresses will be set up; there will be 

 modifications of shape of rock bodies forming members of one or both 

 columns; and there will be changes in the temperatures to which par- 

 ticular bodies are subjected. Locally, these may be manifested more im- 

 portantly than the chief factors in the process. Again, a reasonably ade- 

 quate picture of the whole process of transfer and readjustment would 

 require a book, if indeed any such picture can yet be drawn ; but, again, 

 on the whole the movements near the surface would seem to involve 

 largely shearing along sensibly vertical planes which Leith has empha- 

 sized as a fact of observation. What it is desired to emphasize here is, 

 that the ordinary conception of denudation and deposition in adjacent 

 areas, combined with reasonably complete subterranean transfers and re- 

 adjustments in obedience to the isostatic principle at all stages, will de- 

 velop weak prisms where earlier there were strong ones, and that ulti- 

 mately these weakened prisms may gradually yield under accumulating 

 tangential elastic stress, as discussed above. 



Seismologic Indications of a weak Shell 



One other matter deserves attention here. In many respects it is the 

 most important topic touched upon in this discussion; but an adequate 

 and critical treatment of it would require greater space than the whole of 

 the present discussion, and such a treatment is better undertaken by an- 

 other. It deals with observations of greater precision, from the physicist's 

 point of view, than any ordinary geologic work. It is this : The evidence 

 of seismometry in several independent ways suggests that the material in 



