Uplift and Subsidence. 19 



than the material that is eroded from the surface. If we 

 should have 10,000 feet of material on the average eroded 

 from the mountain mass, we should expect an equivalent 

 mass of material to flow by isostatic adjustment into the 

 column, probably at the base of the column. This 

 material would, undoubtedly, be at least ten per cent 

 heavier than the material eroded from the mountain. 

 Therefore the mountain surface should be lowered at 

 least 1000 feet, if 10,000 feet of material is eroded from it. 

 Aside from these changes in the elevation, I do not know 

 of any others which are caused by isostatic adjustment. 

 They must be due to other causes which, I believe, are 

 decreases and increases of density in the isostatic shell. 

 They are not believed to be due to transference of material 

 horizontally from one region to another. Of course, I am 

 speaking here of comparatively large areas for we do have 

 evidence of transference of material in local areas. 



Zone of horizontal movement. 



Barrell in the ''Strength of the Earth's Crust'' pre- 

 sented arguments in favor of a zone of weakness below 

 the isostatic shell which he calls the asthenosphere. He 

 held that the isostatic movement, horizontally, takes place 

 in the asthenosphere. Willis agrees with Barrell on this 

 point. The writer has no yery clearly defined ideas on 

 this subject, but he believes that the views of Barrell and 

 Willis are reasonable and justifiable. 



If the earth's crust were weak enough to permit isos- 

 tatic adjustment to take place comparatively near the 

 surface, then we should expect that the materials would 

 be so weak that masses of different densities would tend 

 to flatten out and adjust themselves in strata, each with a 

 uniform density. 



That the isostatic compensation extends to considerable 

 depth is an indication of a certain amount of resistance, 

 horizontally, to movements. It seems probable, there- 

 fore, that the horizontal movements necessary to effect 

 the isostatic adjustment take place below the isostatic 

 shell. Whether the flow is very deep seated or in the 

 outer portions of the asthenosphere cannot, of course, be 

 determined, but the writer believes that the flow takes 

 place .just below the isostatic shell, in the outer portion 

 of the asthenosphere. 



