YIELDING OF THE EAETH 's CRUST BOWIE. 245 



There have been great horizontal movements of material in areas 

 of uplift, but these seem to be confined to strata of very moderate 

 thickness, especially as compared with the thickness of the earth's 

 crust. These movements were very probably incidental to the verti- 

 cal movement which caused the uplift and maintained the isostatic 

 balance of the blocks. Or the horizontal movements may have been 

 partly due to distortion and rupture of strata incidental to the giv- 

 ing way or yielding of the earth's crust under sedimentary loads. 

 The amount of material involved in the horizontal movements at the 

 surface, which are undoubtedly local in character, is small as com- 

 pared with that involved in the horizontal movement which takes 

 place below the earth's crust to maintain the isostatic equilibrium. 



THE DEPTH OF ISOSTATIC ADJUSTMENT. 



It is a debatable question as to the depth at which the isostatic ad- 

 justment takes place ; that is, the depth at which the horizontal move- 

 ments occur. If we should draw a diagram showing the stress dif- 

 ferences at various depths below sea level which must obtain be- 

 tween a block under an elevated area and a block whose surface is 

 at or close to sea level, we should find that the stress difference would 

 be directed from the elevated block toward the other one until the 

 depth of compensation is reached. Now, if there should be some 

 erosion of even a small amount of material from the elevated block 

 and a corresponding deposition on the lower one, the stress differ- 

 ence would still be in the same direction for the upper part of the 

 blocks but would be in the opposite direction or from the low block 

 toward the elevated block near their lower ends. A diagram, show- 

 ing these stress differences, indicates that there can be no transfer 

 of material horizontally from the area of sedimentation back toward 

 the area of erosion within the first few miles of the earth's surface. 

 Undoubtedly, the transference can not be much above the depth of 

 compensation, and is probably below. 



It is not known whether the horizontal movements of the mate- 

 rial to maintain the isostatic balance take place near the depth of 

 compensation or whether it is lower down in the earth. This is a 

 matter that is of small moment, geologically or otherwise. The prin- 

 cipal problem is to locate approximately the depth at which the 

 material changes from one which resists horizontal movements due to 

 stress differences to one which is yielding to such stresses. It is some- 

 what uncertain as to whether or not the depth of compensation is actu- 

 ally the boundary between the resisting matter and the matter that 

 is yielding. It is certain that no compensation can be located below 

 the resisting material, but it is possible for the compensation to 

 occur in the upper part of the block of resisting matter, and 



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