236 JOSEPH BARRELL 



which reach above o . 040 are much smaller. The limits of regional 

 isostasy appear then to vary with the amount of the load. Well- 

 defined areas 200 to 250 miles in breadth may stand vertically 800 

 to 1,600 feet on the average from the level, giving isostatic equilib- 

 rium, and their central portions reach still higher values. They 

 represent the limits of regional isostasy discussed in an earher part. 

 But these are superposed on broader areas which may extend for a 

 thousand miles or more and lie as much as 400 to 800 feet either 

 above or below the level for equilibrium. Stresses given by loads 

 of this order are then not restricted in area to the limits set for 

 higher values. 



The size of the areas of intenser stress reveal the capacity to 

 which the earth can carry mountain ranges uncompensated by 

 isostasy. The size of the areas of weaker stress shows the capacity 

 of a considerable portion of a continent to lie quiescent while the 

 surface agencies carry forward their leveling work. This is the 

 present state of this particular continent after a geologic period of 

 world-wide notable vertical movement and adjustment. It is not 

 likely, therefore, that these loads measure the maximum stress- 

 carrying capacity of the earth. They may be more in the nature 

 of residual stresses which the earth can hold through periods of 

 discharge of stress. East of the Cordillera there has been but Httle 

 local differential movement and these areas have lain in crustal 

 quiet for long geologic ages, being subject only to broad and uni- 

 form crustal warping of moderate amount. It is to be presumed, 

 therefore, that the strains which exist in such regions by virtue of 

 the regional departures from isostasy are of ancient date and well 

 within the limits of crustal strength. 



It would seem probable for such conditions, from the stand- 

 point of mechanics, that the zone of compensation is not sharply 

 limited, with its implication of marked lowering of rigidity at its 

 base; nor the distribution of compensation uniform to the base. 

 It seems more probable that the abnorrnalities of density and the 

 resultant strains should fade out through a considerable depth 

 more after the manner suggested by Chamberlin. 



[To be continued] 



