658 JOSEPH BARRELL 



The method of attack is largely one of exclusion. By showing 

 what hypotheses cannot apply, the way is prepared for conclusions 

 in better accord with the fields of fact and theory. 



The results show that conditions of isostatic equilibrium cause 

 the light and high segments to press heavily against the adjacent 

 lower and heavier ones, most heavily above. The tendency is 

 consequently for the high areas to spread with a glacier-like flow 

 over the low areas. This tendency, however, is effectively resisted 

 by the strength of the crust. Upon the disturbance of equilibrium 

 by erosion and deposition there are two kinds of stresses produced 

 which tend to restore equilibrium. The first is a tendency of the 

 heavy column to underthrust the lighter, but it could never produce 

 compression and folding at the surface. This force would be most 

 effective under the hypothesis of great crustal weakness, so that 

 the vertical stresses could be transmitted in a horizontal direction 

 within the lithosphere as in a fluid. Even in that case, however, 

 it would not be the dominating force. The actual isostatic move- 

 ments consist of a rising of the eroded areas, a sinking of those 

 which are loaded. This involves shear or flexure around their 

 boundaries. The columns must be large enough so that the excess 

 or deficiency of mass can become effective in producing deformation. 

 When the accumulating vertical stresses have overcome the strength 

 of the crust, the excess pressure from the heavy area is transmitted 

 to the zone below the level of compensation. This deep zone is in 

 turn the hydraulic agent which converts the gravity of the excess 

 of matter in the heavy column into a force acting upward against 

 the lighter column and thus deforms the crust of the eroded area. 

 By this means even the continental interiors are kept in isostatic 

 equilibrium with the distant ocean basins. This implies a great 

 depth and thickness to the zone of plastic flow. Although it must 

 be plastic under moderate permanent stresses, this does not imply 

 by any means a necessarily fluid condition, and fluidity is disproved 

 by other lines of evidence. 



The zone of compensation, being competent to sustain the 

 stresses imposed by the topography and its isostatic compensation, 

 must obey the laws pertaining to the elasticity of the solid state 

 and is to be regarded therefore as of the nature of rock. Conse- 

 quently there may be extended to all of it the name of the litho- 



