66o JOSEPH BARRELL 



foredeeps show both the greatest depths of water and the steepest 

 descents. The Chilean coast, for instance, at lat. 25° S., slopes 

 from the Andes to a depth of 7,500 meters with a submarine grade 

 of one in eight. Under the hypothesis of nearly perfect isostasy, 

 which will be favored in this discussion, this would be taken to 

 show the contiguity of areas in the crust of markedly unlike density. 



Let the slope between such areas be regarded as a thick parti- 

 tion between two columns, each in isostatic equilibrium. These 

 rest then upon the substratum below the zone of compensation 

 with the same pressure and stand vertically in equilibrium. 



In so far as the rock witliin the crust is subjected to mere cubic 

 compression, equal in all directions and increasing with depth, 

 there is no distortional force. In so far, however, as side pressures 

 in one column are not balanced by equal side pressures from the 

 adjacent columns, there is a stress-difference which does produce 

 a distortional strain. If the stress-difference exceeds the elastic 

 limit a permanent deformation results which reduces the stress 

 and eases the strain. It is the plan of this paper to discuss the 

 nature of the stress-differences on the partition separating two 

 contiguous columns of the crust, of markedly unlike density; 

 first, when these are in isostatic equilibrium, and second, when 

 not in such equilibrium. Fig. 13 is drawn to show graphically 

 these relations. 



The land-column of the crust is marked M; the submarine 

 column is N; O is the earth-shell below the zone of isostatic com- 

 pensation; P is the column of sea- water. The vertical partition, 

 between the unlike columns stops in reality, according to the hy- 

 pothesis, at the bottom of the columns. It is here extended down 

 through the earth-shell 0-0 in order to discuss the deformation 

 which would take place in the latter shell. M and N represent 

 what is here called the lithosphere; 0-0 the zone which it is 

 proposed to call the asthenosphere. 



In case A, isostatic equilibrium is assumed and the pressures 

 of the two lithospheric columns are equal upon the asthenosphere. 

 But, assuming for the moment that the vertical pressures are freely 

 transmitted as lateral pressures, it is seen that a marked horizontal 

 unbalanced pressure is produced by the land-column against the 



