284 B. WILLIS DISCOID AL STRUCTURE OF THE LITHOSPHERE 



angles, which are determined by the relative intensities and to which 

 definite values can not be assigned, the component stresses being indeter- 

 minate and variable; but in general the direction of the resultants is 

 fixed and they form a circuit. 



Let the level of no stress difference in any vertical section be taken as 

 the axis of an ellipse; then the resultants of the horizontal and vertical 

 stresses may be regarded as forming the ellipse itself. Their direction 

 will be upward in the unloaded column and outward toward the loaded 

 column above the axis. In the loaded column the resultants will tend 

 downward and from it inward below the axis. 



To assist in visualizing these forces, assume the position of an observer 

 looking northward along the Atlantic coast of the United States. The 

 elastic stress circuit will then be oriented clockwise around an axis which 

 lies beneath the coastal plain or continental shelf, according to local 

 conditions. 



The depth at which the axis of no horizontal stress difference lies 

 depends on the completeness of isostatic equilibrium. If the equilibrium 

 is perfect according to the currently accepted form of the isostatic theory, 

 the center lies at a depth of 76 miles. Where the unloaded column is 

 deeply eroded, and therefore exhibits a defect of mass, the axis should 

 rise toward or even to the surface. It may have approached the extreme 

 upper position possible during the Jurassic-Cretaceous peneplanation of 

 eastern North America. On the other hand, it might locally have been 

 depressed below 76 miles by the outfiow of the Triassic traps, if the 

 weight of the lava-sheet were added to that of the column of lighter 

 material at a time when it was otherwise in isostatic equilibrium with its 

 surroundings. 



These considerations indicate that the resultant elastic stresses vary 

 in position and intensity with geologic changes, but their general clock- 

 wise relation in a circuit, as presented to an observer looking north on 

 the Atlantic coast, is permanent. On the Pacific coast of North America 

 the general direction of resultant stress would be anticlockwise to an 

 observer looking north. 



The elastic stress circuit in the solid lithosphere represents the unbal- 

 anced non-uniform stress which must direct any mass movement or 

 molecular movement, provided that conditions arise which make such 

 movement possible. 



EFFECTS OF ISOSTATIC STRESS ON FOLIATION 



Uniformly stressed molten rock, crystallizing under hydrostatic pres- 

 sure, assumes a holocrystalline structure like that of granite. Unequally 



