GEOLOGY: B. WILLIS 
379 
to complete isostatic equilibrium was reached. But the present de- 
parture from equilibrium is not of such a magnitude, nor is it likely 
that there has been so great a departure during any stage of geologic 
time known to us. The stresses due to differences in weight of adjacent 
masses of lighter and denser material, even when they are intensified 
by erosion of broad surfaces to a peneplain and by correlative loading 
of basin margins, are regarded as quite incompetent to overcome the 
strength of underlying rock masses, whose rigidity increases with load. 
Nevertheless, the effort toward isostatic equilibrium produces non- 
uniform stresses which are long enduring and persistent in direction. 
In any mass which was lighter than its environment the direction of 
least stress would be vertically upward and the directions of greater 
stresses would be horizontal. Under any mass which was heavier than 
its environment the direction of greatest stress would be vertical and 
the directions of least stresses would be horizontal. If the Hghter and 
heavier masses be in elastic continuity with one another, the upward 
stress in the former and the horizontal stress under the latter would 
combine to form a resultant which would rise in a curve from below 
upward. Thus the zone embracing the adjacent parts of both columns 
would be subject to a curved stress, which would proceed outward and 
upward from under the heavier mass. Stresses of this character appear 
to be a necessary result of a lack of equiHbrium among the masses or 
elements of the elastic lithosphere. 
The law of crystalline orientation in response to stress, as developed 
by Van Hise, Leith, and Bowen, requires that foKation shall grow in 
the direction of least stress. If, then, rocks crystallize or recrystallize, 
through the influence of rising temperature and altering solutions, in 
adjacent masses which are pervaded by the curved stress due to 
isostatic inequilibrium, the resulting foliation should be oriented in a 
curve corresponding to that stress. The foliation should be horizontal 
under the heavier element, should approach verticality under the lighter 
element, and should curve upward continuously from under the former 
toward the surface of the latter. 
The quantitative value of the least stress which will orient foliation 
under the exceedingly complex conditions of metamorphism has not 
been determined. It must not be confused with the differential stress 
required to produce mechanical flowage. Adams has demonstrated the 
latter to be very great. The former is a stress which is within the 
elastic limit and may safely be regarded as relatively very small. 
More important than amount of stress are the characteristics of 
non-uniformity, as distinguished from hydrostatic pressure, persistence 
