LOW-ANGLE FAULTING 37 
the resistance of the faulted strip. The slicing and secondary 
shattering would seem to weaken the superficial sheet which has 
suffered the faulting. It may be perhaps that the superficial shell, 
freer to move as a general mass than it was before slicing, while the 
lower, deeper levels have not been equally affected by what has 
taken place, now finds it easiest to slide bodily forward over the less 
movable lower portion. If this be true, it would make the rupturing 
by the preparatory slice faulting far more important in the develop- 
ment of low-angle overthrusts than the piling up of material. 
Greater resistance and drag below.—With horizontally directed 
compressive stresses in operation rotational strains would also 
tend to be produced by the co-operation of any factor which 
increased the resistance of the deeper portion of the rock mass 
involved, while the resistance of the more superficial portion to 
such stresses remained the same. The far-reaching experimental 
studies of Dr. Adams and his colleagues have shown that,on account 
of the increasing rigidity of the rocks due to cubical compression 
from the weight of overburden, resistance to deformation in the 
earth should increase with increasing depth below the surface.’ 
It is concluded that with ‘increasing depth greater and greater 
stress differences are required to deform the rocks. From this 
principle it would seem to be a legitimate deduction that, for a 
lateral thrust of given magnitude, rock deformation should take 
place more readily near the surface of the earth than at a greater 
depth beneath the surface, and that in any case (barring the effects 
of local heating, or liquefaction) deformation should become less 
with depth, unless the magnitude of the stress differences which 
cause the thrusting increases as rapidly with increasing depth 
as does the resistance offered by the rocks.’ 
t Frank D. Adams, ‘‘An Experimental Contribution to the Question of the Depth 
of the Zone of Flow in the Earth’s Crust,” Jowr. Geol., XX (1912), 97-118. 
Since this was written, the principle of increasing resistance to deformation 
with depth below the surface of the earth has been strongly affirmed by Adams and 
Bancroft as the result of further experimental researches. (See Frank D. Adams and 
J. Austen Bancroft, ‘‘On the Amount of Internal Friction Developed in Rocks during 
Deformation, and on the Relative Plasticity of Different Types of Rocks,” Jour. 
Geol., XXV [1917], 5907-637. Also Louis Vessot King, ‘‘On the Mathematical Theory 
of Internal Friction and Limiting Strength of Rocks under Conditions of Stress Exist- 
ing in the Interior of the Earth,” Jowr. Geol., XXV [1917], 638-58.) 
