LOW-ANGLE FAULTING 35 
in both cases, turned upward, producing a considerably steeper 
angle in the immediate vicinity of the surface. 
Experiments therefore show that the effect of a strong rotational 
strain, even in homogeneous material, is to produce shearing and 
complete rupture, essentially parallel to the direction of the applied 
force. If the thrusting be in a horizontal direction, the plane of 
rupture will approach horizontality. In these experiments it was 
noted that the low-angle shearing required fewer turns of the screw 
and thus the application of less force than the 45° fracture from 
Fic. 16.—Deformation of paraffine blocks (same mold as block in Fig. 15) under 
rotational strain. Pressure applied only to the upper half of the right-hand face. 
The fracturing, though irregular, was not far from horizontal. 
non-rotational strain. This merely bears out the well-known fact 
that the resistance of materials to shearing stresses is much less than 
to direct compressive stress. Hence the disposition to shear if con- 
ditions allow. 
Lessening the resistance above-—Deformation by rotational strain 
may thus be developed in homogeneous material by sufficiently 
increasing the effective differential stress in the upper portion of the 
mass with respect to that in the lower portion. It is the greater 
unbalanced pressure in one portion over another which is effective. 
This unbalancing of pressure may be accomplished in several ways. 
Within the earth it may be produced either by increasing the lateral 
