26 R. T. CHAMBERLIN AND W. Z. MILLER 
thrusting in the upper portion or by lessening its resistance, while 
the lower portion remains unchanged. It may also be developed by 
diminishing the thrust below or by increasing the resistance in the 
lower part, while conditions in the upper portion remain essentially 
the same. Or it may be accomplished by some combination of 
these. The first process facilitates deformation in the upper part; 
the second retards deformation in the lower part. Whatever 
affects the ratio influences the character of the deformation. The 
greater the difference developed the greater the shearing tendency. 
The intensity of thrusts and the resistance at different horizons in 
the earth should therefore be a vital factor in determining shearing. 
At the present time the location and intensity of lateral thrusts 
in the earth are so imperfectly understood that a treatment of that 
topic is reserved for further information. Rather more, however, 
is known concerning the resistance offered. Factors which either 
lessen the resistance above, or increase it below, may play a part 
in overthrust faulting. 
The resistance above may be diminished in several ways. The 
erosion-thrust of Willis and Hayes already discussed is a clear- 
cut illustration of how it may be accomplished in heterogeneous 
materials. Resistance above is here reduced by erosion which 
removes the heavy, competent upper layers from the crest of an 
anticline. The resistance of the remnants of the upper layers to 
forward movement having been sufficiently diminished in this 
way, this more movable portion shears nearly horizontally along a 
bedding plane as a line of weakness (see Fig. 2). Shearing along 
bedding planes and the control of overthrusts by differences in the 
competency of the beds are related phenomena. 
In homogeneous material the resistance above is lessened by 
other means. The experiments of Cadell indicated that before the 
low-angle overthrust occurred there was first slice faulting and the 
piling up of slices. Slice faulting to a remarkable extent was asso- 
ciated with the Scottish overthrusts and to a certain extent with 
those in the southern Appalachians. If the mere piling up of 
materials, as such, does not introduce a rotational element to the 
strain and so lower the angle of fracture, nevertheless the repeated 
slice faulting and moving of fault blocks do have an effect upon 
