LOW-ANGLE FAULTING 43 
best only a part of the lowering of the angle from 45° can be 
explained in this way. 
If the low angle of the great overthrusts were solely a matter of 
load steadily accumulated by piling up slice fault blocks, then each 
successive slice fault should break through at a progressively lower 
angle. ‘There should be a complete gradation from the first-formed 
fault near 45° to the final overthrust approaching horizontality. 
While some progressive lowering of the angle of the successive 
slice faults is to be noted in some Scottish Highland sections and 
elsewhere, nevertheless there appears to be a great final jump from 
the minor slice faults to the great horizontal overthrust. 
F. RESUME 
The great overthrusts which are now coming to be recognized 
as a prevalent and commanding type of mountain structure are the 
result of conditions differing considerably from those which produce 
ordinary reverse faults. The distinguishing features of the over- 
thrusts are the extremely low angle, which often approaches hori- 
zontality, and the very great displacement along the plane of 
slippage. The great displacement is made easier by the gentle slope 
of the fault plane. The low angle of the fault plane is the net 
result of the operation of several factors. Among the factors which 
will lower the angle of faulting from the theoretical 45° may be 
listed the following: 
t. The normal or direct stress which, along planes inclined 45° 
to the line of application of the force, has an intensity as great as 
that of the tangential stress. It acts as a frictional resistance to 
shearing by the tangential stress. The lower the angle of the 
fracture plane, the less will be the frictional resistance due to the 
normal component of the stress. Hence the tendency to fracture 
at angles below 45°. 
2. Rotational strain, which will lower one of the planes of no 
distortion (shearing plane) from 45° in pure non-rotational strain 
to o° in the extreme case of rotational strain. Rotational strains 
may be developed from horizontal compressive stresses: (a) in 
homogeneous material: (1) by any factors which will increase the 
intensity of the tangential stress in the upper portion of the mass 
