LOW-ANGLE FAULTING 15 
ing upon it, and strain as the change in the shape or dimensions 
of the body resulting from stress. Strains may be dilatational, 
in which there is change of volume without change of form, or 
distortional, in which the form of the body is changed without 
changing the volume. Of these the latter is by far the more impor- 
tant in the deformation of rocks. Distortional strains may result 
from three kinds of stress—tensile, compressive, and shearing 
stress—torsion being regarded as shearing by twisting. Various 
combinations of these stresses are of frequent 
occurrence. 
In the problem of thrust faulting under 
consideration, the deformation results from 
the action of compressive stresses and 
shearing stresses, with tension only a very 
subordinate and incidental factor. The op- 
eration of these stresses may be analyzed as 
follows :* 
Consider a rectangular block with pressure 
P applied uniformly to a face to find the 
stress on the oblique section mnop. 
Resolve P into normal (1) and tangential 
(T) components. If the angle between the 
direction of application of force (P) and the 
plane of the oblique section (mnop) be 
designated 6, then 
Fic. 5:—Diagram to 
represent the normal or 
N=P sin 0 direct stress (V) and the 
T =P cos @ tangential or shearing 
stress (1) operating on 
The tangential component 7 tends to cause 4” oblique section 
sliding along the section mnop and is called Mave ya eae force’ (P) 
is applied uniformly on 
the shearing stress. The normal component face 4. 
N is called the normal or direct stress. 
Let a represent the area of the cross-section of the block, or 
column, upon which the force is applied. Then the area of the. 
oblique section mnop equals a csc 0. 
*W. C. Unwin, The Testing of Materials of Construction, 1910, pp. 22-23; R. J. 
Woods, The Theory of Structures, London, 1909, pp. 1-4. 
