DIASTROPHISM AND THE FORMATIVE PROCESSES 423 
in the least the direction of applied force, the mica flakes could be 
made to assume any desired orientation. This illustration is 
introduced here for the reason that geologists so commonly refer 
to the direction of applied force as though it of itself determined 
the result, and relate everything to this direction without con- 
sidering with equal care the lines of resistance. But it is the 
differential stress which is all important in deformation, and in 
this the axis of least compressive stress plays a part of the most 
critical importance. In fact, “lines of least resistance’? might 
well be made the topic of a hortative sermon: 
When a region is subjected to strong compressive stress under 
ordinary conditions, the axis of least stress is the vertical one, and 
the easiest relief is upward. Elongation then presumably takes a 
vertical direction, as does also whatever flow cleavage develops. 
But under conditions of special burden, lateral elongation may be a 
condition precedent to a final vertical one. A notable shear zone 
extending obliquely downward on the 45° principle, such as is here 
postulated between segments, might be expected to exert an 
orienting effect on the direction of most ready relief for some 
distance beyond the point where actual shear ceases, though it is 
uncertain how far beyond. After that influence ceased to be 
effective, the inclination of the ensuing schistosity should theoreti- 
cally become more nearly vertical. If, in the deeper parts, the 
border planes between the segments come into parallelism with 
the elongation under recrystallization, and so with the schistosity, 
they should become steeper below. But it is not certain that the 
parallelism exists, nor do we know the controlling conditions 
sufficiently well to be certain that the elongation of the mass will 
be straight upward. 
In the actual drawing of the border planes, the angle of 45° 
serves largely as a convenient average inclination, suggested by the 
planes of no distortion in the ellipsoid of strain.t It is, however, 
recognized in engineering practice that the angle of fracture under 
compressive stress, even where the strain is entirely non-rotational, 
varies widely from 45°, depending upon the nature of the 
tC. K. Leith, Structural Geology (1913), pp. 16-20. 
