LOW-ANGLE FAULTING Be 
the force per square inch was greater there (owing to the smaller 
cross-section over which it was distributed) than at the other end 
_near the pressure block where it was distributed over a larger 
area of cross-section. Rupture occurred where the intensity 
of stress was greatest, even though it was farthest removed from 
the pressure block. The fault averaged 42° for its whole length. 
The angle shows that it was caused by a non-rotational strain. The 
same was true also of the two previous tests. The shape of the 
block, at least to the extent of the variations tried in these experi- 
ments, apparently does not change the nature of the strain. But 
perhaps, after all, only a non-rotational strain could develop under 
RESISTING PRESSURE 
Frc. 14.—Block similar in shape to that shown in Figs. 12 and 13, but composed of 
paraffine. A 45° fracture developed. 
the conditions of these experiments, since the pressure block is 
guided rigidly forward by the controlling flanges of the machine 
and so cannot turn. But one may conclude, nevertheless, that a 
piled-up mass having a higher standing cross-section to be pushed 
forward, does not, of itself, add a rotational element to the strain 
when laterally compressed, nor, so far as this principle is concerned, 
does it lower the angle of fracture. 
How rotational strain develops fracture-—To show how a rota- 
tional strain will deform such a block as was used in the experiment 
just described, another block of paraffine was cast in the same mold 
and subjected to a rotational strain in the following manner. As 
before, the pressure was applied from the same long side, but instead 
of being applied against the whole surface of that side it was applied 
‘only to the upper half of it. The resisting block, as before, but- 
tressed the whole of the shorter left-hand side. With the opposing 
forces acting horizontally at quite different elevations, a rotational 
couple was developed. As the strain slowly increased the paraffine 
