370 The American Geologist. December, isn:r 
the adjacent sheets. In fact, while the earthquake theory 
yields readily two systems of parallel joints crossing at large 
angles, it does not necessitate actual or mutual intersections, 
but the joints of the later system must, obviously, he often less 
continuous than those of the first system ; while the torsion the- 
ory demands equality between the two systems in this respect. 
It is ;i Legitimate corollary of this paragraph that when one 
joint appeals t<> cut and slip another of a different system it 
is quite possibly the older rather than the younger of the two; 
and the same caution is required in determining the relative 
ages of veins and dikes which may subsequently occupy or 
follow the joint cracks. 
The torsion theory calls for only two systems of joints; and 
the earthquake theory accounts with difficulty for more than 
two systems, although a third system is conceivable under this 
theory and in extreme cases, perhaps, a fourth system. The 
question may fairly be raised, however, as to whether, in 
most cases, the joints apparently referable to third and fourth 
systems are not better explained as the irregular examples, 
which, as the experiments have shown, originate simulta- 
neously with the regular rectangular systems. If strains de- 
veloped as uniformly as possible in homogeneous plates of 
glass yield these irregular fractures, much more should they 
be expected in such a heterogeneous mass as the earth's crust. 
The products of the two theories are, so far, strikingly simi- 
lar; although the joints of unlike directions are simultaneous 
in the one case and successive in the other. The strongest 
contrast between joints due to torsion and those due to vibra- 
tory movements arises from the fact that the torsional strains 
must, at least as a rule, be developed with extreme slowness, 
while the earthquake shock reaches its breaking intensity al- 
most instantaneously. If the torsion were attributed to, that 
is, regarded as a phase or consequence of, the swift, vibratory 
movement of the earthquake, then, of course, the two theories 
would be merged in one. In any other case the torsional 
strains must arise from the secular warping or deformation 
of the earth's crust. No argument is required to show that a 
strain that is suddenly developed must result in more regular 
fractures, and fractures less influenced by the structural in- 
equalities of the medium, than one that is slow!}' developed; 
