158 Transactions of the Academy of Science of St. Louis 
mean that the strike set was younger than the dip set. That 
the two sets of joints have different origins is indicated in a 
number of localities by the presence of dikes in one set and 
not in the other. In most cases the dikes are in the set parallel 
to the direction of compressive stress. 
While the strain ellipsoid may be oriented from the position 
of the structures present, it may be impossible to determine the 
direction in which the forces acted. The same strain ellipsoid 
may be produced by forces acting in a number of directions 
but the types of structures present may give some idea of the 
nature of the stresses. The structures formed will be those that 
will best relieve the stress. 
Application to Missouri structures. The most important 
structural system in Missouri is represented by the N. 40° E. 
and N. 50° W. joints. Many of the dikes in the pre-Cambrian 
rocks follow the northeast set of joints indicating that north- 
east was the direction normal to tensional stress during the 
pre-Cambrian. This set may therefore be called the dip set 
and the northwest set the strike set. Faults strike N. 50° W. 
in both the pre-Cambrian and the Paleozoic rocks, indicating 
that this direction is parallel to the intermediate axis of the 
strain ellipsoid and that compressive stress acted in a northeast— 
southwest direction. That the stress was caused by compression 
from the southwest, rather than the northeast, is indicated by 
_ the fact that fault planes in many cases dip to the southwest. 
The presence of dikes in the northeast set of joints may be 
evidence that this set was formed first and the dikes intruded 
before the second set was formed. It may, however, merely 
indicate that this was the direction normal to tensional stress 
which favored the intrusion of the dikes. The theory advanced 
for this type of jointing would require the northeast set of 
joints to be formed first. The presence of later dikes in the 
northwest set in a few localities may show that this set was 
formed later, although these dikes may be in fault planes rather 
than dikes. Faults trending northwest through the St. Fran- 
cois Mountains are normal faults due to relief of pressure 
