The Pre-Cambrian Structure of Missouri 159 
which elevated the block. This faulting was later than intru- 
sion of the basic dikes and probably of the same age as the 
northwest set of joints. These relations conform to the theory 
advanced for the origin of joints. 
The block structure of the St. Francois Mountains and 
Shannon County areas can be explained similarly to the joint- 
ing. Thrust faults were formed by pressure from the south- 
west and were accompanied by vertical, transverse flaws which 
probably followed previously formed joint planes. Upon re- 
lief of the pressure, normal faults were formed parallel to, and 
southwest of, the thrust faults. This left the blocks outlined 
on all four sides. During the later periods there was com- 
pression alternately from the southwest and southeast which ac- 
centuated the block structure. 
There is little distinction between the N. 70° W. and N. 
20° E. sets of joints, but the former set is much more a- 
bundant and there are dikes parallel to it, thus suggesting that 
it is the dip set. It seems more probable, however, that the 
stress acted from the southwest, parallel to the N. 20° E. set. 
The same sets occur in the Wichita Mountains where structures 
are due to pressure from the southwest. Buckley and a 
ler (9) report a southwest dip for a number of the N. 70° W 
joints which is further indication that the stress acted fone 
the southwest. 
The N.—S. set of joints is more important than the E. 
and there are dikes in the N.—S. set in the pre- Cink 
rocks. The Palmer fault in Washington and Crawford counties 
and the Devonian fault in Ste. Genevieve County strike west 
through part of their length and may both lie above a pre- 
Cambrian fault. 
The N. 70° E. and N. 20° W. system is the least well 
defined. The N. 60-70° E. joints at Graniteville may belong 
to this system or may be of local origin. Joints occurring N. 
50-60" E, may be either of local origin or belong to this system or 
