The Pre-Cambrian Structure of Missouri 161 
Summary and Conclusions 
The dominant structural trend in the pre-Cambrian rocks 
of Missouri is N. 50° W. Pre-Cambrian faults and joints strike 
in this direction, as do bedding planes in the pyroclastics and 
flow lines in the felsites. Second in importance are the N. 40 
E. structures which include flaws and joints, and the majority 
of basic dikes also strike in this direction. Minor joint sys- 
tems in the order of their importance are: N. 70° W. and N. 
20° E., N.—S. and E—W., and N. 70° E. and N. 20° W. It 
is believed that the pre- Castries structures were formed by 
compression from the southwest and the source of the stress 
must have been located in northern Mexico or the Pacific O- 
cean basin as structures in Oklahoma and Texas were appar- 
ently formed from the same stress. 
The first recorded event in the structural history of Mis- 
souri is the extrusion of felsite which was followed by the in- 
trusion of the oldest granite. After the intrusion of the granite, 
joints striking northeast were formed in both the granite and 
felsite. Following the intrusion of basic dikes in the north- 
east joints, faulting occurred and was followed by the intrusion 
of granite and felsite and the formation of iron deposits. The 
final events recorded in the pre-Cambrian include the intrusion 
of the youngest granite, of basic dikes striking northwest, and 
finally, mineralization. 
Structures in the Paleozoic rocks have the same trends as 
those in the pre-Cambrian. The northwest and northeast-trend- 
Ing structures are due to movements along pre-Cambrian fault 
planes. 
