The Pre-Cambrian Structure of Missouri 113 
LOCATION OF AREA STUDIED 
Missouri lies in the two physiographic provinces known as 
the Interior Plains and the Interior Highlands (15). The north- 
ern part of the state is within the Dissected Till plains while the 
southern part is within the Ozark Plateau. The northern end 
of the Gulf Embayment touches the southeastern corner of the 
state. The intermediate location of the state is reflected in its 
structural geology for most structures are parallel to those of 
the region to the southwest but some structural trends are also 
characterisitc of the eastern and southern states. 
The pre-Cambrian rocks are exposed chiefly in the St. Fran- 
cois Mountains region, located at the apex of the Ozark Dome, 
(Plate 1) where they outcrop over an area about 35 miles long 
by 25 wide in Iron, Madison, St. Francois, Reynolds, and Wayne 
counties. Southwest of the St. Francois Mountains, in Shannon 
County, there is an area of pre-Cambrian rocks which is smaller 
in extent than that of the St. Francois Mountains. Small areas 
of pre-Cambrian rocks are found outside of these two main 
regions in Ste, Genevieve, Camden, Crawford, and Carter coun- 
ties. 
Geology of the Pre-Cambrian Areas 
GENERAL 
The distribution of the pre-Cambrian rocks is shown on 
Plate II which is compiled from the geologic maps of the Potosi, 
Edgehill, Eminence, Cardareva, Iron Mountain, Mine La Motte, 
and Bonneterre quadrangles, Ste. Genevieve County, and the 
Crystalline rocks of Missouri, all published by the Missouri Geo- 
logical Survey, and from field observation. The rocks include 
intrusive and extrusive types and pyroclastics. Early writers (23) 
referred to all the extrusive rocks as porphyry and later writers 
have used the term rhyolite but these terms are not suitable be- 
Cause, although practically all the rocks are acidic, some of them 
are not porphyritic and some are not rhyolites. Tolman (46) 
