148 Transactions of the Academy of Science of St. Louis 
The uplifted area is bounded on the northeast by the Shir- 
ley and on the southwest by the Leasburg fault, both of which 
strike northwest—southeast. The Leasburg fault is on the same 
line as the Hogan fault and also a small fault near Berryman. 
The Courtois River flows along this alignment from Brazil to 
the Meramec River. A fault crosses the Gasconade River on 
a continuation of this same alignment to the northwest. These 
minor faults indicate an underlying pre-Cambrian fault 115 
miles in length. 
Decaturville uplift, An outcrop of pegmatite surrounded by 
steeply dipping Paleozoic rocks southwest of Decaturville, has 
long been known as the Decaturville Dome. The pegmatite is 
quite coarse over the small area in which it outcrops. Shephard 
(37) reported a mass of mica schist and boulders of quartzite 
over the pegmatite. The Potosi formation outcrops near the 
igneous rock and it is surrounded by ringlike outcrops of sedi- 
mentary rocks ranging from Bonneterre to Jefferson City in age. 
Rocks close to the pegmatite are nearly vertical but the dip be- 
comes more gentle with distance from the pegmatite. Limestone 
close to the pegmatite is highly silicified and contains galena and 
sphalerite. 
The age of the pegmatite is still in doubt. Winslow (52) 
considers it to be intrusive into the sediments and therefore Pal- 
eozoic or later. Shephard (39) at first believed it to be pre- 
Cambrian and describes the Decaturville pegmatite as being 
“without much doubt a portion of one of the early Archean is- 
lands.” He also believes that there had been later uplift along 
the same lines to account for the present disturbed state of the 
sediments surrounding the pegmatite. In a later paper he ex- 
presses the belief that the uplift is due to intrusion of the 
pegmatite into the sedimentary rocks and points to the presence 
of quartzite and schist as evidence of metamorphism by intru- 
sion (37). 
Drill holes in other localities have entered schists and quartz- 
ites of pre-Cambrian age so that the small amount of metamor- 
phic material in the Decaturville Dome does not indicate intru- 
sion into the limestone. Small pegmatitic bodies occur in the 
