The Pre-Cambrian Structure of Missouri 147 
pre-Cambrian faulting. He calls attention to the mineralization 
along the southwestern side as being indicative of buried mar- 
ginal faults, 
A number of large springs in the region are located along 
the edges of the block. Alley Spring lies to the west of the 
block and Blue Spring is on the eastern edge of the block. Big 
Spring is located near Van Buren, west of an outcrop of granite. 
A line from this locality passing through a projection on the 
south side of the Shannon County block would pass through 
Round Spring to the north of the block, This alignment would 
follow the course of the Current River, along which are a num- 
ber of large springs, for a distance of 30 miles from Round 
Spring to Montauk Spring. The alignment of springs suggests 
a pre-Cambrian fault through the block, along which there has 
been some Paleozoic movement with too small a displacement 
to produce recognizable faults. 
The Lamotte sandstone is absent in this area, the basal 
sedimentary rocks ranging from Bonneterre to Gunter in age. 
This suggests the block was already elevated during Lamotte 
time. Apparently the St. Francois Mountains were uplifted 
later, but the Shannon County block did not move with them. 
robably nearly all of its elevation is due to pre-Cambrian move- 
ment, 
Bourbon uplift. The magnetic map of Crawford County 
(21) shows an area of high vertical anomaly to the east of, 
and parallel to, the Leasburg fault, which seems to indicate a 
buried ridge extending from the St. Francois Mountains, The 
greatest anomaly is immediately west of the town of Bourbon 
and is higher than at localities where the pre-Cambrian is nearer 
the surface, suggesting the presence of a buried basic intrusion 
or a large body of iron ore (21). This anomaly is on a line 
with the Ironton fault and indicates that the fault continues 
under the Paleozoic sediments to this point. Further evidence 
of the existence of the underlying pre-Cambrian block is seen 
in the outcrops of felsite just west of the northwestern end of 
the Shirley fault. 
