The Pre-Cambrian Structure of Missouri 139 
The straightness of the boundaries of the Belleview Val- 
ley can ‘only be explained by erosion along nearly vertical faults. 
The sides of the blocks parallel the two most important sets of 
joints in the area but erosion along joint planes would produce 
narrow, parallel valleys rather than such a large and broad de- 
pression. Streams do not follow a single set of joints for any 
great distance, but tend to follow the set of joints best devel- 
oped at each point along their courses. 
Hogan lies at the north corner of another downdropped 
area similar to the Belleview block. Two granite outcrops 
southwest of Hogan lie on the northwest edge of the area. The 
northeast boundary is an extension of the line forming the 
southwest side of the Belleview block and suggests the con- 
tinuation of the same fault. This line passes through the two 
granite areas beyond the boundaries of the main igneous area, 
suggesting a fault striking northwest through the whole St. 
Francois Mountains block. The assumed fault along this line 
may be called the Hogan fault. 
The East Fork of the Black River continues on a straight 
southwest line beyond the Belleview block to the edge of the 
igneous area. The straight southeast end of Buford Mountain 
and the northwest side of Iron Mountain also lie along this 
line. In the northeast corner of the Iron Mountain quadrangle 
and the southeastern part of the Bonneterre quadrangle, the 
St. Francois River likewise flows along this alignment. Stream 
valleys along this alignment are pre-Cambrian as is indicated 
y the presence of Cambrian sedimentary rocks in the valleys. 
The granite porphyry of the Edgehill quadrangle lies northwest 
of the alignment, but its relation to the faulting has not been 
determined. The fault believed to follow this topographic a- 
lignment may be termed the Munger fault. 
The iron deposits at Iron Mountain may be due to solutions 
following along the Munger fault plane from the Ironton fault. 
The deposits properly owe their location to a minor fault in- 
tersecting the Munger fault at Iron Mountain. Faults mapped 
in the Bonneterre quadrangle(8) as stopping at the southern 
