The Pre-Cambrian Structure of Missouri 115 
Differential weathering and erosion. In the central part of 
the St. Francois Mountains the hills are composed of igneous 
tocks while the valleys are floored with sedimentary rocks, A- 
round the outskirts of the area there are hills of sedimentary 
rock which are chert covered. These are not as steep as the 
igneous knobs for the slope depends partly on the angle of re- 
pose of the chert. On topographic maps the igneous knobs may 
be distinguished by a much greater regularity in the contours 
than is seen in hills of sedimentary rocks. 
As weathering in the igneous rocks is controlled by joint 
planes, there is considerable difference in the forms assumed by 
the granite and felsite which are jointed to a different degree. © 
The wide spacing of joints in much of the granite causes it to 
weather into large boulders which reach their maximum develop- 
ment at Graniteville. These are due to weathering along hori- 
zontal as well as vertical joints thus causing the boulders to re- 
semble erratics although they are actually in place. The felsite, 
due to the close spacing of the joints, is seldom seen in large 
boulders but weathers into small, angular blocks. Near the 
mouth of the Little St. Francois River there are chimneys of 
felsite 15 feet square, which are formed by weathering along 
vertical joint planes. These evidently owe their length to lack 
of horizontal jointing. 
Not all of the differences in weathering are due to jointing. 
Because of the diversity in texture, the felsite tends to weather 
with angular corners while the granite is usually rounded. Gran- 
ite hills are usually low and rounded while the felsite hills are 
higher and often conical, Granite is usually seen outcropping 
from top to bottom of the hills with its surface parallel to the 
Slope of the hill. Felsite, on the other hand, is seen as talus 
near the bottom of a hill with an occasional outcrop on the side. 
Near the top of the felsite knobs there is usually a ring of out- 
€rops surrounding the hill while the top is usually rather flat 
and with few outcrops. 
_Angularity is marked in the felsite weathering, not only in 
Tesidual boulders but also in cliffs which frequently have ver- 
