The Pre-Cambrian Structure of Missouri 131 
The histogram shown in Plate [Ip is based on 25 promi- 
nent joints at Hughes Mountain. In order to test the value of 
using prominent joints to determine the dominant directions of 
jointing, eight areas, each two feet square, were marked off 
and all joints within the areas sketched, Out of the 220 joints 
included, 63 belonged to the N. 70° W. set and 46 to the 
N. 15° E., which is about the same proportion seen in the histo- 
gram. As half of the joints belong to one set or the other, 
most of the jointing in this region must be tectonic rather than 
due to contraction on cooling, 
Inasmuch as the geology of Stouts Creek Shut-in will be 
discussed in detail later, it is advisable to include a histogram 
of joints there. Plate IVr based on 45 joints in the dark felsite, 
shows the great abundance of N. 70° W. joints. This jointing 
closely resembles that of Hughes Mountain which is 18 miles to 
the north. 
Joints in the tuffs are more regular than those in most of 
the felsites and are less closely spaced. The same systems of 
Joints are seen in the tuffs as in the other igneous rocks (Plate 
IV Fe). 
Dikes were intruded into joints and are therefore younger 
than the joints. While most of the basic dikes of the region 
forced their way into the N. 40° E. joints, others are found 
in various directions. The histogram of Plate [Vu is based 
on 50 dikes shown on the Iron Mountain and Mine La Motte 
quadrangles or seen in the field. Apparently all of the joint sys- 
tems were developed before the dikes were intruded. The fact 
that most of the dikes are in the northeast rather than the 
northwest and in the north—south rather than east—west di- 
Tections suggests that north and northeast were the directions 
normal to regional tension which caused the joints to open. 
Aplite dikes are found with various strikes showing no 
apparent system, thus suggesting that jointing originated after 
the intrusion of these dikes. One aplite dike on the Black River 
Toad in the granite, strikes N. 7° E. and is cut by closely spaced 
