130 Transactions of the Academy of Sctence of St. Louis 
The vertical joints fall into eight sets in which can be dis- 
tinguished four systems, each consisting of two sets at right 
angles to each other. The sets strike about N.—S., E—W., 
N. 40" EN. SO" Wi, N- 65°: W., N. 25° EB. No 65° Rand 
N. 25° W. In some localities one set will be dominant while 
in others a different set will be the most significant. Apparently 
there is no regular variation in dominance of the different sets 
with position within a body of granite, or between different 
bodies of granite. 
Joints in the granite are much more regtilar than in the 
felsite but they are farther apart, averaging 20 feet between 
joint planes. Plate Illa is a histogram based on 100 joints in 
the granite of the outcrop south of Graniteville, on the Black 
River road. The N. 40° E. set is the only one of any import- 
ance at this locality although the other sets are present. The 
granite at Graniteville (Plate IlIp) has the N. 65° E. set domi- 
nant with the N. 50° W. set also important and there are some 
N.—S. joints in both localities. While the histograms appear 
considerably different there is less difference apparent in the 
field e western part of the southern granite area has the 
N. 65° W. joints best developed while the northern part of the 
Graniteville outcrop has the N. 40° E. 
Plate IIIc shows a histogram based on 50 joints in the gran- 
ite on Wachita Mountain in the Mine La Motte quadrangle. 
Here the dominant joints belong to the N. 60° E. group with 
minor jointing N. 70° W. and N. 40° E. The N. 70° W 
group merges into the EW. and N. 50° W. groups. 
Jointing in the felsite includes the same systems found in 
the granite but is much more frequent and irregular with joint 
planes only a few inches apart. This may, in part, be due to 
the fact that some of the joints have been formed by contraction 
on cooling. At Hughes Mountain in the Bonneterre quadrangle, 
columnar structure has been developed but even here the main 
systems may be distinguished. The columnar structure is partly 
due to contraction on cooling and suggests that the rock is a 
surface flow. This is also indicated by the abundance of flow 
lines. 
