The Pre-Cambrian Structure of Missouri 157 
types of joints are generally recognized although the mechanics 
of jointing is still not fully understood. The type of joints 
generally called tension joints differs from other types in the 
irregularity of the surfaces and in the fact that they are few 
in number, Joints of this type occur in one set only, normal 
to the regional tension. The other type consists of joints in 
two sets which are mutually perpendicular, highly regular, and 
usually very abundant. They are generally assumed to be 
shearing plane features (50) but in this case they would be 
merely thrust faults of very small displacement and so not 
classified as joints. Nevin (34) recognizes the fact that joints 
are due to movement perpendicular to the walls of the fractures 
and that they are mechanically different from faults. He also 
points out the fact that one set of joints is generally parallel 
to the dip of the rock and the other set parallel to the strike 
and that dikes, when present, occur in the dip set only, The 
division into dip and strike sets of joints has been used in most 
textbooks of structural geology. 
It is possible, in some cases, that joints may occur along 
Shearing planes but, in the majority of cases, a regular joint 
system has a different origin. In regions where jointing has 
been carefully studied, it has been found that one set of joints 
iS parallel to the pressure and the other set normal to it. The 
Set parallel to the stress is opened by tension but, unlike true 
tension joints, the joints in this set are highly regular, 
due to the fact that the active force is compression rather than 
tension. This set of joints will be called the dip set for the 
sake of convenience even in cases where the rocks do not show 
dip. Fairburn (14) states that the dip joints are usually the 
Master joints of a region but, although they are the most abun- 
ant, they are often the least prominent and are therefore over- 
looked. The strike set of joints which is normal to compres- 
Sive stress, is more difficult to explain. An open fracture would 
not be produced by compression. If the joints were merely 
faults of very small displacement, they would dip parallel to 
the faults of the region rather than being vertical. A possible 
explanation is that incipient fractures were formed by compres- 
sion and were later opened up by relief of pressure. This would 
