OBSERVATIONS AND EXPERIMENTS ON JOINT PLANES 55 
Those in one group strike nearly parallel to the axes of the folds 
and form the strike set. 
Those in the other group strike approximately along the dip of 
the rocks and are the dip joints. 
The dip joints form two distinct, similar sets and appear to vary 
with the pressure which caused the folding. 
The strike and dip joints are usually nearly vertical and a single 
joint varies little in strike and inclination at any exposure. The 
joints belonging to each set have a range in strike of only a few 
degrees at any one locality. 
A set of joints making a moderate angle with the strike set is 
locally strong but these are not of the same character as the strike 
and dip joints. They are more variable in strike and inclination 
and usually have a greater hade. 
Joints striking between the major sets are common but are 
usually weak and very variable with little apparent system. Their 
hade is usually large. In some localities the highly inclined joints 
are strong. 
The master joints were evidently formed during the earlier part 
of the folding which took place here during the Appalachian Revo- 
lution. They are not younger than the faults which were formed 
at that time, since they are displaced by the faults. The hade of 
the strike joints shows that they were not formed before the folding. 
The dip joints vary with the folds and the forces active during the 
folding and were apparently formed at that time. 
By compressing blocks of paraffin and resin various systems of 
cracks were obtained. Some were like those obtained by Daubrée. 
The finest cracks were nearly parallel to, and at right angles to, the 
pressure. 
The observations support the shear theory of the formation of 
joints by indicating that the joints here were formed while shearing 
stresses were active and that the joints vary with those stresses. 
The joint planes, however, are nearly at right angles to the fault 
planes and are very unlike them, so that the theory that joints are 
incipient faults is not supported. Some factor like shock may have 
determined the position of the breaking planes which are classed 
as joints. 
