OBSERVATIONS AND EXPERIMENTS ON JOINT PLANES 183 
vertical planes. Slipping may occur in two planes which have the 
same strike. In the encrinal limestone both these slipping planes 
are represented by true faults. 
If there was differential movement of the joint faces during the 
folding, that throw was not more than a fraction of an inch. On 
the other hand, there was displacement of half a foot along the 
fault planes after the formation of the joints. If joints lie in the 
planes where slipping would be expected to occur, it seems strange 
that under forces strong enough to cause noticeable displacement 
along known faults there was little or no movement along the joints 
if the latter are incipient faults. In the actual case found in this 
region the joints are so nearly at right angles to the horizontal 
faults that the force along the fault planes had almost no component 
along the joint planes, hence movement along the faults could 
take place without slipping along the joints even though they 
formed planes of weakness. If pressure is applied to the rocks 
at an oblique angle with the joint planes after the joints are once 
formed, slipping would be likely to result; but such subsequent 
movements are not an essential part of the production of the joints. 
One of the difficulties in explaining joints by pressure theories 
has been the presence of a set of joints apparently parallel to the 
pressure. The Ithaca region shows that here, at least, the dip 
joints really consist of two sets not parallel to the pressure but 
more’ or less symmetrically arranged about the pressure and making 
a small angle with it. 
If the faults lie in the true slipping planes then the small hades 
of the joints are still unexplained. Perhaps they are due to shock 
associated with the strains due to the pressure. The conditions 
under which the joints were formed probably included translation, 
rotation, compression, pure shear, some torsion of the kind 
employed by Daubrée, and shock. To determine the exact manner 
of formation of joints and their relations to stresses, there is need of 
detailed field observations on the relations between joints and the 
forces producing them, as indicated by attendant faults and the 
axes and pitch of folds, and need of experimental work by which 
cracks resembling joints can be produced under such conditions 
that measurements may be made to determine the relations 
between stress and strain. 
