OBSERVATIONS AND EXPERIMENTS ON JOINT PLANES 177 
connected to have been formed by entirely separate forces and the 
strike joints distinctly belong to a single set. 
It has sometimes been assumed that joints at right angles to 
each other were formed at separate times, the forces producing the 
second set being at right angles to the forces producing the first 
set, that is, both sets bore the same relation to the forces which 
caused them but the forces were in different directions the two 
times. Opposed to this is the fact that the strike and dip joints 
are not similar. If a certain set of forces produced the strike 
joints, then a similar set of forces acting at right angles to the 
former forces should give another set much like the strike joints 
except in direction. This is not the case in the Ithaca region. 
Experimentally cracks were produced in all the required directions 
during the same application of pressure and it does not seem 
necessary to assume that strike and dip joints were formed by 
separate applications of pressure in the rocks. 
In most cases the master joints are sharply cut by the faults 
but in some cases there is evidence that some strain existed along 
the fault planes at the time the joints were produced. Near the 
right in Fig. 1 is a dip joint which is strong and normal excepting 
near the fault line where it abruptly breaks into a fanlike set of 
radial small cracks with the point below. This fan is about a 
foot across and the fault crosses its center. Above the fan the 
joint becomes normal again. Evidently when the joint was formed 
there was some strain at the place where movement occurred later 
or perhaps the faulting had already begun. Many of these fanlike 
irregular joints were seen near faults and were evidently due to 
strain at the fault planes, but the others were not examined for 
their relation to the master joints. Investigation would probably 
show that more of these are connected with the master joints and 
in the case described the fan is certainly a part of an otherwise 
normal joint belonging to one of the dip sets. 
The evidence thus indicates that all the master joints were 
formed in the earlier part of the period of folding. Probably when 
the pressure reached a certain value, less than its maximum, the 
joints were formed rather abruptly. Further pressure caused 
faulting, or perhaps some faulting occurred before the formation 
