OBSERVATIONS AND EXPERIMENTS ON JOINT PLANES 175 
synclines. The reverse was found to be true. If they originally 
made a uniform moderate or large angle with the vertical, folding 
would have increased the angle on one limb of the fold and decreased 
the angle or reversed it on the other. Thus angles such as were 
actually found might be produced on one limb of the fold but not 
on the other. If the joints had a varying hade which reversed 
near the axes of subsequent folds the present angles might be 
produced, but that is too improbable. 
The joints might have been formed with uniform, nearly verti- 
cal inclination after the beginning of the folding. Subsequent 
subsidence of the folds would give the angles observed. This is 
not probable, because the faulting indicates that much of the move- 
ment took place after the formation of the joints. This theory 
would require a reversal of all the later folding and enough more 
to reverse the joints. It would not alter the question of the age 
of the joints, since, if their hades were produced in this way, they 
must have been formed during the folding. 
The upper limit for the date of the jointing is set by the faulting. 
The nearly horizontal faults of this region uniformly displace the 
joints which they cross; therefore the joints were formed before 
the faulting, or at least before the end of the movement along the 
fault planes. These faults were presumably formed in the time of 
active folding here, that is, during the Appalachian Revolution. 
This sets the date of the joints as somewhere between the begin- 
ning of the pressure which caused the folds and the climax of the 
folding as indicated by the active faulting. Unless the joints were 
formed by subsidence of the folds their direction of inclination is, 
in most cases, the same now as when they were formed. The 
angles, however, have been altered by later folding and in some 
cases may have been reversed. This may explain some cases 
where the reversal of direction takes place at one side of the axis 
of a fold. 
DIP JOINTS 
The evidence for the dip joints is not so conclusive, since the 
lack of detailed knowledge of the pitch of the folds prevents a 
comparison of the variation of the hade of the joints with the varia- 
tion of the folds. From what is known of the pitch the hade of 
