180 PEARL SHELDON 
the folds, that is, if their planes were produced they would meet 
below the axes of anticlines and above the axes of synclines. The 
reverse was found to be true. It seems difficult to explain the hade 
of the strike joints by the tension theory. Neither does this theory 
account for the double nature of the dip joints nor for the mass of 
minor joints in all directions. ‘The perfection of jointing in some 
parts of this region is out of proportion to the amount of cracking 
necessary to relieve the tension in such low folds. 
EARTHQUAKE THEORY 
W. O. Crosby’ offered an earthquake hypothesis as an explana- 
tion of joints. Later? he emphasized the effect of shock on rocks 
already under strain rather than shock alone. Crosby stated some 
of the more important objections to the formation of joints by 
earthquake shock alone. Such breaks would not become approxi- 
mately vertical for some distance from the epicentrum and then 
the energy of the shock would be largely dissipated. In the Ithaca 
region the inclination of the master joints is nearly vertical and in 
changing from one side to the other passes through the vertical, 
not through the horizontal as would be expected from earthquake 
waves. In order to explain the commonly observed right-angled 
relation of joints, it has been assumed that after one shock had 
produced a set in one direction a large component of a subsequent 
shock would be relieved by slipping along the already existing 
planes, unless it was at right angles to the earlier shock, except in 
case of very rapid vibrations. Only one set of planes would be 
formed by each shock. The direction of the strike joints might 
be accounted for by making the focus of the earthquake a long 
fault parallel to the axis of the fold. An objection to the earth- 
quake hypothesis apparent in this region is the symmetry between 
the joints and folds. It is not probable that the folds would 
influence cracks due to earthquakes to such an extent nor is it 
probable that separate shocks occurred simultaneously in the 
different folds to cause a reversal of the joints near each axis. 
The earthquake theory alone seems awkward, since it requires 
t Proc. Boston Soc. Nat. Hist., XXII (1882), 72-85; XXIII, 243-48. 
2 Am. Geol., XII (1893), 368-75. 
