OBSERVATIONS AND EXPERIMENTS ON JOINT PLANES 181 
an improbable system of separate shocks to account for the 
several sets of major joints and the mass of minor joints. These 
shocks must have been symmetrical with the forces producing 
folding, and the resulting breaks were of different character at dif- 
ferent times. It seems simpler to refer the joints mainly to the 
orogenic forces, but perhaps as in Crosby’s combination of shock 
and torsion the shock attending crustal movements materially 
affects the formation of cracks in strained rocks. 
TORSION THEORY 
In Daubrée’s torsion experiments the material broke in two sets 
of cracks, making angles of go° with each other and 45° with the 
axis of torsion on the large faces of the plates. The inclination as 
shown on the narrow side faces was as high as 50° with the vertical, 
though usually less. Becker repeated these experiments with 
plates of glass of various dimensions. He obtained the same direc- 
tions of outcrop on the large faces as were found by Daubrée and 
he also found that the breaking surfaces were curved. Sometimes 
the outcrop on one of the large parallel faces of the glass was straight 
and on the other S-shaped. 
Many of the minor joints of the Ithaca region are shaped like 
the surfaces Becker obtained by torsion and they are probably due 
to this cause. The most important of the curving joints are those 
which strike north of west. The well-developed joints of large 
hade found near the Shurger Point fold also resemble breaks due 
to torsion. The innumerable small joint faces are probably due 
to local twisting. 
The master joints do not show such curvature in a single expo- 
sure, though perhaps if their full extent were seen they would be 
found to have a twisted surface. The torsion theory has been 
criticized because the breaks make angles of 45° with the axis of 
torsion while joint planes are nearly parallel to the dip and strike 
of rocks. If the ends of a piece of cardboard are twisted in opposite 
directions the resulting ridge runs diagonally between the corners 
or nearly parallel to one of the sets of cracks obtained by Daubrée. 
In order to twist the plates, however, Daubrée applied a couple 
at right angles to the plane of the large faces. In the rocks this 
