OBSERVATIONS AND EXPERIMENTS ON JOINT PLANES 179 
The Ithaca region seems particularly favorable for the study of 
joint planes. The stratigraphy is comparatively simple, showing 
only one time of crustal movements sufficiently great to be accom- 
panied by jointing. The pressure was strong enough to produce 
joints and folds and faults with which the joints could be compared, 
but it did not carry the folding far enough entirely to reverse the 
joints and so complicate the record. In regions of higher folds the 
hades of the joints should be studied with the plane of bedding 
rather than the horizontal as a datum plane. 
CAUSE OF THE JOINT PLANES 
Geologists are not agreed upon the cause of joint planes in 
stratified rocks. On account of the observed relation between 
joints and the strike and dip of rocks it is usually conceded that 
they are connected with movements of the earth’s crust. Some of 
the theories of the cause of joints have been abandoned. Among 
those still recognized are tension, earthquake shock, torsion, and 
shear. 
TENSION THEORY 
According to the tension hypothesis joints are formed during 
folding. As the folds develop there is tension along the upper — 
surface of anticlines and the under side of synclines. The rocks 
then crack in planes whose outcrops are parallel to the axes of 
the folds. At right angles to the axes the pitch of the folds causes 
another set, the dip joints. A general objection to this theory is 
the character of the joint faces. They are smooth and are remark- 
able for passing directly through hard masses like pebbles or 
concretions, instead of around them. The Hamilton shales of this 
region show this very well. The rock is mostly an even shale but 
there are several bands of hard concretions. Some of the joints 
do not pass through the concretions but probably the majority 
cut them in a smooth plane even with the rest of the joint face. 
It does not seem probable that a crack due to tension and separation 
of faces would pass so smoothly through concretions instead of 
around them. ‘The general smoothness of a joint face in shale is 
very unlike the surface caused by breaking rock apart. 
Joints formed by tension would be arranged radially about 
