182 PEARL SHELDON 
would correspond with a set of vertical forces. The pressure form- 
ing the folds of this region was tangential to the beds rather than 
vertical; therefore the conditions in the rocks were unlike the con- 
ditions of the torsion experiments, though there was probably 
some variation in the vertical forces. Moreover, in comparing 
the various theories for the formation of joints it may be considered 
that torsion like that employed by Daubrée is only a special 
case of shear. 
SHEAR THEORY 
Daubrée also obtained a network of cracks by a compression 
which caused shearing stresses. Becker" has given a mathematical 
treatment of strains in rocks and explains joints as the result of 
shear. The field observations show an intimate relation between 
the master joints and the forces which caused the folding. Those 
forces are supposed to have been tangential to the beds. The 
symmetry of the faults in the encrinal limestone about a nearly 
horizontal plane is evidence that, if not vertical, the chief forces 
were nearly horizontal. By applying pressure to the narrow faces 
of blocks of paraffin fine cracks were obtained which bore nearly 
the same relation to the pressure which the joint planes bear to 
the pressure active during the folding. The pressure used in the 
experiments corresponded with a tangential pressure on the strata. 
The evidence points toward the formation of the joints as the 
result of a nearly horizontal pressure, but in the present state of 
knowledge of the relations between stress and strain in rocks the 
relations between joints and the strains caused by this pressure 
are uncertain. Strains in rocks are exceedingly complicated, since 
the forces vary and the materials acted upon are not homogeneous. 
The joints found here do not exactly agree with the breaks dis- 
cussed by Becker. They are similar in some respects but unlike 
in others. He concluded that faults, joints, and slaty cleavage all 
lie in the planes of maximum tangential strain. This implies that 
joints are only faults of little throw. The faults in this region were 
apparently formed under the same set of forces as the joint planes; 
yet the two are very unlike. Of three planes at right angles to 
each other, the faults lie approximately in one, the horizontal, and 
the strike and dip joints lie approximately in the other two, the 
t Bull. Geol. Soc. Am., IV (18093), 13-90; Proc. Wash. Acad. Sci., VII (1905), 
267-75; Eng. and Mining Jour., LX XIX (1905), 1182-84. 
