SOME OBSERVATIONS AND EXPERIMENTS ON JOINT 
PLANES 
PEARL SHELDON 
la 
EXPERIMENTAL WORK 
Systems of cracks at right angles to each other were obtained 
by Daubrée’ by twisting plates of ice and glass and by compressing 
mixtures of beeswax and resin. W. O. Crosby? found that if plates 
twisted not quite to the breaking-point were given a shock they 
would break in cracks at right angles to each other. Systems of 
cracks due to torsion have been studied experimentally by G. F. 
Becker.3 For this reason and because the conditions involved in the 
torsion experiments do not agree closely with the conditions under 
which the joint planes of this region were formed, these experiments 
were not repeated. 
Theoretically and practically work similar to Daubrée’s pressure 
experiments seemed most likely to give satisfactory results. 
Daubrée applied pressure over the square ends of blocks of wax. 
The sides were left unconfined so that deformation could take place 
on all sides. Besides large planes of slipping, the deformed blocks 
showed a network of fine even cracks at right angles to each other 
and parallel to the larger breaks. Daubrée compared the larger 
breaks with faults and the smaller with joints. Unfortunately, the 
outcrops of the cracks, though at right angles to each other, made 
angles of 45° with the direction of pressure, and, judging from his 
figures, the planes of breaking were parallel to the faces of an 
octahedron. This does not agree well with the commonly observed 
angles between joint faces, and the usual strike and dip relation of 
joints indicates that they are nearly at right angles to, and parallel 
to, the pressures acting at the time they were formed. 
« Etudes synthétiques de géologie expérimentale, 300-52. 
2 Am. Geol., XII (1893), 368-75. 
3 Trans. Am. Inst. Mining Engineers, XXIV (1894), 130-38. 
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