OBSERVATIONS AND EXPERIMENTS ON JOINT PLANES 167 
break and slip with a slickensided surface along a plane making an 
angle of 45° with the pressure, that is, a plane whose outcrop on the 
face abef made an angle of 45° with the edges and whose outcrop 
on bd/g was a straight line parallel to the right edge. On account 
of the uneven face some points usually received a greater thrust 
than others so that the material broke in sections, the outcrops on 
the flat faces being parts of ellipses as shown in the figure. These 
sections each moved along a slipping plane of about 45° with the 
pressure, so that where there were several rows of semi-ellipses 
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Fic. 1o.—Photograph of a block of paraffin after compression. X1.4 
the material had been broken along several parallel slipping planes. 
The outcrop of the main slipping plane is shown from h to 7. 
There was a similar tendency for the planes of slipping to break off 
the corners along the edges of the end faces. This is shown at 
h and 7 where the breaking plane extends farther in than it does 
near the center of the long edges. Sometimes this was carried so 
far that the point / was near the center of the shorter edges. 
At the left edge is shown the result of a thrust which was stronger 
along the center of the face than near the edges ac and bd. In 
practice this occurred when the face of the material was wider than 
the jaws of the vise. The effect was usually to split the material 
in a nearly horizontal plane which, farther in, merged into a small 
slipping plane at 45° with the pressure with an outcrop along 7k. 
