OBSERVATIONS AND EXPERIMENTS ON JOINT PLANES 171 
became nearly perpendicular to the line of pressure and appeared 
as a mass of wavy lines as at ad in Fig. o. 
Where the application of the pressure split off part of the block 
as along the left edge of Fig. 11 the cracks usually started farther 
from the edge and omitted the acute-angled portion as along 7 in 
Fig. 11 and ad in Fig. 9. Thus near the outcrop of the slipping 
planes these cracks were nearly parallel to the outcrop and were 
arranged symmetrically about the line of pressure. 
The preceding are the forms of cracks which were most common 
but in the better specimens the central part of the block was usually 
covered with cracks and these were more regular than in the 
rupture portions of the paraffin. They occurred evenly over the 
large faces, commonly making angles of 90° with each other and 
45 with the pressure. At their connection with the oblique- 
angled cracks near the slipping planes the angles changed rapidly. 
These cracks were usually stronger near the center of the end 
edges, as at p in Fig. 11, than at each side of the center. In the 
block shown in Fig. to the breaking lines corresponding with 7& in 
Fig. 11 both occurred near the center so that the cracks across the 
center showed a compromise between the large angles near the 
breaking lines and the right angles of the unbroken center of a 
block. 
In Fig. 11 is shown the effect of a greater thrust near one end 
of the block. This was commonly caused by a greater width of the 
block at one end. Where the thrust died out the cracks were not 
symmetrical about the general line of pressure. This is shown in 
the lower central part of Fig. 11. Sometimes locally, as near an 
irregular breaking edge, a few cracks would occur in nearly the 
shape of a fan, one set forming a few rays and the other set curving 
at right angles to the rays. 
Since the diagonal cracks made angles of 45° with the edges of 
the end faces, the planes of the cracks near the ends were nearly 
parallel to the faces of an octahedron. The inclination of the 
cracks near the center of the block could not be determined, but 
in some cases where the oblique cracks continued to a point near 
the edge the outcrop on the end faces was less than 45° with the 
vertical, as shown at gin Fig. 11. Probably the inclination varied 
