170 PEARL SHELDON 
them with joints. Their behavior under various conditions of 
pressure and breaking of the material is shown in Fig. 11 and the 
photographs. 
Perhaps the most conspicuous result of the experiments on 
paraffin concerned the relation between these cracks and the forces 
at right angles to the active pressure. Whatever the direction of 
the cracks in the inner part of the block, whenever they approached 
the unconfined edges they turned so that their outcrops made angles 
of 45° with the edges. This is shown at 7,/, and in Fig. 11 and at 
ain Fig. to. At ain Fig. 9 the same thing was present but the 
change occurred abruptly so near the edge that it did not show in 
the photograph. The cracks also turned or strengthened when 
they approached larger breaks as shown at m in Fig. 11, at 6 and 
c in Fig. to, and from 6 toc in Fig. 9. Wherever these fine cracks 
approached free edges they immediately turned to the 45° position. 
In practically all cases the cracks became stronger near such edges 
as is shown in the photographs and indicated in the drawing. 
Obviously, in rocks there would be few such places where the 
material acted on ended abruptly. In practically all cases deforma- 
tion would be resisted by strong molecular forces at the sides and 
usually by the influence of overlying beds. Hence the cracks 
obtained by Daubrée and those at the edges and surfaces of the 
paraffin blocks could not be expected closely to imitate joints. 
More satisfactory results would be expected near the center of the 
block where the molecular forces would have a normal effect. 
Near the edges there were often irregular cracks making small 
angles with the pressure like those shown at the left edge of Fig. 11. 
On the smaller semi-elliptic surfaces they were often forked, as at 
n and 0, so that the portions of the break made equal angles with 
the line of pressure. Usually at some distance from the edge and 
beyond these irregular breaks the network of cracks appeared. 
They usually started at a fairly even distance from the edge, 
though often a few extended backward to the edge. At first they 
made oblique angles with each other, the line of pressure bisecting 
the acute angle. Soon they spread to right angles and then to 
larger angles near the outcrop of the slipping plane /z. There the 
line of pressure cut the obtuse angle. The cracks sometimes 
