OBSERVATIONS AND EXPERIMENTS ON JOINT PLANES 173 
occurred in the same places as the larger diagonal cracks, so that 
the surface showed two sets of opaque 45° cracks with these fine, 
wavy lines superposed upon them over the whole surface and 
arranged nearly at right angles to the pressure. Where the thrust 
decreased laterally there was a suggestion of a change in direc- 
tion which would make the average direction of the cracks slightly 
concave toward the thrust. 
When the paraffin was pared down so that a section from the 
interior could be studied by transmitted light it was found that in 
sections several millimeters thick there were opaque diagonal lines, 
but these lines no longer made angles of go° with each other and 
45° with the pressure as at the surface. The angles between the 
two were usually about 70°, with the line of pressure bisecting the 
acute angle. These were commonest where the thrust was 
unusually strong. 
When the paraffin was cut to a thickness of about a millimeter 
these opaque lines disappeared and the material was seen to be 
full of fine, sharp, wavy cracks about a millimeter long like those 
shown in Fig. 13 but parallel to the pressure. Examination with 
a hand lens did not show that they were arranged in diagonal rows, 
but, since it was found uniformly that a thicker section gave broad 
opaque lines and these lines disappeared entirely in a thin section 
and fine wavy lines appeared in their place, it seemed, from analogy 
with the sigmoid lines and diagonals of the upper surface, that the 
breaking within the mass of paraffin consisted of fine cracks nearly 
parallel to the pressure, the cracks arranged in diagonal series so 
that by superposition they gave opaque lines in a thick section. 
Thus in passing from the unnatural surface conditions to the 
interior of the mass where conditions would be more similar to those 
in the rocks the angles between the larger lines decreased so that 
the lines made a smaller angle with the pressure and the smaller 
cracks of which each line was made became straighter and entirely 
separated from each other. Here was a set of cracks uniformly 
present within the mass subjected to pressure, the cracks each 
nearly parallel to the line of pressure and suggesting an arrange- 
ment in two sets with the pressure bisecting the acute angle. 
Unfortunately the fine lines at right angles to the pressure 
