174 PEARL SHELDON 
which were seen abundantly on the surface were not found within 
the mass, though certain fine lines may have been their representa- 
tives. If so, they were smaller than at the surface and too fine 
compared with the grain of the material itself to be studied with 
any accuracy. All of these fmer cracks, both at right angles to, 
and parallel to, the pressure, could be studied only in thin sections 
by transmitted light and required a lens, but they were practically 
the same throughout the large number of specimens in which they 
were found, so that their general character was evident. The 
cracks were too fine for a determination of their inclination, but it 
probably was not large, since the cracks appeared as sharp lines 
instead of broad, opaque lines as would be the case if they ran 
diagonally through the material. Both of the sets of finer cracks 
appeared in the central unruptured portions of the paraffin as well 
as in the broken parts. The material appeared unbroken but when 
it was held before a light the places of greater thrust were found to 
be opaque, and examination with a lens showed the fine lines which 
caused the opaque appearance. 
The experiments were carried about as far as possible under 
these conditions, since the most interesting cracks were too fine 
for study with the naked eye and the irregularities of the material 
were large compared with the size of the cracks. Further study 
should be made with finer-grained material which could be studied 
with a microscope, or the conditions should be such as to give 
larger cracks. 
DEDUCTIONS 
AGE OF THE JOINT PLANES 
STRIKE JOINTS 
It is apparent that the strike joints were formed in connection 
with the low folds of this region. Their average strike corresponds 
too closely with the axes of the folds to be accidental. If they had 
been formed before the folding with a uniform, nearly vertical 
inclination they would now be nearly at right angles to the bedding 
planes; that is, if the planes of the joints were produced they 
would meet below the axes of anticlines and above the axes of 
AL ole 
