172 
PEARL SHELDON 
as much from 45° as the direction did in the parts of the block away 
from the influence of the ends. 
Fig. 11 shows the general appearance of the cracks with the 
exception that the lines were seldom sharp except near the edges. 
Fic. 12 
They were usually blurred by the crushing 
of the material, as shown in the photo- 
graphs. 
Closer examination of some of the 
diagonal cracks on the upper faces showed 
that the lines were not continuous but 
consisted of a series of more or less sigmoid 
lines arranged in diagonal rows, as shown 
on an enlarged scale in Fig. 12. Some- 
times the diagonals consisted of a series 
of steplike fine cracks, as shown in the 
lower left-hand corner. Evidently where 
the thrust was sufficient to cause rupture 
the breaking took place along the diagonals, 
but a lesser thrust left the series of smaller 
cracks which had been formed first. These 
cracks were usually a millimeter or two in length. 
FINEST CRACKS 
When the material was examined by transmitted light with a 
hand lens it was found that still other cracks were present. Over 
the upper surface were many 
lines like those shown enlarged 
in’ Pies 13. 2 hey looked like 
Titin oraniter: whey) awere 
about a millimeter long and 
were fine and sharp. They 
occurred especially where the 
thrust was unusually strong, in 
front of the semi-elliptical 
breaks, though they were often 
present between the slipping 
—a 
— — 
(ea we x Rays 
cam Lie a ae oN a 
Gi aa PSS 
a ee a 
—_— anaes LSS SS 
Fic. 13 
plane and the face where the pressure was applied. They often 
tR.S. Tarr, Am. Jour. Sci., 3d ser., XLI (1891), 267-72. 
