OBSERVATIONS AND EXPERIMENTS ON JOINT PLANES 169 
Along hi slipping has taken place but 72 represents the condition 
just before breaking at the surface. The outcrops of the slipping 
planes occurred on either the face b/gd or aec or both, depending 
upon the application of the pressure. 
With the distance between the jaws of the vise small compared 
with the length and especially with hard material at the lowest 
temperatures, the blocks often broke in planes nearly parallel to the 
end faces abef and cdg. The break through the middle was nearly 
a plane face and the breaks toward the ends were convex outward, 
resembling a pile of thin cards compressed at the ends so that the 
cards bow out. In these breaks the faces separated instead of 
slipping on each other and slickensiding. With material cooled 
rapidly to a low temperature these faces were usually covered with 
even, featherlike markings which were of interest because they 
closely resembled the patterns which J. B. Woodworth’ found 
on the faces of joint planes. The patterns consisted of a central 
smooth axis from which extended scales of the paraffin shaped like 
a half-crescent with the point toward the axis. The scales were 
free along the concave margin and passed into the material along 
the convex edge. The position of the axis of the feather depended 
on the application of the pressure and the homogeneity of the 
material but was usually near the middle of the breaking plane. 
With similar hard material and the pressure applied to points 
as along fg the breaking took place along surfaces shaped much 
like half a bell with the rim in the face bfdg. These bell-shaped 
surfaces were also covered with half-crescent scales with their 
points toward the place where the thrust was applied and their 
broader portions radiating outward. 
Such markings are associated with separation of faces rather 
than slickensiding. Further study of them might be of interest 
in connection with the question of whether joint faces are separated 
at some time in their formation or are always held tightly together. 
Such markings were not seen on the joint faces of the Ithaca 
region. 
NETWORK OF CRACKS 
Besides the large breaks, there was a system of intersecting even 
cracks similar to the fine network found by Daubrée. He compared 
1 Proc. Boston Soc. Nat. Hist., XXVII (1896), 163-83. 
