172 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
At a later stage the cross-fractures c came into existence, marking 
out in a zigzag course the path of the joint-plane proper in the 
adjoining rock. (See Plate 1, fig. 8.) 
Where the plates of rock formed by the b-planes crossing 
obliquely between contiguous blocks, remain unbroken, by the 
failure of e-fractures to form, the incipient joint may offer con¬ 
siderable resistance to rupture. The exposure of the joint-fringe to 
weathering may produce the final rupture which completes the 
joint-plane. Earthquake shocks may do the same thing as Crosby’s 
experiments (’93) would go to show. Becker (’94) is inclined to 
attribute the failure of sojne joints to gape to the force exerted by 
capillary water, but this action can only have more than a temporary 
effect below the zone where the seasonal changes of temperature 
cross the freezing point. 
In most surface stations, the presence of water is undoubtedly by 
reason of freezing a cause of the breaking of the bridges of rock 
which unite the opposite walls of the joint. Since invariably the 
c-fractures have been the last formed in the production of the joint, 
we must suppose the unbroken oblique plates or bridges exerted up 
to the time of the rupture much retaining strength. 
Artificial origin of some c-firactures : — The freshness of some of 
the c-fractures in rocks recently quarried leads to the conviction that 
the fractures are frequently produced in the process of quarrying. 
An examination of many quarries and natural ledges leads me to con¬ 
clude that straight medial cross-fracture in the joint-fringe is the 
normal form and that curved lateral cross-fracture is generally pro¬ 
duced when the joint-fringe is ruptured by a force acting independ¬ 
ently of the jointing process as in the artificial work of the quarry- 
man. Thus curved lateral fracture is very common in the cement 
quarries of the water lime at Buffalo, N. Y. 
Joint interval in the fringe: — The interval between the b-planes 
in the fringe is tolerably constant in the same joint. It will be 
noted in Plates 4 and 5 that this interval at the point of origin of 
the b-planes at the edge of the joint-plane is extremely small, and 
that it increases with the distance from that edge. In the specimens 
which I have studied this interval corresponds closely to that 
between the c-fractures at their point of origin on the edge of the 
joint-plane. In a general way, the interval between the b-planes in 
the fringe increases with the size of the principal joint. In granite 
it is often 12 inches. 
