DEFORMATION OF ROCKS 481 
secting structures may be explained as original developments, 
but the former places the direction of greatest pressure in the 
obtuse angle, while the latter, basing his conclusion on experi— 
ments, places it in the acute angle made by the two structures. 
The phenomena may be partly explained by supposing that after 
the two diagonal structures developed in the shearing planes, 
the pressure continuing to be applied with a force between the 
elastic limit and ultimate strength of the rock, there was a rota- 
tion of the two sets into approximate parallelism, just as there 
is when a net with rectangular meshes is pulled out until the 
intersecting lines are almost parallel. Professor Hoskins offers 
another explanation of this transition, as follows (doc. cit.): In 
an early stage of the development there may have been flowage 
of the rock sufficient to produce a more or less perfect cleavage. 
The rock, as a result of more rapid deformation or of erosion, 
may then pass to the zone of fracture. If the stresses remain in 
the same direction the fractures would not take place along 
planes of greatest shearing stress, but tend to approach the 
planes of flattening. In proportion as a previous cleavage was 
prominent, a secondary intersecting fissility in two directions 
might very nearly approximate in position with the cleavage. 
Doubtless in many cases the different causes above given com- 
bined to produce acutely intersecting fissility. 
FISSILITY SECONDARY TO CLEAVAGE. 
For any given layer in a horizontal position in the zone in 
which fissility is developed, it is probable that horizontal thrust 
is great in proportion to gravity. If gravity were wholly 
neglected the greatest shearing stress would be at Asian COmat ne 
bedding. In the zone in which cleavage is developed it is prob- 
able that gravity is very important as compared with tangential 
thrust. If these are supposed to be equal, the direction of 
greatest normal pressure would be inclined at 45° to the bed- 
ding, and the cleavage planes at right angles to this would also 
be equally inclined to the bedding, but in an opposite direction. 
It therefore follows that fissility developing in heterogeneous 
