470 SLWQLDIGES, JHU SL QUIN TCS, 
shearing extends from these into the harder ones, for when 
motion has once begun along a certain set of planes, that very 
movement weakens the rocks along these planes and makes it 
Parallel fissility on the 
limbs of the folds and 
cross fissility on the anti- 
clines, and gradations be- 
tween the two. After 
Heim. 
The deformation is 
mainly by folding, but on 
the anticlines, where the 
material is partly relieved 
from stress, the deforma- 
tion is partly by multiple 
minor faulting. 
easy for the forces applied in any direc- 
tion to be decomposed into forces pro- 
ducing further movement along the same 
planes. 
The maximum force producing shear 
is composed of horizontal thrust and 
gravity, and its direction will therefore 
usually be inclined to any layer in a 
horizontal position, although if gravity 
is unimportant as compared with thrust 
this inclination may be slight. When 
the layers become tilted this inclination 
is marked, although the problem becomes 
more complicated on account of the 
strength of the individual beds. These 
tend to decompose the forces which they 
receive at one point into components 
parallel and normal to themselves, and 
transmit the longitudinal thrust to an- 
OQuner join,  Woom thre Imos or tne 
folds the beds are, however, never free 
from superincumbent weight, and the rig- 
idity of the overlying beds must also be 
Oitemmovercome. 9 Therefore) ane mearhy, 
all cases of heterogeneous rocks the 
direction of greatest normal 
inclined to each layer. 
stress is 
There are two 
classes of cases—those in which the 
rock bed is in the zone of flowage, and those in which the 
rock bed is in the zone of fracture. 
The first afford conditions 
for the development of cleavage; the second, those for the 
development of fissility. Between the two there may be grada- 
tions. 
