466 STUDIES FOR STUDENTS 
geneous, and as the direction of greatest normal force is always 
compounded of thrust and gravity, the directions of the fissility 
may vary considerably. 
In case the parting is very close, the rock is foliated. Each 
lamina moves slightly over the adjacent lamine. The rubbing 
of the laminz over one another, due to the differential movement, 
gives the slickensided surfaces which are so common on both 
sides of the parted lamine. The more intense the movement, 
the thinner and more brilliant do the folia become. Since part- 
ings are usually inclined to the bedding, this structure may be 
called cross fissility. 
In passing from the zone of fracture to the zone of flow it is 
to be expected that all gradations would be found between the 
development of cleavage in the normal planes and the develop- 
- ment of fissility in the shearing planes. This point is discussed 
later. (See pp. 480-481.) 
Frequently fissility forms in lithologically homogeneous 
rocks in which the property of cleavage had already been devel- 
oped, and which by subsequent denudation are brought so near 
the surface that the superincumbent weight is less than the 
strength of the rocks. When subjected to stress under these 
circumstances numerous fractures develop along the cleavage 
planes. This occurs because fractures take place so readily along 
these planes, and because the chances are always that these are 
shearing planes, although they may not be those of maximum 
shearing stress. 
This may be the case although the direction and force of 
thrust may not have varied. The direction of greatest normal 
pressure, combined of gravity and thrust, would be in a different 
direction when the rocks are in the deep-seated zone of flowage 
and when they are in the superficial zone of fracture. Thus, 
cleavage in the normal planes would pass into the shearing planes 
as denudation progressed. 
In cases in which the fissility develops along a prior cleav- 
age, the fissility parallel to these planes would be marked. The 
fracturing along the other set of diagonal shearing planes, not 
