DEFORMATION OF ROCKS 467 
having the advantage of a previous cleavage, would be much 
less marked, the deformation perhaps largely occurring by con- 
siderable movements along a few planes of shearing. In this 
case the infrequent cracks may properly be called joints. In the 
planes of fissility secondary to cleavage the folia slip over one 
another just as in the case of originally developed fissility inde- 
pendent of cleavage, producing slickensided surfaces, and the 
d ’ 
rock develops a ‘‘fault slip” cleavage or ‘“ausweichungs”’ cleav- 
age. In the majority of cases in which the fissile laminz are 
close together and have a uniform direction for a considerable 
area, it is probable that the structure first developed as true 
cleavage in the normal planes, and that a later movement, 
when the rock was nearer the surface, developed the fractures 
along the shearing planes. The structure is thus a product of 
the forces producing cleavage and those producing fissility work- 
ing successively under different conditions, 
As will be seen below, fissility may form, in a manner simi- 
lar to its development parallel to cleavage, parallel to planes of 
weakness of any other kind, as, for instance, bedding. 
The directions of the fissility are therefore dependent upon 
the direction of the forces, upon whether they are equal or 
unequal, upon the superincumbent load and the consequent fric- 
tion, upon the cleavage and other previous structures, and upon 
previous folding. 
If fissility develop along cleavage, as this structure is 
usually steeply inclined, the fissility will be in the same direc- 
tion. That great thrust faults sometimes develop with nearly 
horizontal hades, or even parallel to bedding, is well known. 
Multiple minor thrust faults or fault slips producing fissility may 
similarly develop with flat hades, upon the same principles as 
cleavage, in a like direction, and thus produce a fissility nearly 
or quite horizontal, or parallel to bedding. It is therefore clear 
that the structure may vary from a vertical to a horizontal atti- 
tude. There are therefore all gradations between cross fissility 
and parallel fissility, described in the following number ( Fig. 
7), just as between cross cleavage and parallel cleavage. In 
