464 STUDIES HOR STUDENTS: 
arrangement not so nearly perfect in the original sediment as in 
a slaty or schistose rock. If a maximum deforming force be 
normal to the bedding, as it may be in some cases, the original 
imperfectly parallel arrangement of the mineral particles, with 
their longer axes in the direction of bedding, would be so much 
done toward producing a cleavage, and the result would be to 
give a more highly developed secondary structure parallel to the 
bedding with a given amount of movement than would result 
from the same deformation in a different direction. 
In some instances, as has been seen, observations show that 
the conjoint action of tangential thrust and friction throughout 
considerable masses of rocks was apparently such as to give 
approximately uniform shortening of the strata in one direction. 
‘The material is confined on all sides, and its deformation is that 
of a plastic solid. The direction of least resistance is toward 
the surface of the earth. The direction of mean resistance is at 
right angles to the minor thrust and parallel to the surface. 
Consequent upon the flow, the mineral particles, new and old, 
at any particular point arrange themselves with their longer axes 
in the plane of these directions. This plane is vertical, or nearly 
so, and almost at right angles to the major thrust. Therefore, 
in approximately homogeneous rocks cleavage is frequently ver- 
tical or highly inclined, and hence intersects the bedding. This 
may be called cross cleavage. 
But supposing the lateral thrust to vary greatly in depth, and 
considering that friction increases with depth, the movement 
may vary, as explained above, from upward plastic flow to differ- 
ential movement in a horizontal direction. In this change we 
have passed from a pure shortening to a simple shearing. If 
the movement was sufficient the cleavage would be nearly paral- 
lel to the bedding, and it may be called parallel cleavage. In the 
case of parallel shear the cleavage would accord in its principle 
of development with that of parallel cleavage in heterogeneous 
rocks, subsequently described (see pp. 472-480). 
These extreme cases rarely occur. Almost invariably there 
is a combination of pure shortening with shearing motion. The 
