DEFORMATION OF ROCKS Ay 
DEVELOPMENT OF CLEAVAGE IN HETEROGENEOUS ROCKS. 
For a given very small area in a stratum the cleavage may 
vary from parallel to perpendicular to the bedding. If at any 
moment the direction of greatest normal pressure isin the direc- 
tion of bedding, the secondary structure will tend to develop 
at right angles to the bedding. If, on the other hand, the direction 
of greatest pressure isat right angles to bedding, the cleavage will 
tend to develop nearly parallel to the bedding. Between these two 
there will beall gradations. For different beds and for different 
areas in the same bed the direction of greatest pressure will, at 
successive times, vary both relatively and absolutely, and there- 
fore the cleavage developing in the normal planes at any given 
moment will gradually vary in direction. For a small, practic- 
ally homogeneous area the final resultant will be the same as in 
homogeneous rocks (p. 457)—that is, the cleavage will be per- 
pendicular to the direction of greatest final shortening. 
Having made this general statement, let us inquire more 
particularly how the forces producing cleavage act in hetero- 
geneous rocks. 
As a result of the folding, combined with denudation, the 
relative amounts of thrust and gravity vary at different times, 
so that the average direction of greatest normal pressure for a 
given area also varies. On account of the differing strength of 
the beds the forces received by them are decomposed to a 
greater or less degree into directions parallel and normal to 
themselves, and thus the direction of greatest pressure is varied 
at each particular point. During the folding process the incli- 
nation of the beds is constantly changing, and as a consequence 
any given small area is being rotated with reference to the 
direction of greatest pressure. The problem is further com- 
plicated by the readjustment between the beds, which constantly 
changes the relative position of the material with reference to 
the pressure. Lastly, the readjustment usuaily takes place 
mainly within the weaker layers, and this changes by a varying 
amount the relative positions of different parts of the beds with 
reference to the greatest pressure. At any given point there 
