472 STUDIES FOR: STUDENTS 
will be a plane in which are the major and mean axes of the 
mineral particles, and this will be the plane of cleavage. It is 
evident that this plane will vary in position from place to place. 
As shown by Professor Hoskins, these conclusions follow from 
the fact that all of the different compressions, acting in different 
directions at different times, are equivalent to some single com- 
pression acting continuously in one direction, and the shearing 
motion of differential movement is equivalent to a compression 
combined with a rotation. 
Very frequently the position of the cleavage with reference 
to the bedding for a given small area will be controlled by the 
readjustment between the beds. The structure will tend at any 
moment to develop in the normal planes, but as a consequence of 
the shearing the material is rotated so that the final direction of 
cleavage is inclined to the direction of greatest pressure (see Fig. 3). 
Since the readjustment is mainly concentrated within the weaker 
layers, the structure may be so rotated in them as to approach 
parallelism with the beds, while in the adjacent stronger beds, in 
which there is less differential movement, the structure may be 
nearly at right angles to the beds ; and at various positions between 
the cleavage will have intermediate directions. There will thus 
be developed a cross cleavage in the more resistant beds, which 
varies by a gentle curve into parallel cleavage in the weaker beds. 
On opposite sides of the layer the curves are in opposite direc- 
tions (Fig. 10). By these curves it is easy to determine the rela- 
tive directions of the movements of the layers. 
Directly following from the above we have the explanation 
of the step cleavage’ of the older authors and of Becker,’ and 
of the differential cleavage of Dale. In this case the change in 
the direction of cleavage, instead of being gradual, as it is in 
rocks of gradually varying plasticity, is somewhat sudden in pass- 
ing from one bed to another differing in rigidity. Asa result of 
tOn Slaty Cleavage and Allied Rock Structures, with Special Reference to the 
Mechanical Theories of their Origin, ALFRED HARPER, Brit. Assoc. Ady. Sci., 55th 
meeting, 1885, Proceedings, pp. 829-830. 
2 Finite Homogeneous Strain, Flow, and Rupture of Rocks, GEORGE F. BECKER, 
Bull. Geol. Soc. Am., Vol. IV, pp. 84-87. 
