DEFORMATION OF ROCKS 453 
From the foregoing it is believed that vock-cleavage 1s due to 
the arrangement of the mineral particles with their longer diameters or 
veadiest cleavage, or both, in a common adtrection, and that this 
arrangement ts caused, first and most important, by parallel develop- 
ment of new minerals ; second, by the flattening and parallel rotation of 
old and new mineral particles; and third, and of least importance, by 
the rotation into approximately parallel positions of random original par- 
ticles. The propriety of the definition of rock-cleavage as first 
given is therefore evident. It is always due to a capacity to 
part, and very frequently the parting does occur by the actual 
cleavage of the mineral particles. 
DEVELOPMENT OF CLEAVAGE IN HOMOGENEOUS ROCKS. 
Becker has recently rediscussed the origin of cleavage, and 
concludes that it always develops in the shearing planes rather 
than in the normal planes. Even in the case of the experimental 
development of a cleavage-structure in wax, which is strictly 
normal to the pressure, the structure is explained as developing 
in the shearing rather than in the normal planes. 
As will be seen below, it is my own conviction that a struc- 
ture develops in the normal planes under certain conditions, and 
that under other conditions structures develop in the shearing 
planes, as advocated by Becker. The first is believed to bea 
deep-seated phenomenon of the zone of flowage; the second is 
believed to be a more superficial phenomenon in the zone of 
fracture. In other words, as already stated, it is thought that 
under the term cleavage two entirely distinct structures of differ- 
ent origins have been confused. Theories which explain or partly 
explain one of these structures have been extended to cover both 
of them, because it was not understood that they are different. 
Rocks when deformed under great weight flow as a plastic 
solid, and under these circumstances, as shown by the geologists 
above cited, the property of cleavage is developed. At all 
times the particles of the rock are welded together. Fissility 
will not form, for by the supposition the rocks are so deeply 
buried that no crevices can exist. In the formation of flow- 
