DEFORMATION OF ROCKS 48 3 
CLEAVAGE AND FISSILITY MAY DEVELOP AT THE SAME DEPTH. 
It is clear that in heterogeneous rocks cleavage and _fissility 
may develop in beds of different character at the same depth, 
for weak beds may be in the zone of flowage at the same depth 
at which strong beds are in the zone of fracture. For instance, 
the fine-grained argillaceous beds within a formation may develop 
cleavage, while the coarser-grained siliceous beds may develop 
fissility. Thus may be explained some of the cases of change in 
direction of the secondary structures in passing from one to the 
other. Also,abed under a certain weight may be in the zone of 
fracture if rapidly deformed, and in the zone of flowage if slowly 
deformed. As has been seen under the subject of deformation 
(p. 212), the middle zone of combined flowage and fracture is 
probably 5000 meters thick, and it may be thicker, and through- 
out this zone either cleavage or fissility may be formed. It is 
only in the zone of fracture that fissility alone can form, and 
only in the deep-seated zone of flowage that cleavage alone can 
form. C. R. Van Hise. 
