OU DIES BOR SLUDENTS. 
DEFORMATION OF ROCKS.—III. CLEAVAGE AND 
LUIS SIUE LADO 
CONTENTS. 
Definition of cleavage and fissility. 
Development of cleavage in homogeneous rocks. 
Development of fissility in homogeneous rocks, 
Development of cleavage and fissility in heterogeneous rocks. 
Development of cleavage in heterogeneous rocks. 
Development of fissility in heterogeneous rocks. 
Relations of cleavage and fissility to each other. 
Gradation between cleavage and fissility. 
Fissility secondary to cleavage. 
Cleavage and fissility may develop at the same depth. 
?Relations of cleavage and fissility to other structures. 
Relations of cleavage and fissility to bedding. 
Relations of cleavage and fissility to thrust faults. 
Relations of cleavage and fissility to thickness of strata. 
Development of cleavage by other causes than thrust. 
Modifications of secondary structures. 
Application to certain regions. 
Relations of cleavage and fissility to stratigraphy. 
DEFINITION OF CLEAVAGE AND FISSILITY. 
THE property of cleavage in rocks is here defined as a 
capacity present in some rocks to break in certain directions 
more easily than in others. By virtue of this property rock 
masses may be split into slabs or into leaves. The term cleav- 
age is taken from a property in minerals, and is here confined 
to a strictly parallel usage. This definition does not agree with 
‘In preparing this section I am greatly indebted to a paper by L. M. HoskIns on 
Flow and Fracture of Rocks as Related to Structure. Without his discussion this sec- 
tion would have been far more imperfect than it is.) PROFESSOR HOSKINS’ entire 
paper will appear in connection with my own full paper in the Sixteenth Annual Re- 
port of the U. S. Geol. Survey. 
2 The part of this paper covered by the contents below this point will appear in 
the following number of the JOURNAL. 
449 
