482 SLODIES HORS RUDEN TLS: 
rocks in the shearing planes and cleavage developing in rocks 
of the same character in normal planes may have like 
directions with reference to the bedding. If cleavage be 
developed in the normal planes inclined to the bedding, 
and by denudation this stratum passes to the zone of fracture, 
as a consequence of the lessening power of gravity, these normal 
planes are now shearing planes, and fissility is controlled in direc- 
tion by the previous cleavage structure. It thus becomes evi- 
dent that it may be exceedingly difficult, if not impossible, to 
discriminate between original fissility in the shearing planes and 
a secondary fissility which has been controlled by cleavage. 
In other cases, as well as these special ones, it is to be expected 
that a rock in which cleavage has developed under deep-seated 
conditions would be ruptured before reaching the surface dur- 
ing the long time it is in the zone of fracture. To this expect- 
ation the facts correspond. In all regions with which I am 
acquainted having well-developed cleavage, fissility is also found 
to a greater or lesser degree. Usually the fissility is more 
marked here and less marked there, for where a fracture or set 
of fractures has been formed, there it is easier for further move- 
ment to occur. Hence belts of strongly fissile rock are sepa- 
rated by others in which there is but slight fissility. In general, 
however, fractures along shearing planes occur near together or 
wide apart within all rock-masses showing cleavage. To this 
the confusion of the two structures is doubtless largely due. It 
may be that the use of the term ‘‘fissility”’ should be restricted 
to a structure developing secondary to cleavage or to bedding, 
and that original fractures, developing along shearing planes, 
independent of any previous structure, should be called joints. 
However this may be, it appears probable that where a rocix is 
broken into very thin laminz in a uniform direction for a con- 
siderable area, the secondary structure originally developed as 
true cleavage in the normal planes. I therefore conclude that 
fissility developing in the shearing planes 1s usually secondary to cleav- 
age which developed in the normal planes. 
