TIIE STRUCTURE OF ROCK MASSES. 
119 
lines of greatest weakness, but, in this case, as is usual in all 
good roofing slate, the tendency to separate or split along the 
lines of bedding has been altogether obliterated by the effects 
of cleavage. 
The section, fig. 1,P1. LVII. (Carnarvonshire), serves to in- 
dicate the mutual relations of the planes of cleavage and strati- 
fication on the large scale. The beds of clay slate, b b, with 
the intercalated grits c?, are disposed in a curve resting on the 
one side against a mass of quartz porphyrite, the boss of the 
mountain being also composed of porphyrite, with however, 
little or no visible quartz : the cleavage of the slate is indicated 
by the lines c c , and is developed at a high angle across the 
stratification. The cleavage planes in North Wales are very 
commonly found to be at angles of from 30° to 40° with that 
of the stratification. 
The development of the cleavage planes, and consequently 
the facility with which such cleaved rock will split up into 
slates, is entirely dependant upon the mineral nature of the 
original beds, for it must be recollected that cleavage is invariably 
a superinduced structure. When the beds are of a coarse cha- 
racter, the cleavage may not be developed at all, but it becomes 
more and more perfect in proportion as the rock constituents 
.are themselves more fine and homogeneous ; when alternating 
beds of grit and slate occur, the cleavage, which may be perfect 
in the latter, is at most very imperfectly developed in the 
former. When, as depicted in fig. 2, PL LVII., a thin bed of 
hard quartzose character intervenes between two beds of cleaved 
slate rock, the cleavage lines are stopped by this bed, but re- 
commence on the other side of it, whilst, at the same time, the 
hard bed is usually more' or less fractured by fine joints, which 
■do not follow the angle of the cleavage, but are, in most cases, 
nearly perpendicular to the plane of the bed itself.* 
A somewhat analogous effect takes place when the slate rock 
is composed of alternating thick and thin beds, all of slate rock, 
but differing considerably in hardness ; in this case, the cleavage 
cuts through the whole, but instead of, as usual, following a 
perfectly straight line, it bends in and out at the junction of the 
soft and harder beds, so that a slate split of such rock may, in 
the length of a foot, show some six or eight bends in and out, 
like a sort of wave line, which commonly is rendered still more 
striking to the eye by being alternately tinted of a greenish and 
* When cleavage lines cut through beds on the surface of which ripple 
marks exist, this structure is curiously affected, being distorted so that the 
ripples are compressed in one direction, and elevated like ridges or waves in 
the other. The attempt made to show this structure in fig. 2 is, unfortu- 
nately, not a success. 
