SLATY CLEAVAGE. 
23 
His figure is here repeated (see fig. 1) with the construction added. « 
The gist of this is that if we knew the dip of the hard beds on either side of a bed 
of slate we could foretell the cleavage dip of the slate. This, however, would be 
applicable only where no secondary disturbance of sufficient force to disturb the 
relations had occurred. 
Relations of cleavage to axes of folds. — Some pre-Cambrian and Paleozoic schist 
masses have two transverse systems of folding which within limited areas interfere 
with one another; both systems are also intersected by a cleavage with a constant 
strike different from that of each.& Where only one system of folding occurs the 
strike of the cleavage is not necessarily parallel to that of the bedding. In such a 
case the cleavage is attributed to a change in the direction of the pressure. <' 
The cleavage planes of the slate rocks of North Wales are always parallel to the main direction of 
the great anticlinal axes, but are not affected by the small undulations or contortions of these lines. 
The strike of the cleavage in a district is far more constant and regular than the strike of the beds.d 
t* !,: 
Fig. 1.— Diagram from Gosselet showing relations of cleavage of slate to dip of inclosing hard heds. 
To these facts should be added this — that in the case of a pitching fold the cleavage, 
although parallel to the axis of the fold, must necessarily intersect the strikes of the 
sides of the fold. Phillips gives a section from Sedgwick,' which lie calls a local 
exception, in which the cleavage planes, while coinciding in strike with that of the 
anticline which they traverse, incline on either side of it toward its axis. Rogers/ 
describes a case of this fan-like cleavage in an anticline. Such a structure could be 
produced by secondary movement creating an anticline in horizontal beds already 
possessing a vertical cleavage, and in the synclinal part of the fold the fan structure 
would radiate downward. 
Sorby figures from Ilfracombe, North Devon// a small, highly plicated bed of coarse- 
grained light-colored sandy slate traversing a mass of vertically cleft shaly slate. 
The gritty beds show a coarse and imperfect fan-like cleavage which curves slightly 
around the anticlines into the synclines. Here the fan structure seems to be due in 
part, at least, to the deflection of the cleavage by the coarser material, ami there is 
no need of supposing a secondary movement. The fine and more plastic material 
has developed a vertical cleavage which in the coarser has become rudely fan like. 
a Gosselet, Les schistes de Fumay, pp. 68. 69, fig. 5. 
bLoretz, Ueber Sehieferung, pp." 69-70. 
elbid., pp. 83,84. 
dSharpe and Phillips, Contrib. to Geology of North W 
1846. 
e Report on cleavage, p. 374. 
/Geol. Surv. Pa., vol. 2, part 2, p. 903, fig. 715. 
a On the origin of slaty cleavage, p. 138. 
tmart. .lour. Geol. Soc. London, vol.2. 
