THE STRUCTURE OF ROCK MASSES. 
121 
the river Wye cut across the anticlinal of the strata, and con- 
sequently cut across the same beds in two different directions,, 
i.e. dipping both with and against the underlay of the beds. 
That the inclination or dip of the cleavage planes varies 
infinitely more than the strike is quite certain, and observation 
shows that, in many cases at least, the cleavage planes on each 
side of an anticlinal may dip inwards towards the axis of eleva- 
tion, consequently in opposite directions, so as to present a fan- 
like arrangement ; this was specially noticed by Darwin in South 
America, and subsequently by other geologists. The late Mr. 
D. Sharpe attempted to generalise on these facts, and brought 
forward the hypothesis that the strike of the cleavage, when 
studied on the large scale, followed a general direction, but that 
the varying lines of dip formed a series of anticlinal and syn- 
clinal “ systems of cleavage,” extending over vast areas like 
a series of immense arches. It is to be feared, however, that 
subsequent researches in the field have not corroborated this 
hypothesis. 
From the direction of the strike of the cleavage in some of 
the districts first examined it was at once taken for granted that 
the normal direction of cleavage planes would be exactly east 
and west of the magnetic north, and this gave rise to nume- 
rous attempts to account for this structure by the action of 
magnetic or electric currents ; it required, however, but a little- 
more extended observation in the field to prove this generalisa- 
tion unfounded. Sedgwick also announced that “the strike of 
the cleavage is nearly coincident with the strike of the beds,” 
which is, no doubt, often the case in districts in which the 
direction of the beds is tolerably uniform, but quite at fault 
when the stratification is of a more complicated and contorted 
character. Professor Phillips, in 1843, arrived at the conclu- 
sion with respect to the slate rocks of Wales, that the cleavage 
planes are always parallel to the (( main direction of the great 
anticlinal axes, but are not affected by the small undulations or 
contortions,” and this conclusion appears to hold good in the 
majority of cases ; yet exceptions are not wanting, as Professor 
Phillips has himself pointed out. 
In summing up the evidence it can only be stated, that it 
appears certain that there is a decided tendency in the cleavage- 
planes to follow, more or less, the strike of the strata or axis of 
elevation. Yet, at the same time, the inclination of the cleavage 
planes does not appear to have any connection with the dip of' 
the stratification, and it appears most in accordance with facts 
to assume, that both the strike and dip of the cleavage planes 
are entirely governed by local circumstances. Since cleavage 
is not known to occur in normal sedimentary rocks, it is 
evidently dependent upon the local disturbance of the strata,. 
