File:The Quarterly journal of the Geological Society of London (12645102823).jpg

114
PROCEEDINGS OF THE GEOLOGICAL SOCIETY. .Nov. 1,
direction. Its tendency to split with different degrees of resistance
in different directions is the consequence of this arrangement and
form of its constituent parts, and is therefore only a secondary cha-
racter.
Two planes of cleavage in slate. — In preparing slates, the first
process is to split them along the planes of cleavage to the thickness
required, they are then to be cut to size ; most slates of good quahty
can be split tolerably straight down the sides of the sheet by apply-
ing the tool at the top, from which we see that besides the true or
principal planes of cleavage, there is also another direction in which
the rock has a tendency to split in a slighter degree ; this may be
called the plane of secondary cleavage ; the workmen call it the " side
of the sheet." The ends of the sheet must be chipped or sawed to
the size required, as the rock will not split at all in this direction,
which is across both the other planes of cleavage : in fact it will give
way in any other direction more easily than in this, which may be
called the plane of greatest resistance. .
A diagram will make this more intelligible, and will also explain
Y\o- 3 . the relation which these va-
rious planes have to the po-
^^^^^-^^czzjj •=. .=ir~' cS'A .^^^^-^h ^ ®^t^^^ ^^ ^^ constituent par-
"'iXly \y i j/ \ ry^j^TlT/V)/ tides of the slate. Let us
suppose fig. 3 to represent a
block of slate, of which ah cd
is one of the planes of clea-
vage, ad and he showing the
dip, a h and d c the strike of
the cleavage ; h cgf is the
plane of secondary cleavage,
a bfe is the plane of greatest
resistance. The component
parts of the rock are repre-
sented enlarged, of an almond
y- shape, with their largest and
flattest sides parallel to the
true cleavage-plane ah cd, and with their longest diameters in the
direction of the dip of the cleavage a d.
It is obvious that wdth such a distribution of its particles, the rock
can offer less resistance to an attempt to split it down the plane of
cleavage ahcd, than in any other direction ; for that plane, and all
planes parallel to it, pass along the flat surfaces of many of the par-
ticles, and intersect a smaller number of them than any other plane
which can be drawn through the block.
Next to the true cleavage-plane, the direction in which the rock
will offer least resistance is along the planes of secondary cleavage
hfg c, for the particles being all lengthened in that direction, it will
more readily separate down that plane than in any cross direction.
Nevertheless the cleavage along these planes is very imperfect, and
the sheets often fly off in a curve instead of splitting straight down.