ON SLATY CLEAVAGE AND ALLIED ROCK-STRUCTURES. 835 
certain degree of flexibility, and to this flexibility the ‘quarry water’ or 
natural moisture of the rock is essential. .The blocks must therefore be 
worked while fresh from the quarry; even an exposure of a day or two 
makes an appreciable difference in the facility of cleavage, and in dry 
weather the men are careful to keep the blocks well watered. The softest 
rock, which is the easiest to cleave when fresh, becomes the most difficult 
if allowed to dry. 
The tendency of the split to ‘run out’ to the face of the slate and the 
manner in which it can be drawn back are instances of the fact which we 
have found to be a necessary consequence of the mechanical theory, viz., 
that the rock will split at a small angle with the true cleavage-planes 
under the action of suitable stresses. It is well known that a ‘false 
cleavage,’ slightly oblique to the direction of the true, is often obtained. 
Such is the case on the faces of the blocks when they are roughly split 
off with a thick chisel and hammer. In blasting, too, the fracture is 
commonly slightly oblique to the true cleavage direction, and often 
curves away from it at a considerable angle in the neighbourhood of a 
‘fast’ foot-joint. In these cases the surface of the false cleavage is not a 
rough surface consisting of portions of successive true 
cleavage-planes, but a surface as smooth as that of an 
ordinary slate. 
With reference to the so-called ‘ secondary cleavage’ 
parallel to the ‘side,’ the mode of treating the blocks of 
slate-rock in the quarries is again instructive. In cutting 
a block lengthwise, parallel to the ‘ side,’ a tendency to 
‘run out’ towards one side may be shown (fig. 10). To 
bring back the crack, a flat bar of iron (6 in the figure) 
is laid across the end, near the groove and towards the 
weaker side, and this is struck with a hammer. The 
effect is to bring back the crack as shown by the dotted 
line. This is the most approved method; the old- 
fashioned plan was to place a lump under the block, immediately below 
the crack (at c), and then to strike with a mallet on the upper end, at 
the corner (@ in fig.) next the stronger side. This was less accurate 
than the other method described, besides presenting the danger of break- 
ing the block at the corner struck. It is easy to see why the deviation 
of the cut in this case should be greater than that of the ‘ false cleavage,’ 
for (referring to fig. 5) though the tendency to split along the ‘side’ isa 
maximum among all planes parallel to the axis c, the maximum is much 
less sharply defined than in the case of the cleavage proper. The face 
of a fracture parallel to the ‘ side,’ also, is much less smooth and regular 
than a cleavage face, and is frequently fluted and grooved. Indeed, it may 
safely be asserted that no evidence of anything that can fairly be described 
as a second cleavage is to be found in the slate workings of North Wales. 
The structure known as ‘ cross-cleavage,’! the gwniad of the quarrymen 
and the ‘ lace’ of slate merchants, which renders worthless much other- 
wise valuable rock, is only a system of secret jointing. 
M. Jannettaz,? however, asserts that the longrain of the Ardennes 
slates is a true second cleavage, but it is not quite clear in what sense he 
employs this term. 
? Davies, Slate and Slate Quarrying, 2nd ed., pp. 25, 48, 49, &c. (1880). 
* Bull. Soc. Géol. Fran. sér. 3, t. xii. p. 211 (1854). Cf. Sedgwick, Synops. Brit. 
Pal. Rocks, Introd. p. xxxvy. (1855). 
Fie. 10. 
3H 2 
