ON SLATY CLEAVAGE AND ALLIED ROCK-STRUCTURES. 833 
lines’ meeting an obstacle, and in fact for a similar reason. Very per- 
fect examples of this peculiarity are sometimes seen in North Wales, e.7., 
near the Dorothea Quarry, Nantlle. 
Again, we may occasionally observe that where slate with a cleavage 
oblique to its bedding adjoins a harder bed or even such obstacle as a 
sheet of quartz along a joint-fissure,! the cleavage of the slate is slightly 
turned aside on approaching the harder bed, so as to make a smaller 
angle with it. Here asimilar explanation may hold, as has been remarked 
by Mr. Sharpe. M. Laugel,? however, ascribed this phenomenon to a 
certain amount of shearing (glissement) along the planes of bedding, sub- 
sequently to the setting up of the cleavage structure ; and there seem to 
be some cases of irregularity in cleavage-surfaces to which this explana- 
tion applies, as suggested by Professor T. McKenny Hughes.* There may 
even be a bending or crumpling of the cleavage-surfaces where they meet 
planes which are not those of bedding,‘ for it is obvious that those sur- 
faces must share all subsequent vicissitudes of the rocks in which they 
occur. In the cases figured by Sir H. De la Beche® it is possible, as he 
suggested, that the beds have shrunk owing to the abstraction of some 
constituent by solution. 
VI. The Mode of Working Slate-Rock in the Quarries. 
The behaviour of slate-rock when broken and split agrees perfectly 
with its intimate structure as described in the foregoing pages. Besides 
the cleavage, the Welsh quarrymen ® recognise in slate a certain ‘ grain’ 
giving the rock different properties along different directions in the 
cleavage-plane. The facts are completely in accord with the deductions 
made above as the results of the mechanical theory. A slate breaks 
across the cleavage more readily parallel to its ‘length’ or ‘side,’ 7.e., 
the cleavage-dip, than it does parallel to its ‘ breadth’ or ‘end,’ i.e., the 
cleavage-strike. In a case where it is difficult to ascertain by merely 
looking at a block which is its length and which its breadth, the quarry- 
man tries to find a chip on its surface: on striking this with a hammer 
it is found to break (stolpio) parallel to the length or side of the block. 
In breaking across the massive blocks or slabs of rock which are 
afterwards to be split into roofing-slates, the same peculiarity of struc- 
ture shows itself. Ifa block is to be broken lengthwise, it is sufficient 
to cut a slight groove at one end, place the edge of a chisel on it, and 
strike a blow witha hammer. The cut travels tolerably straight along 
the length of the block, although its surface often becomes grooved and 
fluted towards the further end. The object of the groove is to steady 
the chisel for the first blow, and in cutting smaller blocks it may be dis- 
1 Sharpe, Quart. Jowrn. Geol. Soc., vol. v. p. 117 (1849). Forbes, tbid., vol. xi. 
p. 170 (1855). 
2 Bull. Soc. Géol. Fran., sér. 2, t. xii. p. 363 (1855). 
3 Lyell’s Student's Elements of Geology, p. 590, 3rd ed. (1878), 
4 Jukes, Quart. Jowrn. Geol. Soc., vol. xxii. p. 359 (1866). 
5 Geological Observer, p. 709 (1851). Geikie’s Textbook of Geology, fig. 248, 
‘p. 522 (1882). 
§ The following remarks apply more particularly to the Lower Cambrian slates of 
_ North Wales. I am greatly indebted to C. W. Rathbone, Esq., Manager of the 
Pen-y-bryn Quarry at Nantlle, who has afforded me every opportunity of examining 
the working of the slate-rock, and to Messrs. J. T. Parry and John Roberts, of the 
‘Same quarry, who have kindly given me the benefit of their large practical ex- 
perience. 
Dy, 1885. 3H 
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