814 REPORT—1885. 
that such solids are not capable of indefinite subdivision into similar 
solids, except in one direction, viz., that of true cleavage.’ The distinc- 
tion here stated between true slaty cleavage and mere jointing is funda- 
mental: slaty cleavage, like the crystalline cleavage of minerals, is a 
property of a definite direction, jomting a property of definite planes in 
the rock. If a rock-mass will split along a certain plane owing to a 
true slaty cleavage structure, it will split with equal readiness along any 
other plane parallel to the first; but although parallel joints may occur 
in a rock at very small distances apart, there is no special tendency in 
the parts of the rock between two successive joints to split parallel to 
them. Moreover, cleavage planes are merely surfaces of weakness, not 
of actual discontinuity, in the rock in which they occur ; while, although 
secret or ‘close’ joints are not uncommon in some kinds of rocks, the 
structural planes properly known as joints are: usually actual fissures, 
which may, however, be of very minute width. 
The structure of slaty cleavage, to which the above remarks are strictly 
applicable, is typically exemplified in ordinary roofing-slate: in some other 
rocks we find a passage from the true cleavage structure to a system of 
close parallel planes of discontinuity (which, however, are not to be with 
propriety regarded as joints), while, on the other hand, it is difficult to 
discriminate the cleavage of slates from the schistosité of certain schists. 
The words clivage and Transversalschieferung, when strictly appropri- 
ated,! are used in nearly the same sense as that in which we have defined 
cleavage : the first-named term is, however, employed by many writers 
with reference to structures which we should describe as jointing. 
It seems desirable, if possible, to limit the name slate to rocks which 
exhibit the true cleavage-structure, although this restriction has not 
always been observed. 
Confining our attention for the present to true slaty cleavage, it will 
be convenient to summarise at the outset the information that had been 
obtained on that subject prior to Professor Phillips’ Report in 1856. 
It is a matter of no small wonder that the fundamental difference of 
cleavage planes from surfaces of original deposition should have met with 
such tardy recognition at the hands of geologists in general. In North 
Wales, for instance, where some of the quarries have been worked for at 
least six centuries,” the cleavage-planes are often seen to cut at high 
angles across alternating bands of different lithological characters, which 
clearly indicate the original bedding of the rocks. The names of Otley,’ 
MacCulloch,! and Phillips may be mentioned as among the first to 
recognise slaty cleavage as a distinct phenomenon. The scientific study 
of the structure must, however, be considered as dating from Prof. Sedg- 
wick’s paper ‘ On the Structure of Large Mineral Masses,’ ° read before 
the Geological Society of London in 1835. In this valuable contribution 
to physical geology the author insisted anew upon the distinction between 
cleavage and stratification, and pointed out clearly for the first time how 
the former structure differs from jointing ; he showed that cleavage-planes 
1 Hg. Heim: Untersuchungen iiber den Mechanismus der Gebirgsbildung, Ba. ii. 
s. 58, 59; 1878, Basel. 
2 Davies, Slate and Slate Quarrying, chap. xxii., 2nd ed., 1880, London. 
3 Kirkby Lonsdale Magazine, 1820. ‘Concise Deseription of the English Lakes,’ 
1823. Keswick. 
4 Western Isles of Scotland, vol. iii. p. 33, and pl. xxii. fig. 6, 1819. Journ. Roy. 
Inst., 1825. System of Geology, vol. ii. 1831. 
5 Trans. Geol. Soc., 2nd ser. vol. iii. p. 461, 1835. 
