828 REPORT—1885. 
We have already remarked that the extent to which the fossils partake 
of the distortion of their matrix depends necessarily on the relative con- 
sistencies of the two at the time of the distortion. Jt should be added, 
for the sake of completeness, that sometimes hard but brittle fossils, such 
as the guards of belemnites, have been able to resist distortion, but have 
yielded by fracture. In such cases the several portions of the fossil are 
found to be separated from one another in the direction of the cleavage 
dip; and we have in these rocks, not only a proof, but a direct measure 
(which cannot be an over-estimate) of the expansion of the rock in that 
direction. Examples are quoted by MM. Heim! and Daubrée;? and 
we may see that at the time of the distortion the rocks must have already 
acquired a degree of firmness and hardness, from the fact that the 
separation of the fragments of the fossils left cavities, which were only 
subsequently filled by crystallised calcite. 
V. Slaty Cleavage in Rocks of various Lithological Characters. 
Slaty cleavage is by no means confined to the rocks of any one geo- 
logical period or era, but owing to its association with earth-movements 
on an extensive scale, the structure is, in the British Isles, characteristic 
of the older Paleozoic formations. The best slates of North Wales are 
procured from the Lower Cambrian strata, as at Penrhyn, Llanberis, and 
Nantlle, and from the Bala and Llandeilo beds, as at Ffestiniog and other 
places. The slates of the Lake District also are referred to the Bala and 
Arenig series, but slates of Devonian age are worked in Cornwall, and 
Carboniferous in Devonshire. In different parts of Europe, North 
America, &c., good slates are obtained from strata of Jurassic, Cretaceous, 
and Tertiary ages. 
But although the structure is not restricted to strata of any special 
geological age, its association with rocks of particular lithological types 
has been noted from an early date. It is met with in perfection in 
argillaceous rocks of fine texture and in fine-grained fragmental rocks of 
volcanic origin. A specimen in the Woodwardian Museum shows fifty 
slates split from a block 3} inches in thickness, and in the Nantlle 
Apansiess where this was obtained, the best slates are usually from } to 
yy inch thick. The Llanberis ‘slates are similar, and the beds. at 
Ffestiniog are even more yielding. The degree of thinness to which the 
slates can be split in working depends, however, not only on the per- 
fection of the cleavage structure, but also on the flexibility and toughness 
of the rock. 
Some argillaceous limestones exhibit cleavage well developed, as do 
others, for example some in Devonshire, which are almost purely cal- 
careous and dolomitic. In some cases a relation is observable hetween 
the percentage of argillaceous matter and the facility of cleavage. An 
excellent instance is furnished by a section in the Lias at Grenoble, 
where several beds of argillaceous limestones of different compositions 
are exposed, and those with the greatest amount of argillaceous matter 
are found to present the best cleavage. M. KE. Jannettaz? has examined 
these different strata by a method which he has applied with success to 
» Mechanismus der Gebirgsbildung, Atlas, Taf. xiv., figs. 1-5 (1883). 
? Htudes Synthétiques de Géologie Lnpérimentale, p- 404 (1879). 
’ «Mém. sur les Clivages des Roches, &c.,’ Bull. de la Soc. Géol. de France, sér. 3, 
t. xii. p. 216 (1884). 
