36 
THE HALL. 
of specimens of rocks which are capable of being split, along the 
planes of bedding, into slabs suitable for use as flagstones, &c. 
The streets of London are partly paved with Yorkshire flagstones, 
which are pale brown micaceous sandstones from the Lower 
Coal Measures of the Yorkshire coal-field. The Caithness flag- 
stones are dark -grey bituminous and calcareous sandstones, from 
the Lower Old Red Sandstone ; whilst the Arbroath flagstones, 
from Forfarshire, are obtained from the same formation. 
With the arenaceous flagstones are placed certain fissile lime- 
stones, which being easily split into thin slabs, are used locally 
for roofing, under the name of “ slates.” These tiles were largely 
employed by Gothic architects, and are still used in ecclesiastical 
architecture, though generally superseded for domestic purposes 
by the highly cleavable and lighter Welsh slates. The principal 
fissile limestones are the Stonesfield slate, which occurs at the 
base of the Great Oolite in Oxfordshire, and the Collyweston 
slates which are found immediately below the Lincolnshire 
Limestone, in the lower part of the Inferior Oolite near Stamford. 
The Duston slates have been obtained from the Inferior Oolite 
(Northampton Sands). Thin slabs of Forest Marble, for roofing, 
are got at Poulton, near Fairford, and other places. It is worth 
noting that many of these stones, after being quarried, are exposed 
to the disintegrating action of the winter’s frost, whereby their 
fissile structure is developed, and they are then easily split up. 
Slate. 
This valuable material is a highly indurated argillaceous rock, 
readily cleaved in certain directions into thin lam hue or plates ; 
and upon this fissile structure depends to a great extent its 
economic value. Most slate contains much finely- divided mica 
of secondary origin. 
The beds, deposited originally as a fine muddy sediment, 
appear to have been subjected, long after consolidation, tc the 
action of intense lateral pressure ; the effect of which was not 
only to contort the beds, but also to induce a re-arrangement of 
the particles of the rock, the flattened sides of these particles 
being forced by the lateral compression into positions transverse 
or at right angles to the direction of the pressure, and hence the 
rock readily cleaves parallel to that direction in which all the 
particles are thus definitely arranged. All contorted strata are 
not, however, cleavable. Experiments on cleavage have shown that 
a similiar, though much less perfect, fissility may be artificially 
developed by simple mechanical compression ; the direction of 
the induced cleavage being always perpendicular to that of the 
applied pressure. 
In some cases the direction of the original bedding may be 
seen on the cleaved surface in the form of bands, known to 
quarrymen as the stripe of the slate. 
A collection of slates, admirably illustrating the cleavage of 
the rock, is arranged against the wall under the west window at 
