SLATES. 
37 
the south end of the Hall, overlooking Jermyn Street. Most of 
these specimens are from North Wales. 
The slate quarries and mines of North Wales are celebrated 
for the excellent character of the slates which they produce. 
The most celebrated quarries are those of Penrhyn and Llanberis, 
which are worked in the Lower Cambrian series of Carnarvon- 
shire. Other Welsh slates, of less importance, are obtained from 
higher horizons in the Cambrian systems. The slates of the 
Ffestiniog and Corris districts are worked in Lower Silurian 
strata ; whilst those of Llangollen are in Wenlock beds, of 
Upper Silurian age. 
The “ green slates ” of the Borrowdale series of the Lake 
district are finely-divided volcanic ashes, more or less cleaved by 
pressure. Slate pencils have been largely made from some of 
the Skiddaw slates. 
The Devonian rocks of the north-eastern part of Cornwall, and 
of neighbouring districts in Devonshire, yield slates, more or less 
valuable for roofing purposes. The Delabole slate quarries, 
situated near Tintagel, in Cornwall, have been long celebrated 
for producing a durable material combining considerable lightness 
with strength. In 1602 Carew, in his Survey of Cornwall , 
speaks of healing stones (in many districts roofing slates are 
still called hailings or healings, probably from hele or hail , to 
hide, and hence the name of helier given to a tiler or slater) : — 
“ in substance thinne, in colour faire, in waight light, in lasting 
long, and generally carrieth so good regards as (besides the 
supply for home provision) great store is yearly conveyed by 
shipping both to other parts of the realme, and also beyond the 
seas, into Britaine and Netherlands Borlase, in 1758, speaks of 
the extent of the workings of Delabole. The Delabole quarries, 
now merged in one huge pit, produce not only roofing slates, 
but flagstones which are employed for pavements in passages, 
courts, yards, &c. and for tombstones. The inscriptions upon 
old tombstones of the Delabole slate remain remarkably perfect, 
showing its durability when exposed to atmospheric influences. 
Slates are subject to considerable variation both in colour and 
texture. Good roofing slate should be hard, tough, and fine- 
grained, emitting a metallic ring when struck ; iF should absorb 
but little water/and be so compact as not to be decomposed by the 
action of the atmosphere. Iron -pyrites, which is by no means 
uncommon in many slates, is apt in some cases to decompose 
and disfigure the slate with rusty blotches ; but certain kinds 
of pyrites are capable of resisting, to a remarkable extent, the 
action even of a town atmosphere. ioj 
Slabs of slate which are not fit for splitting into roofing slates 
are cut with a circular saw into pieces of from half an inch to 
two inches thick, and are used for flooring, landings, steps, 
cisterns, mantel-pieces, &c Slate slabs are also extensively 
enamelled, and employed for ornamental work in the place of 
marble. 
