Slate in these forms is particularly useful on account of its strength, "The strength of slate 1 inch thick is considered equal to that of Portland stone 5 inches thick,"l and "its resistance to shearing is said to be greater than that of any other stone." 2
Slate slabs are easily obtained of any length under 6 or even 8 feet, and containing from 10 to 30 superficial feet.
Their thickness ranges from 1 inch to 3 inches.
Larger slabs may be obtained by paying extra. The Exhibition of 1862 contained one sent by the Llangollen Slate Company which measured 20 x 10 feet, and weighed 41/2 tons; also several from the Ffestiniog quarries of the Welsh Slate Company averaging 14 feet by 7 or 8 feet.3
They may be procured either self-faced - that is, as they are split from the blocks - rough sawn, quarry planed, or polished.
The edges are sawn square, planed, filed, or rounded.
Slate Blocks, containing as much as 2 or 3 cubic feet, can easily be obtained.
Slate is also sent out from the quarries in the form of steps, sills, etc.
Varieties in use. - There are many slate quarries throughout Great Britain and Ireland, on the Continent, also in Canada and the United States.
Some American slates have been imported during late years, but the great bulk of the slates used for building are from home quarries.
The finest slates found in the United Kingdom come from Wales.
1 Papworth, 657. 2 Wray.
3 Hunt's Handbook, Exhibition 1862.
The slates from the Silurian formations of Merionethshire, Montgomeryshire, etc., are generally of a blue or grey colour, and of beautiful cleavage; splitting very thin, and sawn square by machinery. The best-known quarries are those in the Ffestiniog district, such as the Oakeley quarries and those of the Welsh Slate Company.
The slates of the Cambrian formation in Carnarvonshire are of varied colours - blue, purple, green, and dark grey. They are more siliceous than the Lower Silurian slates, and not so easily cleaved. They are therefore thicker and heavier, but they are very hard and ring well when struck. Their edges are not sawn, for the reasons given above. The best-known quarries are those of Penrhyn and Dinorwig.
Many of the quarries produce also slabs of first rate quality.
English Slates are generally thicker and coarser than those from "Wales - hard, tough, and very durable. The best known are the green slates from Westmoreland, and the slabs from Delabole in Cornwall.
The best known quarries are those of Ballachulish, Easdale, and Cullipool. They are generally blue.