The ordinary slate used for roofing and other purposes is an argillaceous rock, compact and fine grained. It was originally a sedimentary rock, but it will no longer divide along the planes of bedding, but splits readily along planes called "planes of slaty cleavage."
This facility of cleavage is one of the most valuable characteristics that slate possesses, as it enables masses to be split into thin sheets, whose surfaces are so smooth that they lie close together, thus forming a light and impervious roof covering.
These planes of cleavage are caused by intense lateral pressure.
Planes of cleavage are either coincident with the layers of deposit or lie at angles with them. When they are in the same plane, or nearly so, the rock is converted into slabs for paving; or planed, if it is soft enough, and made into cisterns, etc. The reason that it cannot be made into roofing slates is that the lamina of the bedding and the lines of cleavage run into each other and render the surface rough and uneven.
There is another line of imperfect cleavage, which will yield to the chisel. Along this line the blocks of slate are split up longitudinally. It is along this line that fracture occurs when a slate is accidentally broken. The split along this line is called by quarrymen the "Plerry."
The rock is worked in "Floors," or tunnels one above another.
Powder is used to detach the blocks, which are plerried into widths suitable for making the best-sized slates; then split into thicknesses of about three inches; cut by circular saws into suitable lengths; split by skilful hands with the aid of thin inch chisels; and squared, either by machinery or by hand.
Cambrian slates are not sawn, because natural joints occur at distances about equal to the length of the slates. They are generally squared by hand.
Slate rock becomes more compact and the blocks are generally larger and more valuable the deeper they are from the surface; but the rule does not always hold good, and there is apparently a limit to it. The blocks are split more easily when fresh from the quarry.