The Probert Encyclopaedia of Rocks & Minerals

CLAY-SLATE

In geology, clay-slate is a rock consisting of clay which has been hardened and otherwise changed, for the most part extremely fissile and often affording good roofing-slate. In colour it varies from greenish or bluish grey to lead colour. The cleavage is independent of the stratification. It rarely lies parallel to the bedding, generally crossing the strata at all angles.
Research Clay-Slate