SLATE (properly ; in M. Eng. slat or sclat, from O. Fr. esclat, a small piece of wood used as a tile; esclater, to break into pieces, whence modern Fr. éclat, the root being seen also in Ger. schleissen, to split), in geology, a fissile, fine-grained argillaceous rock which cleaves or splits readily into thin slabs having great tensile strength and durability. Many other rocks are improperly called slate, if they are thin bedded and can be used for roofing and similar purposes. One of the best known of these is the Stonesfield slate, which is a Jurassic limestone occurring near Oxford and famous for its fossils. Slates properly so-called do not, except on rare occasions, split along the bedding, but along planes of cleavage, which intersect the bedding usually at high angles. The original material was a fine clay, sometimes with more or less of sand or ashy ingredients, occasionally with some lime; and the bedding may be indicated by alternating bands of different lithological character, crossing the cleavage faces of the slates, and often interrupting the cleavage, or rendering it imperfect. Cleavage is thus a superinduced structure, and its explanation is to be found in the rearrangement of the minerals, and the development of a certain degree of crystallization by pressure acting on the rock. Slates belong mostly to the older geological systems, being commonest in Pre-Cambrian, Cambrian and Silurian districts, though they may be found of Carboniferous or even of Tertiary age, where mountain-building processes have folded and compressed these more recent formations. The action of pressure is shown also by the fossils which sometimes occur in slates; they have been drawn out and distorted in such a way as to prove that the rock has undergone deformation and has behaved like a plastic mass. Evidence of the same kind is afforded by the shape of the knots and concretions sometimes present in the slate. If the bedding be traced, either in the slates or in the other rocks which accompany them, flexures will be frequently observed (the folding often being of an isoclinal type), while reversed faulting, or thrusting, is usually also conspicuous.