The original material was a fine clay, sometimes of sand or volcanic dust, and the bedding of the sediment as originally laid down may be indicated by alternating bands, differing in colour or in lithological character, sometimes to be seen on the cleavage faces of the slates. Cleavage is a super induced structure, the result of pressure acting on the the rock at the time when it was buried deep beneath the earth's surface.

On this account slates are found chiefly among rocks of the older geological systems, although some occur in regions where comparatively recent rocks have been folded and compressed as a result of mountain building movements in the earths crust.

Slates may be black, blue, purple, red green or grey; dark slates usually owe their colour to carbonaceous material or to finely divided sulfide of iron, reddish and purple varieties to the presence of oxide of iron in the form of hematite and green varieties to the presence of much chlorite.