Cleavage

Cleavage, Slaty, the division of a rock into layers capable of being indefinitely subdivided, having the same mineral composition, mostly made up of non-crystalline particles, and differing altogether in direction from the planes of bedding or stratification. Cleavage occurs notably in argillaceous rocks, which are known as clay-slates, and belong either to the older geological formations, Cambrian, Ordovian, or Silurian, or are in the neighbourhood of intrusive igneous rocks or other disturbed district. The "strike" of the cleavage-planes, or their direction across the country as laid down on the map, generally remains parallel over wide areas, coinciding with that of the principal folds in the crumpled rocks. Their "dip" or inclination to the horizon is generally either vertical or nearly so, thus cutting crumpled bedding-planes at various angles. Fossils are distorted along the faces of the cleavage-planes, and pebbles often cut through by them. Microscopic examination shows the rock-particles arranged with their longer axes parallel to the planes, thus giving a "grain" to the rock, and obviously compressed. Scales of mica and small garnets are common in slates. These characters and experiments with hydraulic pressure upon wax, clay, etc., made by Tyndall, Sorby, and others, show slaty cleavage to be the result of tangential (horizontal) pressure. It must not be confused with crystalline cleavage, the splitting up of a crystal into smaller crystals.