The original subdivision by George Barrow (1853–1932) of the sequence of mineral changes seen in rocks of pelitic composition in the Glen Esk region of north-eastern Scotland. Each zone is bounded by two isograds, each of which marks the appearance, in the direction of increasing metamorphic grade, of a new index mineral, and is named after the index mineral seen on the lower-grade boundary. (For instance, the kyanite zone has the kyanite isograd as its low-grade boundary and the sillimanite isograd as its high-grade boundary.) Within each zone, no additional minerals appear in the pelites, the constant mineral assemblage representing equilibrium over a range of metamorphic conditions. See barrovian-type metamorphism. Compare buchan metamorphic zones.