A sequence of regional metamorphic mineral reactions recorded by the successive mineral assemblages seen in metapelites (metamorphosed or sandy shales) from the Barrovian terrain around Glen Esk in north-eastern Scotland and characteristic of medium regional metamorphic gradients of temperature and pressure. George Barrow (1853–1932) in 1912 was the first to recognize this sequence of metamorphic mineral assemblages. In pelites, Barrovian-type metamorphism is marked by the development of a sequence of index minerals, starting with chlorite in the lowest-grade rocks, and passing upgrade through biotite, garnet, and kyanite, to sillimanite in the highest-grade rocks (see barrow’s zones).