George Barrow mapped a widespread series of zones of progressive metamorphism, as observed in the Dalradian sequence exposed in Scottish Highlands, in the late 19th century.
Each zone recognized is based on the first appearance of a group of distinctive index minerals as the highest metamorphic grade, along the thermal axis, was approached.
Thermal axis has abundant granitic bodies situated along its length.
Index mineral is one which is characteristic of that zone - any mineral within the zone may be the index mineral.
The Dalradian sequence consists of Precambrian and early Cambrian sediments, overlain by unmetamorphosed Upper Devonian Sandstones.
Within the Dalradian sequence, Barrow recognized the existence of unmetamorphosed shaley (pelitic) sediments which subsequently underwent metamorphism.