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.
So what this means....as one stand on the rocks and moves from places to place and notices different minerals present. The presence and absence of these index minerals infer temperatures and pressures that the package of rocks were at within the crust during an orogoney (mountain building). The really thing about the Scottiah Highlands is one can walk through a progressive series (different temperatures and pressures) via the visible index minerals and see how the same lithology (package of rock) changed with T & P.
I love metamorphic geology but groundwater pays the bills
Originally Posted by Mike1
thing about the Scottiah Highlands is one can walk through a progressive series (different temperatures and pressures) via the visible index minerals and see how the same lithology (package of rock) changed with T & P.
but groundwater pays the bills 
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