The work carried out by Barrow led to his recognizing the following metamorphic zones within the Dalradian Sequence:
Marked by the first appearance of red-brown biotite produced from reaction between muscovite and chlorite.
The characteristic assemblage recognized by Barrow is quartz-muscovite-biotite-almandine (Fe,Mg)3Al2Si3O12-albite or oligoclase.
The assemblage is quartz-muscovite-biotite-almandine-staurolite-(oligoclase)
Assemblage is quartz-biotite-muscovite-oligoclase-almandine-kyanite
Zones were mapped in a small portion of Aberdeenshire and latter extended across the full extent of the Highlands.
Some minerals, e.g., biotite, continue through the higher grade zones, however others, e.g., staurolite, disappear in the next zone.
A line on a map that corresponds to the first appearance of an index mineral is called an isograd (line of equal grade). Isograds can be drawn for each index mineral.
Barrow's work led to our present concept of metamorphic grade, although the interpretation of the significance of the zones varies.