The work carried out by Barrow led to his recognizing the following metamorphic zones within the Dalradian Sequence:

Zone of digested clastic mica

• Now termed chlorite zone - characterized by first appearance of chlorite.
• The characteristic assemblage - quartz-chlorite-muscovite-albite.

Biotite Zone




Marked by the first appearance of red-brown biotite produced from reaction between muscovite and chlorite.

Garnet Zone

The characteristic assemblage recognized by Barrow is quartz-muscovite-biotite-almandine (Fe,Mg)₃Al₂Si₃O₁₂-albite or oligoclase.

Staurolite Zone

The assemblage is quartz-muscovite-biotite-almandine-staurolite-(oligoclase)

Kyanite Zone

Assemblage is quartz-biotite-muscovite-oligoclase-almandine-kyanite

Sillimanite Zone

quartz-biotite-muscovite-oligoclase-almandine-sillimanite

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.