In the progressive thermal metamorphism of siliceous dolomites, tremolite and calcite are usually considered the earliest of the new mineral phases formed by reaction of the primary materials, dolomite and quartz. An early formation of forsterite has also been recorded, as, for example, in the dolomitie zones of the Durness limestone surrounding the Tertiary Beinn an Dubhaich granite of Skye. In this aureole, however, the relation of forsterite to tremolite has hitherto not been closely investigated. As Bowen has pointed out in his systematic study of the progressive metamorphism of siliceous limestones and dolomites, a first formation of forsterite from dolomite and quartz could not rank as an equilibrium reaction since forsterite and quartz can have no stable coexistence. Tremolite has accordingly been regarded as the first metamorphic product in an equilibrium series of reactions of increasing decarbonation affecting siliceous dolomites in an advancing higher temperature environment at any given pressure.