The concept of “mineral facies” of rocks as defined by Eskola is an excellent aid to the study and classification of metamorphic rocks. A mineral facies, in the sense of Eskola, “comprises all the rocks that have originated under temperature and pressure conditions so similar that a definite chemical composition has resulted in the same set of minerals, quite regardless of their mode of crystallization, whether from magma or aqueous solution or gas, and whether by direct crystallization from solution . . . or by gradual change of earlier minerals. . . . ”1
In an area of progressive metamorphism, each successive stage corresponds to a new mineral facies. It also corresponds to a new zone of metamorphism, in the sense used by Barrow2 in his study of the regional metamorphic rocks of the Scottish Highlands. Rocks within the same zone may be called isofacial, or . . .