Summary. Garnets undergo readjustment of ion-assemblages under external mechanical and thermal stresses; this adjustment favours those paths that annul the very cause to which it is due. Metal ions in garnets being replaceable, with rising metamorphism garnets become reluctant to accommodate ions of larger ionic radii.Data from recent literature on chemical analyses of garnets from rocks belonging to different grades of regional metamorphism reveal that the (FeO A-MgO) content of garnet varies linearly with (CaO § content. As the Fe/Mg ratio does not vary appreciably, a fair estimate of metamorphic grade can be made from a knowledge of (CaO § or of (FeO +MgO) individually or even, with less confidence, of FeO or MgO alone. From the study of the available data it is found that (CaO + MnO) and (FeO §247 provide the best indicators of progressive metamorphism. Cell edges of garnets decrease progressively with increasing metamorphism and this may also be used as a fairly good index of metamorphism. When cell edges are plotted against (FeO § a hyperbolic curve is obtained.T HE index minerals chlorite, biotite, almandine garnet, kyanite, staurolite, and sillimanite have long been used for the determination of the grade of regional metamorphism in rocks of pelitic composition. Recently it has been shown that an individual mineral or a pair of diadochic minerals change progressively in composition with rising metamorphism (Engel and Engel, 1958) ; use of this principle permits more minute demarcation of the intensity of regional metamorphism.Miyashiro (1953) also found significant correlation between grade of metamorphism and the MnO, CaO, and FeO content (wt. %) of Ca-poor garnets. The variation in MnO shown by him agrees well with that obtained by Barth (1936). Sturt (1962) has confirmed graphically the continuous variation of (FeO § and (CaO § with metamorphic grade in a diagram in which he plotted (FeO+MgO) content of garnets against (CaO § MnO) content of garnets from pelitic rocks of varying metamorphic grades. Further he was able to correlate the variation of CaO in garnet with the ratio (Cell edge/Refractive index). These results were obtained

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