AbstractThe antipathetic relation between chloritoid and biotite in the lower metamorphic grades of the Barrovian zones and the genesis of the index minerals in the higher metamorphic grades are briefly discussed. The observed mineral assemblages of 13 new analyses of peltic schists are compared with the assemblage required by the mineralogical phase rule; good agreement exists provided allowance is made for magnesia acting as a separate component. Six new biotite analyses covering all of the Barrovian zones show an increase in the degree of replacement of Si**** by Al*** with increase in metamorphic grade. The ratio of FeO: MgO varies according to the associated ferromagnesian minerals and shows no regular relation to metamorphic grade.