The topology of the reaction surfaces for the phases--garnet, chloritoid, staurolite, kyanite, cordierite, chlorite, and biotite--is shown for two limiting sections of Ps, T, P water space. . . . The basic assumption made is that the course of progressive metamorphism is primarily one of progressive dehydration with a progressive restriction of the fields of the most hydrous minerals, notably chlorite. The resulting petrogenetic grids. . . are predominantly theoretical and should serve as a useful check in assigning natural metamorphic sequences to their relative pressure-temperature relationships.