Pelitic and calcareous rocks in the Whetstone Lake area have an unusually wide range of chemical composition. Metamorphic reactions have been deduced that represent the observed ‘discontinuities’ in compatible mineral assemblages, and by plotting the reactant and the product assemblage of each reaction on a map, metamorphic isograds have been delincated ‘from both sides’. For the pelitic rocks, successively higher-grade isograds are based on the following reactions: