"A 500 foot sill of dolerite from Hangnest in the Calvinia division of the Cape Province (South Africa) exemplifies very clearly two well-marked features of the thicker Karroo sills: (1) Very slight amount of crystallization differentiation; (2) Marked tendency to mobilize and react with the associated shales and siltstones. A petrological and chemical study of this sill is given and the mechanism of the various processes discussed."