Summary: The Upington cordierite-rock is thought to be the result of the metasomatism of an original pelitic sediment whereby it gained alumina and magnesia and was converted into a rock containing por-phyroblastic cordierite together with potash-felspar, oligoclase, biotite, andalusite, and quartz. This stage corresponds to the pyroxene-hornfels facies of Turner. At the height of the metamorphism it is possible that some of the andalusite was converted into sillimanite. With falling temperature, increased stress, and in the presence of excess water the above assemblage was unstable and retrogressive metamorphism took place. Equilibrium under the changed conditions was not attained, though there was a partial replacement of potash-felspar and andalusite by sericite and quartz and pinitization of the cordierite. The groundmass minerals took the stress while the large eordierites remained relatively unstrained. The tourmaline and apatite probably belong to this late metasomatic phase.