The association of clay minerals with sillimanite near Williamstown, approximately 23 miles north-east of Adelaide, South Australia, has been noted by the writer in earlier papers (Alderman, 1942, 1950). These papers described the occurrence of quartz-sillimanite and quartz-kyanite rocks and gave the evidences which suggest that they were formed metasomatically by alumina-rich fluids at high temperatures. It was also suggested that as the alumina-concentration and temperature of the fluids decreased the metasomatic activity was followed by a period of pegmatitc formation which, in turn, gave way to a hydrothermal stage. Brief reference was made to the conversion of sillimanite to clay by hydrothcrmal agencies.