An occurrence of a regular mixed-layer clay-mineral

About twelve miles south-east of Burghersdorp in the Cape Province, South Africa, the unusual clay reported in this paper was first discovered on the farms 'Groenvlei' and 'Kuilfontein'. Later deposits were also uncovered on the farms 'Kapokkraal' and 'Kleinhoek'. The clay-mineral appears to be of a very unusual type, namely, a regular mixed-layer clay-mineral (1:1 hydrous mica and montmorillonite). The mode of origin of the clay is also of interest in that it appears to be produced from a shale by hydrothermal alteration, due to intrusion of dolerite sheets.