Abstract: A thick deposit of flint clay in the Permian Illawarra Coal Measures of the southern part of the Sydney Basin, New South Wales, is described. The flint clays which have affinities with those of the Pennsylvanian of North America and with the kaolin coal tonsteins of the Westphalian, are associated with red-brown claystones that contain in addition to hematite, an abundance of a regular mixed layer clay mineral. Pyrophyllite generally forms a minor constituent only of the flint clays but in one part of the deposit it is more abundant than kaolinite. Although there are many puzzling aspects concerning the origin of the deposit, evidence suggests that the kaolinite and the pyrophyllite in the flint clays are detrital.