During a study of the petrography and mineral constituents separated from various shales overlying the coal-measures of South Wales, Dr. Alfred Brammall noticed on several hand-specimens from Llandebie a white infilling to fissures or a coating on slickensided surfaces which could be readily detached from the matrix. Chemical examination showed that this coating, unlike the shales themselves, contained more sodium than potassium. The present note gives an account of subsequent chemical, X-ray, and optical work, which confirms Dr. Brammall's suggestion that the white incrustation contains a new sodium-rich mineral allied to mica. It is not, I hope, an unfitting tribute to his work in this field as well as to his interest in recent advances in mineralogy that I have named this mineral brammallite. A short account of its nature and relationship to illite, which is the chief constituent of the shale itself (see preceding paper, p. 297), will now be given.