Summary: Fine fractions were prepared from six shales overlying anthracite and bituminous coal in certain South Wales collieries. X-ray diffraction and chemical analyses show that these fine fractions contain illite with about 10% of kaolin and 1% of quartz; dehydration, optical, and base-exchange data are given, and the conditions of formation and stability of illite are discussed. The sodium content of the illite decreases slightly with decreasing rank of the associated coal, and a possible connexion with McKenzie Taylor's theory of coal formation is pointed out.