Mineralogical compositions have been calculated from a number of average analyses of shales and clays using the scheme of Imbrie and Poldervaart (1959). The average shale comprises 59 per cent. clay minerals, 20 per cent. quartz and chert, 8 per cent. felspars, 7 per cent. carbonates, 3 per cent. iron oxide minerals, 1 per cent. organic matter and 2 per cent. other minerals. Illite predominates among the clay minerals.