In the Harlech Grit Series pyrite always occurs associated with rocks of practically all grades. It is, on the whole, of least importance in the grits, but even in these it becomes locally fairly abundant. Among the mudstones it occurs concentrated into layers, which are relatively persistent laterally. Immediately below the manganese-ore of the Manganese Shale Group, for example, there occurs a very widespread band of green mudstone which contains abundant pyrite crystals. Although its general characters are the same this band varies slightly, in detail when traced laterally and it is with these variations that the present paper is largely concerned. This band is usually 1 to 2 inches thick and is usually of dark green mudstone, though rarely it is of the normal blue-grey colour. The pyrite crystals vary from very minute to about ⅜ inch in size. In addition, pyrite occurrences from other rocks are described. The account is particularly concerned, not with the pyrite crystals themselves, but with zones of authigenous minerals which almost invariably surround them in these rocks.