Petrological Studies in the Harlech Grit Series of Merionethshire. I. Metamorphic Changes in the Mudstones of the Manganese Shale Group

Abstract

The mudstones of the Manganese Shale Group are very similar in their characters throughout, but in the present work, greatest stress is laid upon the mudstones of one horizon—the Bluestone. This is about 5 ft. 6 in. thick, and occurs immediately above the manganese-ore band, which is 30 feet above the base of the group. From this horizon special metamorphic derivatives of the normal mudstones have been obtained from one locality—the lower end of the workings of the Hendre Mines on the north-west slopes of Moelfre, about 2¾ miles east 15 degrees south of the village of Llanbedr. There is no doubt that the types described belong to this one horizon, which is everywhere else remarkable for its uniformity of character. They always follow immediately on the manganese-ore band in the normal manner, without break, and their relationships with the succeeding strata are apparently the same as with the normal rocks. Also, with the exception of the calcite-bearing rock (p. 372), all the rock types are identical chemically.