The Petrography and Petrology of the Lower Cambrian Manganese Ore of West Merionethshire

Abstract

The Cambrian manganese ore of the Ardudwy district of Merionethshire occurs interstratified in that series of rocks which locally forms the lowest Cambrian and which is known as the Harlech Grit series. This series crops out in a broad, irregularly oval, mountainous tract, with the long axis alined north and south. It extends from Barmouth in the south to within about one mile of Ffestiniog in the north and from the coast on the west to the Ganllwyd valley on the east. Ramsay (1881, p. 19) first described these rocks, but no attempt was made to classify the series until in 1893 Good-child grouped them as follows (Dewey and Dines 1923, p. 59):— 4. Upper Mudstones (Lingula Flags and Menevian) with Upper Manganese Bed at base. 3. Upper or Moelfre Grits. 2. Lower Mudstones; Lower Manganese Bed near base. 1. Lower Grits. He thus recognized two manganese horizons separated by at least 850 feet of rocks. Charles Lapworth and T. Stacey Wilson, recognizing only one ore bed, divided the Harlech Grit series of the Survey as follows (Andrew 1910, p. 161):— 5. Gamlan Shale group: grey, greyish green, and sometimes purple shales, slates and flags with occasional grit bands increasing in number eastward. 750–1200 feet. 4. Barmouth Grits: massive grits and conglomerates. 600 feet. 3. Hafotty or Manganese Shale group: grey and green mudstones with rare grits in west, becoming more frequent