The present work was undertaken to determine the nature and origin of the Managanese-ore bed, which occurs in the lower portion of the Manganese or Hafotty Shale Group of the Harlech Grit Series of West Merionethshire (North Wales). This has involved a detailed study of the associate sediments which were all found to contain abnormal amounts of manganese. The deposits included grits, mudstones, pyrite-bearing rocks and sphaerodialogite developments. The distribution of these rocks has been mapped on the 6 inch scale and an account of their field relationships is given, this leading to a statement on the structure of the area. In the laboratory all the rock types were examined in thin section, and in addition the heavy mineral content of the grits was investigated.