On the Geology of the Gold-Bearing District of Merionethshire, North Wales

Abstract

T hat part of Merionethshire to which this notice refers lies between Dolgelli and the Moelwyn and Manod range, north of Ffestiniog*. A small part of this area has for some years been distinguished by discoveries of gold, of more or less importance. Its geological relations are as follows:—On the N. and W. of the lower part of the River Mawddach lie the lower part of the Lingula-flags and the Cambrian rocks. The latter consist of the coarse, thick-bedded, greenish-grey grits of Barmouth and Harlech. Their upper boundary is marked by a sinuous line which strikes in a north-easterly direction from Barmouth to Rhaiadr-Mawddach, and from thence trends northerly a little E. of Trawsfynydd to the turnpike road about a mile S. of Ffestiniog. The line then strikes S.W. to Morfa Harlech, on the coast of Cardigan Bay. These grits are overlaid by that part of the Lower Silurian rocks known as the Lingula-flags, which here consist mostly of blue slaty beds, generally more or less arenaceous, and partly interstratified with courses of sandstone. A well-marked portion of the series, composed of rusty ferruginous slate, occupies part of the cliffs of Moel Cynwch that overhang Dol-fawr on the Mawddach. Both Cambrian and Silurian rocks have been penetrated by numerous greenstone-dykes. Many of them are of a light grey colour and highly calcareous. Others assume the colour and texture of ordinary greenstone. Some of them are magnetic. Among the Cambrian sandstones they run in all directions, sometimes with, but more generally across