The Harlech Dome, north of the Barmouth Estuary

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Abstract
Summary: The area described, 150 square miles in extent, lies between the Barmouth estuary and the Vale of Ffestiniog and contains some 10,000 feet of Cambrian rocks which range from ?Lower Cambrian to Ffestiniog Flags and include thick formations of grit. The igneous rocks are all intrusive, are of Lower Palaeozoic age (though of more than one date), and are nearly always found in an altered condition; they are " greenstones " of either dioritic or basic composition. Quartz veins and metalliferous lodes are common. The sediments, the materials of which are chiefly derived from an easterly extension of the Mona Complex of Anglesey and south-western Caernarvonshire, have undergone low-grade metamorphism; some beds (especially the Manganese Ore Bed) are rich in authigenic spessartite. The general structure is a broken anticlinorium which has been compressed from west to east. The cleavage is predominantly northerly. Faults are numerous, the latest being an important north-and-south series with displacements up to about 4000 feet.