Summary The Lower Palaeozoic succession consists mostly of slates but contains a large volcanic series, apparently of Bala age. This is composed essentially of rhyolitic pyroclasts but contains some basic lavas in parts. It reaches a thickness of more than 8000 feet of virtually uninterrupted volcanic rocks in the north-east, but fades out rapidly towards the west. The basic structure of the area is a fold of synclinal type. In the north-east this is sharply overturned towards the north-west, but it changes south-westwards to a shallow, symmetrical synclinorium. This fold has been cut through by numerous thrust-planes with displacements averaging two to three miles and by many high-angle faults, some of considerable dimensions. Evidence on the relative ages of intrusions and faults is abundant and allows the complex Caledonian history to be largely deciphered. The initial formation of the synclinorium was succeeded by numerous phases of faulting and a variety of igneous intrusions. Metasomatic events and periods of mineralization are also found in this history.

A sedimentary formation beneath the Bray Series is found to have an extensive outcrop in southern Co. Wicklow. Evidence is given of the Pre-Cambrian age of both series. They are exposed in a Caledonian anticlinorium, which has been cut by thrusts and highangle faults, but they had suffered strong orogenic deformation and prolonged erosion before the unconformable deposition of Lower Palaeozoic strata.

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This area of Wales is a westerly continuation of the northern limb of the Snowdon syncline and stretches from the Pre-Cambrian volcanic series in the north to just above the abbreviated Snowdon Volcanic Series in the south. Large parts of the intervening area consist of Caledonian intrusive rocks, but the middle part of the sedimentary succession has yielded scattered graptolites, most and probably all, of Llanvirn age. The exposed volcanic rocks are largely of ignimbrite character while the Caledonian intrusions are of four types (for which chemical analyses are given). The sequence of these four rocktypes and of the structural events of the district is considered.