Abstract
Summary The paper deals with an area of dolerites intrusive into Llandeilo Shales and forming a prominent ridge near Welfield, north-west of Builth. The area has been mapped on the scale of 25 inches to the mile and an analysis of the relations between the dolerites and the associated strata, usually strongly baked near the contacts, leads to the conclusion that the ridge is occupied by a core of dolerite having the habit of an elongated .laccolith. The inferred main body is accompanied by many lenticles of dolerite on the flanks of the structure and also sheets which appear to spring off from two dyke feeders near the crest of the laccolith. The whole area has been tilted to the north-west in post-Silurian times. The height of the laccolith is inferred to have been about 500 feet and its width about 1500 feet, and it was intruded under a cover of Ordovician and possibly early Llandovery sediments estimated to be at least 4500 feet thick.
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