(1) Introduction North of the little town of Llanwrtyd Wells lies an oval patch of igneous rocks, about 3 miles long and half a mile broad. It has long been known that these rocks occupy the core of an anticline—that of the Vale of Towy—which strikes approximately north-north-east and south-south-west. Some years ago I visited the igneous rocks in the neighbourhood of Builth (described by Mr. Henry Woods in 1894). It has usually been presumed that the igneous rocks of Llanwrtyd, which locality is about 10 miles west-south-west of Builth, are of the same age as those near the latter town. Accordingly, in order to test the truth of this belief, a survey of the Llanwrtyd rocks was begun. It was found that the igneous rocks differed considerably from those near Builth, but at that time no palsæontological evidence of their age was found. In 1920, while studying the Siluro-Devonian junction in Central Wales, I took the opportunity of revisiting Llanwrtyd Wells for the purpose of completing the mapping of the igneous rocks and Ordovician sediments. I was accompanied on this occasion by Mr. Wooldridge, who has undertaken the petrological description of the rocks. (2) Previous work The literature relating to this district is scanty. The only description is that given by Sir Roderick Murchison. He states that the presence of mineral springs at Llanwrtyd Wells led him to suspect the presence of igneous rocks in the neighbourhood, and his investigations proved that such rocks did occur. His account occupies