16 DB. STAMP AND ME. WOOLDEIDGE ON THE [vol. Ixxix, 



2. The Igneous and Associated Rocks o/'Llanwetyd (Beecon). 

 Bj Laueence Dudley Stamp, B.A., D.Sc, A.K.C.(Lond.), 

 F.Gr.S., and Sidney William Wooldbidge, B.Sc, F.G.S. 

 (Read November 9th, 1921.) 



[Plates I & II.] 



Contents. 



Part I. Stratigraphical. (By L. D. S.) 



(1) Introduction 16 



(2) Previous Work 17 



(3) The Succession of the Strata 18 



(4) Description of Typical Sections 25 



(5) Structure and Scenery of the District 28 



(6) Comparison with other Areas 29 



(7) Conclusion 33 



Part II. Petrographical. (By S. W. W.) 



(1) Introduction 33 



(2) The Lower Ashes and Breccias 33 



(3) The Spilite-Breccias and Spilites 34 



(4) The Tuff- Bands in the Hardened Mudstones 38 



(5) The Upper Ashes 39 



(6) The Intrusion 42 



(7) Summary and Conclusions 43 



Paet I. Stbatigbaphical. 



(1) Introduction. 



Noeth 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 palseontological 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. 



