Geological Society. 455 



PROCEEDINGS OF LEARNED SOCIETIES. 



GEOLOGICAL SOCIETY. 



November 9th, 1921.— Mr. R. D. Oldham, F.R.S., 

 President, in the Chair. 



The following communications were read : — 



1. 'The Igneous and Associated Rocks of Llanwrtyd (Brecon).' 

 By Laurence Dudley Stamp, D.Sc, A.K.C., F.G.S., and Sidney 

 William Woolcli-idge. 



PaET I — SXKAXIOBAPHICAL (L. D. S.). 



The igneous rocks of Llanwi'tyd occupy the core of the Towy 

 Anticline of Central Wales for a distance of about 2 miles. 

 The succession proved is as follows : — 



[(7) Intrusion.] 



(6) Black Slates, cleaved and poorly foBBiliferous. 



C FossOiferous ashy shales. 

 (5) The Upper Ashes < Fine banded ashes. 



C. Coarse ashes. 

 (4) Hardened mudstones with a band of ashy limestone. 

 (3) The Spilites and Spilito-Breccias. 

 (2) Hardened sediments with fossiliferous mudstones. 

 (1) The Lower Aehes and Breccia. 



The fossils from the lower horizon (2) include Dicrano(jroptus 

 rectus Hopkinson, Glyptor/r(q)tus terethisculus var. siccutus 

 EUes & Wood, and Climacograptus schiirenhergi Lapworth. Those 

 from the higher horizon (5) include Dicellograptus sextans Hall 

 and var. exilis EUes & Wood, and Glyptograptus terethisculus 

 var. siccatus Elles & Wood. Both assemblages are characteristic 

 of the Dicranograpttis Shales of South Wales, especially of the 

 horizon of the Mydrim Limestone. The volcanic rocks of Llan- 

 wrtyd are therefore of lowest Bala (Suiwey classification) or highest 

 Llandeilo (classification of Miss G. L. Elles) age, and on the 

 same horizon as the tapper Basic and Upper Acid Series of Cader 

 Idris. It had previously been suggested that the Llanwrtyd 

 rocks were of the same age as those at Builth, 10 mUes away, 

 which are Llanvirnian. 



A detailed description of the beds is given, as well as an account 

 of the more important sections, and a comparison with other parts 

 of Wales. The igneous rocks are cut off on the west by a fault, 

 into which an intrusive mass appears to have been forced. 



