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Geological Society. 45 
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.Sec., A.K.C., F.G.S., and Sidney 
William Wooldridge. 
Parr I—Srrarieraputcan (L. D. §.). 
The igneous rocks of Llanwrtyd oceupy 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 fossiliferous, 
Fossiliferous ashy shales, 
(5) The Upper Ashes {Fin banded ashes. 
Coarse ashes. 
(4) Hardened mudstones with a band of ashy limestone. 
(3) The Spilites and Spilite-Breccias. 
(2) Hardened sediments with fossiliferous mudstones. 
(1) The Lower Ashes and Breccia, 
The fossils from the lower horizon (2) include Dicranograptus 
rectus Hopkinson, Glyptograptus teretiusculus var. siccatus 
Elles & Wood, and Climacograptus schérenbergi Lapworth, Those 
from the higher horizon (5) include Dicellograptus sextans Hall 
and var. ewilis Elles & Wood, and Glyptograptus teretiusculus 
var. stecatus Elles & Wood. Both assemblages are characteristic 
of the Dicranograptus Shales of South Wales, especially of the 
_ horizon of the Mydrim Limestone. The volcanic rocks of Llan- 
wrtyd are therefore of lowest Bala (Survey classification) or highest 
Llandeilo (classification of Miss G. L. Elles) age, and on the 
same horizon as the Upper 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 miles 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. 
