

Yol. 62.] IGNEOUS AND SEDIMENTARY ROCKS OF LLANGYNOG. 223 



11. On the Igneous and Associated Sedimentary Rocks of Lean- 

 gynog (Caermarthenshire). By Thomas Crosbee Cantrill, 

 B.Sc. Lond., and Herbert Henry Thomas, M.A., B.Sc, 

 F.G.S. 1 (Read January 24th, 1906.) 



[Plates XXIII-XXVI.] 



Contents. 



Page 



I. Introduction 223 



II. Historical Summary 224 



III. The Sedimentary Eocks 226 



(1) The Tetragraptus-Beds. 



(2) The jDidi/mograpttis-bifidus Beds. 



(3) The Old' Eed Sandstone. 



IV. The Igneous Eocks 232 



(1) The Eocks near Coomb. 



(2) The Eocks at Capel Eethesda. 



(3) The Lambstone Porphyry. 



(4) Detailed Petrography. 



V. Structure, Folding, and Faulting of the District 246 



VI. Sequence and Age of the Igneous Eocks 248 



VII. Summary 249 



I. Introduction. 



In the neighbourhood of the village of Llangynog, some 6 miles 

 south-west of the town of Caermarthen, an elevated tract of ground 

 lying between the River Towy on the east and the Cywyn on the 

 west, presents features of especial interest in that it contains several 

 small masses of igneous rocks among the Ordovician sediments. In 

 the re-mapping on the 6-inch scale of the South Wales Coalfield and 

 the ground adjacent thereto, it fell to us to re-examine this district. 

 In memoirs necessarily devoted chiefly to a description of the Coal- 

 field, it is not possible to enter into such detail concerning the 

 petrology and stratigraphy of these older rocks as they seem to 

 deserve. We therefore sought the permission of the Director of 

 the Geological Survey to lay before this Society the following 

 particulars. 



The district is comprised in the Old Series 1-inch Ordnance-map, 

 Sheet 41 ; in the New Series 1-inch map, Sheet 229 (Caermarthen), 

 and in the 6-inch maps, Caermarthenshire 38 S.E., 39 S.W., 45 N.E., 

 and 46 N.W. It consists of an elevated plateau, which at Pen-y- 

 Moelfre and Castell Cogan attains an altitude of 546, feet and 

 426 feet respectively : it is drained in part by several streams 

 which flow southward into the estuaries of the Taf and the Towy, 

 and in part by streams flowing north-westward into the Cywyn, 

 which itself falls into the Taf. 



1 Communicated by permission of the Director of H.M. Geological Survey. 



R 2 



