part 1] IGNEOUS AND ASSOCIATED EOCKS OF LLANWRTYD. 45 



brought forward that the Llanwrtyd rocks are on a higher strati- 

 graphical level than those of Builth is an important step in the 

 elucidation of the history of vulcanism in Wales ; it gives further- 

 support to the sequence determined in the Cader-Idris area where 

 fossils are not easily obtainable, and it raises interesting questions, 

 as to the underground extension of these rocks in the areas round 

 Llanwrtyd. Another remarkable point is the great thickness of 

 the zonal deposits, even when due allowance is made for the- 

 presence of volcanic rocks. In this respect also, the area allies- 

 itself with the more distant Merionethshire regions, and offers a 

 strong contrast to the nearer Carmarthenshire district, where the 

 Nemagraptus-gracilis Zone is thin, and contains no suggestion! 

 of the existence of volcanic activity. 



He would draw attention to the difficulty caused by the present 

 want of agreement as to the exact delimitation of the Llandeilo 

 and Bala Groups. This difficulty confronted all workers on the 

 Ordovician rocks of Wales, and led to undesirable and quite 

 unnecessary complication of the nomenclature. In view of the- 

 importance of the horizon, he thought that the Geological Society 

 might well establish a Committee which should consider the ques- 

 tion, and lay down a ruling that subsequent writers could follow. 

 He did not know whether there was any precedent for such action, 

 but its advantages seemed obvious. 



Miss G. L. Elles expressed the opinion that the horizon of the 

 rocks as indicated by the graptolites was rather later than that 

 surmised by the Authors. She considered the absence of certain 

 characteristic species as significant, the species recorded being 

 those that commonly survived into beds at a higher horizon. She 

 regarded it as interesting that the assemblage recorded was identical 

 with that in the black shales resting upon the Arenig Mountain 

 Volcanics, which were the base of the Dicranograplus Slates of 

 North Wales. 



In the Builth country there were two fossiliferous horizons above 

 the extrusive volcanic beds, the higher of which contained precisely 

 the assemblage recorded by the Authors from Llanwrtyd. These 

 were, however, cut and altered by the intrusive rocks, and if these 

 were to be regarded as belonging to the same period (as seemed 

 likely), it was hardly correct to say that the rocks of the Builth 

 area were of Llanvirnian age. 



Dr. G. H. Plymen said that it was matter for comment that 

 the volcanic sequence described by the Authors contained a lower 

 keratophyric flow and an tipper spilitic outburst, the normal suc- 

 cession being that of keratophyric or rhyolitic rocks after spilitic 

 or others of intermediate nature ; also, he would have expected 

 the vesicular spilites to overlie those of massive character. The 

 Authors were to be congratulated on registering a case where what 

 might be called ' conventional ' volcanic activity was reversed. 



Mr. G. M. Part enquired whether there was any evidence to 

 be obtained from the chlorite-pseudomorphs, as to the nature of 

 the original ferromagnesian mineral of the spilites. 



