Yol. 62.] IGNEOUS AND SEDIMENTARY ROCXS OF LLANGYNOG. 251 



of the Authors' work. In regard, however, to the sandstone with a 

 conglomeratic base un conform ably overlying the Ordovician Series, 

 described by the Authors as Old Eed Sandstone, he was impressed, 

 when he visited the district, with the idea that it was possibly of 

 Downtonian age. The speaker described a set of beds much 

 resembling Old Red Sandstone, but enclosing marine Downtonian 

 fossils, from the neighbourhood of Llangadock. Remembering that 

 these beds thinned out, changed colour, and became less fossiliferous 

 as they were traced towards the area dealt with by the Authors, it 

 seemed quite possible that the grey sandstones which they described 

 might represent Downtonian deposits that had overlapped the under- 

 lying Silurian, and he would like to ask whether the Authors could 

 give satisfactory proof that this was not the case. 



Mr. J. Y. Elsden said that he had taken particular interest in 

 the igneous rocks described by the Authors, who had suggested a 

 possible resemblance between the Lambstone porphyry and the 

 lime-bostonites and oligoclase-porphyrites occurring farther north,. 

 in Pembrokeshire. So far as he could judge, the Lambstone- 

 rock differed chiefly in having a less felspathic groundmass ; but 

 only a chemical analysis could determine the question satisfactorily. 

 The alternations of andesites and rhyolites, mentioned in the paper, 

 were especially interesting, and suggested the possibilitj^ of the 

 Lambstone intrusion representing part of a later dyke-phase of the 

 same volcanic sequence. A similar explanation was also possible 

 in Pembrokeshire, where the speaker had shown reasons for assign- 

 ing the lime-bostonites to an earlier period than the diabase- 

 intrusions. The absence of rhombic pyroxenes from the Llangynog 

 diabases was significant, in view of their great abundance in the 

 St. David's-Head district. 



Mr. W. G. Pearnsides congratulated the Authors upon their 

 additions to our knowledge of the Ordovician rocks of South Wales. 

 He enquired whether the igneous series described would fall within 

 the zones of Didymograptus extensus and D. hirundo, remarking 

 that in Southern Merionethshire Ramsay had observed very similar 

 rocks within what would now appear to be the limits of these zones. 

 He was glad to hear of one more district wherein later igneous 

 intrusions are practically confined to the immediate neighbourhood 

 of the rigid masses of the earlier volcanic outpourings. 



Mr. Canirill pointed out that the mapping of the Llangynog and 

 adjacent districts clearly establishes the fact that there have been 

 at least two periods of movement along the same general east-north- 

 east and west-south-west lines. That the first antedates the Lower 

 Old Red Sandstone is proved by the manner in which that formation 

 crosses the outcrops of various members of the Cambrian and Ordo- 

 vician systems and of the igneous rocks of Coomb. A subsequent 

 movement is equally clearly later than the Old Red Sandstone, for 

 that formation is not only much folded, but is faulted deeply 

 between some of the Ordovician subdivisions. 



That the green beds at the base of the Old Red Sandstone cannot 

 be assigned to the Downtonian seems clear from the existence of 



