vol. 6s.'] the geology of mynydd-t-gadee. 363 



Discussion. 



Mr. H. H. Thomas congratulated the Authors on the results of 

 their detailed mapping in a complicated region, and he was glad 

 that another series of Welsh igneous rocks had been relegated to 

 their proper position in the Ordovician sequence. In this district 

 the Authors had traced fossiliferous Tremadoc and Lower Llanvirn 

 rocks, but between these there was only a relatively thin series of 

 rhyolites. Did the Authors consider the Rhyolitic Series to form 

 part of the true Arenig, or did they think that it possibly occurred 

 as part of the Lower Llanvirn ? 



He was interested in the extremely close resemblance, both 

 lithological and chemical, which these rhyolites bore to those of 

 Skomer Island, and was glad that the Authors had come to the 

 conclusion that the cavities in the larger spherulites were of 

 secondary origin and were not lithophysal in character. 



Prof. W. W. Watts considered the existence of a volcanic 

 horizon between the Lower and the Upper Dolgelly Beds to be of 

 such importance, that he would be glad to hear whether the Authors 

 had any additional evidence in support of the extrusive character 

 of the igneous rocks. 



Mr. P. Lake, in reply to Mr. Thomas, said that the Rhyolitic 

 Series probably belonged to the Didymograptus-e.vtensus Zone ; but 

 all that was definitely proved was, that it lay between the 

 Dictyonema Zone on the one hand and beds with Didyraograptus 

 hifidus on the other. With regard to the question raised by 

 Prof. Watts, the relations of the andesite to the Lingula Flags 

 were dealt with in the Authors' previous paper, and they had 

 no further evidence to offer. The andesite appeared to lie at a 

 constant horizon, and showed no signs of intrusion. 



