Professor R. M. SHACKLETON said that he accepted the hypothesisof magmatic doming but not that of volcanic collapse, and hedid not think that the Llyn Dinas Breccia could represent materialthat slid off the dome; he preferred Dr Rast’s view thatit was a lahar unrelated to the dome. The breccia undoubtedlycontained blocks of the Pitt’s Head flow; the Pitt’sHead Group was supposed to have been domed with the underlyingsediments and the spectacular local unconformity was supposedto separate the Pitt’s Head Group from the Lower RhyoliticTufts, whereas the speaker thought that the unconformity wasbelow the Pitt’s Head Group; in the Snowdon Volcanic rocksin the syncline between Beddgelert and Moel Ddu there were manyexposures of the lower Pitt’s Head flow, and the speakernow considered that the volcanic rocks in this syncline restedessentially unconformably on the eroded dome, although therehad been movements on many of the contacts. The Pitt’sHead flow, which had been demonstrated by the work of Dr Rastand others to be a welded tuff and was therefore highly mobile,could only have covered ground that was almost flat—thisbefore the breccia was formed. The author’s view dependedupon two correlations; first, of the two Lower Lapilli-tuftson Yr Arddu with the Pitt’s Head flows on Moel Hebog,and secondly, of the disconformity below the third of the YrArddu Lapilli-tuffs with the unconformity below the tufts andbreccia on Moel Dyniewyd. One or other
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