part 1] AIs'NlVEESAET ADDEESS OF THE PEESIDEXT. Ixxvil 



The widespread vulcanicit^^ of Arenig times in South Wales 

 had died out, and the new theatre of activity lay farther north. 

 Eruptions broke out on the flanks of the new central anticline, 

 both in Radnorshire and in Shropshire. In Merioneth the vul- 

 canicit}^ prolonged through the whole of Llandeilo time, M'as on a 

 larger scale. It is represented by a great thickness of fragmental 

 accumulations, together with andesitic sills which, as Prof. Fearn- 

 sides has shown, were closeh'' bound up with the volcanic episode. 

 In Caernarvonshire, in addition to explosive outbursts, there was a 

 great outpouring of lavas. The district there affected, centreing 

 in Snowdonia, extends in one direction beyond Conw}^ and in the 

 other into the Lleyn peninsula. The maximum energy of this age 

 in Wales was thus displa3^ed along a belt immediately fronting the 

 Llanberis ridge ; but at that boundary it came abruptly to a stop. 



The several volcanic dist]-icts show nearly the same petrographical 

 types, folloAving one another in the same order of decreasing basi- 

 city, but with significant points of difference. In the Wells 

 district of Radnorshire basalts were succeeded by hypersthene- 

 andesites, and these finally b}' rhyolites. About Arenig Fawr, in 

 Merioneth, the earliest term of this sequence is wanting. First 

 come agglomerates and tuffs of hypersthene-andesite composition, 

 then, after an interval, more acid andesitic material, followed by 

 dacitic and finall}^ rhyolitic. In Snowdonia, not merely are basalts 

 lacking, but p3a'oxene-andesites are only feebly represented. The 

 great pile of lavas, with subordinate tuffs, is first dacitic and then 

 rhyolitic in composition, reaching also a higher degree of acidity 

 than is found elsewhere. The earlier types of the sequence are 

 thus better represented in the south, and the later in the north. 

 In the Shropshire district, lying somewhat aside from the main 

 line of thrust, the volcanic record is m^ueh scantier, and is also 

 spread over a longer time. After the Stapeley andesitic ashes, 

 which seem to correspond with the lowest group of Arenig FaAvr, 

 the Llandeilo strata here show no sign of contemporaneous vul- 

 canicity, and the completion of the sequence is represented in the 

 Bala Series by the Hagley and Whittery ashes, the admixture of 

 rhyolitic with andesitic material in the latter marking the final 

 phase. 



The change from an alkaline to a calcic facies, which marked 

 the Llandeilo epoch, did not necessarily take effect simultaneously 

 in different areas. In the Cader Idris district the spilitic type 

 seems to have persisted while andesitic eruptions were breaking out 



