part 1] ANNITERSART ADDEESS OF THE PRESIDENT. Ixxiu 



history. Although there are no data for correlating the rocks of 

 the isolated areas, it is clear that the^^ all belong to a time when 

 the main Ai-chaean crust-movements were past. It does not follow 

 that the conditions as regards crustal stress were the same in 

 the several disti-icts, and the observed petrogra])hical dilferences are 

 probabh^ not without significance. In Pembrokeshire there seems 

 to be no noteworthy break between the volcanic series and the 

 succeeding Cambrian, and indeed the basic intrusions which repre- 

 sent the latest igneous episode sometimes penetrate the Lower 

 Cambrian strata. The old lavas and tuffs here have been studied 

 onh^ in part, but soda-rh^^olites seem to be the most cliaraeteristic 

 type. The associated intrusions in the Brawdy and St. David's 

 districts, as described by Dr. H. H. Thomas & Prof. 0. T. Jones, 

 include especially albite-granites, and the plutonic complex of 

 Johnston is also composed of rocks rich in sodic felspars. In this 

 area, not affected by any contemporaneous disturbance, the rocks 

 have, then, a pronounced alkaline facies. Herein they differ from 

 the probably homotaxial rocks of the Wrekin and Malvern, Bangor 

 and Llanberis, areas which lay close to centres of Lewisian in- 

 trusion, and were perhaps still under the influence of special stress. 



With the Paheozoic era we enter upon a time concerning which 

 we possess much fuller information. Some leading features of the 

 broad structure of our country were already roughly blocked out, 

 and have exerted a dominant influence upon subsequent events. To 

 the north- we^t was the large cr^^stalline mass of which the Scottish 

 Highlands and North- Western Ireland are relics, consisting of great 

 plutonic cores with their aureole of metamorphosed sediments. 

 This, with a general north-easterly and south-westerly extension, 

 marks rovighly one limit of the geosyncline in which our LoAver 

 Palaeozoic strata were laid down. As regards the southern limit, 

 there are many indications of an important Archaean tract on the 

 site of the English Channel ; and it may well be that what we 

 regard as the Armorican trend was already impressed on the 

 southern part of the British area, as the Caledonian trend Avas on 

 the northern. The geosyncline thus bounded was not a simple 

 one, but contained within it minor features, likewise of Archaean 

 age, which continued to make their influence felt at later times. 



During the deposition of the Cambrian and earlier Ordovician 

 sediments this area was undergoing a general subsidence. The 

 dowuAvard movement, however, was neither equable in extent nor 



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