ANNIVERSARY ADDRESS OF THE PRESIDENT. I 57 



of true bedded tiifFs among them is a proof that volcanic action had 

 been in o])eration there hnig before the outbreak of the vents which 

 are now hiid bare along the cliffs. 



In the south-east of Ireland there is the usual association of acid 

 and basic sills with the evidence of a superficial outpouring of 

 lavas and ashes. But these intrusive masses pluy a much less 

 imposing part than in Wales. They may be regarded, indeed, as 

 bearing somewhat the same proportion to the comparatively feeble 

 display of extrusive rocks in this region that the abundant and 

 massive sheets of Merionethshire and Caernarvonshire do to the 

 enormous piles of lavas and tuffs which overlie them. 



Among the acid intrusive sheets the most conspicuous are those 

 mapped by the Survey as " elvans." These rocks, as they occur in 

 Wicklow and Wexford, have been examined by Dr. Hatch, who 

 iinds them to be microgranitic in structure, occasionally exhibiting 

 micropegmatitic or granophyric modifications *. The true strati- 

 graphical relations of these rocks have not yet been adequately 

 investigated. Those of them which occur on the flanks of the great 

 •granite ridge are not improbably connected with that mass. 



The basic sills, or " greenstones," consist largely of diabase, 

 frequently altered into epidiorite ; they include also varieties of 

 diorite t. That they were intruded before the plication and 

 cleavage of the rocks among which they lie is well shown by 

 their crushed and sheared margins where they are in thick mass, 

 and by their cleaved and almost schistose condition where they 

 are thinner. The intense compression and crushing to which 

 they have been subjected are well shown by the state of their 

 component minerals, and notably by the paramorphism of the 

 original augite into hornblende. 



The scarcity of dykes associated with Silurian volcanic action 

 is as noticeable in the south-east of Ireland as it is in Wales. I 

 have observed a considerable number, indeed, but they are confined 

 to the line of old vents on the Waterford coast, and, but for the 

 clear cliff-sections cut by the sea, they would certainly have escaped 

 observation, for they make no feature on the ground in the interior. 

 They are sometimes distinctly columnar, and vary from less than 

 a foot to many yards in width. They traverse both the agglo- 

 merates and the intrusive felsites. Most of them are of felsite, 

 sometimes cellular ; but in some cases they are dolerites. There is 

 obviously no clue to the relative dates of these dykes. 



* Explanation of Sheets 138 and 131), p. 53. t Hatch, op. cit. p. 49. 



