ANNIVERSARY ADDRESS OF THE PRESIDENT. IO3 



already expressed, and from wliicli I have as yet seen no renson to 

 depart. 



liolow the lowest fossiliferous Cambrian strata at St. David's 

 there lies a volcanic group consisting of tuifs and breccias, with 

 contemporaneously erupted and subsequently intruded mas-ivo 

 rocks. About 1800 feet of these various materials are visible, but 

 the bottom of the group is nowhere reached. The tuffs belong to 

 two groups— ono derived from the explosion of diabase-lavas and 

 containing about 50 per cent, of silica, the other arising from tho 

 discharge of felsitic fragments and consequently showing as much 

 as 70 to 80 per cent, of silica. The basic and acid detritus arc 

 mingled in some bands, producing materials of intermediate compo- 

 sition. The texture of the tuffs varies from fine silky schists to 

 somewhat coarse breccias or agglomerates. Most of these fragment al 

 rocks are basic in character, the more acid varieties occurring as 

 occasional interstratifications, but rather predominating in tho 

 upper parts. Thus, while the eruptions usually dischaiged diabase 

 detritus, there would seem to have been occasional explosions from 

 felsitic lavas, and apparently a prevalence of the latter towards tho 

 close of the volcanic period. 



The interbedded lavas, unlike those of Caernarvonshire, include 

 divine-diabases, sometimes strongly amygdaloidal or presenting 

 slaggy characters. These are dull, fine-grained, sparingly porphy- 

 ritic rocks, and range in colour from epidote-green to dull blackish 

 green and dark chocolate-brown. They contain less than 46 per 

 cent, of silica and about 10 per cent, of magnesia. Their abundant 

 augite is minutely granular, and their olivine is in great part 

 replaced by haematite. There occur also sheets of porphyrite 

 among the tuffs. These lavas form but a small proportion of the 

 whole volcanic group. An intrusive boss of granite rises among 

 tho rocks, and around it are quartz-porphyries (granophyres), pro- 

 bably its apophyses, which traverse the volcanic group, while later 

 still come intrusive sheets and dykes of diabase. Only a small 

 fragment of a volcanic district has survived at St. David's. The 

 centre of eruption probably lay somewhere to the south-west. 



AVhile volcanic action, though never so vigorous as in later geolo- 

 gical times, manifested itself over most of Wales and probably across 

 what is now the centre of England, there is, I think, evidence that 

 its extreme western limits reached as far as the east of Ireland. 

 The rocks of Howth Island to the north of Dublin and of Bray 



VOL. XLVII. h 



