ANNIVERSARY ADDRESS OF THE PRESIDENT. 131 



affected the volcanic rocks is precisely similar to that which the 

 sedimentary strata immediately beyond them have suffered. 



But though the required evidence may not be obtainable from this 

 district of Anglesey, there is, I think, in other parts of the county 

 sufl'icient proof that these volcanic rocks have no connexion with 

 either of the two pre-Cambrian series to which I have already 

 referred as existing in Anglesey, but belong to the Lower-Silurian 

 groups. Seven miles to the south-east of Holyhead, in the basal 

 Lower-Silurian conglomerates which Mr. Selwyn found lying un- 

 conformably on the green schists, there occur abundant fragments of 

 volcanic rocks, besides the prevalent detritus of the schists of the 

 neighbourhood. Some of the bands have somewhat the character of 

 volcanic breccias or tuffs, and they show an evident resemblance to 

 portions of the Bangor group and the rocks of Llyn Padarn, though 

 they are doubtless of much later age. That these volcanic fragments 

 were not derived from the waste of rocks of a much earlier period i& 

 made tolerably certain by the intercalation of true tuffs among the 

 black shales higher up in the order of succession. Here, then, we 

 have evidence of contemporaneous volcanic action in the very 

 basement Lower-Silurian strata of Anglesey, which by their fossil 

 contents are shown to be on the horizon of the lowest Arenig 

 or even Tremadoc group. The Llangefni agglomerates may thus 

 be connected with a Silurian volcanic period, and may have been 

 brought into their present position by the general plication of the 

 district. 



But still further and more striking corroboration of this inference 

 is to be obtained by an examination of the northern coast-line. I 

 have already referred to the elliptical fault which is marked on the 

 Survey map running from the north-western headland to the eastern 

 coast beyond Amlwch. The necessity for inserting this fault, apart 

 from any actual visible trace of its occurrence, arose when the con- 

 clusion was arrived at that the rocks of the extreme north of 

 Anglesey were essentially altered Cambrian strata. For immediately 

 to the south of these rocks black shales, obviously Silurian, were 

 seen to dip to the north — a structure which could only be accounted 

 for by a dislocation letting them down into that position. The 

 same necessity for a fault has of course been felt by all writers who 

 have subsequently treated the northern area as pre-Cambrian. But 

 it is deserving of notice that in the original mapping of the Survey 

 no continuous abrupt hiatus is shown by the line which was 

 afterwards marked as a continuous line of fault. On the contrary. 



