CHAP, xm 
ANGLESEY 
231 
at once apparent. They seem to have been plicated, ruptured and thrust 
over each other, the harder parts surviving longest, but being eventually 
broken into small fragments. Every stage may be traced from a recogniz- 
able band of grit down to the rounded or elliptical jrebbles of the same 
material entirely isolated in this phyllitic matrix of crushed shale. 
But while the volcanic origin of these coarsely -fragmental masses 
cannot be maintained, there is elsewhere evidence that the older Paheozoic 
rocks of Anglesey include relies of contemporaneous volcanic eruptions. 
Seven miles to the south-east of Holyhead, in the basal Lower Silurian 
conglomerates which, as before referred to, 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 neighbour- 
hood. Some of the bands have sornewlrat 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 is 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 liorizon of the lowest Arenig 
or even Tremadoe group. 
But still further and fuller evidence of Silurian volcanisni 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 Geological Survey 
map as 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 conclusion 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 origmal 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, on one of the field-maps 
in, I believe, Mr. Selwyn’s handwriting the remark occurs : — “ The gradual 
passage from the black shale to the upper green gritty slates of Llanfechell 
is best seen at Bothedd, on road from Llanfaethlu to Llyn-llygeirian.” ^ 
It is no part of my aim to disprove the existence of faults along the line 
1 I have fully considered the evidence adduced by Dr. Callaway and Professor Blake, and 
have examined the gi’ouud, and can come to no other conclusion than that stated in the text. 
But see Mr. Blake’s remarks, Gcol. Mag. 1891, p. 483. 
- There is no continuous section now visible at this place, but the two groups of rock can be 
traced to within a few feet of each other, both inclined as usual in the same direction, and the 
black shales appearing to pass under the others. 
