[223] Quart. Journ. Geol. Soc.
[224] Op. cit.
[225] Presidential Address Geol. Soc. vol. xlvii. (1891), p. 130.
[226] Quart. Journ. Geol. Soc. vol. li. (1895), p. 563. See Geol. Mag. 1896, p. 481.
But though their present coarse, agglomerate-like structure is, I think, entirely due to the mechanical crushing of the rocks in situ and not to volcanic explosions, it does not follow that the rocks which have been broken up do not contain evidence of volcanic action contemporaneous with their original formation. Obviously, pyroclastic materials may be subjected to deformation and disruption as well as any other components of the earth's crust, and may be equally converted into crush-conglomerates. And in Anglesey it can, I think, be shown that some of the rocks which have been broken up were originally tuffs and volcanic breccias.
Throughout Anglesey the stratified rocks present evidence of having undergone very great compression, deformation and rupture. Thus at Llanerchymedd thick-bedded Lower Silurian grits, with their intercalations of shale, have been broken up by numerous small faults, and have been pushed over each other in large irregular blocks, the shales being now pinched out, and now pressed up into the interstices between the dislocated harder and more resisting grits. This condition of rupture may be regarded as one of the stages towards the formation of a conglomerate by the crushing together of rocks in situ. A few miles further south at the beginning of the railway cuttings of Llangefni, green, red and purple slates and grits appear in a rather more crushed state, and immediately beyond these strata come the coarse breccias. Neither in their composition nor in their structural condition do these Llangefni strata appear to be marked off from the undoubted Lower Silurian rocks as parts of a different system.
The railway cuttings at Llangefni reveal a series of rocks which appear to have been originally shales, with thin bands of siliceous grit. The argillaceous portions of this series are now green and phyllitic, and remind one of the finer parts of some basic tuffs among the older Palæozoic systems. They include, however, pale flinty bands, such as might have been derived from fine felsitic dust. The grits are for the most part fine-grained and highly siliceous, but they include also coarser varieties with clear quartz-grains. The enormous deformation which these strata have undergone is 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 recognizable band of grit down to the rounded or elliptical pebbles 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 Palæozoic rocks of Anglesey include relics 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 unconformably 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 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 horizon of the lowest Arenig or even Tremadoc group.
But still further and fuller evidence of Silurian volcanism 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.[227] 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, 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."[228]
[227] I have fully considered the evidence adduced by Dr. Callaway and Professor Blake, and have examined the ground, and can come to no other conclusion than that stated in the text. But see Mr. Blake's remarks, Geol. Mag. 1891, p. 483.