[119] The occurrence of flinty or cherty deposits, in association with volcanic rocks of Lower Silurian age, is well established in Britain, and will be more particularly referred to in the sequel.
A cursory examination of the contents of the conglomerates, breccias and grits shows them to consist largely of different felsites, with fragments of more basic lavas. Some of these might obviously have been derived from the rock of the porphyry ridge, but, as at Llyn Padarn, there is a far greater variety of material than can be found in that ridge. Some of the fragments show perfect flow-structure. Professor Bonney has described the microscopic characters of some of these fragments, and has especially remarked upon their glassy character. Among the slides prepared from specimens collected by myself, besides the abundant fragments of felsite (rhyolite), there are also numerous pieces of different andesitic lavas and fine tuffs, as well as grains of quartz and felspar, and sometimes a good deal of granular iron-ore.
That a large proportion of the material of the so-called "Bangor beds" was directly derived from volcanic explosions can hardly be doubted. There appears to have been a prolonged succession of eruptions, varying in intensity, and somewhat also in the nature as well as in the relative fineness of the material discharged. On the one hand, coarse massive agglomerates were probably accumulated not far from the active vents, as the result of more violent or transient explosions; on the other hand, exceedingly fine and well-stratified tuffs, which attain a great thickness, serve to indicate a phase of eruptivity marked by the long-continued discharge of fine volcanic dust. Ordinary sediment was doubtless drifted over the sea-bottom in this district during the volcanic episode, but the comparative infrequence of distinct interstratifications of shale or sandstone may be taken to imply that as a rule the pauses between the eruptions were not long enough to allow any considerable accumulation of sand or mud to take place.
No satisfactory proof has yet been obtained of any interstratified lavas among the tuffs of the Bangor district. Some rocks, indeed, can be seen on the road between the George Hotel and Hendrewen, which, if there were better exposures, might possibly furnish the required proof; but at present little can be made of them, for their relations to the surrounding rocks are everywhere concealed.
From what I have now adduced, it is obvious that while both felsitic and andesitic lavas existed within the volcanic foci, and were ejected in fragments to form the tuffs and breccias, the lavas poured out at the surface during the Cambrian period in Caernarvonshire were mainly, if not entirely, felsites (rhyolites) in which the chief porphyritic constituent was quartz. These lavas thus stand entirely by themselves in the volcanic history of Wales. Though felsites of various types were afterwards poured out, nothing of the same quartziferous kind, so far as we yet know, ever again appeared. Further south, in Merionethshire, as will be shown in [Chapter xii.], the Cambrian volcanic eruptions appear to have been on the whole less acid, and to have begun with the outpouring of andesitic lavas.
I have now to consider the relation of the volcanic group of Bangor to the strata which overlie it. The geological horizon of these strata is not, perhaps, very definitely fixed. It may be Arenig, possibly even older. But for my present purpose it will be sufficient to consider the strata in question as lying at the bottom of the Lower Silurian series. Professors Hughes and Bonney have taken as their base a marked but impersistent band of conglomerate. Mr. Blake, however, has more recently shown that, as this band is succeeded by tuffs like those below it, it cannot be claimed as marking the upper limit of the volcanic group. He therefore classes it in that group and traces what he thinks is an overlap or unconformability at the bottom of the Lower Silurian strata to the east. Mr. B. N. Peach, who accompanied me in an examination of this ground, agreed with me in confirming Mr. Blake's observation as to the position of the conglomerate, which is undoubtedly overlain by the same flinty felsitic tuffs as are found below it. But we were unable to trace any unconformability. According to the numerous observations which we made, there does not seem to be any discordance in strike or dip between the flinty tuffs and the overlying shales and grits. The two groups of rock appeared to us to be conformable and to pass into each other, as at Llyn Padarn.[120]
[120] See Mr. Blake on this point, Quart. Journ. Geol. Soc. vol. xlviii. (1892), p. 252, note. I retain the opinion expressed above.
An unconformable junction here would, in some respects, have been welcome, for it would at once have accounted for the superposition of Lower Silurian strata directly upon the Cambrian volcanic series, and for the disappearance of the Llanberis slates and grits which form so conspicuous a feature above the tuffs and conglomerates at Llyn Padarn. In the absence of such a structure we must accept the order of succession as apparently unbroken, and rely on some such explanation as was proposed by Sir Andrew Ramsay to account for the overlap of the Arenig rocks on everything older than themselves as they are traced northwards.[121] But this explanation will not entirely remove the difficulties of the case. The inosculation of the volcanic group of Bangor with the base of the Lower Silurian series cannot be accounted for by any such overlap; it seems only explicable on the supposition that the volcanic activity, which ceased in the Llyn Padarn district about the time that the Llanberis Slates were deposited, was continued in the Bangor area until Arenig time, or was then renewed. The thick volcanic group of Bangor would thus be the stratigraphical equivalent not only of the thin volcanic group of Llyn Padarn, but of the overlying mass of strata up to the Arenig rocks. In confirmation of this view, I shall show in a later chapter that volcanic action seems to have been prolonged in Anglesey to a still later geological period, that it appeared during the deposition of the Arenig strata, and that it attained a great development throughout the time of the Bala group. That a series of volcanic rocks, with associated cherty strata, may be the stratigraphical equivalent of a great thickness of ordinary sediments in other districts will be dwelt upon in the description of the Lower Silurian volcanic geology of the Southern Uplands of Scotland.[122]
[121] Mem. Geol. Surv. vol. iii. 2nd edit. p. 252.
[122] A group of cherts and mudstones not more than 60 or 70 feet thick appear in that region to be stratigraphically equivalent to the great depth of sedimentary material which elsewhere constitutes the Upper Arenig and Lower and Middle Llandeilo formations. See Annual Report of the Geological Survey for 1895, p. 27 of reprint.