Fig. 61.—Blue shale or slate passing into volcanic breccia east of Porth Padrig, near Carmel Point, Anglesey.
The breccias south of Carmel Point, though they are chiefly made up of felsitic detritus, sometimes show a preponderance of fragments of shale. They vary also rapidly in texture and composition. These variations may indicate that the vent or vents from which their materials were derived stood somewhere in the near neighbourhood, if indeed they are not to be recognized in some of the boss-like eminences that rise above the shore. At the same time, the enormous amount of crushing and shearing which the rocks of this region have undergone has doubtless introduced crush-conglomerates into the structure of the ground. And some patient labour may be required before the nature and origin of the different fragmental masses are determined.
Certain remarkably coarse, tumultuous breccias, exposed on the coast at Mynyddwylfa and Cemmaes, were formerly regarded by me as volcanic agglomerates. But more recent examination has satisfied me that these, like the breccias at Llangefni, are not of volcanic origin but are crush-conglomerates.[234]
[234] Presidential Address, Quart. Journ. Geol. Soc. vol. xlvii. p. 134; Rep. Brit. Assoc. 1896, Section C; Geol. Mag. 1896, p. 481.
While the lower breccias are sometimes tolerably coarse, the volcanic detritus becomes much finer in the higher parts of the Amlwch slates. Above the limestones and black shales of Cemmaes volcanic breccias and ashes, with limestone, quartzite, conglomerate and thin seams of black shale, continue to the extreme northern headlands. The amount of fine volcanic detritus distributed through these strata is very great. We can clearly make out that while ordinary sedimentation was in progress, an almost constant but variable discharge of fragmental materials took place from the vents in the neighbourhood. Sometimes a special paroxysm of explosion would give rise to a distinct band of breccia or of tuff, but even where, during a time of comparative quiescence, the ordinary sand or mud predominated, it was generally mingled with more or less volcanic dust.
Some bands of conglomerate in this group of strata deserve particular notice. The most conspicuous of these, already referred to as seen at Porth Wen, is made up of quartz and quartzite blocks, embedded in a reddish matrix largely composed of ashy material, and recalling the red spotted tuffs of Llyn Padarn. The occurrence of strong conglomerates near the top of a volcanic series has been noted at St. David's, Llyn Padarn and Bangor. In none of these localities, as I have tried to show, do the conglomerates mark an unconformability or serious break between two widely-separated groups of rock. The Anglesey section entirely supports this view, for the conglomerates are there merely intercalations in a continuous sequence of deposits; they are succeeded by tuffs and shales like those which underlie them. The interposition of such coarse materials, however, may undoubtedly indicate local disturbance, connected, perhaps, in this and the other localities, with terrestrial readjustments consequent upon the waning of volcanic energy.
The detailed geological structure of Anglesey is still far from being completely understood. Besides the serious crushing here referred to, there is reason to suspect that considerable plication, perhaps even inversion, of the strata has taken place, and that, by denudation, detached portions of some of the higher groups have been left in different parts of the island. The occurrence of Upper Silurian fossils in several localities adds to the perplexity of the problem by indicating that, among the folds and hardly distinguishable from the older slates, portions of Upper Silurian formations may have been caught and preserved. These difficulties, moreover, involve in some obscurity the closing phases of volcanic activity in Wales; for until they are, to some extent at least, removed, we shall be left in doubt whether the vents in the north of Anglesey, which were in eruption probably during Bala time, were the last of the long succession of Welsh volcanoes. If the black shales of Parys Mountain are really referable to the horizon of the Mayhill Sandstone, the two great igneous bands between which they lie would seem to mark an outbreak of volcanic energy during Upper Silurian time. No other indications, however, of eruptions of that age having been met with in Great Britain (though they occur in the south-west of Ireland and possibly in Gloucestershire), more careful investigation is required before such a position can be safely assigned to any rocks in Anglesey.
Putting these doubtful rocks aside for the present, we may, in conclusion, contrast the type of eruption in Anglesey with that of the great Snowdonian region. While the Caernarvonshire volcanoes were pouring forth their volumes of felsitic lava, and piling them up for thousands of feet on the sea-floor, the northern Anglesey vents, not more than some five-and-twenty miles away, threw out only stones and dust, but continued their intermittent explosions until they had strewn the sea-bottom with detritus to a depth of many hundred feet.
There is yet another feature of interest in this independent group of submarine vents in Anglesey. Their operations appear to have begun before the earliest eruptions of the Bala volcanoes in Caernarvonshire. Their first beginnings may, indeed, have been coeval with the explosions that produced the older Arenig tuffs of Merionethshire; their latest discharges were possibly the last manifestations of volcanic energy in Wales. They seem thus to bridge over the vast interval from Tremadoc to Upper Bala, possibly even to Upper Silurian time. But we may, perhaps, connect them with the still earlier period of Cambrian volcanism. I have referred to the evidence which appears to show that the vents whence the lavas and tuffs of Moel Trefan and Llyn Padarn were erupted gradually moved northwards, and continued in eruption until after the beginning of the deposition of the black slates that are generally regarded as Arenig. The Anglesey tuffs and breccias may thus be looked upon as evidence of a still further shifting of the active orifices northward. In this view, while the Aran and Cader Idris volcanoes broke out in Upper Cambrian and continued through Arenig time, and the Snowdonian group was confined to Bala time, a line of vents opened to the north-west in the Cambrian period before the epoch of the Llanberis slates, and, dying out in the south, continued to manifest a minor degree of energy, frequently discharging fragmental materials, but no lava, over the sea-bottom, until, towards the close of the Bala period, possibly even in Upper Silurian time, they finally became extinct.