There can be no doubt regarding the serious amount of crushing which the rocks of this coast-line have undergone. Some of the bands might even be described as "crush-conglomerates." Yet the intercalation of seams of black shale and limestone, and the occurrence of the exactly similar but thicker group of black shales at Porth Prydd, which are admitted to be Lower Silurian, unite the whole series of strata as parts of one formation.
It thus appears that the area coloured "altered Cambrian" on the Survey map, and regarded as pre-Cambrian by some later observers, is proved by the evidence of fossils at its base, towards its centre and at its top, to belong to the Lower Silurian series, probably to the Bala division. That this was the geological horizon of part at least of the area was recognized by Sir A. Ramsay, though he confessed himself unable "precisely to determine on the north coast of Anglesey how much of the strata are of Silurian and how much of Cambrian age."[232] Professor Hughes was the first to suggest that the whole of these rocks should be referred to the Bala group.[233]
[232] Mem. Geol. Surv. vol. iii. 2nd edit. p. 242.
[233] Proc. Camb. Phil. Soc. vol. iii. (1880), pp. 341-348.
Fig. 59.—Section of intercalated black shale in the volcanic series at Porth yr hwch, south of Carmel Point.
Fig. 60.—Green slates overlain with volcanic breccia, Carmel Point, Anglesey.
I have dwelt on the determination of the true geological age of the rocks of the north of Anglesey because of the diversity of opinion respecting them, and because of their great interest in regard to the history of volcanic action in Wales. These rocks contain a record of volcanic eruptions, probably contemporaneous on the whole with those of the Bala period in Caernarvonshire, yet independent of them and belonging to a different type of volcanic energy. Some of the vents probably lay in the north-western part of Anglesey. The materials ejected from them were, so far as we know, entirely of a fragmentary kind. Vast quantities of detritus, largely in the form of fine dust, were thrown out; but no trace has yet been found of the outflow of any lava. The lower part of this volcanic series consists of bedded breccias which are sometimes remarkably coarse. Their included stones, ranging up to six inches or more in diameter, are usually more or less angular, and consist mainly of various felsites. Layers of more rounded pebbles occasionally occur, while the bedding is still further indicated by finer and coarser bands, and even by intercalations of fine tuffs and ashy shales. Towards their upper limits some of these volcanic bands shade off into pale grey or greenish ashy shale, followed by black sandy shale of the usual kind. The relation of the peculiar greenish shale of the Amlwch type to these tuffs and breccias is well shown east of Carmel Point. This shale is interleaved with tuff and contains frequent repetitions of finer or coarser volcanic breccia, as well as occasional seams of black shale. An illustration of this structure is given in [Fig. 59], where some yellow decomposing breccias (1), cut by a fault (f), are overlain by about 40 or 50 feet of black shale (2), above which lies a flinty felsitic rock (3) that appears to run in bands or dykes through the agglomerate. At Carmel Point ([Fig. 60]) a similar structure may be observed to that at Llyn Padarn already referred to ([p. 163]). The cleavage, which is well developed in the green slates (a), is much more faintly marked in the overlying breccia (b), but the bedding can still be detected in both rocks running parallel to their mutual boundary-line. Beyond Porth Padrig, which lies east from Carmel Point, the section may be seen which is shown in [Fig. 61]. Here the blue or lead-coloured shale or slate (a) marked as Silurian on the Geological Survey map passes up into a mass of fine yellowish felsitic tuff and breccia (b). The shale at the junction intercalates in thin leaves with the tuff.
