[110] I can find no evidence of unconformability beneath any of the conglomerates. The section described by Professor Green, Quart. Journ. Geol. Soc. vol. xli. (1885), p. 74, merely shows the difference between the effects of cleavage on the fine tuffs and the more massive resisting conglomerate which overlies them. This section is represented in [Fig. 43]. At first sight the conglomerate appears to be lying on the vertical edges of an older group of slates, but any one acquainted with cleavage can trace this structure from the tuffs into the conglomerate and resuming its course again in the finer sediments above. The whole series of deposits in the section is continuous and conformable. The section on the slate railway has deceived Mr. Blake as well as Professor Green (Quart. Journ. Geol. Soc. vol. xlix. (1893), p. 445). The correct interpretation is given by Professor Bonney and Miss Raisin (op. cit. vol. l. p. 592).
One of the first features of these detrital deposits to arrest attention is the amount and variety of the fragments of igneous rocks in them. Some of the conglomerates, though enclosing pebbles of quartz, quartzite, granite and other rocks not found in situ in the immediate district, are mainly composed of the debris of the quartz-porphyry of the ridge. Indeed, this latter material appears to have contributed a large proportion of the detritus of which the general body of strata here is made up. But there are to be noticed among the contents of the conglomerates and breccias pieces of many volcanic rocks not to be found on the porphyry ridge. Among these, besides felsites showing sometimes beautiful flow-structure (rhyolites) and various quartz-porphyries, there occur abundant fragments of less acid lavas (andesites) and pieces of older tuffs. Some of the fragmental rocks are green in colour, probably from the abundance of fine basic volcanic dust in them. Certain bands are full of large angular pieces of shale, similar in character to the Cambrian slates, and doubtless due to the disruption of pre-existing Cambrian strata by volcanic explosions. It is clear that from vents in this neighbourhood there continued to be an abundant discharge of dust and various andesitic and other lapilli, which, falling on the sea-floor, mingled there with the ordinary mechanical sediment that was being deposited at the time.[111]
[111] On the composition of the conglomerates or breccias, see Professor Bonney and Miss Raisin, Quart. Jour. Geol. Soc. vol. l. (1894), p. 598.
Fig. 44.—Section of Clegyr on the north-east side of Llyn Padarn, near the lower end.
But we have evidence that, during the period when these showers of volcanic detritus were thrown out, streams of lava, though on a greatly diminished scale, continued to be poured forth. The hill of Clegyr ([Fig. 44]), near the lower end of Llyn Padarn, on the north-east side, consists mainly of cleaved tuffs (t) and slates with conglomerates (c), overlying the quartz-porphyry (p). Near the summit a band of felsite is intercalated in these rocks.
Still more striking are the sections on the south-west side of the lake.[112] Starting from the porphyry of the ridge, we cross a zone of conglomerate and grit largely composed of the debris of the porphyry, until we reach a band of felsite or quartz-porphyry, which at its eastern end is about ten feet thick, while it seems to increase in dimensions westwards.[113] In the centre the rock is dark purplish-red, exceedingly compact or flinty, sprinkled with a variable proportion of quartz-blebs and felspar crystals. Towards its southern or upper edge (for the rocks, though nearly vertical, dip southwards) it has been cleaved into a variety of purple slate, and would there at once be classed among the ordinary slates of the neighbourhood. But the fissile character is merely a marginal structure which the rock shares with the highly-cleaved tuffs that follow it. Traced westwards, this bed is found to enclose a core of quartziferous porphyry, which, though it has escaped from the most complete results of crushing, is nevertheless cleaved along its margin as well as partially in its interior. It would not be possible to distinguish parts of this intercalated less crushed core from portions of the porphyry of the main ridge. The difference of colour does not count for much, for even in this core the purple tint gives place to greenish grey, and what in the centre at the east end is a solid dark purplish-red felsite passes westward into a greenish slate, like that already noticed on Mynydd y Cilgwyn.
[112] For various readings of these sections, compare Mr. Blake (Quart. Jour. Geol. Soc. vol. xlix. (1893), p. 450) with Professor Bonney and Miss Raisin (op. cit. vol. l. (1894), p. 581).
[113] See Professor Bonney and Miss Raisin, op. cit. p. 593 et seq.
The microscopical examination of this rock shows it to be a true felsite of the rhyolitic type, which in the central uncleaved part exhibits a wavy flow-structure like that found in the quartz-porphyry of the ridge. So intense has been the cleavage in its upper part that the original structure of the rock is there effaced. The immediately overlying tuffs, which are likewise so thoroughly cleaved that it is not easy to draw a sharp and continuous line between them and the intercalated lava, precisely resemble those found below the conglomerate on the opposite side of the lake. They include bands of coarse volcanic breccia as well as fine compact material, showing the varying intensity of the volcanic discharges. Their included stones consist of various felsites, andesites and slates.[114]