[114] I see no reason to doubt that the less acid igneous fragments were ejected during the closing phases of volcanic action, even though no such rocks have been found at the surface in situ. We must remember how frequently mixtures of acid and basic materials are to be found in the same continuous series of volcanic ejections and even in the same vent, of which illustration will be given in subsequent pages. Nor should we forget what a mere fragment of a volcanic group is exposed at the surface in the Llanberis district. See Professor Bonney and Miss Raisin, op. cit. p. 596, footnote.
The thin sheet of interstratified quartz-porphyry here described is not the only one to be found in the section. Others thinner and more intensely cleaved lie among the tuffs higher up. They have been sheared into mere pale unctous slates, but the remains of their quartz-blebs may still be detected in them.
The tuffs, with their interstratified bands of porphyry, become more and more mingled with ordinary argillaceous and sandy sediment as they are followed in upward succession. Among them occur bands of grit and fine conglomerate containing pebbles of porphyry and pieces of slate. Some of these grits are mainly composed of white felspar, felsite and clear grains of quartz, evidently derived from the disintegration of a rock like the porphyry of the main ridge. As the ordinary sediment of the Llanberis group sets in, the tuffs are restricted to thinner and more widely-separated bands. Some thin layers of felspathic breccia, seen among the slates close to the Glyn Peris Hotel, probably mark the last discharges of the slowly-expiring vents of this region. Here, as at St. David's, from the most ancient of our volcanic records, striking evidence is furnished of the gradual extinction of volcanic action. Through many hundreds of feet of strata which now supervene, representing the closing ages of the Cambrian and the earlier ages of the Silurian period, no trace of volcanic material has been found in this district until we reach the Bala lavas and agglomerates of Snowdon and the Pass of Llanberis.
In the neighbourhood of Bangor another area of similar rocks wraps round the northern end of the western porphyry ridge. The Geological Survey map, in conformity with the ideas that governed its representation of the older rocks of Anglesey and Caernarvon, colours these as altered Cambrian. That this error should have been made, or, when made, should not have been speedily corrected, is all the more surprising when we consider the thorough mastery which the surveyors had acquired of the aspects and the interpretation of ancient volcanic rocks in Wales, and when, moreover, we remember that as far back as 1843, long before the Survey of Caernarvonshire was published, Sedgwick had pointed out the true volcanic nature of the rocks. That great pioneer recognized the presence of "trappean conglomerates" and "trappean shales (Schaalstein)" among these deposits at Bangor; but he could not separate them from the Cambrian series of the rest of Wales.[115] And in his section he represents them as undulating towards the east and passing under the great mass of the Caernarvonshire slates and porphyries.
[115] Proc. Geol. Soc. vol. iv. p. 212; Quart. Journ. Geol. Soc. vol. iii. (1847), p. 136.
This interpretation, which I believe to be essentially accurate, was modified by Professor Hughes, who, fixing on a conglomerate as the base of the Cambrian system, regarded all the rocks below it, or what he termed his "Bangor group," as pre-Cambrian.[116] He has been followed in this view by subsequent writers;[117] but Mr. Blake has argued that here, as in the Llanberis district, there is no evidence to separate the volcanic detrital deposits above the porphyry from the Cambrian system.[118]
[116] Quart. Journ. Geol. Soc. vol. xxxiv. (1878), p. 137.
[117] Prof. Bonney, op. cit. vol. xxxv. (1879), p. 316; Dr. Hicks, ibid. p. 296.
[118] Op. cit. vol. xliv. (1888), p. 278.
A little southward from Bangor the quartz-porphyry is overlain by a most interesting group of fragmental rocks, the "Bangor group" of Professor Hughes. Largely of volcanic origin, they must be some hundreds of feet thick, and pass under the dark shales and grits of the Lower Silurian (Arenig) series. Some of the most persistent bands among them are conglomerates, which differ from each other in composition, but most of which consist largely of fragments of various igneous rocks. Some of the coarser masses might be termed agglomerates, for they show little or no trace of bedding, and are essentially made up of blocks of volcanic material. There are abundant beds of grit, sometimes pebbly or finely conglomeratic, alternating with tuffs and with bands of more ordinary sediment. Courses of purple shale and sandstone, green shale and dark grey sandy shale occasionally occur to mark pauses in the volcanic explosions. Perhaps the most striking feature in the pyroclastic materials is the great abundance of very fine compact pale tuffs (hälleflintas of some writers), sometimes thinly laminated, sometimes occurring in ribbon-like bands, each of which presents internally a close-grained, almost felsitic or flinty texture.[119]