[101] Quart. Journ. Geol. Soc. vol. xliv. p. 271. For subsequent papers by Mr. Blake, see op. cit. vols. xlviii. (1892) p. 243, xlix. (1893) p. 441.
That the igneous rocks of the Llyn Padarn area mark a volcanic period has been recognized by most writers since Professor Bonney pointed out the flow-structure of the quartz-porphyry, and other proofs of active volcanic eruptions have been traced by him, as well as by Professor Hughes and Mr. Blake, in the stratified rocks which stretch north-eastwards to Bangor. The extent and persistence of these ancient volcanic phenomena, and their probable connection with the remarkable northward attenuation of the Cambrian sedimentary rocks, deserve special attention.
It is generally agreed that the rocks variously termed quartz-porphyries, felsites or rhyolites form the oldest members of this volcanic series.[102] They come to the surface in two long ridges, one running from Caernarvon to near Bangor, the other from Llanllyfni to Ann's Chapel, at the mouth of Nant Francon (Map. II.). Whether the materials of these two ridges are parts of one originally continuous sheet or group of sheets, or, if different protrusions, whether they belong to the same geological horizon, or whether, as Mr. Blake believes, they are distinct masses, separated by a considerable thickness of detrital material, cannot in the present state of our knowledge be positively decided. It seems to me probable that they are connected underground, as a continuous platform beneath the overlying pyroclastic materials.
[102] Whether the granitic rock of Twt Hill, Caernarvon, is connected with the porphyry or belongs to an older eruption is immaterial for my present purpose.
These acid rocks have been regarded by some observers as intrusive sheets, by others as lava-streams that were poured out at the surface. If account be taken simply of their petrographical characters, they find their nearest analogies among the intrusive quartz-porphyries of older geological periods. The presence of flow-structure in them has been thought to indicate that they were superficial streams. But this structure may be found in dykes and intrusive sheets as perfectly as in lava-flows, so that it cannot by itself be taken as proof of a surface-discharge of lava. It must be confessed that, both in the main mass of quartz-porphyry and in the abundant fragments of it in the overlying conglomerates and breccias, there is an absence of such scoriform portions as one would naturally look for in a superficial lava-stream;[103] while, on the other hand, the rock generally presents the tolerably uniform flinty texture so familiar in intrusive sheets of similar material.[104] My own impression is that these igneous masses were probably erupted to the surface as long banks which rose above the waves; that they were thus exposed to prolonged subærial and marine denudation; that by this means any upper more cellular portions of the lava which may have existed were broken up and pounded down into detritus, and thus that what is now visible is a part of the eruptive rock which originally lay at some depth within its body. This view is confirmed by a study of other lavas which are found on different platforms in the detrital deposits that overlie the Llyn Padarn quartz-porphyry.
[103] But the Llyn Padarn rock, like many acid lavas, may never have possessed a vesicular structure in any portion of its mass. The sheets of felsite which occur among the overlying tuffs are not cellular, but present the closest resemblance to the main mass below.
[104] Mr. Blake brought forward the evidence of a section on the north or under side of the Llyn Padarn ridge to show that the rock has there been intruded into the Cambrian strata (Quart. Journ. Geol. Soc. vol. xliv. (1888), p. 283). But the rock supposed by him to be altered slate has been shown to be intrusive "greenstone" (Miss Raisin, op. cit. vol. xlvii. (1891), p. 336).
That the material of each of the two main ridges is the result of more than one eruption has been inferred from the supposed intercalation of bands of slate and of breccia in the rock.[105] Considerable lithological differences may be detected in each mass, but they are not greater than may be observed in single sills and bosses. In some parts of the Llyn Padarn porphyry a distinct nodular structure appears which shades off into bands and lenticular streaks, reminding one of the characters of some of the Bala rhyolites. Other portions are markedly brecciated, the separated fragments being surrounded in a matrix of the rock, which shows flow-structure sweeping past them. On Moel Gronw angular fragments of a dark pinkish tint are scattered through the general mass. Again, some parts are crowded with quartz-grains, while others are comparatively free of these, and occasionally a spherulitic structure has been observed.[106]
[105] See for example, Prof. Bonney, Quart. Journ. Geol. Soc. vol. xxxv. (1879), p. 312; Mr. Blake, op. cit. vol. xliv. (1888), pp. 277, 287. But some at least of the supposed "slates," as stated in a previous footnote, have been since shown to be dykes.
[106] Mr. Blake, ibid. p. 277.