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] [Footnote 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.] [Footnote 106: Mr. Blake, _ibid._ p. 277.] The microscopic structure of this ancient eruptive rock has been studied by Professor Bonney, who found that the general type was a compact dull grey felsite, with porphyritic crystals of felspar and grains of quartz, closely resembling some modern rhyolites. Though unable to detect any actual glass in the base, he had no doubt that the rock was originally vitreous, and he found abundant and fresh examples of the most perfect flow-structure.[107] [Footnote 107: _Op. cit._ vol. xxxv. p. 312.] [Illustration: Fig. 42.--Basic dyke traversing quartz-porphyry and converted into a kind of slate by cleavage. West side of Llyn Padarn. _p_ _p_, quartz-porphyry; _d_ _d_, dyke and connected veins.]