Sir Andrew Ramsay, in his great Monograph on the geology of North Wales, has described the Merionethshire volcanic district in considerable detail. He seems finally to have come to the conclusion that the eruptions of that area were included within the Arenig period.[134] He shows, indeed, that on Rhobell Fawr the ejected materials lie directly on disturbed Lingula Flags without the intervention of the Tremadoc group, which is nevertheless present in full development in the near neighbourhood.[135] And in trying to account for this remarkable fact he evidently had in his mind the possibility that volcanic eruptions had taken place long before as well as after the beginning of the deposition of the Arenig grit and slates.[136] He seems eventually, however, to have looked on the Rhobell Fawr sections as exceptional and possibly to be accounted for by some local disturbance and intrusion of eruptive rock.[137] He clearly recognized that there were two great epochs of volcanic activity during the Silurian period in Wales, one belonging to the time of the Arenig, the other to that of the Bala rocks, and he pointed out that the records of these two periods are separated by a thick accumulation of sedimentary strata which, being free from interstratifications of contemporaneous igneous rocks, may be taken to indicate a long interval of quiescence among the subterranean forces.[138] [Footnote 134: _Mem. Geol. Survey_, vol. iii. 2nd ed., p. 96.] [Footnote 135: The ashes and agglomerates of Rhobell Fawr can be seen in various places to rest on the highest members of the Lingula Flags. See Messrs. Cole and Holland, _Geol. Mag._ (1890), p. 451.] [Footnote 136: _Op. cit._ p. 72.] [Footnote 137: He was disposed to regard Rhobell Fawr as one of the great centres of eruption of the district. See _Memoir of A. C. Ramsay_, p. 81, and _Geology of North Wales_, 2nd edit. p. 98.] [Footnote 138: _Op. cit._ pp. 71, 96, 105.]