CHAP. XII 
A RE NIG ER UP TIONS—MERIONE TH SHIRE 
177 
But short of actually resurveyiug the whole of that rugged tract, it was 
impossible to bring the maps abreast of the onward march of science. 
They consequently remain, as a whole, very much as they were some thirty 
or forty years ago. 
Sir Andrew Eamsay, 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.^ He shows, indeed, 
that on Bhohell 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.^ And in trying to 
account for this remarkable fact he evidently had in his mind the possi- 
bility that volcanic eruptions had taken place long before as well as after 
the beginning of the deposition of the Arenig grit and slates.^ He seems 
eventually, however, to have looked on the Khobell Fawr sections as excep- 
tional and possibly to he accounted for by some local disturbance and 
intrusion of eruptive rock.* 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 inter- 
stratifications of contemporaneous igneous rocks, may be taken to indicate 
a long interval of quiescence among the subterranean forces.'' 
The lower limit of the Arenig rocks has been fixed at a band or bands 
of grit or conglomerate (Garth grit) which can be followed with some slight 
interruptions all round the great dome of Cambrian strata from Llanegrin 
on the south to the shore at Criccieth on the north. The volcanic group 
doubtless lies, generally speaking, above that basement platform. But, 
besides the sections at Ehobell Fawr just referred to, where the volcanic 
materials lie on the Lingula Flags, the same relation may, I think, be 
observed on the north Hank of Cader Idris. Messrs. Cole, Jennings, and 
Holland have come to the conclusion that the eruptions began at a rather 
earlier date than that assigned to them in the Survey Memoirs, and my 
own examination of the ground led me to accept their conclusion.'’ f 
inferred that the earliest discharges in the southern part of the region took 
place in Cambrian time, at or possibly before the close of the deposition 
of the Lingula Flags, and that intermittent outbursts occurred at many 
intervals during the time when the Tremadoc and Arenig rocks were 
deposited. 
’ Mem. Ocol. Survey, vol. iii. 2nd ed., p. 96. 
- 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. 
^ Op. cit. p. 72. 
■» He was di.spo.sed to regard Rhobell Fa-wr as one of the great centres of eruption of the district. 
See Memoir of A. G. Jtamsay, p. 81, and Geology of North Wales, 2nd edit. p. 98. 
“ Op. cit. pp. 71, 96, 105. 
® Qiuirt. Journ. Geol. Sue. vol. .xlv. (1889), p. 436 ; Geol. Mag. (1890), p. 447. Pres. Address 
Geol. Soc. 1890, p. 107. 
VOL. I 
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