CHAP. XII 
A REN/G £R UPTIONS—MERIONE THSHIRE 
87 
Merionethshire, we may speculate on the existence of a group of submarine 
vents, coming into eruption at successive intervals, from some epoch during 
the period of the Lingula Flags up to that of the Bala rocks, and covering with 
lavas and ashes a space of sea-bottom at least forty miles from east to west 
by more than .twenty miles from north to south, or roughly, an area of some 
800 square miles.^ 
Besides the materials ejected to the surface, the ancient volcanic region 
of Merionethshire was marked by the intrusion of a vast amount of igneous 
rock between and across the bedding-planes of the strata deep underground. 
One of the most prominent features of the Geological Survey map is the 
great number of sills represented as running with the general strike of the 
strata, especially between the top of the Harlech grits and the base of the 
volcanic series. On the north side of the valley of the Mawddach, between 
Barmouth and Ehaiadr Mawddach, in a distance of twelve miles the Survey 
mapped “more than 150 intrusions varying from a few yards to nearly a 
mile in length.” ^ This zone of sills is equally marked on the south side of 
the valley. It may be traced all round the Harlech anticline until it dies 
out, as the bedded masses also do, towards Towyn on the south and about 
Tremadoc on the north. 
The presence of such a zone of intrusive sheets at the base of an ancient 
volcanic series is a characteristic feature in the geology of Britain. It is 
met with again and again among the Paheozoic systems, and appears on a 
striking scale in association with the Tertiary basaltic plateaux of Antrim 
and the Inner Hebrides. But nowhere, perhaps, is it more strongly de- 
veloped than beneath the Arenig group of lavas and tuffs in North Wales. 
Abundant as are the protrusions marked on the Geological Survey map, they 
fall short of the actual number to be met with on the ground. Indeed, to 
represent them as they really are would require laborious surveying and the 
xise of maps on a far larger scale than one inch to a mile. 
The vast majority of these sills are basic rocks, or, in the old and con- 
venient terminology, “ greenstones.” Those of the Cader Idris district have 
been examined by Messrs. Cole and Jennings, who found that, notwith- 
standing the considerable alteration everywhere shown by the abundant 
epidote and calcite, the coarser varieties may be recognized as having originally 
been dolerites approaching gabbro, with a well-developed ophitic character, 
the general range of structure being from dolerites without olivine and 
aphanites to andesitic rocks with an originally glassy matrix. Dr. Hatch 
confirmed this diagnosis from slides prepared from my specimens. The 
ophitic structure is usually characteristic and well preserved, in spite of the 
alteration indicated by epidote, chlorite, urahte, and leucoxene. 
That this zone of “ greenstone ” sills belongs to the period of the 
Merionethshire volcanoes may be reasonably concluded. The way in which 
they follow the line of the great escai’pment, their almost entire absence 
from the Cambrian dome to the west, their cessation as the overlying lavas 
^ The Berwyn Hills, however, will be describeil in later pages as a distinct volcanic district. 
^ J/cOT. (ii’ul. Siirv. vol. iii. ]>. 26. “ Quart. Jnitrii. (real. f!oc. vol. xlv. (1889), p. 432. 
