2I8 
THE SILURIAN VOLCANOES 
BOOK IV 
region, for they share in the foldings and cleavage of the rocks among which 
they he. The terrestrial movements that produced this disturbance have 
een proved to have occurred after the time when the uppermost Bala rocks 
were deposited, and before that of the accumulation of the Upper Silurian 
torinations.* The epoch of intrusion is thus narrowed down to some part of 
the Upper Bala period. With this subterranean manifestation, volcanic 
action in this part of the country finally died out. 
iv. THE VOLCANIC CENTRE OF THE BERWYN HILLS 
Among the thick group of sedimentary formations which overlies the 
great volcanic ridge of the Arans and Aren ig, and undulates eastwards across 
the Bala Valley, occasional thin intercalations of tufi’ point to the existence 
ol another centre of volcanic activity which lay somewhere in the region of 
t le Berwyn Hills. The structure of this ground, first indicated by Sedgwick 
was investigated in detail by J. B. Jukes and his colleagues, whose work 
was embodied in the Maps, Sections and Memoirs of the Geological Survey.'^ 
I he distinguishing characteristics of the volcanic rocks of this district are 
t le occiUTcnce of both lavas and tuffs as comparatively thin solitary bands 
in the midst of the ordinary sediments, and the persistence of these bands 
for a distance of sometimes more than 24 miles. The position of the vent 
or vents from which this extensive outpouring of volcanic material took 
place has not been revealed. As the bands tend to thin away eastwards, it 
may be surmised that the chief focus of eruption lay rather towards the 
west, perhaps under the trough of Upper Silurian strata somewhere in the 
neighbourhood of Llandderfel. There was probably another in the Hirnant 
custrict. 
Ihe mapping of the oflicers of the Survey showed that in the Berwyn 
Hills there are representatives of both the great volcanic periods of ISTorth 
Wales A lower series of “felstones and greenstones” probably belongs to 
the older period, which began towards the end of Cambrian time and lasted 
111 some districts even into the time of the Llandeilo formation. An upper 
group of tuffs, lying among the Bala rooks, is evidently equivalent, on the 
whole, to the much thicker volcanic series of the Snowdon region. 
The lowest visible volcanic rocks occur among the hills to the north- 
west of Llanrhaiadr yn Mochnant. They are described as consisting of 
e stone of a pale greenish -grey colour and compact texture, like those of 
Arenig, and ashes distinctly interstratified with the slates. Ho exact 
peti ographical examination of these rocks has yet been made. From the 
account given in the Survey Memoir there appears to be here a group of 
avas and tufts intercalated in Llandeilo perhaps partly in Upper Arenic, 
strata which form the broken dome of the Berwyn anticline. The lavas 
are represented as lying on four or five platforms, a single band reaching a 
1 Mem Gcol.SuT j,Ii. iii. 2nd edit. p. 326. See also Mr. Harker’s Bala Volcanic Scries, p. 76. 
one-mcli map ; Sheets 32, 35, 37 and 38 of tlie Horizontal Sections ; and 
chapter .xxxi. of the Memoir on the Geology of North Wales. 
