CHAP, xm 
LLANDEILO AND BALA ERUPTIONS— BUILTH 
203 
area, we must be content to allow a wide margin for error. It is hardly 
possible to adhere strictly to the stratigraphical arrangement, for the geo- 
logical record shows that in the volcanic districts the sedimentary formations 
by which the chronology might have been worked out are not infrequently 
absent or obscure. It will be more convenient to treat the rest of the Lower 
Silurian formations as the records of one long and tolerable definite section 
of geological time, without attempting in each case to distinguish between 
the eruptions of the successive included periods, so long as the actual 
volcanic sequence is distinctly kept in view. I will therefore take the 
history of each district in turn and follow its changes from the close of 
the Arenig period to the end of Upper Silurian time. The stages in the 
volcanic evolution of each tract will thus be clearly seen. 
Above the Arenig group with its voluminous volcanic records comes the 
great group of sediments known as the Llandeilo formation, in which also 
there are proofs of contemporaneous volcanic activity over various parts of the 
sea-floor within the site of Britain. We have seen that in the south of 
Scotland the eruptions of Arenig time were probably continued into the 
period of the Llandeilo rocks, or even still later into that of the Bala group. 
But it is in Wales that the history of the Llandeilo volcanoes is most fully 
preserved. A series of detached areas of volcanic rocks, intercalated among 
the Llandeilo sediments, may be followed for nearly 100 miles, from the 
northern end of the Breidden Hills in Montgomeryshire, by Shelve, Builth, 
Llanwrtyd and Llangadock, to the mouth of the Taf river. But some o5 miles 
further west another group of lavas and tuffs appears on the coast of Pembroke- 
shire, from Abereiddy Bay to beyond Fishguard. The want of continuity in 
these scattered outcrops is no doubt partly due to concealment by geological 
structure. But from the comparative thinness of the. volcanic accumulations 
and their apparent thinning out along the strike it may be inferred that no 
large Llandeilo volcano existed in Wales. There would rather seem to 
have been a long line of minor vents which in the south-east part of the 
area appear to have only discharged ashes. Certainly, if we may judge 
from their visible relics, these eruptions never rivalled the magnitude of the 
discharges from the Arenig volcanoes that preceded, or the Bala volcanoes 
that followed them. 
i. THE VOLCANO OF BUILTH AND ITS NEIGHBOURS 
So far as the available evidence goes, the most important volcanic centre 
down the eastern side of Wales during the Llandeilo period was one which 
lay not far from the centre of the long line of vents just referred to. Its 
visible remains form an isolated tract of hilly ground, some seven miles long, 
and four or five miles broad, immediately north from the town of Builth. 
This area is almost entirely surrounded by uneonformable Upper Silurian 
strata, so that its total extent is not seen, and may be much more consider- 
able than the area now laid bare by denudation. 
