CHAPTEE Xm 
THE ERUPTIONS OF LLANDEILO AND BALA AGE 
i. The Builth Volcano ii. The Volcanoes of Pembrokeshire — ill. The Caernarvonshire 
Volcanoes of the Bala Period — iv. The Volcanic District of the Berwyn Hills — ■ 
V. The Volcanoes of Anglesey — vi. The Volcanoes of the Lake District ; Arenig 
to close of Bala Period — vii. Upper Silurian (?) volcanoes of Gloucestershire. 
The stratigraphical subdivisions of geology are necessarily more or less 
arbitrary. The sequence in the sedimentary deposits of one region always 
differs in some degree from that of adjoining regions. In drawing up a 
table of stratigraphical equivalents for separate countries, we must be content 
to accept a general parallelism, without insisting on too close an identity 
iu either tlie character of the strata or the grouping of their organic remains. 
We need especially to guard against the assumption that the limit assigned 
to a geological formation in any country marks a chronological epoch which 
will practically agree with that denoted by the limit fixed for the same 
formation iu another countiy. The desirability of caution in this respect is 
well shown by the vagueness of the horizons between the several subdivisions 
of the Lower Silurian system. So long as the areas of comparison are near 
each other, no great error may perhaps be committed if their stratigraphical 
equivalents are taken to have been in a broad geological sense contemporary. 
But in proportion as the element of distance comes in, there enters with it 
the element of uncertainty. 
Even within so limited a region as the British Isles, this difficulty makes 
itself strongly felt. Thus, in the typical regions of Wales, the several sub- 
divisions of the Lower Sdurian strata are tolerably well marked, both by 
lithological nature and by fossils. But as they are followed into other parts 
of the country, they assume new features, sometimes increasing sometimes 
diminishing in thickness, changing their sedimentary character, and altering 
the association or range of their oiganisms. The subdivisions into which the 
geologist groups them may thus be vaguely defined by limits which, in 
different parts of the region, may be far from representing the same periods 
of time. 
Hence, in trying to ascertain how far the volcanic eruptions of one area 
during the Silurian period may have been contemporary with those of another 
