2o8 
THE SILURIAN VOLCANOES 
BOOK IV 
pages not having extended apparently into N'orth Wales. When the next 
outbreaks took place, the vents are found to have shifted northwards into 
Caernarvonshire, where they fixed themselves along a line not much to the 
east of where the Cambrian porphyries and tuffs now- appear at the surface. 
The lavas and ashes that were thrown out from these vents form the highest 
and most picturesque mountains of North Wales, culminating in the noble 
cone of Snowdon. They stretch northwards to Diganwy, beyond Conway, 
and southwards, at least as far as the neighbourhood of Criccieth. They 
die out north-eastwards beyond Bala Lake, and there can be but little doubt 
that they thin out also eastwards under the Upper Bala rocks. The lavas 
and tuffs that rise up on a similar horizon among the Bala rocks of the 
Berwyn Hills evidently came not from the Snowdonian vents, but from 
another- minor volcanic centre some miles to tlie east, while still more 
remote lay the vents of the Breidden Hills and the sheets of andesitic tuff 
that probably spread from them over the ground east of Chirbury (Map II.). 
The Caernarvouslnre volcanic group extends from north to south for 
lully thirty miles, with an extreme breadth of about fifteen miles ; while, if 
we include the rocks of the Lleyn peninsula, the area will be prolonged some 
twenty miles farther to the south-west. 
The general stratigraphical horizon of this volcanic group has been 
well determined by the careful mapping of Eamsay, Selwyn and Jukes on 
the maps of the Geological Survey. These observers brought forward 
ample evidence to show that the lavas and tuffs were erupted during the 
deposition of the Bala strata of the Lower Silurian series, that the ^Bala 
Limestone is in places full of ashy material, and that this well-marked fos- 
siliferous band passes laterally into stratified volcanic tuffs containing the 
same species of fossils.^ But the progress of stratigraphical geology^ and 
the increasing value found to attach to organic remains as marking even 
mnior stratigraphical horizons, give us reason to believe that a renewed and 
still more detailed study of the Bala rocks of North Wales would probably 
furnish data for more precisely defining the platforms of successive eruptions 
and would thus fill in the details of the broad sketch which Sir Andrew 
Eamsay and his associates so admirably traced. Besides the Bala Limestone 
there may be other lithological horizons which, like the Garth grit and the 
pisohtic iron-ore of the Arenig group, might be capable of being followed 
among the cwms and crests as well as the opener valleys of Caernarvon- 
shire. Until some such detailed mapping is accomplished, we cannot safely 
advance much beyond the point where the stratigraphy was left by the 
Survey. ^ 
From the Survey maps and sections it is not difficult to follow the general 
volcanic succession, and to perceive that the erupted materials must altogether 
be several thousand feet in thickness from the lowest lavas in the north to 
the highest on the crest of Snowdon. In that mountain the total mass of 
volcanic material is set down as 3100 feet. But this includes only the 
higher part of the whole volcanic group. Below it come the lavas of Y 
1 Mem. Geol. Siirv. vol. iii. 2nd edit. pp. 126, 128, 131, 139, etc. 
