THE SILURIAN VOLCANOES 
BOOK IV 
1 86 
upper agglomerate, fragments of cleaved slate containing LAwjida have 
been observed. 
The name of “felstoue” is restricted by Messrs. Jennings and Williams 
to certain fine-grained varieties of rock, of which a thin hand lies at the 
base of tJie lower agglomerate, while another of considerably greater im- 
portance occurs in the middle of the upper agglomerate. These bands 
consist of a fine compact greenish base, and weather with a dull white crust ; 
sometimes, as in the thicker sheet, a columnar structure shows itself. 
Whether these rocks are to be regarded as lavas or sills, or even as finer 
varieties of tuff, is a cpiestion that awaits furtlier inquiry. But it is clear, 
from the investigation of tlie two observers just cited, that the pyroclastic 
constituents must vastly preponderate in the volcanic series over the 
northern part of the region. All these rocks, whether coarse or fine-grained, 
appear to be rather acid in composition, and no evidence has yet been 
obtained of a sequence among them from a more basic to a more acid series, 
as in Cader Idris. 
. 1 he higliest agglomerate bed of the Manod and Moelwyn area is covered 
by slates which contain Llandeilo graptolites. In this way, by means of 
pakeontological evidence, the upward and downward limits of the Arenig 
volcanic series in this part of Wales arc definitely fixed. 
Hardly any information has yet been obtained as to the situation and 
character of the vents from which the lavas and ashes of Merionethshire 
were discharged. In the course of the mapping of the ground, the Geological 
Survey recognized that, as the greatest bulk of erapted material lies in the 
eastern and south-eastern parts of the region, the chief centres of emission 
were to be looked for in that quarter, and that possibly some of the intrusive 
masses which break through the rocks west of the great escarpment may 
mark the site of vents, such as Tyddyn-rhiw, Gelli-llwyd-fawr, Y-Foel-ddu, 
Rliobell Fawr, and certain bosses near Arenig.^ The distribution of the 
volcanic materials indicates that there were certainly more than one active 
crater. Wliile the southward thickening of the whole volcanic gi’oup points 
to some specially vigorous volcano in that quarter, the notable thinning 
away of the upper tuffs southward and their great depth about Arenig 
suggest their having come from some vent in this neighbourhood. On the 
otlier hand, the lower tufls are absent at Arenig, while on Aran Mawddwy, 
only nine miles to the south, they reach a depth of 3000 feet. Still farther 
to the south these voleanic ejections become more and more divided by 
intercalated bands of ordinary sediment. One of the most important 
volcanoes of the region evidently rose somewhere in the neighbourhood of 
what is now Aran Mawddwy. There seems reason to surmise that the 
sites of the chief vents now lie to the east and south of the great escarp- 
ment, Imried under the tliick sedimentary formations which cover all that 
region. 
If we are justified, on stratigraphical and petrographical grounds, in 
connecting the lowest A'olcanic rocks of tlie Berwyn range with those of 
' Mem. deal. Sure. vol. iii. 2iid edit. p. 98 ; see also pp. 44, 54, .58, 71. 
