206 
THE SILURIAN VOLCANOES 
BOOK IV 
tainiiig Eidyiuofji ojptus J\IuTchiso7iiy the tiifi's themselves being sometimes 
fossiliferoiis. A second great volcanic belt, composed of felsitic lavas, 
breccias and tuffs, lies at the base of the Upper Llandeilo strata and shows 
the maximum of volcanic energy. The breccias are partly coarse agglom- 
erates, which probably represent, or lie not far from, some of the eraptive 
vents of the time. A higher band of lavas and breccias appears to 
be referable to the Bala formation. The whole volcanic series is stated to 
thin out towaids the south-west, so that the chief focus of eruption prob- 
ably lay somewhere in the neighbourhood of Fishguard. 
The lavas may all be included under the general term felsite. Their 
specific^ gravity ranges from 2'60 to 2-76, and their silica percentage from 
G8 to t'2. Mr. Cowper Heed observed among them three conspicuous types 
of structure. Some are characterized by a distinct arrangement in fine 
light and dark bands which rapidly alternate, and are sometimes thrown 
into folds and convolutions. A second structure, observed only at one 
locality, consists in the development of pale grey or whitish ovate nodules, 
about half an inch in length, with a clear quartz-grain in their centre, or 
else hollow. The third type is shown by the appearance of perlitic structure 
on the weathered surface.^ 
The tuffs and breccias are chiefly developed at the base and top of each 
volcanic group. Some of them contain highly vesicular fragments, as well 
as pieces of slate and broken crystals of quartz and felspar. 
A characteristic feature of this volcanic district is the occurrence in it 
of sills and irregularly-intruded masses of “ gi-eenstone.” Under that name 
are^ comprised basalts, dolerites, andesitic dolerites with tachylitic modifi- 
catmns, as well as diabases and gabbros.^ Some of these rocks exhibit a 
variolitic structure. As regards age, some of the intrusions appear to 
have taken place before the tilting, cleavage and faulting of the strata. They 
have not been noticed in the surrounding Upper Silurian strata, and we may 
perhaps infer that here, as at Builth, they are of Lower Silurian date. Mr 
Cowper Feed, however, is inclined to regard the large Strumble Head masses 
as later than the tilting and folding of the rocks.'* 
A lew miles to the south-west of the Fishguard district, on the coast of 
Abereiddy Bay, good sections have been laid bare of the volcanic rocks of 
this region. Dr. Hicks has shown that the bands of tuff there displayed 
are intercalated among the black slates of the Lower Llandeilo group, and 
that there was probably a renewal of volcanic activity during the deposition 
of the upper group.** But the volcanic history of this area still remains to 
be properly investigated. 
In southern Pembrokeshire two conspicuous bands of eruptive rocks 
have long been known and described. Their general characters and dis- 
1 Mi\ Cowper Reed enters into a detailed aoeouut of the microscopic structures and chemical 
composition ot tlieseyocks. They have rather a high percentage of alumina, potash and soda, 
and are obviously akin to the keratophyres of other districts. 
^ Mr. Cowper Reed, op. cit. p. 180. '■> 
* QiimH. Jowrn. Geol. Soc. .\-xxi. (1875), p. 177. 
Op. cit. p, 193. 
