CHAP. XIII 
BALA ERUPTIONS OF CAERNARVONSHIRE 
.207 
tribution were sketched by De k Beebe, ^ and furtlier details were afterwards 
added by Murchison.- As traced by the officers of the Geological Surv’^ey, 
they were represented as consisting of “ greenstone,” “ syenite ” and “ granite.” 
The more northerly band was shown to run in a nearly east and west line 
from Lawrenny to the Stack Eoek, west of Talbenny, a distance of about 
fourteen miles. The second band, placed a short way farther south, stretches 
in the same general line, from Milford Haven at Hall Eoad into Skomer 
Island, a distance of about seven miles. 
ihe relations of these rocks to the surrounding formations and their 
geological age have been variously interpreted. He la Beche regarded the 
different masses as intrusive, and probably later than even the adjoining Coal- 
measures.® Murchison, on the other hand, considered the bedded eruptive 
rocks of Skomer Island to be undoubtedly lavas contemporaneous witli the 
strata among which they are intercalated.^ 
The rocks have been studied petrographically by various observers. Mr. 
Eutley gave a full description of the remarkable nodular and banded felsites 
of Skomer Island. Mr. Teall has also noticed these rocks, lilcewise “a 
magnificent series of basic lava-flows ” in the same island, and a number of 
“ porphyrites.” The basic lavas seemed to him to contain too much felspar 
and too little olivine to be regarded as perfectly typical olivine-basalts, and 
he found tliem to lie sometimes in very thin and highly vesicular sheets. 
The “ porphyrites ” he placed “ on the border-line between basic and inter- 
mediate rocks.” ® 
More recently this southern district of Pembrokeshire has been examined 
by Messrs. F. T. Howard and E. W. Small, who have obtained further 
evidence of the interbedded character of the igneous series. Below an 
upper basalt they have noted the occurrence of bands of felsitic con- 
glomerate, sandstone, shale and breccia lying upon and obviously derived 
from a banded spherulitic felsito, below which comes a lower group of 
basalts. The age of this interesting alteration of basic and acid eruptions 
has not been precisely determined, but is conjectured to be that of the 
Bala or Llandovery rocks.^ 
iii. THE CAEENAEVOXSHIRE VOLCANOES OF THE BALA PEEIOD 
Owing to the effects partly of plication and partly of denudation, the 
rocks of the next volcanic episode in Wales, that of the Bala period, occupy 
a less compact and defined area than those of the Arenig group in Meri- 
onethshire. From the latter they are separated, as we have seen, by a con- 
siderable depth of strata,® whence we may infer, with the Geological Survey 
that the eruptions of Arenig, the Araus and Cader Idris were succeeded by 
a long period of repose, the Llandeilo outbreaks described in the foregoing 
1 Tnmis. Gcol. Soe., 2iid ser. vol. ii. (1823), p. 6 et seq. 2 System, p. 401 c.t scq. 
» Mem. GcoL Survey, vol. i. p. 231. Silurian System, p. 404. 
•'i “The Felsitic Lavas of England and Wales,” Mem. Gcol. Survey (1885), pp. 16, 18 
« British Petrography, pp. 224, 284, 336. ’’Rep. Brit. Assoc. ]S93,p. 766 ; Gcol. May. 1896. p'. 481. 
® Estimated at from 6000 to 7000 feet, Me7ii. Gcol. Surv. vol. iii. 2nd edit. p. 131. 
