CH.xiii LLANDEILO AND BALA ERUPTIONS—PEMBROKESHIRE 
205 
section runs across tins ground, and the band of rock does not appear to 
have been described.^ 
Seventeen miles to the south-west a still feebler display of intercalated 
volcanic material occurs in the Llandeilo formation near the village of 
Llangadock. The Geological Survey map represents one or more bands of 
ash associated with Hmestone, and thrown into a succession of folds. In 
the Horizontal Section (Sheet III. Section 3) a band, 100 to 200 feet thick, 
of “ trappeau ash ” with fossils is shown among the shales, limestones and grits, 
and in the Gataloyue of Rock-qjecimens the same rock is referred to as 
brecciated ash in connection with specimens of it in the Museum, which are 
described as not purely ashy, but containing many slate - fragments and 
broken felspar-crystals together with organic remains.^ 
About twenty -four miles still further in the same south-westerly 
direction, two patches of “ ash ” are shown upon the Survey map, near the 
mouth of the river Taf hTo description of these rocks is given.® 
ii. THE VOLCANOES OF PEMBKOKESHIEE 
In north-western Pembrokeshire, the observations of Murchison, lie la 
Beche and Eamsay showed the existence of an important volcanic district, 
where numerous igneous bands are interstratified among the Lower Silurian 
rocks, over an area extending from St. David’s Head for thii’ty miles to 
the eastward.'* On the maps of the Geological Survey, lavas, tuffs, sills 
and bosses were discriminated, but no description of these rocks was 
published. Since the publication of the Survey map very little has yet 
been added to our information on the subject. 
There appear to have been at least three principal groups of vents. 
One may be indicated by the bands of “ felspathic trap ” which have been 
mapped as extending from near St. Lawrence for fourteen miles to the east. 
Another must have existed in the neighbourhood of Fishguard. A third is 
shown to have lain between Abereiddy Bay and Mathry, by the abundant 
bands of lava and tuff and intrusive sills there to be seen. 
Of these areas the only one which has yet been examined and described 
in some detail is that of Fishguard, of which an account has recently been 
published by Mr. Cowper lieed.^ This observer has shown that the 
eruptions began there during the deposition of the Lower Llandeilo rocks, 
and continued intermittently into the Bala period. The earliest consisted 
of felsites and tuffs intercalated between Lower Llandeilo black slates con- 
^ The locality is referred to by De la Beclie, Mem. Oeol. Surv. vol. i. ji. 31, and by Ramsay 
in the Descriptive Calaloi/ue of Rock-specimens in the Museum of Pradual Geology, 3rd edit, 
p. 38, but no .spoeiiuons from it are in the collection. ^ Op. eit. p. 38. 
® One of the patches was shown by .1. Phillips in Ilonzontal Section, Sheet III. Section 6, 
as a “ felspathic trap,” near which the shales are bleached. The map, however, was subsequently 
altered, so as to make the igneous rocks pyroclastic. 
^ See Silurian System, p. 401 ; Sheet 40 of the Geological Survey Map ; Memoir of A. C. 
Ramsay, p. 232 et seq. ; De la Beche, Trans. Geol. Soc. 2nd series, vol. ii. part i. (1826), p. 3. 
® Quart. Jouru. Geol. Soc. vol. li. (1895), p. 149. 
