ii. THE VOLCANOES OF PEMBROKESHIRE

In north-western Pembrokeshire, the observations of Murchison, De la Beche and Ramsay 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 thirty miles to the eastward.[184] 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.

[Footnote 184: 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.]

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 Reed.[185] 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 containing _Didymograptus Murchisoni_, the tuffs themselves being sometimes fossiliferous. 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 agglomerates, which probably represent, or lie not far from, some of the eruptive 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 towards the south-west, so that the chief focus of eruption probably lay somewhere in the neighbourhood of Fishguard.

[Footnote 185: _Quart. Journ. Geol. Soc._ vol. li. (1895), p. 149.]

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 68 to 72. Mr. Cowper Reed 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.[186]

[Footnote 186: Mr. Cowper Reed enters into a detailed account of the microscopic structures and chemical composition of these rocks. They have rather a high percentage of alumina, potash and soda, and are obviously akin to the keratophyres of other districts.]

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 "greenstone." Under that name are comprised basalts, dolerites, andesitic dolerites with tachylitic modifications, as well as diabases and gabbros.[187] 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 Reed, however, is inclined to regard the large Strumble Head masses as later than the tilting and folding of the rocks.[188]

[Footnote 187: Mr. Cowper Reed, _op. cit._ p. 180.]

[Footnote 188: _Op. cit._ p. 193.]

A few 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.[189] But the volcanic history of this area still remains to be properly investigated.

[Footnote 189: _Quart. Journ. Geol. Soc._ xxxi. (1875), p. 177.]

In southern Pembrokeshire two conspicuous bands of eruptive rocks have long been known and described. Their general characters and distribution were sketched by De la Beche,[190] and further details were afterwards added by Murchison.[191] As traced by the officers of the Geological Survey, 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 Rock, 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 Dall Road into Skomer Island, a distance of about seven miles.

[Footnote 190: _Trans. Geol. Soc._, 2nd ser. vol. ii. (1823), p. 6 _et seq._]

[Footnote 191: _Silurian System_, p. 401 _et seq._]

The relations of these rocks to the surrounding formations and their geological age have been variously interpreted. De la Beche regarded the different masses as intrusive, and probably later than even the adjoining Coal-measures.[192] Murchison, on the other hand, considered the bedded eruptive rocks of Skomer Island to be undoubtedly lavas contemporaneous with the strata among which they are intercalated.[193]

[Footnote 192: _Mem. Geol. Survey_, vol. i. p. 231.]

[Footnote 193: _Silurian System_, p. 404.]

The rocks have been studied petrographically by various observers. Mr. Rutley gave a full description of the remarkable nodular and banded felsites of Skomer Island.[194] Mr. Teall has also noticed these rocks, likewise "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 them to lie sometimes in very thin and highly vesicular sheets. The "porphyrites" he placed "on the border-line between basic and intermediate rocks."[195]

[Footnote 194: "The Felsitic Lavas of England and Wales," _Mem. Geol. Survey_ (1885), pp. 16, 18.]

[Footnote 195: _British Petrography_, pp. 224, 284, 336.]

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 conglomerate, sandstone, shale and breccia lying upon and obviously derived from a banded spherulitic felsite, 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.[196]

[Footnote 196: _Rep. Brit. Assoc._ 1893, p. 766; _Geol. Mag._ 1896, p. 481.]