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.]