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.] iii. THE CAERNARVONSHIRE VOLCANOES OF THE BALA PERIOD