178 ME. H. H. THOMAS ON THE [May 1911, 



The following is a list of the chief works and papers which refer 

 to, or deal solely with, the geology of Skomer Island and the 

 Skomer Volcanic Series, arranged in chronological order : — 



1823 [1826]. II. T. De la Bechf. Trans. Geol. Soc. ser. 2, vol. ii, pt. 1, p. 8. 



He mentions the 'trap' of Marloes and Skomer Island, and states that the 

 mass consists principally of 'greenstone,' but contains rocks intermediate in 

 character between 'cornean ' and 'greenstone.' On the map published with his 

 paper Skomer Island is coloured as consisting entirely of igneous rocks, but it is 

 evident that he had noted the sediments which are exposed on the eastern side 

 of North Haven. 



1839. R. I. Mukchison. ' Silurian System ' pp. 401 & 405. 



Refers to the stratified and unstratified ' trap-rocks ' of Skomer. He makes 

 the statement that felspar-breccias and conglomerates alternate conformably in 

 thick parallel masses with regularly stratified purple-green and yellow sandstone 

 and schist of the ' Upper Cambrian System.' Presumably he suggested a 

 Cambrian age for these rocks, on account of their colour and their unfossiliferous 

 nature. 



He evidently thought that some of the Skomer rocks were" intrusive, for he 

 remarked that ' the intrusive trap cuts in vertical dykes through the bedded trap 

 and sandstone.' This statement, however, cannot be substantiated by reference 

 to any known section, and it is presumed that he was misled by the isolated 

 exposures occurring in the interior of Skomer. 



PI. xxxv, fig. 10 gives a somewhat misleading section, and on the map the 

 Upper Llandovery rocks to the south of Wooltack are lettered as Llandeilo 

 Flags. 



1845 to 1857. Publication of the Geological Survey Map, Old Series, 1 inch to 



1 mile, Sheet 40. Surveyed by II . T. De la Eeche, A. C. Ramsay, W. T. 



Aveline, and J. Rees, .Tun. Revised in 1857, with the addition of Silurian 



and igneous lines bj r W. T. Aveline. 



In this map all the igneous rocks are colom-ed as 'intrusive greenstone,' and 



the sediments of the Skomer Series are lettered as 'Llandeilo Flags.' 



1867. R. I. Murchison, ' Siluria ' 4th ed. pp. 53, 143. 



He here, presumably relying on the work of the geological surveyors, assigned 

 the igneous rocks to the Llandeilo or Bala Series, but the evidence for this corre- 

 lation is not given. 



1881. F. RrjTLEY. Appendix to a paper on the Microscopic Structure of Devi- 

 trifled Rocks from Beddgelert and Snowdon. Q. J. Gr. S.vol. xxxvii,p. 409. 



He gave the first petrological description of any rocks of the Skomer Series, 

 but unfortunately without any locality other than 'Skomer Island.' The four 

 rocks described b} r him, which had been collected some time previously bj r Sir 

 Andrew Ramsay, were named (1) devitrified banded obsidian, (2) devitrified 

 banded and spherulitic obsidian, (3) basalt or andesite, and (4) quartz- 

 oligoclase trachyte (?) . 



These slides are preserved in the Geological Survey Collections, and corre- 

 spondingly numbered. 



1885. F. Rutley. ' Felsitic Lavas of England & Wales' Mem. Geol. Surv. pp. 18-20. 



He here described in greater detail the spherulitic and banded obsidians 

 already mentioned, his remarks being evidently based on the specimens collected 

 b}' Ramsay and preserved in the Jermyn Street Museum. 



These rocks can now be referred with certainty to the spherulitic and banded 

 rhyolite of Tom's House on the west side of Skomer Island (see p. 187). 



1888. J. J. H. Teall. ' British Petrography ' pp. 224, 284, & 336. 



He described the less acid rocks of Skomer as a ' magnificent series of basic 

 lava-flows,' and referred them mainly to basalts and porphyrites. He noted that 

 they possess characters of their own, which rendered it difficult to class them 

 with rocks already described by other authors. 



1893. F. T. Howard & E. W. Small. Rep. Brit. Assoc. (Nottingham) p. 766. 

 ' On some Igneous Rocks of South Pembrokeshire, &c.' 

 In this is given a preliminary account of the geology of Skomer Island. 



