THE ORDOVICIAN SYSTEM 151 



Thus the cherts and black shales of the Arenig Series may be 

 compared with the gaize and black clays of the Oxfordian, but have 

 no resemblance to modern oceanic deposits, which are red, yellow, 

 grey, and white, but never black. There are no fine red clays nor 

 foraminiferal limestones among the Scottish Arenig and Llandilo 

 Series, like those which are associated with Tertiary oceanic deposits 

 in Barbados and elsewhere, and the utmost that can safely be 

 inferred from the character and small thickness of the Scottish 

 deposits is that they were formed in clearer water and at a greater 

 distance from land than the Arenigs of Wales. 



Finally, it should be noted that in the British area this was a 

 period of great volcanic activity, volcanic vents being opened from 

 time to time on the sea-floor, and probably in most cases they were 

 built up into volcanic islands from which subaerial eruptions took 

 place. This happened in one part of the region or another 

 throughout the whole period. Thus there are volcanic rocks of 

 Arenig age in Pembroke, Merioneth, Scotland, and Ireland (Mayo) ; 

 of Llanvirn age in Merioneth and Cumberland ; of Llandilo age in 

 Merioneth, Carnarvon, and Cumberland ; of Bala age in Waterford 

 and Dublin. 



REFERENCES 



1 H. Hicks, Quart. Journ. Geol. Soc. vol. xxxi. p. 167 (1875). 



2 H. Hicks, Pop. Sci. Review for 1881, p. 289 ; see also Proc. Oeol. 

 Assoc. vol. vii. p. 291 (1882). 



3 Marr and Roberts, Quart. Journ. Oeol. Soc. vol. xli. p. 476 (1885). 



4 A. Strahan, "Geology of the Country round Carmarthen," Mem. Oeol. 

 Suro. 



6 W.G. Fearnsides, " North and Central Wales," in Otology in the Field, 

 published by Geol. Assoc. (Stanford 1910), p. 786. 



6 W. G. Fearnsides, Quart. Journ. Oeol. Soc. vol. Ixi. p. 608 (1905). 



7 Miss G. L. Elles in Oeol. Mag. for 1904, p. 199. 



8 C. Matley, Quart. Journ. Oeol. Soc. 



9 See Memoirs of Geol. Survey on Llandilo, Ammanford, and Carmarthen. 



10 Marr and Roberts (op. cit. above). 



11 T. Ruddy, Quart. Journ. Oeol. Soc. vol. xxxv. p. 200. 



12 Groom and Lake, Quart. Journ. Oeol. Soc. vol. xlix. p. 426. 



13 G. L. Elles, Quart. Journ. Oeol. Soc. vol. Ivi. p. 169 (1909). 



14 J. E. Marr, Geol. Mag. for 1894, p. 122. 

 18 J. E. Marr, Oeol. Mag. for 1892, p. 97. 



18 C. Lapworth in Quart. Journ. Oeol. Soc. vol. xxxiv. p. 25, and vol. 

 xxxviii. p. 538 ; and in Geol. Mag. for 1889. pp. 20 and 69. 



17 Messrs. Egan and M 'Henry, Sum. Prog. Oeol. Survey for 1899 and 

 1900. 



18 Gardiner and Roberts, Quart. Journ. Oeol. Soc. vol. liii. p. 520. 



19 Reynolds and Gardiner, Quart. Journ. Oeol. Soc. vol. lii. p. 523. 



20 Summary Prog. Oeol. Survey for 1896, p. 48. 



21 Carruthers and Muff, frisk Naturalist for 1909, p. 7. 



