72 UPPER CAMBRIAN (ORDOVICIAN). 



Asaphiis Poivisii, Calymene hrevicapitata, Ampyx tuviidiis, Phacops 

 BrojigJiiartii, Lichas laxatus, Trinucleus concentrictis, Illmius Bow- 

 77ianni, Beyrichia complicata, Lingula ovata, Orthis vespertilio, 0. 

 elegantula, O. porcata, O. Actoni(Z, 0. calligramma, Strophoinefia 

 dipressa, S. expansa, S. grandis, LeptcBiia sericea, Modiolopsis, Holopea 

 concmna, BeUerophon pertxirbatiis, Orthoceras vagans. Starfishes of 

 the genus Palceaster are found at Bala, Cystideans also occur. The 

 Coral Favositt'S fibrosa is met with in the upper beds. Orthis 

 alternata characterizes a zone near the base of the series. 



Graptolites are abundant, chiefiy Diplograptidce and Dicj-ano- 

 grapiida;, and these, as Prof. Lapworth has pointed out, are distinct 

 from the forms met with in the Arenig series. 



The Polyzoan Pinnaiopora Sedgivickii {GlaucoJiome disficha, in part) 

 has been described by Mr. G. W. Shrubsole from the Bala Beds of 

 Glyn Ceiriog, in Denbighshire, and is regarded as the oldest known 

 representative of its class.' 



The typical Bala beds lie in the Bala district, between Dinas- 

 Mowddwy, Bettws-gwerful-goch, and Bettws-y-Coed. They con- 

 sist of black and blue slates, and grey and brown sandstones ; the 

 Bala limestone, generally very impure, lying about the middle, and 

 averaging from 20 to 30 feet in thickness. Between the limestone 

 and the lower eruptive rocks of the Arenigs and Llyn Conwy, two, 

 and sometimes three thin and imperfect beds of volcanic ashes 

 represent the whole of the vast volcanic accumulations of Moel 

 Hebog, Snowdon, and Carnedd Llewelyn. The middle part of the 

 Bala beds, including the limestone, is most fossiliferous, the black 

 slates below, and the slaty and sandy interstratifications above, 

 being comparatively barren.^ 



Sir A. C. Ramsay states that north of Moehvyn and the Manods 

 the slaty rocks dip north and north-west, plunging under the 

 felstones and calcareous ashes of Dolwyddelan and of the equivalent 

 traps of the Snowdon region south and south-east of Llyn Gwynant 

 and Llyn-y-ddinas. The slates are associated with numerous dykes 

 of hornblendic greenstone, which for the most part lie more or less 

 between the beds, and are proved to be intrusive by the alteration 

 of the strata in contact with them both above and below. Towards 

 the upper part near the felstones, the strata are often sandy, and 

 yield numerous fossils of the ordinary Bala species. The same 

 author adds that both physically and pal^ontologically the proof 

 is clear that these are equivalents of the Bala beds, and of the 

 Caradoc sandstone of Shropshire, and that the calcareous ashes of 

 Dolwyddelan are the actual representatives of the Bala limestone ; 

 and this leads to the startling conclusion that all the vast masses 

 of ashes that crown the felstones of Snowdon and Moel-Hebog 

 are but an enlarged development of the same strata. Orthis 

 flabeUulum occurs in shales near the little Inn on the summit of 

 Snowdon. (See Fig. 10, p. 58.) 



Proc. Chester Soc. Nat. So. 1884, p. 98. 



Ramsay, Geology of North Wales, edit. 2, pp. 12, 123. 



