ANNIVERSARY ADDRESS OP THE PRESIDENT. IO3 



the elucidation of the true position of the Arenig group in North 

 Wales remained a doubtful matter ; probably the slates of Angers 

 are of the same age. Mr. Salter did much in 1853 to unravel the 

 structure of the Portmadoc area, and was enabled to correct the 

 nomenclature and distribution of the so-called Lower Llandeilo 

 rocks of the Penrhyn promontory, between Traeth-bach and Traeth- 

 mawr, and of the country to the east, towards the Arrans. The 

 succession across the Penrhyn promontory, from the Lingula-flags to 

 the Arenig beds, is complete ; and it is here that the Tremadoc slates 

 and Arenig beds are typically developed in North Wales, the close 

 of the Tremadoc and base of the Arenig being everywhere marked 

 by the presence of a well-defined band of 41 grit," varying in thick- 

 ness. The position of this grit had long been fixed by the late Mr. 

 Salter in his Penrhyn section ; but it was left for Professor Ramsay, 

 in 1874, with Professor Hughes, Mr. Homfray, and myself, to trace 

 this zone from Ogof-dua, near Criccieth, to Drws-dwgoed, under 

 the Dolbenmain alluvium, and on to Tai-hirion ; and in 1875 the late 

 Mr. Ward and Mr. Herbert continued and completed the survey of 

 it west of the Arenig and the Arrans on to Cader Idris, until last 

 seen passing under the sea at Towyn. This line is the base of 

 Sedgwick's Arenig slates and porphyries, and of the great roofing- 

 slate series of the Ffestiniog area. Although occasionally obscured 

 by faults, it is never lost, and must be traced by all who would 

 understand the succession in North Wales between the Tremadoc and 

 Arenig series. The fossils, when seen, at once determine the posi- 

 tion — Psilocephalus innotatus, Niobe Homfrayi, Asaphus Homfrayi, 

 Lingulella Davisii, and Dictyonema sociale illustrating the lower part 

 of the Tremadoc, and Angelina SedgiuicJcii, Asaphus Homfrayi, and 

 Ogygia scutatrix at Garth and Penclogwyn the upper ; whereas, suc- 

 ceeding the grit-band above named, at Ty-obry, Tai-hirion, &c, Galy- 

 mene Murchisoni, Asaphus affinis, Dionide atra, and JEglina caliginosa 

 are the characteristic forms, being Arenig fossils. Only about 16 

 species connect the two groups. This physical line, when once deter- 

 mined, is a marked feature at the base of the Arenigs, dividing the 

 two formations. Beneath the grit at Garth Hill and Deudreath occur 

 the upper beds of the Upper Tremadoc slates (hard grey flags), rich 

 in fossils, including Asaphus Homfrayi, Angelina JSedgwicJcii, Ogygia 

 scutatrix, Liagulocaris lingulcecomes, Cheirurus Frederici, and Oo- 

 nularia corium. These hard grey beds at the top, some would 

 separate from the Tremadoc group, purely upon palaeontological 

 grounds, or because certain species above named are common to the 

 two horizons. Some regard must, however, be paid to stratigra- 

 phical evidence. 



There is no distinct passage-bed containing fossils between the 

 Upper Tremadoc and the Arenig. Through the Portmadoc district 

 the two groups are separated by a peculiar band of felspathic grit, 

 which intervenes to cut off the upper (volcanic) series from the true 

 Tremadoc slates below. This felspathic grit, variously conditioned, 

 may be traced along Tr-allt-wen, above Tremadoc, and across the 

 estuary of Treath Maw ; it constitutes the brow of the Garth, and 



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