ANNIVERSARY ADDRESS OP THE PRESIDENT. 



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occur with this form, furnishing strong evidence that the Tremadoc 

 rocks extend as far N.W. in Wales as Anglesey : the above-men- 

 tioned species has not occurred in the classical Tremadoc area, cither 

 in the lower or the upper division of the group. The presence of 

 the Arenig beds in Anglesey favours the belief that the Tremadoc 

 may also occur in the island. 



The Tremadoc slates, now so well known in the two classical 

 Welsh localities, viz. Tremadoc in North, and St. David's in 

 South Wales, are so rich in conspicuous forms and well-determined 

 species that we readily recognize and place them in two distinct or 

 separate horizons or zones. Their true position in North Wales, 

 between the Lingula-flags below and the Arenig group above, was 

 clearly shown and fixed by the distinguished Cambridge Professor 

 in 1851. Their base was an easier line to trace than their passage 

 above into what was then a group of rocks believed to be almost 

 unfossiliferons. Small indeed was the fauna known to Sedgwick 

 and Salter in 1813, when, at Tai-hirion, west of Arenig Mountain, 

 they collected the two characteristic fossils Ogygia Sdwynii and 

 Calymene parvifrons. These two species f ormed the mental basis 

 of Sedgwick's Arenig, and determined the top of his Tremadoc 

 group till better worked out. Now the Tremadoc fauna of North 

 Wales consists of 42 species, of South Wales 30, and of Shropshire 

 (Shineton) 16. The Arenig group numbers 62 genera and 149 

 species; or 97 species occur at St. David's, 74 in the Skiddaw beds 

 of Westmoreland, 36 in the Stiper-stones area, 34 in North Wales. 

 These have nearly all been described since the far-seeing eye of 

 Sedgwick determined the stratigraphical place of the groups on 

 physical grounds. We owe the description of the species to Dr. Hicks, 

 Mr. Salter, Professor M'Coy, and Messrs. Hopkinson, Lapworth, 

 Nicholson, and Davidson. Guided by mineral or physical characters 

 alone, the line of separation between the Lingula-flags and Lower 

 Tremadoc would probably be drawn immediately above the hard 

 grey arenaceous so-called Middle Lingula-flags, in the typical Tre- 

 madoc area, where Lingulella Bavisii ceases to be an abundant and 

 characteristic fossil ; but zoological conditions forbid this ; the " black 

 slates " that succeed the " hard grey series," possess a recurrent 

 generic fauna from the lower zones, including Conocoryphe depressa, 

 Dikelocephalus ?, and Agnostus princ&ps, with Orihis lentieularis, 

 the two latter in millions. The cliff of Ogof-ddu, near Criccieth, 

 and the jutting promontory of Craig-ddu exhibit a complete section 

 from the Lingula-flags to the Upper Tremadoc slates, the character- 

 istic Orthis lentieularis marking by myriads the junction of the 

 Middle Lingula-flags and the Upper Black pyritous slate. No Upper 

 Lingnla-flag section in Wales is more prolific in life : Conocoryphe 

 abdita, Salt., and two other species, Dikeloe&phalus celiicus, Salt., 

 D. discoidalis, Salt., Agnostus princeps, and Orthis lentieularis 

 dispel all doubts as to age or horizon ; but the Tremadoc fossils do 

 not occur at Ogof-ddu ; the fossiliferous zones must be sought for in 

 the sections at Penmorfa, Borthwood, Moel-y-gest, &c. near Port- 

 madoc (places rich in characteristic species), and also across the 

 estuary from Aber-ai, by Dudreath, to Cae-Iago, near Maentwrog. 



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