14 



ME. PHILIP LAKE ON THE DENBIGHSHIRE [Feb. 1 895, 



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These slates are followed by a grit, which forms the upper part 

 of the quarry, and which may be traced southward along the ridge 

 of Moel-y-Gwynt to the top of Moel Ferna. There it bends round 

 eastward and forms the crest of the watershed between the Dee and 



the Ceiriog until it is lost in the 

 heather, which covers everything east 

 of the road from Glyndyfrdwy to 

 Nantyr. No doubt it still continues 

 under the heather, probably in a 

 somewhat attenuated form, for a grit 

 appears in the same stratigraphical 

 position a few miles farther east, a 

 little way north of Llansaintffraid 

 Glyn Ceiriog. 



On Moel Ferna the grit dips be- 

 neath slates, which are exposed a 

 very short distance east of the summit 

 and lower down on the eastern flanks 

 of the hill are worked in the quarries 

 known as the Moel Ferna slate- 

 quarries. They closely resemble the 

 Peu-y-glog Slates, consisting of well- 

 cleaved dark banded slates ; but grap- 

 tolites are not so abundant. The only 

 forms found here were Monograptus 

 priodon and M. Memingi. 



Proceeding down the valley of Nant 

 Arddau these slates are succeeded by 

 the well-known flags of the Denbigh- 

 shire series, which are so frequently 

 quarried, and to which Prof. Hughes 

 has given the name of Nantglyn 

 Flags. They are dark in colour, with 

 numerous very thin, pale-coloured, 

 gritty, or siliceous bands, and they 

 usually split along the bedding. In 

 this valley they are extensively 

 worked at the Deeside slab-quarries, 

 on the flanks of a prominent hill im- 

 mediately south of the stream. Here, 

 as elsewhere, the commonest fossils 

 are Orihoceras primcevum, Forbes, 

 0. Seclgivicki, Forbes, 0. ventricosum, 

 Sharpe, and — most important of all 

 — Monograptus colonus, Barr. 



The flags cover a very extensive 



area, and the characteristic fossils 



may be found in many places. Continuing our section towards the 



east, they form the whole, except perhaps the top, of the hill between 



Nant Arddau and Nant-y-gro. They also occur along the western 



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