■206 



G. L. Elks — GvaptolUc Zones, in Arenicj Rochs. 



Dtplograptus foliacens, var., and Diplogrnplvs angnstifolius, Hall, are 

 not of the tyi)e commonly met with in tlie Arenig beds of other 

 localities, but, on the contrary, are of the type characteristic of the 

 beds of other areas now generally regarded as being of Llandeilo age ; 

 the occurrence of Dicellogrnplns Moffatensis, Carr, and a Dicranograpttis 

 points the same way. With this shale band, therefore, I am not 

 at present concerned, though its fauna is clearly of interest as bearing 

 on the age of the volcanic group as a whole ; it is possibly the 

 equivalent of the middle and upper slates of the Moelwyn district.* 

 The lower shale band underlies the main volcanic group and 

 contains two zones, the zone of D. bifidus above and the zone of 

 D. hirtuido below. The succession may be studied on the slopes 

 of Arenig Fach and Arenig Fawr. 



On the side of Arenig Fach there rises a little stream, Nant rhos ddu, 

 which flows at first in a S. 15° W. direction, then bends abruptly, 

 running appi'oximately south-south-east for a short distance, but 

 soon resuming its former direction, crosses the Bala-Ffestiniog road 

 about 160 yards below Taihirion Farm, and ultimately runs south 

 to join the river Tryweryn. A series of shales, impure limestones, 

 and gritty beds are exposed in the little gorge made by the stream 

 when it runs south-south-east, and also where it resumes its south- 

 west direction above and below tlie road. 



The general succession is as follows (Figs. 2 and 3) : — 



Arenig. 



8. Felspathic ash of main volcauic group. 

 / 7. Bauded slates with light -coloured ashy hands (fossiliferous) 

 i 6. Blue-hlack slates with graptolites in hands ... 

 ^ 5. Impure limestones with few shale partings containing a few 

 I gi-aptolites 



\ 4. Grit with shale ^visps ... 



3. Calcareous flags. 

 TllEMADOC. I 2. Ash. 



1. Calcareous slates and flags (fossiliferous). 



feet. 



4i 

 4" 



Fig. 3. — Diagrammatic Section across Xant Rhos Ddu. Vertical scale exaggerated. 



The Arenig beds dip E. 25° N, at 20°, they are strongly cleaved, 

 the cleavage making with the bedding an angle of about 20°. Some 

 of the ashy bands of the uppermost series have yielded Ogygia 

 ^elwyni (Salt.) and other fossils, and the blue-black slates which 

 miderlie them have yielded graptolites characteristic of the zone 

 of I), hifidus, namely, D. hifidus, Hall, Di2jlograptus dentatus, Brong., 

 and CUmacogroptiis confertns, Lapw. The only fossiliferous band 

 among the impure limestones is at the very base where it rests 



' Quart. Journ. Geol. Soc, 1891, p. 368. 



