378 MESSRS. A. V. JENNINGS AND G. J. WILLIAMS 



In the same waj' the Nyth-y-Gigfran slate-bed is obviously the 

 same as that worked at AVrysgan (the Middle-Moelwyn bed), or 

 a coalescence of thnt and the narrow band above (Upper-Moelwyn 

 bed), owing to the thinning oub of the Middle Agglomerate. The 

 Upper Igneous series, over the Nyth-y-Gigfran Quarry, corre- 

 sponds to that over the Wrysgan Quarry, but the felstone is less 

 in thickness. 



We find in fact that the strata north of Cwm Orthin can be 

 correlated with those south of it, in consecutive order and without 

 involving the difficulties raised by the interpretation given on the 

 Geological-Survey Map. 



Proceeding farther northwards, the escarpment of the igneous 

 rocks is seen to dip downwards towards the railway-tunnel and the 

 road to Dolwyddelen. There is a curious break in continuity of the 

 upper series for a short distance just south of the conspicuous rock 

 known as Craig Flaen Llym, owing to which the Upper-Slate bed 

 seems to come into contact with the Llandeilo slates above. It is 

 also of importance to notice that the upper series becomes entirely 

 ashy, while in the lower there is a development of a grey felstone. 



The great Efestiniog andTrawsfynydd fault cuts through the strike 

 just to the east of the railway, so that the continuation of the strata 

 under consideration has to be looked for in Gareg Ddu, half a mile 

 to the south. 



Thus the Garth Grit is found altogether south of Blaenau Ffestin- 

 iog, to the ]^.E. of Cwmbowydd, and the Lower-Slate bed aj)pears by 

 the side of the High Street. 



Gareg Ddu itself consists below of grey felstone like that of Manod, 

 columnar in parts. It corresponds in position with the Lower Ag- 

 glomerate of Moelwyn, and though coloured as felstone on the map, 

 its upper portions consist entirely of agglomerate. Along a strip of 

 marshland behind runs the equivalent of the Upper-Slate bed of 

 JSTyth-y-Gigfran, covered as before by an upper agglomerate. It is 

 this latter bed that we regard as equivalent to the " Upper Ashes " 

 of Arenig, and not the thin band occurring here, as at Foel Eydd, in 

 the Llandeilo slates above *. 



Bounding Gareg Ddu on the east is another fault with downthrow 

 to the west : that is, the Gareg-Ddu mass lies in a " trough " between 

 the lines of fracture. The strata with which its east side was con- 

 tinuous lie just outside the sketch-map accompanying this paper, 

 and occupy a small area between this fault and the next. The 

 greatest fault in the district is that which, running along the east 

 side of this area, has thrown down to the south the mass forming 

 Manod Mawr and Manod Bach. 



Thus the Garth Grit, last seen above the road near Plas-isaf, 

 appears again behind Manod School. 



Above it occur the spotted flags with the secondary cleavage re- 

 ferred to as found near Tan-y-grisiau ; and a little higher a bed of 

 slate, evidently the same as that at the foot of Gareg Ddu. The 



* Ramsay, op. cit. p. 95. 



