RAMSAY PHYSICAL STRUCTURE OF MERIONETHSHIRE. 173 



It was long ago proved by Professor Jukes and Mr. Selwyn, that, 

 from the physical structure of the country, the fossiliferous calcareous 

 ash in the valley of Dolwyddelan, is the precise equivalent of the 

 Bala limestone, and the great mass of felspar-porphyry that lies below 

 this ash, between Dolwyddelan and Yr Arddu, is clearly connected with 

 the same set of volcanic causes that produced the thin volcanic beds 

 underneath the limestone at Bala. The western end of the Dolwyddelan 

 porphyry is only separated from the great masses of porphyry that form 

 the chief components of the Snowdonian range by a narrow anticlinal axis 

 of sandstone and slate, full of the ordinary Bala fossils. The rocks of 

 Dolwyddelan lie, indeed, in an elongated basin, which is an outlier 

 of the greater basin of which Snowdon forms the centre. This 

 larger basin is well worthy of notice. If we follow the geological 

 lines from Moel Hebog to Carnedd Llewelyn, we find, on a 

 great scale, that the structure of the country precisely corresponds to the 

 minor basin of Dolwyddelan. On the south, east, and west of 

 Moel Hebog, there are fossiliferous sandstones of slates of the Bala 

 beds, overlaid by felspathic porphyry, on which are thick beds of 

 calcareous ash, forming the summit of the mountain, and cor- 

 responding precisely to the ash of Dolwyddelan. It is therefore, in 

 part, at least, the equivalent of the Bala limestone. The porphyry of Moel 

 Hebog ranges north to Snowdon in an unbroken line, and there, again, 

 between Snowdon and Twll-du, below Y-Glyder-fawr, it is overlaid by 

 the very same ash, which in places, as on the top of Snowdon, becomes 

 partially slaty and fossiliferous. Another patch of the same strata lies 

 in the valley of N"ant Gwynant, surrounded by the same porphyry. The 

 basin of volcanic rocks is here over five miles wide, from the hills on the 

 east side of Nant Gwynant, where the rocks dip iN". W. to the Pass of 

 Llanberis, where they dip S. E. All these Snowdonian porphyries 

 are true Silurian lava-beds, accompanied by volcanic ashes of the 

 same period. They are perfectly interbedded with fossiliferous 

 strata ; and it is worthy of remark that the slates on which the 

 porphyries rest have been altered at points of contact by the over- 

 flowing melted masses, whereas the slaty beds that rest upon them, hav- 

 ing been deposited on a cooled surface, are unchanged by heat. 



Between Snowdon and Y-Glyder-fawr and Menai Straits, blue slaty 

 Silurian strata rise to the north-west. The base of these, interbedded 

 with grit, forms the Lingula-flags, beneath which lie the Cambrian 



