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 Dolwyddelau and Yr Arddu, is clearly connected with 
the same set of volcanic causes that produced the thin volcanic beds 
underneath the limestone at Bula. The western end of the Dolwyddelau 
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 surarcit of the mountain, and cor- 
responding precisely to the ash ot 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 TwU-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 Nant Gvvynant, suri'ounded by the same porphyry. The 
basin of volcanic rocks is here over five miles wide, from the hiUs on the 
east side of Nant Gwynant, where the rocks dip N". 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, 
