1853.] RAMSAY — LOWER PALAEOZOICS OF N. WALES. 167 



and, the whole rolling over in an anticlinal, the same traps and ashes 

 are thrown across to Y-Foel-goch at the upper end of the Valley of 

 Dolwyddelan*. Hence the highly calcareous ashes of that valley 

 (almost a hmestone in some places) are the precise equivalents of 

 the Snowdon rock of the same kind, and also of the ashy beds towards 

 Llanrwst ; and all of these are, in parts, the equivalents of the Bala 

 limestone and of the rocks immediately associated therewith. South- 

 east of Bettws-y-Coed this hmestone in its normal state actually 

 appears, and, together with a thin associated bed of trap, it seems to 

 bear the same relation, as regards superposition, to the Glyn Lledr 

 traps, that the traps and ashes of Dolwyddelan and the neighbour- 

 hood of Llanrwst bear to the same rocks. There is, therefore, no doubt 

 that the uppermost Snowdon rocks are represented by the limestone 

 and its immediately associated strata, and the two extremely thin 

 ashy beds that lie at some distance beneath it in the Bala country 

 are all that remain to represent the thick igneous masses that spread 

 across Caernarvonshire from Moel Hebog to Conway and Llanrwst, 

 and which Section No. 2f proves to be about 6000 feet above the 

 igneous rocks of Moel-wyn, just as the Bala limestone lies about 

 6000 feet above the equivalents of Moel-wyn, — viz. the igneous 

 masses of Arenig and the Arans. 



To return to the rocks that lie between Snowdon and the Menai 

 Strait I . On the sides of the Pass of Llanberis and of Nant-Francon, 

 dark blue slates dip under the traps at angles of from 70° to 80°. 

 Towards their base, sandstones and ferruginous slates occur, in 

 which, at Marchlyn Mawr, I found Olenus micrurus and Lingula 

 Bavisii. These are the Lingula beds. The rocks that lie betwixt 

 them and the Snowdon trap are therefore the representatives of the 

 Bala beds that he beneath the Bala limestone. No fossils have been 

 found in them, neither does there appear in these sections any trace 

 of the great igneous interstratifications of the Arans, the Arenigs, and 

 Moel-wyns§. These have, therefore, entirely thinned out under the 

 great Snowdon trough. As already stated, they dip northerly at 

 Moel-wyn and the Manods, underneath 6000 feet of the lower Bala 

 beds. Then the Snowdon trap and ashes come on, dipping to the 

 north-west. They rise again in the Pass of Llanberis, but the igneous 

 masses that underlie the Bala beds in the meanwhile have entirely 

 thinned away underground; and this impossibility of tracing the 

 beds is the reason why it has been heretofore supposed that the Snow- 

 don traps were the equivalents of those of the Arans and of Cader 

 Idris. The whole of the stratified series is much thinner on the 

 Llanberis side of the trough than on the side of their outcrop towards 

 Harlech and Ffestiniog. The Lingula flags, especially, are probably 

 not of half the thickness ; their upper limit, however, is uncertain. 



From beneath the Lingula beds, both at Nant-Francon and Llan- 

 beris, sandstones crop out, which, it is well known, are the equiva- 

 lents of the Barmouth and Harlech grits of Professor Sedgwick. 



* Geological Map 75 N.E. 



t Sheet 28 of the Horizontal Sections of the Geological Survey of Great Britain. 



X Maps 78 S.E. and S.W. 



§ Moel-wyn is in Map 75 N.E. 



