166 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 20, 



The Snowdon igneous rocks can be fol- 

 lowed to Camedd Dafyd, where certain 

 lower beds of trap come in, interstratified 

 with bands of fossiliferous slates and sand- 

 stones ; these also form parts of Camedd 

 Llewelyn. Most of these rocks may be 

 traced from Carnedd Llewelyn to Nant- 

 francon, on the sides of the steep hill west 

 of Llyn Ogwen called Braich-du. There 

 they dip easterly at a high angle, and rise 

 again with a westerly dip in the great peak 

 of Y-Tryfan . From thence they roll over 

 in a large anticlinal and form the heights 

 of Gallt-y-gogo, on the south side of the 

 Bangor road, within two miles of Capel- 

 Curig. The two bands of slate that 

 separate the igneous mass into three, cease 

 in the valley of the Llugwy, and the trap 

 becomes one thick band. This great band 

 of felspathic trap can then be traced by 

 Ffynnon Llugwy and Llyn Dulyn almost 

 to Conway. It is broken here and there 

 by faults. Its dip is invariably easterly. 

 Above it, on the east, are beds of sandstone, 

 slate, and felspathic traps and ashes. The 

 sandstones, especially, are often fossilife- 

 rous, and contain the fossils common to 

 Snowdon and Bala. These, vvdth igneous 

 interstratifications, extend with many con- 

 tortions to the vale of the Conway. Their 

 finest development lies between Ffynnon 

 Llugwy and Llanrwst, around Llyn Cwlyd, 

 Llyn Crafnant, and Llyn Geirionydd. 

 Their position above the rocks of Y-Tryfan 

 and Carnedd Llewelyn proves them to be 

 the equivalents of the Snowdon traps and 

 ashes, and their fossils are also the same. 

 The calcareo- felspathic quality of some of 

 the ashy beds nearly resembles some of the 

 rocks of Snowdon and Y-Glyder-fawr. The 

 slates and traps of Glyn Lledr (two miles 

 south-west of Bettws-y-Coed) dip under the 

 rocks that lie between Ffynnon Llugwy 

 and Llanrwst. The Glyn Lledr rocks also 

 dip under those in the Dolwyddelan Valley. 

 A reference to the published maps of 

 the Geological Survey shows that the 

 Snowdon trap bounds Nant Gwynant on 

 the east. Together with the slates that 

 immediately underlie it, it dips westerly. 



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