164 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 20, 



Cader Idris. The Lingula beds lie underneath them between Aran 

 Mowddwy and the Dolgelli and Bala road. They dip easterly at 

 angles of from 50° to 60°, The section is unbroken, and the total 

 thickness of these flags exposed is rather more than 6000 feet. They 

 are cut off by a great fault, which passes from the borders of the 

 Coal-measures, about six miles south-west of Chester, by Corwen and 

 Bala lake, through Tal-y-llyn to Cardigan Bay. It appears to be 

 covered up by the New Red Sandstone near Chester. From thence 

 to the point where it enters the sea, the line is about sixty-five miles in 

 length. It is invariably a downthrow on the north-west. On that 

 side of the fault in the Bala country, Bala beds, traps, ashes, and 

 Lingula beds are repeated in the same order in which they occur on 

 the flanks of Aran Mowddwy. This constancy in the section proves 

 the fault. From measured thicknesses, on Careg-llysog the top of 

 the Aran trap must be thrown down to the level of the road, and the 

 amount of the throw cannot be less than about 12,000 feet. The 

 Barmouth sandstones must lie about 1000 feet beneath the surface 

 at the Bala road. The supposed angles of the fault would scarcely 

 aff'ect these figures*. 



Beyond the western outcrop of the ash-beds of Y-Dduallt, the Lin- 

 gula flags crop out and undulate towards Gors -goch, near Trawsfynydd. 

 Here and there they are pierced by masses and dykes of grey green- 

 stone, often very felspathic. Near Trawsfynydd they abut against the 

 Barmouth and Harlech sandstones. This fault cannot be less than 

 4000 feet. Near Trawsfynydd is the apex of the Merioneth anti- 

 clinal. Beyond that point towards Moel-wyn the rocks dip to the 

 north-west. The boundary of the Barmouth and Harlech sand- 

 stones and the Lingula beds runs near the Tan-y-bwlch and Traws- 

 fynydd road. The sandstones are succeeded by about 8000 feet of 

 Lingula flags, if under this name we include the 3000 feet of flaggy 

 mottled rocks that lie between the Ffestiniog syenite and Moel-wyn. 

 These mottled rocks, however, on the south-west frequently pass 

 into ash, and on the north-west by Manod-Mawr into sandstones 

 and slates. The Ffestiniog syenite alters the rocks all around, in 

 this respect being readily distinguishable from the contemporaneous 

 felspathic traps which only alter the slates that underlie them. 

 Above the mottled rocks there are beds of ash, conglomerate, fel- 

 spathic slaggy-looking traps, and slates, all regularly interstratified. 



On the south-east flank of Moel-wyn there are, in ascending 

 order, first — 350 feet of solid felspathic trap, then 120 feet of ashy 

 conglomerate, then 130 feet of slate, which is succeeded by 180 feet 

 of ashes. The ash is followed by 125 feet of slate, over which lies 

 nearly 900 feet of slaggy felspathic trap, in some places columnar. 

 The slates beneath these traps are altered by heat ; those above are 

 unchanged. This series occupies the same general geological horizon 

 with the traps of Aran Mowddwy and the Arenigsf. Where unbroken 

 by faults, they may be traced, as a mass, continuously about the top 

 of the Lingula flags, circling round with the Merioneth anticlinal. 



* See Horizontal Sections of the Geological Survey, sheet 29. 

 t See Horizontal Sections of the Geological Survey, sheet 28. 



