"5 -2 < 



SHARPE ON NORTH WALES. 295 



along the Holyhead road. The synclinal axis near its southern end 

 is formed of a high ridge of greenstone, flanked on both sides by 

 nearly vertical beds of metaraorphic quartz-rock, and these form the 

 rugged hill of Yr Arddu. West of this is metamorphic schist of little 

 thickness ; and below the schist lies a very hard grey chert or horn- 

 stone, faintly affected by cleavage, dipping to the south-east at an 

 angle of 60°. This is the bed of chert already noticed as containing 

 fossils which belong to the upper part of the Lower Silurian system. 

 The Rhaiadr-cwm section No. 3 passes to the north of Yr Arddu ; 

 and on this line the greenstone is divided into two ridges, which are 

 separated by vertical beds of schist. The western of these green- 

 stone ridges forms the very remarkable crag, Craig-llyn-llagi. From 

 this point in the synclinal axis to Rhaidr-cwm, which lies to the 

 south-east, the beds which here constitute the eastern side of the 

 Carnarvonshire trough are traced by the author in the following 

 descending order : — 



' 28 Greenstone of Craig-llyn-llagi. Dip of beds N.W. Dip of cleavage 



65° N.W. 

 27 Schist. 



26 Greenstone, forming a narrow ridge. 

 25 Slate which runs up to Cynicht. This perhaps belongs to the same 



bed as the slate at Llanrhwchwyn, behind Gwydir Park. 

 24 Semi-crystalline slaty grit. 

 23 Greenstone. 



22 Hard, blue, slaty flagstone. 

 21 Dark grey slate. 

 20 Coarse schist. 



19 Greenstone, forming a narrow ridge. Strike of beds N.E. by E. 

 18 Dark slate, of fair quality, near Bryn-gelinnen. It seems identical with 



one of the beds worked in the great quarries of Dolwyddelan. Dip 



45° north-west, of cleavage 65° W.N.W. — Below this point the slates 



become lighter in colour, and the beds in general become harder than 



they are above. 

 ^17 Slaty flagstone. Dip of beds 65° N.W. Dip of cleavage 85° N.W. 

 16 Hard slaty flagstone. Dip of beds 25° N.W. Dip of cleavage 45° N.W. 

 15 Blue slaty rock. 



14 Semi-crystalhne chert or hornstone. 



13 Blue slate of Moel Wynn. The same bed is worked in the great quar- 

 ries of Rhiw Brefder, of which see the description, p. 296. 

 12 Hard schist. Dip 20° N.W. 

 1 1 Greenstone interstratified with the schists. To the south it becomes half 



a mile wide; at the foot of Moel Wynn it thins ofi"; at Rhiw Brefder 



it is very narrow. 

 10 Hard schist. Dip 45° to 50° N.W. 

 9 Thick series of slaty beds alternating with chert or hornstone, seen in 



the hill above Tan-y-Bwlch. Dip 20° N.N.W. 

 8 Semi-crystaUine quartz-rock, behind Maentwrog. 

 7 Coarse schist. Dip 30° N.W. 

 6 Slate of inferior quality, between Ffestiniog and Pant-llwyd. This is 



the lowest bed of slate worked in this line of section. 

 5 Coarse schist. 

 4 Greenstone, interstratified, forming a ridge of low crags. Strike of beds 



N.E. 

 3 Hard blue schist. Dip 15° N.E. 



2 Schist, which continuas to Rhaidr-cwm. Dip 40° north-west. 

 1 Schist, intersected by quartz veins, at Rhaidr-cwm. The bods and 



cleavage jilanes are shattered and in great confusion. Dip 20^ N., 



30" N., and 40" N.E. 



PU 



c^ 



^ 



If 



o 

 -5= S 



