w. 



SHARPE ON NORTH WALES. 301 



Section 7. Valley of Llanllyfni (north side). 



Gravel. 



Greenstone and schist. 



Schists and Slate. 



south-east, and does not vary more than 10° on one or the other 

 side of the vertical plane. The most easterly part of this slate dips 

 north-west 65°, and it there rests conformably on the schists of the 

 Snowdon chain. 



The purple slate extends about two miles to the south of the 

 valley of Llanllyfni, but it is not worked beyond that valley. The 

 slate, where it terminates, is cut oiF by a system of rocks striking 

 east and west. 



Notwithstanding the repetitions shown in sections 4, .5 and 7, the 

 author considers the purple slate as forming only a single bed, which 

 reaches from Aber on the coast above Bangor to Llanllyfni, and 

 overlies, along that line, the schists of the Snowdon chain. The 

 section of Bettws Garmon might lead one to regard the slate as lying 

 below the schists of Snowdon, but that inference would be at vari- 

 ance with all the other sections; and the beds in that locality are 

 so much displaced by the greenstones, that they cannot afford us 

 any sure guidance. The slate is usually unconformable to the 

 Snowdon schists, and the only exception to this is at Llanllyfni. As 

 from the central axis of the Snowdon chain to the purple slate on 

 its west flank there is an ascending series of beds ; as the fossils 

 found on that flank of the chain belong to the Caradoc or upper 

 portion of the Lower Silurian system, and as no other Lower Silu- 

 rian rocks are found above the purple slate, the author concludes 

 that the purple slate is the uppermost of the Lower Silurian rocks. 



This purple slate is probably identical with some one of the beds 

 of slate on the east side of the anticlinal of Snowdon, but there is 

 no bed of slate on that side of the ridge which resembles the pur- 

 ple slate in colour. Throughout the greater part of North Wales 

 the top bed of the Lower Silurian formation is a good roofing-slate, 

 but of a dark grey colour. The purple colour ought therefore to 

 be regarded as a local peculiarity *. 



Along the whole line of the purple slate on the west of Snowdon, 

 the cleavage planes are very constant in their direction ; for, with 

 the exception of a slight deviation at Llanberris, they uniformly 

 strike north-east, and do not vary from the vertical plane on either 

 side more than 10°. 



Northern continuation of the Snoicdon Chain. — The author made 

 only a slight examination of the district north of Glyder-fawr. 

 What he saw of it was analogous in structure to tiiat mountain ; 



* The purple slates with their green spots have much resenihlance to the green 

 slates of Cumberland, which are green with purple spots. These arc the only 

 slates in North Wales which have any resemblance to the green slates of the 

 Lakes. 



