300 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



These beds are seen on both sides of the quarry. The trough of slate 

 crosses the Holyhead road, but is not worked to the north of it. 



To the west of the Penhryn quarries the same purple slate is re- 

 peated three times between ridges of greenstone : 



1. On the west of Bethesda, where it dips north-west. 



2. At Coet-mor and at Tan-y~Bwlch, where it dips south-east 50°, 

 and is underlaid, as at Penrhyn, by green slate. The quarries at 

 these places are of considerable extent. 



3. At Pont-y-Coetmor and Bryn, where it dips south-east 30°. 

 At all these places the planes of cleavage strike, with the beds, 



north-east, and dip south-east from 80° to 85°. 



The purple slate is worked on a large scale near Llanberris, but 

 the beds worked on the opposite sides of the valley are in different 

 positions, as a considerable fault runs along the pass*. 



On its northern side are the great Dinorwig quarries, which are 

 worked on three apparently different beds, which may be only one 

 bed repeated owing to local disturbance, all dipping W.S.W. 60° ; 

 the cleavage dipping E.S.E. 70°. On the same side of the pass, op- 

 posite the middle of Lake Padarn, and again below the middle of that 

 lake, the purple slate is seen lying between great eruptive masses of 

 greenstone. 



On the south side of the pass, the slate is first seen at the seventh 

 milestone, dipping west 20°. Here it is separated from the Glyn 

 quarries by a narrow dike of greenstone. These quarries are worked 

 upon two beds, which are separated by some irregular masses of 

 greenstone. The beds dip south-east 55° ; and the cleavage is ver- 

 tical, and strikes, with the bedding, north-east. The slate rests on 

 metamorphic rock ; and below that are greenstones and other igneous 

 rocks, forming a band two miles wide, the western edge of which is 

 covered by gravel. 



Section 6. Valley of Bettws Garmon. 

 N,W. (Scale i inch = 1 mile.) S.E. 



Gravel. TT s tt Schist. tt Schists. 



TT. Greenstone. s. Slate. 



At Bettws Garmon, on the road from Beddgelert to Carnarvon, 

 the purple slate occurs only once ; it lies between two beds of green- 

 stone. The beds dip south-east 45°, that is to say, towards the 

 Snowdon chain : the cleavage dips north-west 85°« The slate is of 

 the usual purple colour with green spots, but is of indifferent qua- 

 lity ; the workings are on a very trifling scale. 



Further south, in the valley of Llanllyfni, the purple slate is 

 thrown up several times, owing to faults and to eruptions of green- 

 stone ; in consequence of which it is worked very largely, on no 

 less than six different lines. The colour and quality of the slate are 

 the same as at Penhryn and Llanberris. The beds strike north- 

 east, and dip either north-west or south-east. The cleavage strikes 



* See Section 5. 



