24 E. W. Hughes — Geology of Part of Carnarvonshire. 



reached. Exposures of purple slate also occur in the meadow below 

 Coch-y-big, and also near the outhouses atLlech-y-dwr. 



The dip of the slate cannot be ascertained, but the position of the 

 three exposures gives some indication that the strike here is similar 

 to that of the Cambrian slates in the Nantlle Valley — namely 

 N.jST.E.-S.S.W. If this were the case, purple slates would be 

 present at a point within 300 yards of the grits at Craig-y-Dinas, in 

 the exact position in which they should stratigraphically occur. 

 Unfortunately the exposures are not sufficient to verify this. The 

 Survey memoir mentions Llwyd-coed (2 miles to the north-east) as 

 the most southerty exposure of the Cambrian purple slates. 1 



(iv) Green Slate. — One hundred yards due east of the exposure of 

 purple slate at Llech-y-dwr the one already referred to at Pentwr is 

 found. This exposure shows a band of fine grit in green slate, both 

 macroscopically very similar to the grits and green slate that overlie 

 the purple slate both in the Nantlle and Llanberis quarries. Still 

 further east, 60 to 70 yards away, near Ysgubor-Wen, there is also 

 an exposure of green slate. As already mentioned, the weathered 

 surface shows the cleavage to strike E.N.E.-W.S.W., but the dip of 

 the beds cannot be determined in such a small exposure. 



No further exposures of any description are to be seen in this 

 neighbourhood until we come to the hill of Y Eoel. The whole of 

 this hill consists of black slate showing poor cleavage and containing 

 iron pyrites in large quantity. These slates are put down as of 

 Ordovician age in the Geological Survey map, but no fossils have 

 been obtained here. They closely resemble the black slates in the 

 quarry to the south-west of Cil-y-Coed. 



EXPLANATION OP PLATES. 



Plate I. 



Fig. 1. — Quartz-rbyolite, Cil-y-Coed. Showing two large crystals of corroded 



quartz (a) and (b) ; fluxion structure (c) winding round a crystal of 



muscovite on the left and of orthoclase (d) on the right. Natural light. 

 x 18. 

 ,, 2. — (Fig. 1 above, under crossed nicols.) Showing microspherulitic 



structure. The quartz crystal (a) and the orthoclase crystal (d) are the 



same as in Fig. 1. The flow-structure (c) is still evident. x 18. 

 ,, 3. — Rhyolite, Cil-y-Coed. Showing very small spherulites (d) and fluxion 



structures (b) ; also porphyritic crystals of biotite (a) and orthoclase (c). 



Natural light, x 18. 

 ,, 4. — (Fig. 3 under crossed nicols.) Showing microspherulitic structure 



(a) and (b). x 80. 

 ,, 5. — Rhyolite, Cil-y-Coed. Showing slight flow-structure (6) ; phenocrysts 



of plagioclase (a) and quartz (c), corroded and invaded by the groundmass. 



Crossed nicols. x 18. 

 ,, 6. — Conglomerate, Cil-y-Coed. Showing a chip of felsite (a) ; quartz (d) ; 



and an angular chip of tuff (b) containing a quartz crystal (c) and a 



decomposed felspar (e). Natural light, x 18. 



Plate II. 

 ,, 1. — Matrix of conglomerate, Cil-y-Coed. Containing subangular quartz 

 crystals (a) and grains of felsite (b) and (d) in an abundant sericitic 

 matrix (c). Natural light, x 18. 



1 See extract on p. 21. 



