16 E. Wynne Hughes — Geology of 



the area in a very general way. The village of Llanllyfni is 

 mentioned as the most southerly extremity of the quartz-porphyry 

 of the St. Annes-Llanllyfui ridge, 1 and the conglomerate is said to 

 extend in this south-westerly direction no further than Mynydd 

 Cilgwyn. 2 The exposures at Bryn-mawr, Cil-y-Coed, and Craig-y- 

 Dinas are not referred to at all by name; but in the 1 in. Geological 

 Survey map published in 1850 the area containing Bryn-mawr and 

 Craig-y-Dinas is designated as V — Lower Cambrian — and Cil-y-Coed 

 is mapped as an intrusive felspathic rock. Hicks in a map accom- 

 panying his paper on the Pre- Cambrian rocks of Carnarvonshire 

 indicated that he considered the rocks of Cil-y-Coed, as also of Pen-y- 

 gaer, Tre-Ceiri, Yr Eifl, Gryn-ddu, Gryn Goch, Bwlch Mawr, and 

 Mynydd Cenin, to be of Pre -Cambrian age. 3 The igneous rocks at 

 all those localities, however, have been claimed by Harker as of 

 Ordovician age. 4 Again, in a further paper, Hicks claimed that the 

 rocks at Craig-y-Dinas were of Pre-Cambrian age, 5 and with regard 

 to Cil-y-Coed he writes as follows : — 



"In the absence of true Lower Cambrian rocks in this area, the evidence of 

 their age has to be frequently based on the general character of the rocks 

 themselves and the behaviour of the beds in contact with them . . . The 

 small mass to the north of Clynogg Fawr (presumably Cil-y-Coed or Pen-y- 

 garreg) is of the same character as the above-mentioned masses — a quartz- 

 porphyry — and is also surrounded by Upper Cambrian rocks, and on one side 

 by even Lower Cambrian beds also unaltered in contact." 6 



Tawney also visited this district, and in his paper on " The Rocks 

 of Caernarvonshire ", after a general discussion of the rock exposures 

 in the county, he remarks : — 



"It seems to me that there is sufficient evidence that these are igneous rocks 

 intruded through Cambrian shales in the manner delineated in the Geological 

 Survey Map. To begin with, the mass about 1 mile north-east of Clynnog, at 

 Cil-y-Coed, is a quartz-porphyry of pinkish grey colour." 7 



Harker, in his Sedgwick Essay on " The Bala Yolcanic Series 

 of Carnarvonshire", refers to tbe mass at Cil-y-Coed in the following 

 words : — 



" Proceeding south-westerly we come to Clynnog district, where a number of 

 distinct intrusions occur, which unfortunately have not all been studied in 

 detail. There is a chain of four large hills due to four sets of connected 

 intrusions. The other intrusions in the neighbourhood are quartz-porphyries 

 of various characters, the Pen-y-gaer rock being a beautiful granophyre, while 

 the little bosses of Cil-y-Coed, north-east of Clynnog-fawr and Moelfre near 

 Llanhaelhaiarn, show Rhyolitic affinities." 8 



III. Exposures. 



The country for some distance to the south-west of the St. Aunes- 

 Llanllyfni ridge is comparatively flat and low-lying. It is mainly 



1 Mem. Geol. Surv., 1886, p. 140. 



2 Op. cit., p. 143. 



3 Q.J.G.S., vol. xxxv, p. 297, 1879. 



* Harker's Bala Volcanic Series of Carnarvonshire, 1888, p. 44. 



5 Q.J.G.S., vol. xxxv, p. 296, 1879. 



6 Op. cit., p. 298. 



7 Geol. Mag., Vol. IX, p. 552, 1882. 



8 Bala Volcanic Series of Carnarvonshire, 1888, pp. 44-5. 



