Part of Carnarvonshire. 79 



Old Cilgwyn and Veingoch quarries. The former of these two 

 quarries lies only 250 yards away from the conglomerate, hut does 

 not expose the grit. 



On Mynydd Cilywyn, moreover, the quartz-porphyry is exposed in 

 several places, always to the west of the conglomerate, but the exact 

 line of junction could not he seen. Several microscopic sections of 

 the quartz-porphyry were examined, and here again the resemblance 

 to the rhyolitic rocks of Bryn-mawr and Cil-y-Coed is most marked. 

 The rocks contain porphyritio crystals of quartz and felspar embedded 

 in a cryptocrystalline matrix which grades to microcrystalline in 

 a patchy mauner. It will be interesting to record here Bonney's 

 description of microscopic sections from this ridge. He states : — 



"The general type is a compact dull felsite with porphyritic crystals of 

 felspar and grains of quartz closely resembling some modern rhyolites. On 

 Moel Gronw angular fragments of a pinkish tint are scattered through the 

 general mass. Again, some parts are crowded with quartz grains, while others 

 are comparatively free of these, and occasionally a spherulitic structure is 

 observed. The rock was probably originally vitreous, and there are abundant 

 fresh examples of the most perfect flow-structure in the rock." l 



This description compares very closely with that already given of the 

 Cil-y-Coed and Bryn-mawr rhyolitic rocks. 



C. Clogwyn Melyn to Caer Engan. — On the southern slopes of 

 Mynydd Cilgwyn the conglomerate stops abruptly. All the lower 

 slopes of the hill on this side are composed of rhyolitic rocks very 

 badly sheared. A careful examination of the area disclosed no 

 exposure of either conglomerate or grit, although a large amount of 

 solid rock is exposed, all of which is massive rhyolite becoming more 

 and more sheared towards the east. As indicated in the Geological 

 Survey Map the rhyolite in this area is brought up against the 

 purple slates. The rhyolite was traced through Pare Pant-dy to the 

 lower roadway going from Pen-y-groes to Talysarn. Here it stops 

 abruptly. But a small area of the rhyolite is exposed again at Caer 

 Engan, 500 yards to the south-east of the nearest exposure in the 

 main mass. In addition, a very narrow band of the conglomerate 

 is found on the south-east side. It is badly crushed and the pebbles 

 almost unrecognizable. The matrix is similar to that at Moel 

 Tryfan, being argillaceous rather than felspathic. The greater 

 portion of the hill, however, is composed of a felsitic rock, which 

 both in hand specimens and in microscopic sections resembles the 

 light variety at Cil-y-Coed ; but whatever fluxion structure it may 

 contain is obscured by shearing. 



Furthermore, 200 yards to the east of Caer Engan there is a small 

 quarry in purple slates, hut there is no exposure of grit in the 

 intervening area. 



D. ScnniAiiY of the SUCCESSION. — We have, then, on the St. Annes- 

 Llanllyfni ridge — 



(i) Rhyolitic rocks, conglomerate, and quartz grit at Moel Tryfan. 

 (ii) Conglomerate and quartz grit at Bwlch-y-llyn. 

 (iii) Rhyolitic rocks and conglomerate at Mynydd Cilgwyn. 

 (iv) Rhyolitic rocks and conglomerate at Caer Engan. 



(v) Purple slates all along the eastern side of the ridge. 



3 Q.J.G.S., vol. xxxv, p. 312, 1879. 



