Part of Carnarvonshire. 77 



(iii) To the west and north-west the country is flat and low-lying. 

 No solid rock was found anywhere in this direction. 



(iv) The most northerly exposure of the Pre-Camhrian rhyolite 

 series is within 400 yards of the boundary assigned to the St. Annes- 

 Llanllyfni ridge in the Geological Survey map (1850). The nearest 

 locality on this ridge where solid rock is exposed is at Pare Pant-dy, 

 800 yards due east of Pen-y-groes and a mile and a half east of 

 Bryn-mawr. Referring to the rocks at this end of the ridge, the 

 Survey memoir states :— 



"Further south the conglomerate forms the highest points of Moel Tryfan 

 and Mynydd Cilgwyn, where it is partly metamorphosed into a sort of talcose 

 schist and conglomerate. Beyond this it has been either completely obliterated, 

 or, curving round to the east near the crest of the hill, it is cut off by a fault 

 which throws the superincumbent purple slate directly against the porphyry. 

 It is seen that the grits and lower conglomerates disappear at Mynydd Cilgwyn, 

 but the purple slates that are interstratified with these follow an unbroken line 

 to the neighbourhood of the turnpike road near Llanllyfni. The general 

 character of the porphyry is that of a felsitic rock with an amorphous grey 

 felspathie base containing small crystals of quartz, which are often somewhat 

 granular, sometimes hexagonal, and sometimes they seem to be four-sided 

 prisms. 



" . . .It also contains small distinct crystals of glassy felspar. The base 

 of the conglomerate is highly felspathie and sometimes crystalline, enclosing 

 pebbles of felspathie trap, quartz, quartz rock, purple and black slate, and 

 jasper. The whole mass is altered, and it is easy to note first : the disappearance 

 of the granular structure in the conglomerate or sandy matrix and its gradual 

 assumption of a porphyritic character, with small crystals of felspar embedded, 

 while the enclosed pebbles still retain their distinctive form ; and again, 

 approaching the recognised porphyry the hard outlines of the pebbles in the 

 conglomerate gradually melt away till they become undistinguishable in the 

 general fusion of the rock, and the view that the porphyry is not an intrusive 

 mass is aided by the fact that it is impossible to define any line of demarcation 

 between conglomerate and porphyry." l 



Evidently, then, we have on the St. Annes-Llanllyfni ridge 

 a succession very similar to that at Craig-y-Dinas and Cil-y-Coed. 

 In consequence, the south-west portion of the ridge between Moel 

 Tryfan and the village of .Llanllyfni was carefully examined. 



3. The St. Annes-Llanllyfni Ribgk. 



A. Moel Tryfan. — The conglomerate at the top of Moel Tryfan 

 seems outwardly identical with that at Cil-y-Coed. The pebbles are 

 well rounded and of various sizes, though seldom more than 3 inches 

 long. They are mainly of volcanic origin and are enclosed in an 

 argillaceous matrix. There are also quite ,a number of quartzose 

 pebbles of a type which is of rare occurrence in the conglomerate at 

 Cil-y-Coed and Ci'aig-y-Dinas, and the matrix is distinctly more 

 argillaceous. On the top of Moel Tryfan there are no exposures of 

 the quartz-porphyry or the quartz grit, but not more than 200 yards 

 to the east of the conglomerate we find, at the Alexandra Slate 

 Quarry, a great thickness of purple slate. 



Prom this quarry an adit has been driven east and west right 

 through the mountain. An examination of the rocks in this adit 

 shows that a quartz grit occurs here on the eastern side of the 



1 Mem. Geol. Surv., vol. ii, p. 143, 1866. 



