226 



C. CALLAWAY ON THE ARCHAEAN 



tain. The dip is usually to the south-east; but the beds frequently 

 roll over to the north-west, as may be seen south of Porth twyn- 

 mawr, and at Caer ceiliog on the Holyhead road. There is no doubt 

 that these schists represent the dark green band which in the 

 central zone underlies the granitoidite. 



2. Holyhead District. — Chloritic schists arc well seen at many 

 points in and round the town. A typical specimen from Porth felin 

 (Note 36, p. 234) is of the same character as the schist at Porth y 

 defaid. The strike is to the south-west ; and undulations are numerous. 

 These rocks are separated from the quartz group by faults : one, on 

 the south-west, runs from the south coast, at Porth y corwgl, to the 

 south end of Holyhead Mountain ; and another, on the north-west, 

 lies along the south-eastern base of the mountain and cuts the coast 

 at Porth yr Ogof. 



Porth-yr-Ogof fault. — Prof. Ramsay * states that the ' ; quartz 

 rock" dips under the " foliated schists ;" and he gives a section in 

 support of his opinion. This view would have greatly simplified 

 my work ; but on visiting the spot I was disappointed to find that the 

 two groups were brought together by a fault. There was a notch 

 in the cliff; on one side was quartz rock, and on the other green 

 schists (Note 35, p. 234), with no signs of a passage. The break 

 was filled in with fallen rubbish. It was very fortunate that this 

 dislocation was discovered, as it compelled me to seek the true 

 succession elsewhere. 



Holyhead Mountain. — The microscopic examination (Note 31, 

 p. 233) of a typical specimen of the quartz rock from the great quarry 

 shows that it is a true quartz-schist of the same type as tho Bodafon, 

 Gwalchmai, and Mynydd-baen quartz groups. The structure of the 

 mountain is interesting. Ascending the south-east slope, separation- 

 planes are seen to dip to the north-west at about 80° ; but on the 

 north-west side the dip changes to south-east at the same angle. The 

 planes are not close and even, as in slaty cleavage, but they cause 

 the rock to split in large thick flat flakes. This difference is doubtless 

 due to the coarseness of the material ; and the planes must represent 

 true cleavage. The cleavage-strike agrees with the strikes of the 

 contorted bedding and of the ridge. 



The Holyhead district is thus brought into clear comparison with 

 other parts of Anglesey. The chloritc-schists undoubtedly repre- 

 sent the dark schist which underlies the granitoidite in the central 

 zone ; and the quartz-schists may with the highest probability be 

 placed on the same horizon as the foliated quartz rocks of Bodafon 

 and Gwalchmai . The absence of the grey gneiss is easily explained. 

 On both the west and south the junction between the quartz and 

 chloritic groups is a fault ; but the two formations dip in the same 

 direction, and the downthrow is apparently on the side of the chlo- 

 rite-schist. As the latter group is repeated in numerous shallow 

 curves, there is no reason why the grey gneiss should appear at the 

 surface, as it does amidst the deeper undulations of the Menai anti- 

 line. 



* Geology of North Wales, p. 183. 



