224 



C. CALLAWAY ON THE ARCFLEAN 



that the Amlwch and Cemmaes groups are on the same horizon ; 

 but the evidence appears to point in that direction. The limestone 

 (Note 52, p. 236) is very similar to a rock at Llanfaethlu ; and the 

 associated slaty beds are alike in both localities ; but it would be 

 rash to correlate the groups. 



The northern area (omitting Paris Mountain) is thus seen to 

 consist of chloritic schists, felspatho-quartzose grits, felspathic shales, 

 chloritic slates, quartz-conglomerates, and grey limestones, with some 

 limestone bands on more than one horizon. In such a shattered 

 district a true succession can hardly be indicated. North of Paris 

 Mountain some of the horizons are probably faulted out of sight, 

 and it is difficult to correlate the rocks of this district with those 

 further west. Most of the groups appear to pass into each other, 

 either vertically or laterally ; and I cannot avoid the conclusion that 

 they all belong to the same epoch. 



V. North-western Area. 



In Western Anglesey are two well-marked groups of strata, which 

 I regard as the equivalents respectively of the Gneissic and Slaty 

 series. They are brought together by a fault. 



Porth-y -clef aid Fault. — This dislocation is situated on the west 

 coast, about midway between the northern and southern extremities. 

 It is well seen on the shore, striking inland to the east. Greenstone 

 is erupted along the line of junction, and alters the strata for a 

 short distance on each side. South of the fault are thoroughly crys- 

 talline quartzo-micaceous and chloritic schists dipping north-west ; 

 while close at hand on the north side green ashy shales dip north- 

 east. This fault is the boundary between the two series ; for true 

 schists with a south-west strike occur everywhere to the south, 

 while a comparatively unaltered group, with a prevailing northerly 

 dip, stretches right up to the north-west corner of the island. 



Section from the Fault to Pen bryn'r Eglwys. — Proceeding north- 

 ward, we find at Trefadog ashy shales dipping north-east. In the 

 quarries north of Llanfaethlu church is a good section of grey lime- 

 stone and pale-green ashy slate with easterly dip. The former is com- 

 pact, and apparently as unaltered as any Palaeozoic limestone. A 

 careful search revealed no trace of fossils. The slaty rock is of a 

 common Anglesey type ; and the whole group is like the Cemmaes 

 series. About Llanrhyddlad exposures are numerous. At Porth 

 Swtan, on Church Bay, is a considerable thickness of yellowish fel- 

 spathic slate and breccia. The dip is north-east, with slight contor- 

 tion on a small scale. Higher beds are seen in quarries at Ehyd 

 N gharad ; the rocks are brecciated in part and very felspathic. 

 Greenish colours predominate. One variety, a pale-green slate, 

 contains cubic pyrites. On about the same horizon, at Ogo Lowry, 

 are green brecciated rocks more highly indurated. 



The district between' Ogo Lowry and Penbryn 'r Eglwys is one of 

 the most faulted parts of the island ; the section consists of alterna- 

 tions of Archaean and Palaeozoic rocks, repeated by faults, and dipping 

 to the north. The wedges of dark shale let in amongst the older 

 group are very clearly seen in the sea-ciifTs. The younger shale, of 



