212 



C. CALLAWAY ON THE AECHJEAN 



a few yards to the north-west we find the dark-green schist in its 

 usual position above, both dipping to the north-west. 



Following the strike to the north-east, the gneiss is seen to con- 

 tain angular pieces of quartz, some of which are flattened in the 

 foliation-planes, so as to appear like ordinary folia. 



2. Mynydd Llwydiarth. — This elevated ridge, trending to the 

 north-east for nearly a mile and a half, is composed of highly con- 

 torted gneiss. The general dip is north-west ; but in some places 

 the beds undulate to the south-east ; and the thickness cannot be 

 great. The rock is more quartzose than usual ; and the quartz frag- 

 ments become much more numerous, — the two facts suggesting the 

 proximity of quartz land. The gneiss, rolling in frequent contor- 

 tions, is finely exposed in low vertical cliffs ; and the angular pieces 

 of white quartz studding and projecting from the surface produce a 

 very striking effect. Both types of gneiss appear to be present ; 

 but the felspar being less abundant, the rock is brought into closer 

 resemblance to the micaceous and chloritic schists of Holyhead. On 

 the north-western slope the gneiss is extensively brecciated, and 

 the presence of a fault is further inferred from an abrupt transition 

 to younger formations. At the foot of the slope, near the fault, the 

 gneiss becomes somewhat granitoid, suggesting the commencement 

 of the passage into granitoidite, which is seen so clearly in the Craig- 

 yr-allor anticline. At the extreme north-east of the ridge, on Red 

 "Wharf Bay, the green schist is exposed for some distance across the 

 strike to the east ; but south of Wern we come onto the grey gneiss, 

 all the dips being north-west. Further to the east, at Bryn y Castell, 

 the grey type is again seen, but with the dip reversed ; and at Haf odty, 

 still further to the south-east, the dark schist comes in again. This 

 is a clear anticline, the grey gneiss exposed at the apex throwing off 

 green schist in opposite directions. The structure of this district is 

 shown in PI. VIII. fig. 3. 



3. Gaerwen to Menai Bridge.— Crossing the anticline from Gaer- 

 wen, we find at the railway -junction that the dark schist has rolled 

 over to the south-east ; but half a mile to the south-east, near 

 Llanddaniel, the series, represented by contorted quartzose chloritic 

 schist, dips north-west. Exposures then are rare till we come to 

 the Anglesey column, where dips are in opposite directions, but 

 usually to the north-west. This rock is of the ordinary dark green 

 type, occurring everywhere above the grey gneiss in both the Menai 

 and Craig-yr-allor areas. It has been described by Prof. Bonney *. 

 On the Straits, near the west end of Menai Bridge, green schists 

 occur of the ordinary varieties. 



From the above facts it is clear that the rocks of the anticline 

 belong to the gneiss series, "Well-foliated gneiss occurs at numerous 

 localities ; and the green schist is so intimately associated with it, 

 as an upper band, as to remove all shadow of doubt that the two 

 types form one unbroken group. In this area I have not found a 

 trace of any thing but true crystalline schists. 



The thickness of the group is not great. The foldings are nume- 

 * Quart. Journ. Geol. Soc. vol. xxxv. p. 308. 



