440 Geological Society : — 



conglomerates on Holyhead Island it was inferred that the previous 

 denudation had been great. The rocks of the eastern district, how- 

 ever, are proved to he Pre-Camhrian from the basal Cambrian rocks 

 of Bangor type lying on them unconforraably near Beaumaris and 

 near Redwharf Bay. The rocks described are found in six distinct 

 districts in Anglesey. 



1. The Western District. — The lowest rocks are the great quartz- 

 ites of Holyhead, which pass at Porth-yr-ogof and inland into chlo- 

 ritic schists, which are foliated in planes of lamination, and thus 

 is produced a tough rock which will not cleave or break, but 

 bends into minute contortions. Towards the east this becomes 

 finer in grain, and may be distinguished as chloritoid schist. On 

 the side of the Straits near the valley it may be seen passing into 

 purple slate. Further north the rocks are confused, especially at 

 Porth-y-defaid, but there is no well-defined fault. The material 

 becomes irregular and forms rocks described as " marbled slate," 

 " lenticular pelites," and soft tuffs. Amongst such are found two 

 special features, viz. masses of quartz in the form of knobs, and 

 lenticular patches of limestone. These it is suggested were pro- 

 duced by the agency of springs rising through and into the ashy 

 rocks. They are specially characteristic of this part of the series. 

 The granite of Pen-bryn-yr-Eglwys is intrusive, and its junction may 

 be seen in several places, the surrounding rocks developing mica. 

 It is here therefore the youngest rock. 



On the south-west side of a fault in Holyhead Island and the 

 neighbouring mainland occurs a distinct group of rocks, continuous 

 upwards from the chloritoid schists, and equivalent to the volcanic 

 facies of the north. These are called the South Stack series. They 

 are characteristically hedded, thrown into large folds, sporadically 

 cleaved, and possess cleavage-foliation. They contain great beds of 

 quartzite, and others of light dusty material. When not cleaved 

 they are almost entirely unaltered. 



The spot near Tywyn, supposed to show fragments of granite 

 contained in a rock of the upper series, and hence the conformity of 

 the two, shows only an intrusive diabase which has caught up 

 granitic fragments. 



2. The Central District. — This is divided into two parts by a 

 fault. That on the east consists of grey gneiss, considered to be 

 the lowest rock of the whole series, with the quartzite at Bodofon as 

 an episode, followed after a fault by chloritoid schists, so intimately 

 connected with the overlying volcanic facies as to be inseparable. 

 The principal features of the latter are the agglomerates of Llangefni, 

 the quartz knobs, and the more or less bedded sporadic limestones. 

 The portion on the west consists of ashes and fine halleflintas, toge- 

 ther with gneissose rocks of no great similarity to the grey gneiss. 

 These have been so interfered with by intrusive rocks that it is 

 difficult to ascertain their true original character. These intrusions 

 consist of (1) Diorite, often foliated, with the folia contorted, and 

 affording by its brecciation some portion of the surrounding rocks. 

 (2) Granite, seen everywhere to be either intrusive, as at Porth-y- 



