Geological Society of London. 185 
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 bedded, 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 hilleflintas, 
together 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 con- 
torted, and affording by its brecciation some portion of the sur- 
rounding rocks. (2) Granite, seen everywhere to be either intrusive. 
as at Porth-y-dee, Ceryg-defaid, Craig-y-allor, and Maen-wyn, or 
absorptive, as near Llyn-faelog. This granite is of various kinds, 
often passing into a felsite. 
3. The District west of Traeth Dulas shows granite intrusive into 
erey gneiss, and also passing into a felsite ; it is correlated with the 
western half of the central district, of which it appears to be a con- 
tinuation. 
4. The Eastern District.—The lowest portion here is the grey 
gneiss, which is very compact towards the west, especially near the 
igneous rocks, but becomes more micaceous and chloritic towards 
the east, passing through chloritoid rocks into others of the volcanic 
facies, with the usual quartz knobs and sporadic limestones, but 
here, on the whole, more slaty. The complete unity of the whole 
system is here well seen. The most remarkable feature is the 
intrusive foliated diorites, which are coarse and non-foliated near 
Holland Arms, but became finer towards the east, where also 
glaucophane takes the place of hornblende. They are seen intruding 
into and contorting the grey gneiss in the Llangaffo cutting. 
At the southern end at Careg-gwladys is a remarkable volcanic 
group, with a spherulitic diabase breaking into and surrounding the 
baked blocks of calcareous slate. There are associated great masses 
