Vol. 58. | PLUTONIC COMPLEX OF CENTRAL ANGLESEY. 669 
The granite has been more than once described.’ It is normally 
a haphite. I suspect that the mica which it sometimes contains is 
secondary. As compared with the haplite associated with the 
diorites of Malvern, its felspar is more largely plagioclastic. 
The quartz-felsite is of anormal type. Four slides of this 
rock were described’ by Prof. Bonney ? in 1880. One of these (6), 
which at that time he thought might be a trachyte-tuff, he is now 
inclined to regard as cataclastic. Two new slides from other 
localities have also been submitted to him, and identified as of the 
same class, one of them with a ‘ granophyric’ structure: both of 
these he considers as probably devitrified glasses. I need hardly 
remark that the originally vitreous condition of the rock is no proof 
that 1t was ever erupted at the surface. 
III. Toe Revations oF tHE Rock-MassEs, 
(1) The Relations between the Diorite and the Granite. 
(a) The Craig-yr-Allor anticlinal dome.—This ellipsoidal 
dome of dark gneiss is described in my first Anglesey paper.? I 
have recently reviewed the ground, and inserted on the 6-inch 
Ordnance Survey-map a large number of additional outcrops of the 
gneiss and the granite. The result is to confirm amply my original 
account of the structure of the dome; but the more minute detail 
shows that the granite has sometimes invaded the area of the 
gneiss, producing irregularities in the outline of the ellipse. More 
clearly, however, than ever comes out the main fact that, at the 
margin of the dome, the gneiss, wherever I have seen it, dips 
towards the granite. A few additional details will here be given. 
The major axis of the ellipse strikes north-east and south-west, 
in accordance with the general trend of the band of granitic and 
gneissic rocks which passes diagonally across the island, and is 
about 34 miles in length, the breadth of the ellipse being about 
13 miles. The curve of the south-western end (see sketch-map, 
p. 664) lies south of the Holyhead Road, west of Gwalchmai. 
Working along this line from east to west, we find in the quarry 
on the south of the road, just opposite Cerig-y-Cathod, and in the 
old quarry near the 5% milestone, green altered diorite with traces of 
schistosity, the dips being a little to the east of south. At Boleyn, 
the strikes, seen in the road, are due east and west ; and, in a field 
a little farther to the south-west, there is a good section of micaceo- 
chloritic gneiss (578) p. 665, with clear dip to south-south-west. 
The granite crops out within 50 yards to the south. The strike 
of the gneiss, therefore, swings round in a curve, with its con- 
vexity facing southward. 
On the western side of the ellipse, we begin at the Holyhead Road, 
where there is a long section of rotten schist dipping to the west- 
1 T. G. Bonney, Quart. Journ. Geol. Soc. vol. xxxv (1879) pp. 306, 3807; J. F. 
Blake, Rep. of Committee on Microscop. Struct. Older Rocks of Anglesey, Brit. 
Assoc. 1888 (Bath) p. 397. 
2 Geol. Mag. 1880, pp. 125, 126. SOU eps hl: 
