Dr. C. Callaway—Pre-Cambrian Geology of Anglesey. 121 
highest beds of the schistose arch. The ordinary dark gneiss is 
overlain by a band of granitoid gneiss, a rock like granitoidite, but 
with hornblendic or micaceous folia developed in sufficient abund- 
ance to convert it into a true schist. This passage bed is overlain 
by ordinary granitoidite, which also lies in mass to the N.E. 
Fic. 2.—Anticlinal N.E. of Pentre’r felin. 
a=Dark gneiss. b=Granitoid gneiss. c=Granitoidite. 
East of the anticlinal, the dip, exclusive of minor undulations, is 
regular to the S.E. The best section is seen to the 8.E. of Gwyndy, 
which occupies a position corresponding to Llechyn farwy on the 
opposite side of the axis. Just opposite the junction of the two 
roads, green schist dips at a low angle to the S.E. This is overlain 
by passage beds (granitoid gneiss), which undulate for about 200 
yards, and then plunge to the S.E. under massive granitoidite. 
Green schist appears at intervals to the 8.E., but the band between 
Gwyndy and Bodwrog is mainly granitoid. Skirting the S.E. 
margin of the schist anticlinal, we find, about a mile S. of Gwyndy, 
the granitoidite resting on mica-gneiss; the latter dipping to the 
S.E. The junction is evidently a fault, for (1) there are no passage 
beds, (2) the mica-gneiss is not the summit of the schistose series, 
and (8) greenstone breaks up through the Dimetian a few yards to 
the E. of the junction. 
Between the last spot and Bwleyn, the point from which we 
started, the ground is occupied by the broken ridge of Craig yr Allor. 
These rocks consist of black and dark-green gneiss, sometimes 
chloritic, dipping S.E. To the W. of the ridge, the gneiss is massive, 
and contains epidote. At one spot, unrounded fragments of quartzite 
are included, and weather out sharply on the surface. Granitoid 
concretions are not uncommon, composed mainly of felspar, with a 
little quartz and some chlorite. The Holyhead Road passes across 
the anticlinal, about half a mile S. of Craig yr Allor, and rocks 
of the ordinary types are exposed at intervals on both sides. 
These details clearly show that the gneiss passes under the Dime- 
tian to all points of the compass, and that it forms an elliptical 
quaquaversal anticlinal of Pre-Dimetian age. The presence of beds 
of passage wherever the section is unbroken is of great interest, 
proving that the Dimetian granitoid group is simply the continua- 
tion of an older series. 
(b). Section at Gaerwen in the Menai anticlinal.—On the Survey 
Map, two bands of “ gneiss” are indicated, the more easterly passing 
