¢ ROCKS OF CAERNARYVON. 689 
the Bangor-road faults into the city, where it appears again below 
the Mount, from which it can be traced at intervals across the end 
of the hill by the EK. shaft of the tunnel, and 8S. and 8.W. to Nant 
Gwtherin. 
The area of two square miles lying 8.W. of Bangor is a difficult 
bit of mapping, from the number of faults and dykes, and the partial 
metamorphosis accompanying them ; but the beds can be pretty well 
identified. I think the district could be satisfactorily worked out, 
especially if we had six-inch maps. 
Felsites occupy the whole area between the Carboniferous fault and 
the Trawscanol fault as far N.E. as Penychwynfa. Cambrian grits 
cross the road near Beulah Chapel, and are seen in the small road- 
cutting in the field opposite the entrance to Belmont. Here also 
there is a greenstone dyke exposed, and on the E. side of it, close to 
the gate, the conglomerate occurs. To the EK. pale mudstones are 
seen along the road from the George Hotel to Bangor, and near the 
four road ends by the “f” of Gosphwysfa. . of this the con- 
glomerates are repeated as described above, being seen almost 
uninterruptedly running from Gored Gith to Plas Lodwig. On the 
8.E. side of the Trawscanol fault Cambrian grit crops out along the 
top of the ridge by Bryn Adda, and then we have the interruption 
of a dyke running along the lane from S. of Maesmawr to the lower 
Bangor road. 
A short distance 8.W. of this, by the ‘‘ dd” of Tafarnnewydd, a red 
erit and fine conglomerate, like the Careg-Hwfa beds, come to 
the surface, and are quarried in places. 
Along the roadside by 'Tyddyndu sandstones and mudstones are 
seen traversed by a dyke which comes behind Coedty. These beds 
dip at a high angle (50° and 60°) to the N.E., and must be cut off 
from the felsites of Cilmelyn by the Trawscanol fault. 
There are some hornstones and baked-looking banded slates on 
the rugged hill near the “ w” of Tafarnnewydd, which at first sight 
I should be inclined to throw in with the Bangor group; but I have 
not worked out that bit, and there are beds very like them caught 
in along the dykes 8.E. of the conglomerate of Tymawr near 
Llanddeiniolen, and these may either be Cambrian, altered by 
the Coedty dyke, or be older rocks faulted in. 
The actual base of the conglomerate and grit I have not seen in 
this part of the district. Fig. 8 shows the section near Garth 
Point, and fig. 9 those made out on either side of Bryniau Bangor. 
The conglomerate is less conspicuous, and the coarse grit more so, 
as we trace the series to the 8.W. The conglomerate consists of 
large pebbles, almost all of which are fragments of felsite with a 
few of quartzite. The grit is generally a tolerably pure siliceous 
rock with but little felspathic matter. The conglomerate is gene- 
rally weathered to a considerable depth. 1t is much decomposed to 
the bottom of all ordinary quarries; but some fragments brought 
up the shaft, probably from the same bed deep down, are of tough 
purplish rock, containing pebbles of felsite similar to those in the 
more weathered conglomerate. 
