ROCKS OF CAERNARVON. 687 
On the mass 8. of Brithdir a purple bed is seen near Garth; and 
this seems to be developed to the N.K. into the Brithdir rock, which 
is a red quartziferous felspar porphyry. To the 8.W. of this, 
running parallel to it, is a mass of purple brecciated rock seen in the 
ravine near Y Ganol, and running by Cae Cerig to the HE. of Brithdir 
till it is lost under the overlapping grits and conglomerates. 
A similar sequence of beds can be made out in the mass on the 
W. of the Careg-goch fault. A purplish bed of quartz felsite is seen 
trending N.E. near Tanyperthi; and at Pantyfallenfach a purple 
brecciiform felspar porphyry can be traced for some distance with a 
similar strike. These are probably equivalents of the Brithdir 
porphyry and Brithdir breccia respectively, thrown back by the 
Careg-goch fault (see section, fig. 6). 
III. Bungor Beds (=Voleanic series). 
The ground N.E. and E. of Brithdir is much disturbed, and I 
have not worked out the details ; but the sequence is clear further to 
the N.E., and by reference to it the leading features of the rest of 
that district can be made out. The volcanic beds are well exposed 
Fig. 7.—Section from Menai Bridge to Bryniau Bangor. 
Ww. : E. 
Menai Bryniau 
Bridge. Station. Bangor. 
! 
«x. Carboniferous. a. Cambrian. 
B. Bangor Beds (Volcanic series, agglomerates, &c.). 
D. Dinorwig Beds (Felsitic series). d. Dykes. F. Faults. 
on Bryniau Bangor, from the W. shaft of the tunnel to near the 
path above the station, and are seen, as already elsewhere described, 
to consist of hornstones, fine green banded slates, and agglomerates, 
including fragments of rock precisely similar to the beds among 
which the agglomerates occur. They contain also small faults and 
contorted broken-up beds, lying between others apparently undis- 
turbed. From such indications we may infer that we have here a 
volcanic series, consisting chiefly, in this part, of fragmentary ejecta- 
menta. Following the bands across their edges in descending order, 
we come to purplish somewhat felsitic-looking rocks; and it is not 
improbable that we have here traces of small later lava-streams, of 
which the earlier greater flows are preserved in the underlyiug 
Dinorwig beds. 
If the Brithdir and Pantyfallen brecciated rocks are lava-flows, 
owing their brecciiform appearance to half-molten fragments in 
the lava, they would have tio be bracketed with the Dinorwig beds ; 
and this ejection, with imperfect fusion, would seem to indicate the 
commencement of the conditions under which the Bangor beds were 
accumulated, in which the fragments appear to have been thrown 
out unmelted, forming great masses of agglomerate and ash. 
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