Fig. 6.—NSection near Llanddeimniolen. 
W.S.W. 
PROF. T. M‘KENNY HUGHES ON THE PRE-CAMBRIAN 
YA * in the same strike on the brow of 
Z the hill above the station at Port 
/ L) 
Dinorwig. Here it occurs between 
aS grey felspathic rocks; and these can 
hardly be distinguished from parts 
of the great mass of felsites which 
occupy almost all the high ground 
above Port Dinorwig, and, repeated 
by faults, stretch away to Lland- 
deiniolen on the 8.E. and Brithdir 
and Beulah Chapel on the N.E. 
Tymawr. 
Il. Dinorwig Beds (=Felsitic 
series ). 
b. Brecciiform beds. 
2\e 
al 
{p. Purple felsite. 
D. Dinorwig beds (Felsitie series) I 
F, Faults. 
The next series consists chiefly of 
great masses of grey quartz felsite. 
No distinct bedding can be made out 
in any section; but the strike can be 
inferred, in the lower part of the 
series, from the manner in which the 
rock occurs with reference to the 
boundary of the underlying forma- 
tion, and more clearly by following 
certain beds of marked character in 
the middle and upper part. It is 
seen with a subordinate purple bed 
near Tanyperthi, on the 8. of which 
it is overlapped by a set of grits very 
similar to some of those which flank 
the Twt-Hill beds at Caernarvon (see 
map, Pl. XXXVI.). It projects at 
intervals all along the ridge to near 
Brithdir, and then can be followed 
on the other side of the Bangor road 
to the fault N.E. as far as Beulah 
Chapel. 
A fault runs 8.W. and N.E. under 
Careg goch W. of Llanddeiniolen, 
dropping the grits on the N.W.; and 
passing between Cefn and Tanyr- 
wylfa,crosses the valley near the dyke 
under Tynycoed, repeats a great part 
of the felsitic series by bringing 
them up again on the S.H., and so 
runs into the Bangor-road fault N. 
of Brithdir. 
me This repeated mass of felsites is 
= overlapped near Llanddeiniolen by 
grits and conglomerates, which can 
be traced at intervals to the Bangor 
road N.K. of Brithdir (see map and 
fig. 6). 
Fachell. 
W 
SE \ 1 
J Aes 
STZ 
< 
Qo 
Parc y gors. 
PAL 
Soh gS 
TV9 
(\ \ Sey 
= 
?Dd 
ce 
—S 
= ny sealteheds: | Caernarvon beds. 
e". Crug group. 
A 
d. Dyke. 
wN 
cen 
D 
= San 
\ 
Newnes 
?Dp 
Careg goch. 
a’. Sandstone and grit. 
a, Conglomerate. 
a‘, Black slates. 
a. Grit. 
TIN 
716 
YP f) 
/ 
NG lel 
/ 
Git 
Carboniferous. 
x. 
a. Cambrian | 
Griffith’s Crossing. 
