122 Dr. C. Callaway—Pre-Cambrian Geology of Anglesey. 
through the word “Caerwen,” the other about half a mile west, at 
the great fault which brings down Permian rocks against the schist. 
I have run a section across this area from §.E. to N.W., with the 
following result :— 
(1). At the windmill at “Caerwen.” Chloritic mica-schist or gneiss 
with thick quartz folia. Strike N. and. Dip vertical, or a little to 
the W. Small quarry a few yards N. of windmill. Hornblende- 
gneiss! (Note 3, p. 126), indistinguishable from a variety at Craig yr 
Allor, foliation planes N. and 8., passing into a rock in which folia- 
tion is obliterated, and which therefore might easily be taken for 
diorite. The same passage is also seen in the Craig yr Allor area. 
(2). At a quarter of a mile N.N.W. of (1), N.W. of “ Post Office,” 
is an exposure of grey quartzose mica-gneiss, with a high N.W. dip. 
This rock is continued for over a quarter of a mile to the N.W. It 
is well exposed at Bryn sisyr in a road section, where folia of pink 
felspar are very distinct. At this locality some of the schist is very 
quartzose. Further on, near Bryn disgwyl, the gneiss, very con- 
torted, is exposed in quarries on both sides of the stream, and similar 
rock is seen in the road and fields nearly up to Y graig. The grey 
gneiss dips uniformly rather high to the N.W., and, if there is no 
repetition, it must be over 1000 feet thick. 
(3). At Y graig, the dark-green gneiss comes in again, dipping 
N.W. at 50°. It includes, as in the Craig yr Allor district, some 
bands of a pinkish quartzo-felspathic rock, suggesting a transition 
towards the granitoid series: and contains a great deal of chlorite. 
It extends up to the Malldraeth fault (throwing down Ordovician 
shales and Carboniferous rocks), and is several hundred feet thick. . 
We have then two bands of dark-green gneiss, with a broad 
middle zone of grey gneiss, the whole dipping at a moderately high 
angle (say 50°) to the N.W., except at the base of the section, where 
the strike is N. and §., and the dip vertical or a little W. I have 
seen no reason to believe that there are two dark bands in the dis- 
trict. I am disposed to think that there is a repetition by folding, the 
anticlinal falling over to the east, so as to give a general westerly 
dip. The grey gneiss is not seen S8.E. of Gaerwen windmill, and 
a little to the S.E., at Gaerwen station, we have chloritic schist 
dippmg 8.E. Granitoid rock is slightly exposed N.H. of Gaerwen, 
near Cefn du, but I was not quite sure that it was in place. Whether 
the repetition is by folding or faulting, or even if there is no repeti- 
tion, my main conclusion is not affected, viz. :—that at Gaerwen we 
have a considerable thickness of grey gneiss passing up into dark 
gneiss, which in all respects resembles the dark gneiss of the Craig 
yr Allor anticlinal, and which may fairly be identified with that 
series. This section thus serves to bring the Pre-Dimetian gneiss 
of Craig yr Allor into relation with the schistose group W. of the 
Menai Straits, and proves that the dark gneiss is but the summit of 
a great gneissic series. 
* It will be seen that Prof. Bonney considers this a diorite, though his determina- 
tion is not very positive. There is apparent foliation in the rock, and I have left my 
observation to stand for what it is worth. ‘ 
