198 H. HICKS ON CAMBRIAN CONGLOMERATES IN 
basal conglomerate, from its position under the lowest purple slates, 
as worked in the Alexandra quarry, in the district. The con- 
glomerate is marked as altered Cambrian in the Survey maps. I 
examined the conglomerate also on the west side of the ridge about 
Glyn-Llifon, and there also recognized clearly that the conglomerates 
were mainly the result of the denudation of the Pebidian and quartz- 
felsite rocks of the area, with occasional fragments of Dimetian. 
That there may be no possibility 
of doubt as to the meaning of the 
interpretation given by the Sur- 
veyors in their latest remarks on 
the so-called intrusive masses 
marked on their maps which we 
claim to be of Pre-Cambrian age 
in Caernarvonshire, I quote the 
following explicit passage from the 
last edition of the Survey Memoir, 
vol. 13. p. 200 (1881) :—* On Twt 
Hill the rock consists of a mixture 
of felspar and quartz, forming a 
distinct binary compound such as 
was once called granitella, and is 
now often called aplite or granu- 
lite. But for the absence of mica 
it would be a true granite. 
‘‘ Near Brithdir, a mile and a 
half south of Menai Bridge, the 
continuation of the crystalline 
mass is a purple quartz-porphyry, 
and nearer the bridge a quartz- 
porphyry much resembling that of 
Llyn Padarn in the pass of Llan- 
beris. Between these points and 
Caernarvon, though much of it is 
a quartz-porphyry, it is not always 
easy to give it a definite name; for 
though still composed of felspar 
and quartz, it sometimes passes 
into a rudely foliated rock rising 
here and there in bosses through 
the glacial débris that fills the 
hollow. And yet though it never 
perhaps deserves the name of 
eneiss, to me it sometimes con- 
= veys the impression of stratified 
Ai recks in a very advanced stage 
of metamorphism, which for the most part may have originally 
consisted of the Cambrian strata that lie between Bangor and Dol- 
deilo, and perhaps of some of the Silurian strata that border it from 
Dol-deilo to Caernarvon. This may possibly be explained on the 
8.1, 
Llyn 
Padarn. 
(Scale 1 inch to 1 mile.) 
Clegir. 
Llys 
Dinorwig. 
1. Basal conglomerate of Cambrian. 
2. Cambrian grits and slates. 
3. Black flaggy beds 
Dinas 
Dinorwig. 
niolen. 
| 
H 
I 
| 
| 
I 
a. Pre-Cambrian. 
6. Cambrian. 
Ff. Faults. 
Llanddei- 
Fig. 2.—Section from N.W. of Llanddeiniolen to Llyn Padarn. 
