682 PROF. T. MSKENNY HUGHES ON THE PRE-CAMBRIAN 
52. FurtTHER OBSERVATIONS on the Pre-Camprian Rocks of CaEr- 
narvon. By Prof. T, M*Krnny Hugues, M.A., F.G.S. (Read 
May 14, 1879.) 
[Puate XXXVI. | 
On a former occasion (Quart. Journ. Geol. Soc. vol. xxxiy. p. 137) 
I offered a sketch of the Pre-Cambrian rocks which form the long 
ridge stretching from Bangor to Caernarvon. The classification 
I adopted was in descending order :— 
i. The Volcanic series, consisting of voleanic fragmentary ejectamenta, 
agglomerates, ash-beds, and slates, occurring near Bangor, and referred 
to as Bangor beds. 
ii. The Felsitic series, consisting chiefly of quartz felsites, and probably also 
of volcanic origin, occurring near Llanddeiniolen and Dinorwig, and 
now referred to as the Dinorwigq beds. 
iii. Zhe Granitoid series, occurring near Caernarvon, and referred to as the 
Caernarvon beds. 
Endeayouring to correlate these groups with the divisions pro- 
posed by Dr. Hicks, I considered that the lowest group included less 
than the Dimetian, from which I proposed to cut off every thing 
above the brecciated bed below Bryngarn, on the left of the valley 
running down from St. David’s to Porthclais. The upper group 
I divided into two. I do not know how far Dr. Hicks, in the 
modified classification recently offered to the British Association 
(Report, 1878, Trans. Sect. p. 536), adopts the above threefold 
classification. I considered that there was no unconformity visible 
between the groups, whether we accepted the brackets drawn by 
Dr. Hicks or those which I proposed. 
Since this paper was first sent in much additional information has 
been brought forward by Professor Bonney, Dr. Hicks, and Mr. 
Houghton, in communications made to the Society, Feb. 5, 1879. A 
threefold division was adopted; but the authors disagreed entirely 
with the views I had expressed of the structure of the districts as 
regarded dips, strikes, faults, and the sequence of rocks, and affirmed 
the existence of a visible unconformity between the groups *. 
I shall now describe the rocks of the district in ascending order. 
I. The Caernarvon Beds (= Dimetian in part). 
ii. Upper Caernarvon = Crug Beds. 
i. Lower Caernaryon = T'wt-Hill Beds. 
I. i. Zhe Twt-Hill Beds. 
The coarse crystalline rock of Twt Hill is exposed over a small 
area on Twt Hill and in quarries along the north side of it. In a 
* Having gone over the ground again with my friend Mr. Tawney, I am able 
to offer a fuller description of the district, and to bring forward much addi- 
tional evidence on those points respecting which there has been a difference of 
opinion. 
