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PRE-CAMBRIAN ROCKS IN CAERNARVONSHIRE AND ANGLESEY. 295 
24, On the Pre-CamBriAN (DimeErrAn, ARVonIAN, and Pusrptan) Rocks 
in CAERNARYONSHIRE and AneLEsrY. By Henry Hicks, M.D., 
E.G.S. With an Arprnvix, by Prof. T. G. Bonney, M.A., 
F.R.S., Sec.G.S. (Read February 5, 1879.) 
Introduction. 
Wuewn exploring the districts in North Wales described in my 
paper communicated to the Geological Society in December 1877*, 
i visited several other areas, and observed, in some of these, facts 
tending to show that the Pre-Cambrian rocks had a far wider dis- 
tribution there than was then supposed ; and in a note to one of my 
papers I ventured to suggest the position which some of these, viz. 
those in Anglesey and South-west Caernarvonshire, were likely to 
occupy. With the hope of being able to arrive at a satisfactory 
conclusion as to the age and position of these, and also as to the 
true nature of some of the other great masses which were marked 
on the Geological Survey maps as intrusive in Silurian rocks, but 
which I had previously suspected to be of Pre-Cambrian age, I 
visited North Wales again last August, and was fortunate in being 
accompanied during most of the time by such experienced geologists 
as Professor Torell of Stockholm, Mr. Tawney, F.G.S., of Cambridge, 
Professor T. M‘Kenny Hughes, of Cambridge, and Dr. Sterry Hunt, of 
Montreal. Some of the facts to be brought forward in this paper 
were communicated to the British Association at the late meeting 
in Dublin ; but as the results at that time had not been fully worked 
up, and some even were obtained subsequent to that meeting, I 
have thought it advisable to bring this paper before the Society, 
as embodying not only the results then given, but many additional 
facts and observations. As I propose also to commence the descrip- 
tion with the immediately adjoining area to that treated of in my 
paper last year, it will appropriately appear as a continuation of that 
paper. 
Glynllifon and Craig-y-Dinas. 
Along the west side of the areas of Moel Tryfaen and Pen-y-Groes 
described in my former paper, there is in the Geclogical Survey 
map an extensive area coloured as altered Cambrian. It extends 
westward as far as Glynllifon, and the line to the south is marked 
as occurring at Craig-y-Dinas. On examining these rocks at various 
points, I could see nothing to indicate that they were altered Cam- 
brian beds; and on carefully tracing the line of junction between 
them and the unaltered Cambrian rocks, I found distinct evidence 
that the latter rested unconformably upon them, and hence the 
* Quart. Journ, Geol, Soc. vol, xxxiy. p. 147. + Op. cit. p63. 
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