ON SOME PRE-CAMBRIAN ROCKS IN CAERNARVONSHIRE. 147 



13. On some Pre- Cambrian (JDimetian and Pebidian) Rocks in 

 Caernarvonshire. By H. Hicks, Esq., P.G.S.* With a Note 

 by Thomas Davies, Esq., E.G.S. (Read December 5, 1877.) 



In consequence of the remarks made by Prof. Hughes in the dis- 

 cussions following the reading of my paper on the Pre -Cambrian 

 rocks of St. David's on Nov. 22, 1876, in which he expressed the 

 opinion that there was much reason for considering some beds 

 passed through in the adit to the slate-quarry at Moel Tryfaen in 

 i Caernarvonshire, to be Pre-Cambrian, and also of the statement made 

 1 >y Mr. Maw in the discussion of my paper read on. Dec. 2, 1874, 

 i hat under the Cambrian rocks at Llanberis there are unconform- 

 ible beds which may be the equivalents of the Pre-Cambrian rocks 

 v of St. David's, I determined last summer to examine these areas 

 for the purpose of seeing whether there was sufficient evidence to 

 support the above-mentioned supposition. 



I was assisted in these examinations by Prof. Hughes, Mr. 

 Hudleston, and Mr. Homfray, of Portmadoc, and am greatly indebted 

 to each for much kindly help and advice. We first visited the neigh- 

 bourhood of Moel Tryfaen, since it seemed probable that, by the aid 

 of the adit, the basal beds of the Cambrian would be seen in direct 

 contact with the underlying rocks. 



Moel Tryfaen. 



In the Geological-Survey maps the mountain of Moel Tryfaen is 

 coloured as unaltered Cambrian on the east side, and as altered 

 Cambrian on the west, the line of junction being marked by a great 

 fault running from N.E. toS.W. The adit passes through the north 

 side of the mountain from the Alexandra slate- quarries in the Cam- 

 brian beds on the east, through the altered beds, and then opens out 

 on the west side. 



The following were the beds seen from the opening inwards, as 

 near as could be made out with the aid of the rather imperfect light 

 at hand, some half dozen dip candles. At the entrance and for 

 some distance inwards we found a greenish, sometimes pinkish or 

 flesh-coloured, schistose rock containing disseminated quartz grains ; 

 further in, a more porphyritic-looking rock, mostly dark-coloured, 

 with spots of highly vitreous quartz, in a base of felsitic matter, un- 

 doubtedly a greatly metamorphosed rock, and probably of ashy origin. 

 About halfway in the tunnel we came upon some of the Lower 

 Cambrian beds in an unaltered state, chiefly red and purple slates. 

 About the junction of the Cambrian beds with the metamorphosed 

 series there appeared to be several faults, which have undoubtedly 

 reduced the thickness of the Cambrian rocks at this point, but have 

 not altered the general order of succession. At the entrance of the 

 adit to the quarry is a thick band of a chloritic and talcoid rock, one 



* For the discussion on this paper, see p. 165. 



