On some Pre-Cambrian (Dimentian and Pebidian) Rocks in Caernarvonshire

I n consequence of the remarks made by Prof. Hughes in the discussions 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 Caernarvonshire, to be Pre-Cambrian, and also of the statement made by Mr. Maw in the discussion of my paper read on Dec. 2, 1874, that under the Cambrian rocks at Llanberis there are unconformable beds which may be the equivalents of the Pre-Cambrian rocks 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 neighbourhood 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. to S.W. The adit passes through the north side of the mountain from the Alexandra slate-quarries in the Cambrian beds on the