[Footnote 56: The Pembrokeshire area is of interest as the probable existence of Precambrian rocks in Britain was first indicated on good evidence in this county. The general structure of the district is fairly simple, consisting of Cambrian rocks beneath which Precambrian rocks are exposed in at least two ridges of which the northerly and more important one runs through St Davids. The rocks of the St Davids ridge consist of a binary granite (granitoidite), felsites, and volcanic ashes and breccias of intermediate composition. Much diversity of opinion has existed, and to some extent still exists as to questions of detail, and a very extensive literature has been devoted to these rocks. Amongst the numerous papers which treat of them, the student may consult the following:--Hicks, H., _Quart. Journ. Geol. Soc._, vol. XXXIII. p. 229, XXXIV. p. 147, XXXV. p. 285, XL. p. 507, XLII. p. 351, Geikie, A., _ibid._ vol. XXXIV. p. 261, Blake, J. F., _ibid._ vol. XL. p. 294, and Morgan, C. Ll., _ibid._ vol. XLVI. p. 241. Much of the matter contained in these papers is controversial, and need not be fully read by those who merely wish to obtain a general account of the rocks of the district.] In Caernarvonshire two ridges are found, the one running from Bangor to Caernarvon, and the other through Llanberis lake. The rocks of these are generally similar to those of St Davids, and as the lowest Cambrian rocks of the area closely resemble those of St Davids, the Precambrian age of the rocks of these ridges is rendered highly probable, though until the discovery of the _Olenellus_-fauna in the area, it cannot be regarded as proved[57]. [Footnote 57: These rocks are described by T. M^{c}K. Hughes, _Quart. Journ. Geol. Soc._, vol. XXXIV. p. 137, and XXXV. p. 682; by Prof. T. G. Bonney, _ibid._ vol. XXXIV. p. 144; and by Dr Hicks, _ibid._ vol. XXXV. p. 295.] The actual position of the similar rocks of Anglesey has not been so clearly fixed, as the rocks associated with them are of Ordovician age, but their resemblance to the rocks of the adjoining regions renders their Precambrian age highly probable. It is interesting to find in association with the rocks which resemble those of Caernarvonshire, others which Sir A. Geikie recognises as quite similar to some existing amongst the crystalline schists of the north-west Highlands of Scotland, and when these ancient rocks of Anglesey have been mapped in detail, they will probably be found to present greater variety than is afforded by any Precambrian rocks of Great Britain occurring S. of the Scotch border[58]. [Footnote 58: Papers upon the old rocks of Anglesey will be found in many volumes of the _Quarterly Journal of the Geological Society_; see especially Hicks, vol. XXXV. p. 295, Callaway, vol. XXXVI. p. 536, XXXVII. p. 210, and Blake, XLIV. p. 463.]