Note 56 — J. E. MARR, M.A., F.R.S. (continued 3)

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.