V.—On the Base of the Sedimentary Series in England and Wales

Abstract

North-west Carnarvonshire.—There are here two or three separate areas which, at all events historically, must be considered more or less independently, viz. the district between Bangor and Carnarvon, the Llyn Padarn and Moel Tryfaen range, and the Lleyn Peninsula. Except in reference to the last of these, it is certain that the belief in Precambrian rocks in Carnarvonshire began with Prof. Hughes. This author, indeed, quotes Prof. Sedgwick as recognizing such rocks here, but the passages he quotes have a very different meaning in the original. The slates near Bangor and Carnarvon, which Sedgwick says are amongst the oldest of North Wales, are those which occur “along the shores of the Menai Straits from Bangor to Carnarvon,” which he describes as “dark earthy-coloured slates which, were we to judge only by mineral structure, might easily be confounded with Upper Silurian rocks,” and these are “cut through by a great intrusive rib of syenitic porphyry of a different epoch, which ranges nearly with the beds.”