The Arvonian rocks of Arvon

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Abstract
Summary: The Arvonian formation consists, in situ, of a granite, a quartz-rhyolite, and a pyroclastic series, but other members were formerly present. The granite, very coarse at Caernarvon, 1 slowly grows finer eastward, until it graduates into the rhyolite. There are coarse agglomerates, and three groups of finer tuffs. In all, nine members can be distinguished, and there is an order of superposition. Is the formation Cambrian, or is it somewhat older, with a measure of unconformity ? Balance of evidence seems to be in favour of unconformity. Correlation is considered, though very cautiously, with Uriconian and Pebidian. The term " Arvonian " was proposed by the author in 1930 ( Geol. Mag. lxvii, 1930, p. 287) to denote the ancient volcanic series of the country between the Menai Strait and Snowdonia, which comprises the well-known rhyolite of Llyn Padarn, the rhyolite between Bangor and Caernarvon, and the pyroclastic rocks of Bangor itself. When originally described, the formation was referred to as the " Bangor Volcanic Series ", but in the course of the recent detailed survey it was found to be very much more extensive than was formerly recognized, occupying indeed as much of Arvon as its other three major formations combined (Fig. 1). The thickness of the Arvonian formation is great, and its importance in the early volcanic history of Britain is second only to that of the Mona Complex. The formation appears to be entirely igneous, for no sedimentary rocks have been found. In situ at the present time it

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