The Arvonian rocks of Arvon

Abstract

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.