Notes on the Rhyolites of the Hauraki Goldfields (New Zealand)

Abstract

N ew Zealand , during the Secondary period, seems to have been singularly free from volcanic disturbances, but after the advent of the Eocene it again became the scene of outbursts which have prevailed, with only short periods of cessation, up to the present time. The eruptions of Tertiary date were generally more widespread than those of recent times, their ejections sometimes affecting many hundreds of square miles. The ejections of the early Tertiary eruptions were principally basic; those of the middle Tertiary, semi-basic; of the older Pliocene, acidic; and of the newer Pliocene and recent, acidic, semi-basic, and basic. The Pliocene acidic eruptions devastated a wide area in the central portion of the North Island, smothering thousands of square miles with rhyolitic lavas, tuffs, and pumice, which now form barren plains and bare, flat-topped ranges. The chief centres of eruption lay in the Taupo, Rotorua, and Tauranga districts and the Hauraki peninsula. It is in connexion with the rhyolites of the last-named district that these notes are written. The Hauraki peninsula is situated in the province of Auckland, and lies between the Firth of Thames, on the west, and the Pacific Ocean on the east. Altogether it covers an area of about 1,000 square miles. Its surface is diversified with rugged, forest-clad mountain-ranges, cut by deep ravines. The basement-rocks consist of Palaeozoic (probably Devonian) slates and grauwacke, which generally occur in rapidly alternating thin beds or layers. Near Cabbage Bay a small isolated patch of richly fossiliferous marly clays, and limestone