Petrographic examination of drill cores demonstrates that the flat-lying tuffaceous and arenaceous rocks of the Wairakei hot spring area, central North Island, New Zealand, have been and are still being altered by hydrothermal solutions. Interbedded argillaceous rocks remain unaltered. Alteration is progressive with depth and comprises a surficial zone of sulfuric acid leaching and zones of argillization, zeolitization, and feldspathization. The presence of an underlying zone of primary acid leaching extending down to the interface of the magmatic emanations is inferred.

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