Poor Knights Islands, at the north-west extremity of an Upper Cenozoic chain of rhyolitic volcanoes, consist of tuffs and breccias with minor interbedded flows and sediments. Intense hydrothermal alteration has resulted in almost complete replacement of primary minerals by quartz, adularia and albite, minor sericite, cristobalite, pyrite, leucoxene and sphene, and retrograde smectite and kaolinite. The alteration took place at temperatures up to about 240°c and resulted in addition of silica and potash and the leaching of lime. Except for the near absence of calcium bearing minerals and the presence of retrograde minerals, the alteration is very similar to that occurring in the active thermal areas of Taupo Volcanic Zone today.