In the extreme northern part of North Island, New Zealand, keratophyres occur interbedded with spilites. Petrographic study reveals textures indicative of a high-temperature, extrusive origin. The keratophyres are thought to have been partly glassy flow-rocks and breccias although they are nowmade up of a low-temperature mineral assemblage. Some have been enriched in soda and others in potash. No special magma is postulated, nor the introduction of alkalis from outside. Deep burial and sufficiently high temperature to allow recrystallization and mobility of alkalis and silica are the conditions which brought about the changes. Chemical analyses are included.