Petrographic, chemical, and X-ray study of indurated argillites associated with the Nelson ultramafic belt has indicated marked soda-metasomatism. The hardness of the argillites is due largely to albitisation, but additional toughness has been imparted, particularly in those specimens used by the Maoris for tool manufacture, by a felted crystallisation of minute tremolitic fibres. Hochstetter's hypothesis that the indurated argillites were included and altered by the ultramafites appears the most probable and is supported by comparison of the amphibolitisation of the argillites with nephritisation.