Altered tuff having the composition of quartz keratophyre is intercalated within a Jurassic formation composed dominantly of black marine mudstone near Izee in central Oregon. The tuff was deposited originally as a thin yet extensive accumulation of rhyodacitic vitric ash. Shortly after burial, the ash was devitrified and altered, with loss of soda and acquisition of lime, to heulandite and associated diagenetic minerals. After deep burial, heulandite and volcanic plagioclase were recrystallized locally to laumontite. Still later, soda metasomatism that was apparently accomplished by connate waters expressed from nearby mudstones by compaction, and that was accompanied by dehydration and leaching of lime, magnesia, and potash, converted most of the tuff to quartz keratophyric felsite composed dominantly of albite and quartz. Local potash enrichment was a terminal(?) phase of the metasomatic alteration. Relict textures and stratigraphic continuity clearly demonstrate the sequence of alteration.