Phase-equilibrium relations have been determined at 1000 kg/cm² water pressure for compositions within the system NaAlSi₃O₈-KalSi₃O₈-NaAlSiO₄-KAlSiO₄ in the area adjacent to the temperature minimum. The composition and temperature of the minimum are Ne₅₀Ks₁₉Qz₃₁ and 750° ± 7° C respectively. The compositions of 102 plutonic rocks and 122 extrusive rocks, from Washington's tables, that carry 80% or more of normative Ab + Or + have been plotted; the areas of high density show a marked similarity to the positions of the low-temperature regions of the synthetic system and suggest that many undersaturated rocks are derived by fractional crystallization from a trachytic magma.