Summary: The compositions of the groundmass and of the plagioclase and sanidine phenocrysts of a series of over- and under-saturated trachytes indicate that plagioclase will precipitate at some stage in the crystallization interval of all calcium-bearing potassic trachytes. Using this data obtained from natural salic liquids, a concept of crystallization within the system CaA₂Si₂O₈-NaAlSiO₄-KAlSiO₄-SiO₂ may be obtained.