The experimental data available are sufficient to justify a tentative diagram for the system Or‐Ab‐An‐SiO2at 5,000 bars Ph2o. In this diagram the SiO2‐saturated plane is projected onto the Or‐Ab‐An face of the tetrahedron. When the normative feldspars of granitic rocks are plotted on this diagram they fall into a low‐temperature trough, whereas when they are plotted on the Or‐Ab‐SiO2diagram the majority fall into the Or field.