The effect of magnesium on the relative stabilities of ferrosilite-rich orthopyroxene and fayalitic olivine + quartz has been determined with reversed experiments in piston-cylinder apparatus. Reaction reversals to within 0.2 kbar indicate instability of Fs95En5 below 9.8, 11.0, and 12.4 kbar, Fs90En10 below 8.2, 9.5, and 10.8 kbar at 800°, 900°, and 1000°C, respectively; Fs85En15 below 7.8 and 9.1 kbar, and Fs80En20 below 6.1 and 7.3 kbar at 900° and 1000°C, respectively. Fa95Fo5 + quartz and Fa90Fo10 + quartz react to orthopyroxene + olivine + quartz above 7.4, 8.7, and 10.0 kbar and 5.4, 6.7, and 8.1 kbar at 800, 900 and 1000°C, respectively. Electron microprobe analyses of run products indicate at 900° and 1000°C and 2.2 ± 0.7 at 800°C. Each mole % MgSiO3 extends orthopyroxene stability by ~0.34 kbar, relative to FeSiO3 (Bohlen et al., 1980a). These data combined with data for manganese (each mole % MnSiO3 extends orthopyroxene stability by ~0.12 kbar, Bohlen et al., 1980b) allow inference of equilibration pressures for Fe-rich orthopyroxene and orthopyroxene–quartz–olivine bearing assemblages in numerous locahties throughout the world. Pressures determined from assemblages in the Adirondack Mountains, New York, southern Labrador Trough, Canada and Lofoten-Vesterålen, Norway indicate that thick (~70 km) continental crust was not uncommon mid-to-late Proterozoic.