Rocks that were once present in the deep continental crust can be found in surface exposure as xenoliths, as parts of high-grade metamorphic and plutonic terrains, and as abducted lower crustal cross sections. A review of magnetic and mineralogic studies of samples representing all such occurrences shows that magnetite is the only magnetic phase present in significant amounts in these rocks. Thermodynamic considerations indicate that Ti-poor magnetite is the expected magnetic oxide under equilibrium conditions that exist in the lower crust, regardless of the primary magmatic composition of the spinel phase. Furthermore, it seems unlikely that serpentinites with metal alloys contribute significantly to the magnetization of the lower crust or mantle. Thus magnetite in the deep crust is most likely candidate for the source of the long-wavelength magnetic anomalies observed in regional aeromagnetic and satellite magnetometer surveys. If so, the entire crust should be magnetic in most continental areas, including shields. In areas of high heat flow the bottom of the magnetic crust may lie considerably above the Moho.