Magnetic susceptibility is a technique useful for differentiation of magnetite-rich sandstones from those deplete in magnetite. Analyses of fluvial sandstones of the Upper cretaceous La Bocana Roja Formation near El Rosario, Baja California, yielded susceptibility values ranging from 50 X 10 to 700 x 10s, averaging 320 X 10 SI units. These values indicate that the sandstone is rich in magnetite, and the values are similar to those of the magnetite-rich western zone of today's Peninsular Ranges batholith. The submarine canyon sandstones of the overlying Upper Cretaceous (Campanian) Punta Baja Formation show an order of magnitude drop in magnetic susceptibility. These values range from 5 X 10 to 50 X lOs, averaging 20 X ls SI units. The difference in susceptibilities suggests a change in provenance; the values for the Punta Baja Formation resemble the present-day values of the eastern, ilmenite-rich zone of the Peninsular Ranges batholith.