Twenty-seven areas of basement outcrop in California were systematically examined in an attempt to correlate rock type and magnetic susceptibility with observed magnetic response. The investigation was performed by traversing selected areas of basement outcrop with a portable total intensity magnetometer, mapping lithologic contacts along the traverses, and sampling the different rock types for later petrographic identification and susceptibility determination in an inductance bridge.

In general, results from 275 samples support the established relationships between magnetic susceptibility and rock composition; i.e., acidic igneous rocks and metasediments are less susceptible than basic igneous rocks. However, magnetic susceptibility is primarily a function of magnetite and ilmenite content, and susceptibilities of individual samples within a given rock type were found to vary widely and in direct relation to the percentages of these minerals.

Good over-all correlation is shown between magnetic response and rock magnetic susceptibility, but theoretical susceptibility contrasts calculated from the magnetometer profile gradients are usually very much higher than the laboratory measured susceptibilities. This discrepancy is most probably due either to weathering effects on the surface rocks or to an insufficient number of samples. Serpentinite yielded the highest mean susceptibilities both by laboratory measurement and analysis of gradients measured in the field.

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