Susceptibility determinations have been made on 200 outcrops of 11 rock types, using a three-coil induction instrument. Measurement is effective over a hemisphere of 50 cm radius. Data are given to show that the instrument is much less sensitive to rock surface irregularities than any previously described. Measuring circuits consist of 975 cycle oscillator, mutual inductance bridge, and two-stage amplifier. Investigation of errors shows that surface irregularities remain most important. Calibration by calculation is checked against ferric chloride standards through small samples taken at each outcrop. Data are given to show: 1) rock variability makes the check inconclusive, 2) variation of susceptibility within outcrops for gabbro and basalt is of the same order as variation between outcrops, 3) small-sample measurements show greater variation than large-sample measurements.

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