A high-sensitivity superconducting magnetometer has been used to measure geomagnetic activity at Stanford, California, in the 0.1- to 14-Hz frequency range. The measurements, which covered the 2-month interval January 26 to March 26, 1974, consistently show a minimum of activity in the interval 3--7 Hz, where conventional measurement systems are likely to be limited by their internal noise. At frequencies below the minimum, in the range 0.1--3 Hz, there is typically a monotonic decrease of background activity with frequency f. The variation is approximately of the form f-1.25 for the Pc 1 frequency range (0.2--5 Hz). At frequencies above the minimum, in the range 7--14 Hz, the activity is dominated by the first Schumann resonance. The peak amplitude of this activity is usually low in comparison with the amplitude of the background activity at 1 Hz. No splitting of the first Schumann resonance peak into a triplet could be detected, but a doublet structure was observed on 7 days in the 2-month measurement interval.