Magnetic field time variations were observed in September 1967, with a two-dimensional array of 42 three-component variometers between latitudes 36" and 43" N and longitudes 101" W and 116" W. Fourier analysis of a polar substorm and of a storm shows that the former has a smooth spectrum and the latter a complex spectrum with many maxima. Upper mantle conductivity structure can be seen qualitatively in the original variograms, but is far more sharply defined in maps of Fourier spectral component amplitudes and phases. A ridge of high conductivity runs at a depth no greater than 200 km under the Southern Rocky Mountains between the Great Plains and the Colorado Plateau, which marks a low-conductivity region within the Cordillera. A strong conductivity anomaly runs north-south along the Wasatch Front through central Utah, and indicates the presence of an upwelling of highly conductive material at depth no greater than 120 km along the edge of a step structure which brings the conductive mantle to shallower depth under the Basin and Range Province than under the Colorado Plateau. Long-period maps from the storm suggest a rise in the conductive mantle between the northsouth structures, from the Colorado Plateau southward to the Basin and Range. The daily variation shows the conductivity structures and indicates their great extent in depth. The geomagnetic deep sounding anomalies are found to be in excellent agreement with existing heat flow data, and this supports correlation of electrical conductivity with temperature. There is also good correlation with the available seismic velocity information for the upper mantle.

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Heat flow was determined at sixteen stations regularly spaced over a region of the North American basin between Bermuda and the Bahama Banks. The results show a remarkable uniformity of heat flow in this area, which measures about 800 by 900 km. The mean heat flow at the sixteen stations is 1.14 μcal/cm2 sec, with a standard deviation of 0.06. When allowance is made for experimental errors the actual standard deviation of heat flow in the region is estimated to be only 4½ per cent. Another measurement made at the southern edge of this area, near the Puerto Rico trench, showed a heat flow of 1.76 μcal/cm2 sec, while three measurements made a few hundred km to the northwest, close to the continental slope, gave values of 117, 0.94, and 0.81 μcal/cm2 sec.

Two small sets of oceanic heat‐flow measurements are described. One is a line of six stations across the mid‐Atlantic ridge; the other is a pair of stations close together, which makes it possible to compare the results of measurement with a cylindrical probe and an outrigger apparatus. No high heat flows were found on the ridge, and there is a discrepancy of 34 per cent in the temperature gradient as measured with the different types of apparatus.