The electrical conductivity of a synthetic basalt has been determined at atmospheric pressure in air at temperatures between 700° and 1600°C. The conductivity increases by about 2 orders of magnitude in the melting interval between 1130° and 1263°C. It is suggested that partial fusion in the seismic low‐velocity zone of the upper mantle may account for the correlation of increased electrical conductivity with development of this zone.

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.

Time-varying magnetic fields were recorded during the summer of 1969 with a two-dimensional array of 46 variometers in the north-western United States and south-western Canada between latitudes 44" and 51" N and longitudes 100" and 121" W. Magnetograms and maps of Fourier spectral components of three magnetic events are used to describe conductive structures in the upper mantle and crust. The most prominent of the anomalies are found in the North American Central Plains and over the Northern Rockies. The Central Plains anomaly runs from the eastern edge of the Black Hills northward along the boundary between Montana and the Dakotas to the Williston Basin. Its large magnitude, with anomalous fields larger than the normal fields, and its small half-width indicate a crustal conductor which concentrates current induced in a large region. In the western part of the array, attenuation of the vertical variation fields is attributed to a westward rise in the highly-conducting mantle. Two small anomalies in the vertical and horizontal fields, over the eastern front of the Northern Rockies and the Rocky Mountain Trench, may be associated with ridges or steps on the upper-mantle conductor or with crustal features.

The variation field of a polar substorm was recorded by an array of 42 variometers in the western United States. This field has been separated by surface integral methods into parts of external and internal origins, at four times in the time domain and at four periods in the period domain. It is shown that the anomalies in the vertical and east-west horizontal variation fields are of internal origin and that the external fields vary smoothly over the array. The separated fields in both domains show internal currents induced in north-south striking conductive structures by the east-west horizontal field. Phase differences between the normal and anomalous fields are about 30°, and indicate large, highly-conducting structures in which self-induction controls the currents. The in-phase normalized anomalous fields at period 89 minutes have been approximated by two-dimensional models made up of upheavals of semi-circular section and a step in the surface of a perfectly-conducting half-space. A semicircular upheaval of radius 150 km from an unperturbed level at depth 360 km models the anomaly related to the Southern Rockies. A step of height 120km at the Wasatch Front, together with a semi-cylinder of radius 100 km under the Wasatch fault belt, give a good approximation to the observed anomaly at the Wasatch Front. The actual structures may be somewhat shallower and smoother. The real conductivity is estimated at 2-10-12 e.m.u., a value which would be expected at temperatures near 1500 "C. Such temperatures are reasonable at the depths concerned.