Twelve magnetotelluric soundings were made along a 460-mile traverse in the southwestern United States. Spectral analysis of digitized graphic records was used to determine apparent resistivities and phase differences for periods in the range 10-1,000 sec. The geoelectric section to depths of the order of 300 km was found to be divided into three principal layers. The layer nearest the surface has a resistivity of 200 ohm-m or less. The next layer, which begins in Paleozoic carbonate rock or near the top of the Precambrian rocks, has a resistivity of 500 ohm-m or more. The resistivity of the lowest layer is in the range 5-100 ohm-m and the depth to the top is in the range 20-150 km.

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