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Resistivities in the earth's crust have been evaluated by direct observation using dipole-dipole resistivity soundings and magnetotelluric resistivity methods, supplemented by laboratory measurements of the electrical properties of appropriate rock types at elevated temperatures. Assuming a three-layer crustal model, the field observations indicate an attenuation of one to ten decibels per kilometer in the resistant middle layer. It is pointed out, however, that the resistivities in this zone are biased toward the lower side. Higher resistivities are expected on the basis of the laboratory experiments and if they occur to any significant extent, lower transmission losses would be obtained than are predicted by the field measurements.