Abstract

Near Ironton, Missouri, measurements of the relative vertical intensity of the earth's magnetic field were made along an 1,800-ft line over an intrusive rhyolitic rock unit. Sharp variations of the order of 2,500 gammas were obtained along the sample line. These variations cannot be explained on the basis of induced magnetization alone. However, paleomagnetic measurements made on samples collected coincident to the line of magnetic measurements reveal great inhomogeneities in the magnetic properties: the magnetic susceptibility varies between 0.76 X 10 (super -3) and 2.03 X 10 (super -3) cgs units; the ratio of the intensity of remanent magnetization to the intensity of induced magnetization varies from 0.21 to 26.91; the direction of remanent magnetization varies from sample to sample and is significantly different from the direction of the present geomagnetic field; and the polarity of the samples is both normal and reversed. A theoretical magnetic profile, calculated on the basis of the direction of total magnetization in the individual samples along the sample line, has the same gross features as the measured profile. The improved correlation which results upon consideration of the total magnetization properties of the rock unit emphasizes the necessity for considering the effects of remanent magnetization when interpreting magnetic intensity measurements.

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