A 2000-gamma anomaly at Cottoner Mountain, Madison County, Mo., is caused by thermoremanently magnetized black Precambrian rhyolite contained in a volcanic neck. The black color of the rhyolite is imparted by tiny needles of magnetite which may approximate single-domain grains. The direction of remanent polarization in the rhyolite is S. 30 degrees W., inclined 35 degrees , and the remanent component is approximately 40 times greater than the natural susceptibility magnetization. A total intensity curve calculated on the assumption that the black rhyolite body is an inclined dike of limited depth extent is a good fit to the observed anomaly. Irregularities in thesurface magnetic properties were probably caused by lightning and were acquired after the thermoremanent magnetization.

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