Search for Tracks of Massive, Multiply Charged Magnetic Poles

Massive magnetic monopoles of $\mathrm{charge}\ge \frac{\hslash c}{e}$ (twice Dirac's value of the minimum pole strength and half of Schwinger's) would leave tracks in natural minerals, provided they arrive at the earth with sufficient energy. In order to test Porter's hypothesis that the cosmic-ray particles of energy > ${10}^{17}$ eV are (or include) magnetic monopoles, samples of mica and of obsidian have been scanned for the distinctive, long particle tracks which would have been stored over geological times. The failure to find monopole tracks sets an upper limit (90% confidence) of 1.2×${10}^{-19\mathrm{}}$ monopoles per ${\mathrm{cm}}^2$ sr sec for penetrating monopoles incident upon the earth and rules out Porter's hypothesis for the charge and mass range within which poles would have been detected.