Discordant 'fission-track ages' on mica may be caused by (a) non-uniform uranium distribution, (b) addition or loss of uranium during the existence of the mineral, (c) thermal annealing of tracks, (d) inefficient etching of fossil tracks as compared to induced tracks in the mica, and (e) the effect of water pressure on track stability. The result of the combined effect of these factors causing discordant 'fission-track ages' on mica is at present unpredictable. Therefore, in the case of mica, interpretation of such ages is suspect.