Cosmic ray muon-induced 26Al was measured in terrestrial silicate rock by low-level &ggr;&ggr; coincidence spectrometry with improved statistics. A positive result was obtained for the first time. The measured activity of 0.0068¿0.0028 dpm 26Al/kg SiO2 is well reproduced by a theoretical estimate of the production rate at a depth of 5 meters water equivalent underground. The result is discussed with respect to the erosion rate of the geological sample during the last millions of years.