Color differences in the thalli of Lecanora cascadensis Magn. correspond well with copper mineralization. This green-rimmed lichen was found associated with rocks possessing blebs of disseminated bornite or other evidence of copper mineralization. The intensity and the size of the lichen's green perimeter increase under the influence of copper mineralization; thus, 1,000 ppm and greater than 2,000 ppm anomalies can be distinguished. Apparently the lichen's anomalous green color is due to its ability to absorb and concentrate copper. Biogeochemical data support this hypothesis, as the ash of the anomalous green L. cascadensis assayed 4 percent copper. This concentration is approximately three times that of associated species collected at the same site. This evidence suggests that L. cascadensis could be a valuable geobotanical prospecting tool.