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The weathering phenomena resulting from the encrustation of basalt by Pertusaria corallina have been studied by scanning electron microscopy and a variety of other techniques. The rock consists largely of plagioclase felspar (labradorite) and ferromagnesian minerals, the latter often being replaced by hydrothermally formed ferruginous clay minerals. Lichen weathering results in the extensive etching of the primary rock-forming minerals, particularly labradorite, and in the degradation of the clay minerals to yield a thin ochreous crust of poorly-ordered ferruginous (ferrihydrite) and alumino-silicate materials. These changes are brought about principally by the oxalic acid secreted by the mycobiont, a conclusion supported by observations following experimental mineral alteration.