Lichens are conspicnons pioneers on rocks, and frequently initiate stages of snccession (8). Their seniority as pioneers is illustrated by the cosmopolitau distribntion of certain species (14). It \vas shown that many lichenic acids can complex appreciable ainounts of cations from powdered minerals and rocks (9,10) and also it has been demonstrated that certain lichenic compoimds can altor minerals mider laboratory conditions at the striictural level, giving rise to new minerals (2). The role of licliens in pedogenesis, as related to their ability to colonise rock siirfaces has been studied (13) and more recently it has been demonstrated that lichens can induce chemical and morphological changes in the mineral composition of certain rocks (granite and gneiss) in the zone just below a lichenic thallus (3). Kecause of the importance of lichen-rock interactions for the formation of primitive soils from altered rock minerals and particles of the lichenic thalli, it seems interesting to continué investigating the alteration capabilities of lichens on ernptive rocks and to extencl such stiidies to the alteration of methamorphic rocks.

We wish to thank Dr. L. GARCÍA-CACHO for his contribution regarding petrography of the rocks and to ilr. F. PINTO and Mrs. T. CARNOTA for technical assistance.

14 pages, figures, and tables statistics.

Peer reviewed