Solution channels in quartz, microcline, and plagioclase of a granite were produced by exposure of the rock to an artificial hydrothermal system. Electron microprobe analyses, beam-scanning photographs, and element profiles show that the mineral grains are homogeneous and unaltered within a few micrometers of the channels. The channels, which are 3 to 7 µ in diameter, appear to have formed by removal of material in solution. In the sample material, no new minerals produced by alteration or deposition were detected by microprobe analysis. The apparent absence of such alteration products (e.g., clays) may be due to (1) sample preparation which removed fine-grained alteration products during polishing; or (2) concentrations of such minerals in such small amounts that they are below the limits of detectability with the microprobe. The granite is seen to be preferentially attacked by the hydrothermal solution along grain boundaries and cracks. Similar solution effects are observed in amphibolites exposed to the artificial hydrothermal system. Plagioclase crystals in the amphibolite are partially dissolved so that delicate porous boxwork structures remain