Secondary silicification of country rock is commonly observed in many geological environments, and it is generally considered that this metasomatism was effected by hydrothermal siliceous solutions permeating the host rock. Silicification may be localized in comparatively narrow zones alongside the channelway used by the solutions, or it may result in a regional alteration of a large mass of rock. The former type of occurrence is commonly observed alongside veins of presumed hydro-thermal origin (1, 2) whilst the latter is sometimes seen in rocks, e.g. silicified limestones (1, 3) with no evidence of a path followed by the solutions. In such cases a mechanism of sub-capillary soaking must be assumed; the solutions may act selectively by favouring certain bands or types of rock.