The replacement of limestone by supergene copper minerals provides a key to some of the mechanisms of metasomatism at low temperatures and pressures. The replacements studied were of the type in which the host and guest have no ions in common. Although numerous physical and chemical variables were studied, pH is noted to be the major control of metasomatism. The reaction of hydrolyzed, slightly acid solutions provided replacement always essentially volume-for-volume and with duplication of textures. The replacement of limestone by sulfides probably is analogous. Access of mineralizing solutions again is shown to be a function of secondary permeability. Movement outward from secondary openings is indicated to be mainly by diffusion rather than by forced flow. Under special conditions an impenetrable replacement layer, resembling crustification, may form as a result of diffusional replacement. The replacement layer formed where forced flow is a major factor commonly is more irregular than layers formed by diffusion alone. Natural replacement textures resemble the synthetic. The composition of natural replacement products is more complex than those formed synthetically as a result of oscillation in ionic content of the natural solutions.