The name jasperoid has been applied to rocks that consist mainly of silica and that have formed by replacement. This paper considers only those jasperoids formed by replacement of limestone. Major problems involved in the origin of such jasperoid include: source of the silica; nature of solutions that dissolve, transport, and precipitate silica; and the mechanism of replacement of limestone by silica. The answers to these problems are of practical as well as scientific interest because many jasperoid bodies are closely related to mineralization. Silica may be derived from: juvenile silica of magmatic origin; silica leached from underlying rocks by hydrothermal solutions; silica locally derived from enclosing rocks by circulating solutions; and silica carried downward in ground water from the weathering of overlying rocks. The nature and the concentration of other substances in solutions influence,in a complex manner, the ability of these solutions to dissolve, transport, and precipitate silica. Nevertheless, the following generalizations can be made. The solubility and rate of solution of silica in water at moderate pressure increase slowly with temperature up to 200 degrees C.; from 200 degrees to 360 degrees C. they increase rapidly; above 360 degrees C. solubility is pressure dependent, increasing steadily at high pressure and decreasing slightly at moderate pressure due to the formation of a vapor phase. The pH has slight effect on the ionic solubility of silica in the range from pH 1 to ph 9 at low temperature. The effect of other components on the solubility of silica is probably subordinate to that of temperature above 200 degrees C., but becomes increasingly important as the temperature falls below that point. Most jasperoid bodies form by both replacement and silica deposition in voids, with replacement dominant during the early phase, and precipitation dominant during later phases. Replacement of limestone by silica is favored by relatively low temperature acid solutions, and the presence of CO ² . As limestone dissolves, Ca ions are released to promote the precipitation of colloidal silica. Acid solutions then diffuse through this gelatinous film to continue dissolving limestone behind it; the Ca ions diffusing outward cause the precipitation of more colloidal silica at the solution-gel interface. As the gel mass ages, it shrinks, hardens, and ruptures. More silica is then deposited in the fractures. Eventually the gel crystallizes to a dense mass of aphanitic quartz and chalcedony, with shrinkage cracks and vugs filled or coated with younger coarse-grained quartz and other minerals that have been deposited directly from solution. The theory that relatively low temperature favors the formation of jasperoid replacement bodies in carbonate rocks, and high temperature inhibits their formation, offers an explanation for the gap that is observed in some districts between contact metasomatic lime silicates and siliceous replacement of limestone. This gap is characterized by the lack of any reaction between limestone and silica-bearing solutions moving through it.