The mineralogy of thirty-three samples of silicified wood was examined by X-ray diffraction, and each sample was found to consist of one of the following: opal-A, opal-CT, or quartz. The correlation between ordering of the silica phases and sample age suggests that the opal-A --> opal-CT --> quartz conversion sequence occurs in much the same manner and at approximately the same rates as in biogenic siliceous marine sediments. In addition, a poorly crystalline clay phase (illite?) was identified in most of these samples. Groundwater leaching of volcanogenic sediments may provide aluminum and other cations that are deposited simultaneously in wood tissue; consequently, silica recrystallization may be accompanied by authigenic clay formation.--Modified journal abstract.