Fig. 11. Silicified bituminous wood. In this specimen the siliceous matter occurs in yellow semi-pellucid globules; the colour is supposed to have been derived from the bitumen.
The silicified woods delineated above, belong to the division which Mr. Parkinson denominated opaline; he conceived their peculiar characters to have resulted from an infiltration of fluid silex into the ligneous tissue, which, having previously undergone bituminization, was in a permeable state; hence originated the conchoidal fracture and peculiar resinous lustre which these specimens exhibit.
The specimen, fig. 7, Mr. Parkinson describes as corroborating the opinion that the ligneous tissues were converted into a bituminous substance, and subsequently impregnated with siliceous matter. "In that fossil there is a knot of wood which differs not the least in appearance from that in a recent piece, but it is perfectly impregnated with opaline silex. Is it possible that the change this knot has suffered could have been effected by an abstraction of the greater part or of the whole of its constituent molecules, and a substitution of particles of a different nature? Its hardness and closeness of texture oppose an insuperable bar to the supposition: whilst the mysteriousness of the change is entirely dispelled by admitting of the softening operation of bituminization, and consequent admission of silex in a fluid state."—Mr. Parkinson.
Plate III.
PLATE III.