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_. [Illustration: Plate III.] PLATE III. Petrified Stems and Leaves. Fig. 1. A portion of the trunk of the fossil vegetable called _Stigmaria ficoides_ (of M. Alex. Brongniart); it is the root of a tree common in the coal deposits; see _Supplementary Notes_, Art. _Stigmaria_, p. 198, for a description of the nature and mode of occurrence of these fossils. Fig. 2. Impressions of dicotyledonous leaves in travertine; a modern calcareous deposit; from Campania.[8] [Footnote 8: Medals of Creation, vol. i. p. 193.] Fig. 3. Part of the stem of a reed-like plant (_Calamites dubius_, Brongniart); from the coal deposits of Yorkshire. See description of _Calamites_. Fig. 4. Appears to be a fragment of the stem of a species of _Lepidodendron_. Fig. 5. Fragment of the leaf of a Cycadeous plant, from the oolite of Stonesfield. (_Zamia pectinata._) Fig. 6. Portion of an ironstone nodule, split asunder, showing part of the terminal branch of a _Lepidodendron_, from Coalbrook Dale. See description of _Lepidodendron_. Fig. 7. "A pebble that appears to have been partly enveloped in a leaf while in a soft state, which has produced the markings on its surface."--_Mr. Parkinson._ Fig. 8. "Ligniform pitchstone;" fossil wood having a resinous transparency; supposed by Mr. Parkinson to have originated from an intermixture of silex and bitumen; the internal part is opalized. Fig. 9. Fragment of calcareous coniferous wood from the Lias of Charmouth, Dorsetshire: the vegetable structure is well preserved. [Illustration: Plate IV.] PLATE IV. Fossil Fern Leaves. Figs. 1, & 2. An ironstone nodule, split asunder, showing an inclosed fern-leaf (_Alethopteris lonchitidis_, of Sternberg); from the coal-beds of Newcastle. Figs. 3, & 4. The corresponding parts of another nodule, containing a fern-leaf of a different kind (_Neuropteris_).