The fossil represented in fig. 1 (p. 10), from the collection of Dr John Woodward, affords a good example of a well-defined impression. The surface of the specimen, of which a cast is represented in fig. 1, shows very clearly the characteristic leaf-cushions and leaf-scars of a _Lepidodendron_. The stem was embedded in soft sand, and as the latter became hard and set, an impression was obtained of the external markings of the _Lepidodendron_. Decay subsequently removed the substance of the plant. [Illustration: FIG. 11. _Equisetites columnaris_ Brongn. From a specimen in the Woodwardian Museum, Cambridge. ⅓ nat. size.] In fig. 11 some upright stems of a fossil Horse-tail (_Equisetites columnaris_) from the Lower Oolite rocks near Scarborough, are seen in a vertical position in sandstone. On the surface of the fossils there is a thin film of carbonaceous matter, which is all that remains of the original plant substance; the stems were probably floated into their present position and embedded vertically in an arenaceous matrix. The hollow pith-cavity was filled with sand, and as the tissues decayed they became in part converted into a thin coaly layer. The vertical position of such stems as those in fig. 11 naturally suggests their preservation _in situ_, but in this as in many other cases the erect manner of occurrence is due to the settling down of the drifted plants in this particular position. [Sidenote: FOSSIL CASTS.] An example of _Stigmaria_ drawn in fig. 12 further illustrates the formation of casts[97]. The outer surface with the characteristic spirally arranged circular depressions, represents the wrinkled bark of the dried plant; the smaller cylinder, on the left side of the upper end (fig. 12, 2, _p_) marks the position of the pith surrounded by the secondary wood, which has been displaced from its axial position. The pith decayed first, and the space was filled in with mud; somewhat later the wood and cortex were partially destroyed, and the rod of material which had been introduced into the pith-cavity dropped towards one side of the decaying shell of bark. [Illustration: FIG. 12. _Stigmaria ficoides_ Brongn. 1. Side view, showing wrinkled surface and the scars of appendages. 2. End view (upper) showing the displaced central cylinder; _p_, pith, _x_, xylem, _r_, medullary rays. 3. End view (lower). From a specimen in the Woodwardian Museum. ½ nat. size.]