smooth as possible by flower of emery, freed from its coarser parts. The surface may then be polished by friction, with _crocus_ or rotten stone, on a transverse section of any soft wood."[45] [45] Mr. Witham, Observations on Fossil Vegetables. CHAPTER V. ON PEAT-WOOD, LIGNITE, AND COAL. [Illustration: Lign. 3. Nodule of Ironstone inclosing a Fern-leaf. _Coalbrook Dale._ Fig. 1.--The nodule in its natural state. 2, 3.--The same, split open longitudinally. The leaf remains attached to fig. 2, and the impression of its upper surface is seen on fig. 3. 4.--Outline of the form of the leaf, which is a species of Pecopteris. ] [Sidenote: PEAT-WOOD, LIGNITE, AND COAL.] Before entering upon the examination of the specific and generic characters of fossil plants, and the natural relations of the extinct forms with those of the existing Floras, it will be requisite to notice those vast beds of vegetable matter, in various states of carbonization, which occur in the palæozoic, secondary, and tertiary formations. Submerged Forests. Peat.--The phenomenon of extensive tracts of marsh-land, with layers of prostrate trees of all ages, lying but a few feet beneath the common alluvial soil, is of frequent occurrence, both inland, and in many places along the shores of our island. (_Geol. S. E._ p. 18). These submerged forests are generally situated below the level of the sea, and afford unquestionable proof of subsidences of the land. The trees are of the kinds indigenous to the districts in which they occur; and leaves and seeds of the hazel, beech, elm, &c. are often preserved in the silt in which the prostrate forests are imbedded. On the Sussex coast there are accumulations of this kind, at Bexhill, Pevensey levels, Felpham, &c. The extensive subterranean forests exposed in the Fens of Lincolnshire by the operations carried on for draining that district, must be familiar to those who travel by the Great Northern Railway: the protruding upright stems, broken off at a short distance above the primitive soil, will remind the geological observer of the petrified forest of the Isle of Portland. The wood in these cases has undergone no change but that of being dyed black, by an impregnation of solutions of iron; and many trunks are in so sound a state as to be employed in building. The oak timbers of the Royal George, lately raised up from off Portsmouth, after being immersed in silt about sixty years, closely resemble in colour and texture the wood of the submerged forests. Skeletons of deer, horse, swine, &c. are occasionally found imbedded in these subterranean accumulations of vegetable remains; and sometimes canoes, formed of the trunk of an oak, constructed by the aboriginal inhabitants of Britain, with stone implements called celts, are met with at considerable depths.