In another place, in speaking of the trees in the Great Dismal Swamp, Lyell writes:—“When thrown down, they are soon covered by water, and keeping wet they never decompose, except the sap wood, which is less than an inch thick[99].” We see, then, that trees may have resisted decay for a sufficiently long time to allow of a considerable deposition of sediment. It is very difficult to make any computation of the rate of deposition of a particular set of sedimentary strata, and, therefore, to estimate the length of time during which the fossil stems must have resisted decay. [Sidenote: PLANTS AND COAL.] The protective qualities of humus acids, apart from the almost complete absence of Bacteria[100] from the waters of Moor- or Peat-land, is a factor of great importance in the preservation of plants against decay for many thousands of years. From examples of fossil stems or leaves in which the organic material has been either wholly or in part replaced by coal, we may pass by a gradual transition to a mass of opaque coal in which no plant structure can be detected. It is by no means uncommon to notice on the face of a piece of coal a distinct impression of a plant stem, and in some cases the coal is obviously made up of a number of flattened and compressed branches or leaves of which the original tissues have been thoroughly carbonised. A block of French coal, represented in fig. 13, consists very largely of laminated bands composed of the long parallel veined leaves of the genus _Cordaites_ and of the bark of _Lepidodendron_, _Sigillaria_, and other Coal-Measure genera. The long rhizomes and roots below the coal are preserved as casts in the underclay. In examining thin sections of coal, pieces of pitted tracheids or crushed spores are frequently met with as fragments of plant structures which have withstood decay more effectually than the bulk of the vegetable débris from which the coal was formed. The coaly layer on a fossil leaf is often found to be without any trace of the plant tissues, but not infrequently such carbonised leaves, if treated with certain reagents and examined microscopically, are seen to retain the outlines of the epidermal cells of the leaf surface. If a piece of the Carbonaceous film detached from a fossil leaf is left for some days in a small quantity of nitric acid containing a crystal of chlorate of potash, and, after washing with water, is transferred to ammonia, transparent film often shows very clearly the outlines of the epidermal cell and the form of the stomata. Such treatment has been found useful in many cases as an aid to determination[101]. Prof. Zeiller informs me that he has found it particularly satisfactory in the case of cycadean leaves. [Illustration: FIG. 13. Part of a coal seam largely made up of _Cordaites_ leaves. _Stigmaria_ and _Stigmariopsis_ shown in the rock (underclay) underlying the coal. (After Grand’Eury [82] Pl. I. fig. 3.)] [Sidenote: FOSSILS IN HALF-RELIEF.]