[49] See Dr. Bunker's Mon. Norddeutsch. Weald. Many interesting facts relating to the carbonization of vegetables, came under my observation during my researches in the Wealden strata; and it is a subject of regret to me, that circumstances prevented my following up the investigation of those still imperfectly explored deposits. Small nodular portions of coal, in which no structure is apparent, often occur in the calciferous grit of Tilgate Forest; and sometimes large masses of lignite, fissured in every direction, and having the interstices filled with white calcareous spar.[50] Some of the sandstones are discoloured by the abundance of minute particles of lignite, produced by the disintegration of ferns peculiar to the country of the Iguanodon. [50] A fine specimen of this kind is in the British Museum. The original structure and composition of a plant doubtless affected its carbonization; for in the same layer of stone, the stems of Endogenites, hereafter described, invariably possess a thick, outer crust, of coal; while those of Clathrariæ, plants allied to the Cycads, have not a particle of carbonaceous matter, but are surrounded by a reddish brown earthy substance. The nature of the stratum in which the plants were imbedded, must also have influenced the process of bituminization. Masses of vegetables buried beneath beds of tenacious clay, by which the escape of the gaseous elements set free by decomposition was prevented, must have been placed under the most favourable conditions for their conversion into lignite and coal. That the production of lignite is still going on there can be no doubt; and the following instance of a bed of recent origin, affords an instructive illustration of the subject. Near Limerick, in the district of Maine, one of the States of North America, there are peat-bogs of considerable extent, in which a substance similar to cannel-coal is found at the depth of three or four feet from the surface, amidst the remains of rotten logs of wood, and _beaver-sticks_:[51] the peat is twenty feet thick, and rests upon white sand. This coal was discovered on digging a ditch to drain a portion of the bog, for the purpose of obtaining peat for manure. The substance is a true bituminous coal, containing more bitumen than is found in any other variety,[AZ] Polished sections of the compact masses exhibit the peculiar structure of coniferous trees, and prove that the coal was derived from a species allied to the American fir. [51] Pieces of wood fashioned by the beavers for the construction of their dams. [52] An analysis of 100 grains gave the following results:--Bitumen 72; carbon, 21; oxide of iron, 4; silica, 1; oxide of manganese, 2; = 100. [Sidenote: COAL.]