Our subterranean exploration would be incomplete if it did not illustrate the Vegetable as well as the Animal Life of the ancient world. Let the reader then descend in fancy into the celebrated quarries of Portland on the south coast of England, and he will see the fossilized remains of a long past vegetation exhibited in a very striking manner. In one, of these quarries a vertical section, extending from the surface downward to the depth of about thirty feet, presents the following succession of strata arranged in horizontal layers:—first, a light covering of vegetable soil, beneath which are thin beds of cream-colored limestone, forming a stratum of solid rock ten feet thick; then a bed of dark-brown loam, mixed with rounded fragments of stone, and varying in thickness from twelve to eighteen inches. This is known to the quarrymen by the name of Dirt-bed, and seems, in former ages, to have supported a luxuriant vegetation; for all around are scattered the petrified fragments of an ancient forest. The prostrate stems and shattered branches of great trees are met at every step; but what is most striking and peculiar is, that, in many cases, the petrified stumps are still standing erect, with their roots fixed in the thin stratum of loam, and their trunks stretching upward into the hard limestone rock. Immediately below the Dirt-bed is another thick stratum of limestone, and below this again is a stratum of the famous Portland stone, so highly prized for building purposes. As the quarries of Portland are worked chiefly for the sake of this building stone, little attention is paid to the Dirt-bed and its contents, which are commonly thrown aside by the quarrymen as rubbish.
| Fig. 23.—Section of a Quarry in the Island of Portland. Total thickness about thirty feet. | Vegetable soil. Fresh-water Limestone. Clay. Laminated fresh-water Limestone. Dirt-bed with fossil trees and plants. Fresh-water Limestone. Bed of Clay. Portland building-stone full of marine shells. |
The scene of this petrified forest is thus described by Doctor Mantell:—“On one of my visits to the island the surface of a large area of the Dirt-bed was cleared preparatory to its removal, and the appearance presented was most striking. The floor of the quarry was literally strewn with fossil wood, and before me was a petrified forest, the trees and plants, like the inhabitants of the city in Arabian story, being converted into stone, yet still remaining in the places which they occupied when alive! Some of the trunks were surrounded by a conical mound of calcareous earth, which had, evidently, when in the state of mud, accumulated round the roots. The upright trunks were generally a few feet apart, and but three or four feet high; their summits were broken and splintered, as if they had been snapped or wrenched off by a hurricane at a short distance from the ground. Some were two feet in diameter, and the united fragments of one of the prostrate trunks indicated a total length of from thirty to forty feet; in many specimens portions of the branches remained attached to the stem.”[80]
The Coal Measures of Europe and America offer to the student of Geology a boundless field for the investigation of Fossil Plants and Trees. We have already had occasion to notice the Sigillaria. This ancient tree, remarkable for its beautiful sculptured stem, has no exact representative in the vegetable kingdom of the present day. But it abounds everywhere in the Coal Measures; and there seems little doubt that several great seams of Coal are composed almost entirely of its carbonized remains. Indeed the ancient soil, which commonly constitutes the floor on which the bed of Coal reposes, is often as thickly crowded with the branching roots of the Sigillaria, as the soil of a dense forest with the roots of the trees by which it is covered. The stem itself, when converted into Coal, generally assumes the form of long narrow slabs; having been flattened by pressure during the process of mineralization. Sometimes, however, it is found uncompressed and erect. In this case the interior of the trunk is usually observed to have been filled up with sand or clay: and thus the forest tree, still retaining its external shape and character, is transformed into a cylindrical shell of carbonized bark without, and a solid cylinder of sandstone or shale within. An interesting example is exhibited in our illustration, Figure 11.
Every Coal mine, too, is adorned with the imprint of the graceful Fern, which constitutes one of the most attractive features in the Flora of the ancient world. Not unfrequently it assumes a tree-like character, as it often does even now in tropical countries; and then, indeed, it is an object of striking beauty, reaching to a height of forty or fifty feet, and expanding at the summit into an elegant canopy of foliage.
Fig. 24.—Calamites Nodosus. From the Coal Measures of Newcastle.
The Calamite is another plant in which the Coal abounds. Its true botanical character is not yet clearly ascertained; but it bears a general resemblance, except for its gigantic dimensions, to the common Horse-tail of our swamps and marshy grounds. It is a reed-like, jointed stem, sometimes thirty feet in length, hollow within, and curiously jointed without.
Fig. 25.—Lepidodendron Sternbergii; a Fossil Tree, 39 feet high. From a Coal Mine near Newcastle.