CHAPTER IV.

FOSSIL VEGETABLES.

THE remains of the vegetable kingdom are presented to the notice of the geologist, in various conditions; in some instances but little changed in their aspect, as in the recent accumulations of mud and silt, at the bottoms of lakes and rivers, and in morasses, and peat bogs; in tufaceous incrustations, as decayed wood, with the imprints of the leaves and stems, preserved on the solid masses of concretionary, or crystalline limestone.

In the ancient deposits, vegetables are found in two different states. In the one their substance is completely permeated by mineral matter; it may be calcareous (lime), siliceous (flint), ferruginous (iron), or pyritous (sulphuret of iron); and yet both the external characters, and the internal structure be preserved. Such are the fossil trees of the Isle of Portland, fragments of which so closely resemble decayed wood, as to deceive the casual observer, until by close examination of their texture and substance he finds that they possess the weight

and hardness of stone. In the silicified wood (that is, petrified by silex or flint,) which abounds in many of the tertiary strata, the most delicate tissue of the original is generally preserved, and by microscopical examination (see Pl. V.) may be displayed in the most distinct and beautiful manner. Calcareous wood also retains its structure; and in many limestones, leaves and seed-vessels are well preserved.

The ligneous coverings, or the husks and shells of nuciferous fruits, and the cones or strobili of firs and pines, are frequently in an excellent state of preservation; and in some rare instances indications of flowers have been observed. The parts of fructification, in some of the fern tibe, (Lign. 18 and 20.) occur in coal-shale, and in the grit of Tilgate Forest (Wond. p. 372.): and even the pollen of coniferæ has been found in tertiary marls, associated with animalculites.* The resinous secretions of pines and firs, are also found in a mineralized state. Amber is too well known to require further notice in this place, than that its vegetable nature is unquestionable; this substance has been observed in its natural position, in trunks of coniferæ. (Wond. p. 637.) The fossil resin of the London clay, discovered at Highgate, and the Isle of Sheppey, has had a similar origin. In the Clathraria (Wond. p. 374.) of Tilgate Forest, indications of a resinous

*Animalculites-fossil animalcules.

secretion have been detected. The diamond, which is pure charcoal, is probably a vegetable secretion, that has acquired a crystalline structure by electrochemical action (Wond. p. 638.). When the microscope is more extensively employed in investigations of this kind, it is probable that the siliceous spines and stars, so abundant on the foliage of many plants (as the Deutzia, Lithospermum officinale, &c.) will be found fossil, for they are as indestructible as the spines of sponges and other animal remains, so commonly imbedded in flint and chalcedony.

But vegetables occur not only as petrified stems, leaves, and fruits, associated with other remains in the strata, but in beds of great thickness and extent, consisting wholly of plants transmuted, by that peculiar process which vegetable matter undergoes when excluded from atmospheric influence, and under great pressure, into carbonaceous masses, called Lignite, and Coal. And there are intermediate stages of this process, in which the form and structure of the trees and plants are apparent; and a gradual transition may be traced, from the peat-wood and submerged forests of modern epochs, in which leaves, fruits, and trunks of indigenous species are preserved, to those accumulations of the extinct species of an ancient Flora, whose vegetable origin the eye of science can alone detect.

For the collection and preservation of vegetable fossils, with the exception of those which are permeated with pyrites, (as those of the Isle of

F

Sheppey, &c.), but few instructions are required. The silicified and calcareous stems are generally easy of extraction, even when imbedded in hard stone, and if broken can be repaired with glue. When the stems bear the imprints of leaf-stalks, (as in Lign. 30 and 44.) the surrounding stone should be carefully examined, with the view of detecting impressions, or other indications of the foliage. Delicate leaves in clay, or shale, must not be washed; a thin coat of mastic varnish, applied with a camel-hair pencil, will preserve them, and render them more distinct. When a leaf, fruit, seed-vessel, or other fragile object is attached to clay or friable sandstone, it is advisable to glue the specimen to a piece of thin wood or pasteboard, of suitable proportions.

ON THE INVESTIGATION OF THE FOSSIL REMAINS OF VEGETABLES.

VEGETABLE ORGANIZATION. As fragments of the stems or trunks, and branches, are very often the only vestiges of many species of fossil plants, a knowledge of the characters by which the principal divisions of the vegetable kingdom may be distinguished by their internal structure, is indispensable to the successful investigation of the Flora of the ancient world. Although I have treated of this subject in the Wonders of Geology, (Wond. p. 622.)

it will here be necessary to present the student with more ample details. The excellent introductory botanical works of Dr. Lindley, and Professor Henslow, convey full information on this, and every other department of the science, and should be consulted by those, who intend to make this branch of Geology their particular study. For the general reader, and amateur collector, the following notice of a few obvious essential characters of vegetable organization, will probably afford sufficient information.

Every plant is essentially an aggregation of cells; and the most simple forms of vegetation consist of a congeries of cells (cellular tissue) of the same kind, and have no visible fructification; such are the sea-weeds (alga, confervæ, &c.), mosses, and lichens. In the more complex tribes the cells become variously modified, are elongated into tubes or vessels (vascular tissue), some of which possess a spiral structure, and others have their sides studded with little glands. The vascular tissue consists of two kinds of vessels. 1. The spiral or tracheæ: these are membranous tubes, with conical extremities, having within, a coil of elastic fibre spirally twisted, and capable of being unrolled (Lign. 1, b.). 2. The ducts; which are a modification of the structure of the spiral vessel; their extremities are rounded or conical, and their sides marked with transverse lines, rings, or bars. Their functions appear to be different from those of the spiral vessels, and