remote from each other in time. The organic remains of the Silurian and Carboniferous periods are more like to each other than they are to those of the chalk or tertiary; and the deeper we descend in the strata, the more unlike to the existing races are the fossils we obtain. In the earliest fossiliferous rocks the simplest forms of organization predominated, but representatives of all the classes are found with them. The relative number of species and genera of these classes has continually varied in the successive formations.

FOSSIL BOTANY.

140. All vegetables, fossil or recent, may be arranged under two grand divisions, cellular and vascular.

I. Cellular plants present the simplest forms of vegetation, consisting of an assemblage of cells of the same kind, without regular vessels, and having no visible organs of fructification. Such are the sea-weeds (confervæ, algæ,) mosses and lichens.

II. In vascular plants the cells are elongated into tubes or vessels, called vascular tissue, and form organs of nutrition and fructification. Four classes are formed in this division, in accordance with the structure of the organs of fructification.

1st. Cryptogamiæ, having neither perfect flowers nor visible seed vessels; to this belong the fern, equisetum or marestail, and calamites. Great numbers of these occur fossil.

2nd. Gymnospermous Phanerogamiæ, having flowers; but their principal characteristic is the nakedness of their seeds. To this class belong the Cycades, or pine apple tribe, sago and the Coniferæ, or firs.

3d. Monocotyledonous Phanerogamiæ, flowering plants whose seeds have only one cotyledon, or seed-lobe, as the lily, the palm, and the cane. These are also called Endogenous, because they receive their growth wholly within, by the formation of new bundles of vessels at the center, crowding out and condensing those which lie toward the edge. They have no pith, concentric circles of woody fibre, nor true bark.

4th. Dicotyledonous Phanerogamiæ, flowering plants whose seeds have two cotyledons or seed-lobes, as the bean. These are exogenous, receiving annual deposition of woody fibre, upon the previous growth, and have true bark and pith. A cross section of the oak exhibits the pith at the center, concentric circles of woody fibre, connected by fine straight lines--the medullary rays-which radiate from the central pith. To this class belong our forest trees, and shrubs. 141. As fragments of trunks and branches are often the only specimens we have of a fossil species, we are obliged to resort to the study of their anatomical structure.

Fig. 71.

These

are quite distinctive and are frequently very perfectly preserved. A fragment of fossil wood cut very thin, will, with the aid of a microscope, show the form and position of the vessels so clearly as to enable the observer to determine the nature of the plant.

The leaves, either entire, or impressions of them are very common, and enable us to distinguish between endogenous and exogenous structures. If the veins of the leaf be all parallel, or connected only by little transverse bars, the plant was endogenous; but if they are unequal in thickness or arranged in net-like meshes, they belonged to an exogenous plant. The flowers of some liliaceous plants have been found with their corollas, and calyxes preserved, but the anthers and pistils could not be recognized. The preceding figure, 71, presents a portion of a coal plant, discovered at Tallmadge, Ohio, by Charles Whittlesey, Esq., which very closely resembles the corollas of some plants. The fruits Fig. 72.

Fossil Nuts.

of many plants occur fossil in great profusion, as cones, nuts and seeds.

Some trunks of trees have been found of great length; as the Norfolk Island pine, forty feet long, in the carboniferous limestone of the Craigleith quarry near Edinburgh, and the "petrified forest" near Cairo in the Egyptian desert, where the silicified trunks of trees from forty to sixty feet in length and three feet in diameter, cover the ground. The resinous secretions of pines and other coniferous plants, are sometimes found fossil, as amber, which has been found in its natural position in trunks of trees. Some specimens of amber are black, showing the process of bituminization they have undergone. This also is supposed to be the origin of the diamond.

142. The cellular plants, as the algae, occur most abundantly in the most ancient fossiliferous strata; in some of the Silurian rocks, entire layers are formed of a large species of sea-weed. Of vascular plants, the cryptogamous-fern, equisetum, and calamite-are found predominating in the carboniferous rocks, but existed earlier than that period. As we ascend in the series of rocks from the secondary class to the alluvial, the fossil ferns diminish in number, as they do when we travel from the equator toward the poles at the present day, when of the one thousand five hundred species, one thousand two hundred are within the tropics.

The gymnospermous plants-Cycades and Coniferous— were most abundant in the lias, oolite, and tertiary periods. The endogenous class including the palms flourished during the tertiary period, though they are found to some extent in earlier periods. In the Illinois coal field a group of fossil palm trees has been discovered with their roots in the

50.000 -2000)'

clay, and their trunks in the coal and sandstone above. The geological position of the exogenous plants is in the tertiary and recent deposits. The lignite or brown coal is almost entirely composed of dicotyledonous trees, the poplar, willow, elm, chestnut, and maple.

143. We have reason to presume that the history of fossil vegetation is not complete, since probably many wing bosfamilies were too frail to be preserved, and many of the dicotyledonous class were not in situations favorable to their Europe fossilization. The grasses, so abundant at the present time,

2000-500

seem to have been almost entirely absent from the secondary period, when plants allied to them were flourishing. The exuberant growth of vegetation during the coal period has led M. Brogniart to conjecture that carbonic acid was more abundant in the atmosphere at that time than at other periods. The number of fossil species of plants at present recognised and described, is about eight hundred.

144. The various forms which the carbonaceous matter accumulated by plants presents-anthracite coal, bituminous coal, jet, lignite or brown coal, and peat-depend upon the original structure and composition of the plants; upon the nature of the mineral deposits in which they were imbedded; and upon their more or less complete seclusion from the air. The conversion of wood and peat into lignite and coal, is still occurring. In the state of Maine, true bituminous coal has been found at the depth of four feet from the the surface, in a peat-bog of twenty feet thickness. Sections of the coal show that it is the remains of coniferous trees, immersed in the peat. The timbers of the Royal George, an English ship of war sunk off Portsmouth, and raised after sixty years immersion in the silt of the ocean, resembled in texture and color, the wood of submerged forests.