fronds fell off as the trees advanced in growth. More than two hundred species of ferns have been found in the coal the princely palm-tree in beauty. The ferns are vascular cryptogamous plants, having their organs of fructification on their leaves, as seen on the leaves of the polypody or the brake. Moisture and a warm climate are favorable to their development. The names of the numerous fossil genera of ferns are derived from pteris, a fern, to which is pre pointed leaves are found at Fig. 112. tached to it, and scaly cones posed to be its seeds. tree is regarded by botanists as intermediate between the Coniferæ and the Lycopodiaceæ or club-mosses. 215. The other genus Sigillaria, (Sigillum, a seal,) was characterized by Fig. 113. elegant flutings with rows of symmetrical scars or impressions, disposed with such regularity, as sometimes to have induced the belief that they were engraved stones. These sigilla are scars left in the bark by the separation of the leaves from the stem or the trunk. These trees are usually found in a horizontal position and much flattened by the pressure to which they have been subjected, but are occasionally found erect with their roots in the soil in the positions in which they grew, and their cylindrical forms are then preserved. The Sigillaria grew sixty feet in length, and five feet in diameter, gradually tapering toward the summit. The bark, which is sometimes an inch thick, is frequently converted into coal, while the interior is sandstone or shale. The fossils called Stigmaria (Stigma, a mark,) formerly sup posed to constitute a separate genus of plants, are now shown to be the roots of the Sigillaria; a specimen of the Sigillaria has been found in Eng land in an upright position, Fig. 114, with Fig. 114. Sigillaria and Stigmaria. its roots still attached and extending in their natural directions. These stigmariæ are usually found upon the clay beneath the coal beds. 216. The fossil trunks of trees are found in many instances, erect or inclined in the coal-bearing strata. The mine of St. Etienne, in France, exhibits numerous vertical stems traversing all its strata appearing like a forest of plants petrified where they grew. In the Craigleith quarry near Edinburgh, a coniferous tree fifty-nine feet long was found lying at an angle of forty degrees, and traversing ten or twelve sandstone strata. This would seem to have been due to sudden subsidence or inundation, by which the sand and mud were brought oyer the trees; or in the cases of inclined stems, the greater weight of the roots may have brought them in that position into the sediment of the water in which they floated, as the snags of the Mississippi are known to work their way many feet into the bed of the river. 217. The origin of the succession of coal-beds with their intervening strata, is not yet satisfactorily ascertained. Some geologists suppose that dense Fig. 115. Araucaria.. forests and peat-bogs subsiding beneath the water, were covered with the mud and sand, which constitute the shale and sandstone strata over the coal; the land rising again and accumulating vegetable growth, was again submerged—a submergence and elevation occurring for each bed of coal. The number of alternations thus required renders this mode very improbable. Another mode of accounting for the phenomena attributes the coal strata to successive deposits of sand, mud, and vegetables made by rivers in lakes or estuaries, in accordance with the specific gravity of the materials. This mode of action is supposed to be illustrated on a small scale in the deltas of our large rivers, as the Mississippi and Ganges, where immense rafts of vegetable matter are invested by the silt of the rivers, especially during inundations. The perfect state of preservation of many very frail plants of the coal strata however, seems hardly consistent with such violent action. Fig. 117. 218. The igneous rocks invading the carboniferous strata are trap rock and metaliferous veins. The trap is usually dark colored, presenting itself in dikes, and flattened masses resembling strata, appearing to have been erupted at the bottom of an ocean. The variety of trap called toadstone, is abundant in the carboniferous limestone with an aggregate thickness in some pla ces of one hun dred and eighty feet. The dislocations of the coal strata |