carbon, but the venation is distinctly preserved. In the Stonesfield Slate, and in the Portland and Wealden strata, remains of this tribe are met with. The Museum contains many beautiful specimens of the leaves and fruits or cones of the ordinary species, which are arranged in the lower part of the Case before us-Case F of the plan, p. 10.

Of these the most striking is a well-known fossil plant of the Scarborough Oolite, whose leaves and fruits occur in profusion in some of the strata. This species has been described under the names of Zamia gigas, and Z. Mantelli, and has lately formed the subject of an interesting paper read before the Yorkshire Philosophical Society, by Professor Williamson of Manchester. Several specimens of the fossils locally termed "collars," are in the case before us: these bodies Professor W. has shown to be a zone formed by a scaly bud which originally enclosed the germ of these plants: in the progress of development the fruit burst through the upper part of the investing sheath, and, as it grew to maturity, rose above the incurved elongated scales, till the latter literally formed a zone or "collar" around the pedicle of the cone.

Zamia lanceolata.-Case F.-On a slab of sandstone there is a beautiful example of the foliage of this plant, with a detached cone imbedded immediately above one of the leaves.

TREES AND CYCADEOUS PLANTS OF PORTLAND.-Case FIn this Case, and on the top of the same, are many subcompressed, spheroidal, and sub-cylindrical silicified bodies, having the surface covered with lozenge-shaped scales; these are fossil plants closely allied to the recent Zamiæ, and were obtained from a remarkable stratum in the Isle of Portland, named the dirt-bed, which occurs in the quarries on the north of the island, a few feet above the layer of building-stone for which Portland has so long been celebrated. These fossils are found associated with the erect stems and prostrate trunks and branches of large coniferous trees, of which there is an example twelve feet long on the top of Case D. The circumstances under which these petrified trees and plants occur are so extraordinary, as to warrant a brief notice of the phenomenon in this place.

The Isle of Portland is a bold headland to the south of Weymouth, about four and a half miles in length and two in breadth, and is united to the mainland by a bar of shingle, called the Chesil Bank. It presents on its northern aspect a precipitous escarpment about three hundred feet high; and, declining towards the south, appears when viewed from the east or west, as an inclined plane rising abruptly from the sea. The base of the island consists of Kimmeridge clay, which is surmounted by beds of sand and thick layers of the oolitic limestone or Portland-stone. The strata dip to the south at an angle corresponding with the outline of the surface. The coasts are steep; the base of Kimmeridge clay forming a talus surmounted by perpendicular crags of oolite. The southern extremity consists of low limestone cliffs, which are worn into numerous caverns by the constant action of the

waves.

The summit of the northern brow, to a depth of about thirty feet, is composed of beds of laminated calcareous shale, locally termed "the Cap ;" and sections of these strata are exposed in the quarries that are opened for the extraction of the building-stone which lies beneath.

Immediately upon the uppermost bed of limestone, which is a coarse rock, full of cavities and imprints left by the decay of the usual species of marine univalve and bivalve shells of the Oolite, are layers of calcareous shale a few feet in thickness, in which no vestiges of marine fossils have been observed;

and whose laminated structure, and the presence of horizontal seams of carbonaceous earthy matter, with interspersions of vegetable remains, indicate a fluviatile or fresh-water origin. Upon these deposits is a layer, from one to two feet thick, of a dark brown friable loam abounding in lignite, and so similar in appearance to common vegetable earth or mould, as to have acquired the name of dirt-bed from the quarrymen. In and upon this bed are numerous petrified stems and branches of coniferous trees, and plants allied to the Zamiæ. Many of the trees and plants are standing erect, as if petrified while growing on the spot; the trunks of the trees extending upwards into the limestone above, and vestiges of the roots being traceable into the dirt-bed. The upright stems are in general a few feet apart, and but three or four feet high, and are broken and splintered at the top as if they had been wrenched off at a few feet from the ground. They are from a few inches to three or four feet in diameter; portions of prostrate trunks have been collected, indicating a total height of the originals of thirty or forty feet. In many instances fragments of branches remain attached to the stem. The cycadeous plants occur in the intervals between the upright trees, and the dirt-bed is so little consolidated that specimens, evidently standing in the position in which they originally grew, may be dug up with a spade. The strata above the dirtbed consists of finely laminated cream-coloured shaly limestone, in which casts of the fresh-water crustaceans (Cyprides) so abundant in the Wealden, are the only organic remains hitherto noticed. These deposits are covered by the modern vegetable soil, which but little exceeds in depth the ancient one above described, and instead of supporting cycades and pine-forests, barely maintains a scanty vegetation. Here, then, we have the remains of a petrified forest of the ancient world, the trees and plants, like the inhabitants of the city in Arabian fable, being changed into stone, yet still retaining the places they occupied when alive.1

MANTELLIA (M. nidiformis and M. cylindrica).-Case F. -Such are the remarkable conditions under which the fossil cycadeous plants named Mantellia by M. Ad. Brongniart

'For geological details see "Wonders of Geology," 6th edit. p. 385; or, "Geology of the Isle of Wight," 2d edit. p. 393.

usually occur, and which invest them with a peculiar interest. These vegetables are from one to two feet in height, the circumference of the largest not exceeding three feet. The stem is sub-cylindrical, without a distinct central axis, and the surface is scored with rhomboidal scars, which are widest in the horizontal direction. There are two species, distinguishable

by the form of the stems and the size of the cicatrices of the petioles. In one the stem is short and spheroidal, and the leaf-scars are relatively broader. (M. nidiformis): this species is named "crow's nest" by the workmen, who believe these plants to be nests built by crows in the trees with which they are collocated, and that the trees and nests have become petrified together. The other species (M. cylindrica) is sub-cylindrical, and relatively higher than the former, and the cicatrices of the petioles are much smaller, indicating a more delicate foliage. The fruit of these plants is unknown; one cone has been found which it is supposed belonged to the Mantellia, or some allied species; it is figured in the Fossil Flora of Great Britain as Zamia crassa. 1

LIGN. 19.

FOSSIL WOOD AND TREES OF AUSTRALIA.-Table-case a.In the recesses of the east windows, and in the table-cases beneath, there are many choice specimens of the wood, and two portions of very large trunks of coniferæ, from Van Diemen's Land and New Holland. This fossil wood is partly calcified, and partly silicified; some portions being very earthy and friable, and effervescing strongly with acid, while other parts of the same stem are converted into chalcedony and semi-opal; in both states the organic structure may be detected by the aid of the microscope.

The trees from which the specimens brought to England

1 "Medals of Creation," p. 160, lign. 38.

The fossil plants of the Isle of Portland are admirably described and illustrated by Dr. Buckland, in Bridgewater Essay, p. 497, and pl. 60, 61. The species described in the text under the names assigned to them by M. Brongniart, are the Cycadites megalophyllus and C. microphyllus of Dr. Buckland.

were obtained, appear to occur under similar conditions, and to have been subjected to the same changes, as those of the Isle of Portland above described. They are found with the trunks erect, to the height of a few feet, in a bed of arid sand, apparently on the spots where they grew; the branches and upper part of the stems being scattered around. They so entirely preserve their natural ligneous appearance, that an agricultural colonist mentioned as among the extraordinary sights he witnessed on his first arrival in New Holland, the burning of trees into excellent lime to manure the ground.

A forest of these silicified trees occurs on the eastern coast of Australia under the following circumstances. At the base of a mountain range composed of conglomerates and sandstones, with subordinate beds of lignite, terminating on one side of Lake Macquarrie, an alluvial flat extends to the water's edge, covering the sandstone rock, which lies in situ beneath. Over this plain stumps of petrified trees project a few feet above the soil, presenting the appearance of a forest in which the trees are all cut or broken off at the same level. At the distance of a few yards from the shore, a reef is formed by vertical rows of stems, which project above the water. Many of the fossil trees on the shore have the remains of roots extending into the sandstone below the alluvial deposit; and, like those in the Isle of Portland, are in some instances surrounded by an accumulation of stone that forms a mound of a higher level than the surface of the ground. These trees are of a large size; often six feet in diameter. The concentric annular rings, and the medullary rays and the coniferous ducts, are beautifully preserved in silex and chalcedony; in several examples, from 60 to 120 annual circles of growth were observable.

In the valley of the Derwent in Van Diemen's Land, opalized coniferous trees of a similar character were observed under conditions yet more extraordinary, by the distinguished philosophical traveller, Count Strzelecki. Truncated stems of trees are standing erect in a bed of porous and scoriaceous basalt, and trachytic conglomerate but in some instances these are only casts of trunks that were consumed by the melted basalt when first ejected. This curious phenomenon can only be explained by supposing that the silicified stems were able to resist the intensity of heat of the incan