Imágenes de página
PDF

relative numbers of other families of plants, in each of the successive geological epochs, which are thus connected with our own, by a new and beautiful series of links, derived from one of the most important tribes of the vegetable kingdom.

[graphic]

* See PI. 1, Figs. 31 to 39.

t M. Ad. Brongniart, in his arrangement of fossil plants, has formed a distinct group out of the few species which have been found in the Redsandstone formation (Gres bigarre) immediately above the Coal. In our division of the strata, this Red-sandstone is included, as an inferior member, in the Secondary series. Five Algae, three Catamites, five Ferns, and five Coniferse, two Liliacea;, and three uncertain Monocotyledonous plants form the entire amount of species which he enumerates in this small Flora.

See also Jseger ober die Pflanzenverslcinerungen in dem Bausandstein von Stuttgart, 1827.

t We again refer to Witham's Account of Coniferse from the Lias, in his observations on Fossil Vegetables, 1833.

§ A very interesting account, accompanied by figures, showing the internal structure of the stems of fossil arborescent Ferns of the Secondary period, is given in Cotta's Dendrolithen, Dresden, 1832; these appear to be chiefly from the New red sand-stone of Chemnitz near Dresden.

land plants in the Secondary formations, (from the Keuper to the Chalk inclusive ;) one half of these are Coniferae and Cycadae, and of this half, twenty-nine are Cycadeae; the remaining half are chiefly vascular Cryptogamiae, viz. Ferns, Equisetacea;, and Lycopodiaceae. In our actual vegetation, Coniferae and Cycadeae scarcely compose a three hundreth part.*

The family of Cycadeae comprehends only two living Genera; viz. Cycas, (PI. 58.) and Zamia. (PI. 59.) There are five known living Species of Cycas and about seventeen of Zamia. Not a single species of the Cycadeae grows at the present time in Europe: their principal localities are parts of equinoctial America, the West Indies, the Cape of Good Hope, Madagascar, India, the Molucca Islands, Japan, China, and New Holland.

Four or five genera, and twenty-nine species of Cycadeae, occur in the fossil Flora of the Secondary period, but remains of this family are very rare in strata of the Transition, and Tertiary series.f

* The fossil vegetables in the Secondary series, although they present many kinds of Lignite, very rarely form beds of valuable Coal. The imperfect coal of the Cleaveland Moorlands near Whitby, and of Brora in Sutherland, belong to the inferior region of the Oolite formation. The bituminous coal of Buckeberg near Minden, in Westphalia, is in the Wealden formation.

The coal of Hoer in Scania is either in the Wealden formation, or in the Green-sand (Ann. dee Sciences Nat. torn. iv. p. 200.)

f I learn by letter from Count Sternberg, (Aug. 1835.) that he has found Cycadeae and Zamites in the Coal formation of Bohemia, of which he will publish figures in the 7th and 8th Cahier of his Flore du Monde primitif. This is, I believe, the first example of the recognition of plants of this family in strata of the Carboniferous series.

During a recent visit to the extensive and admirably arranged geological collection in the Museum at Strasbourg, I was informed by M. Voltz that the stern of a Cycadites in that museum, described ;by M. Ad. Brongniart, as a Mantellia, from the Muschelkalk of Luneville, is derived from the Lias near that Town. M. Voltz knows no example 'of any Cycadites from the Muschelkalk. Stems and leaves of Cycadese occur also in the Lias at Lyme Regis. (Lind. Foss. Fl. PI. 143.)

The Cycadeae form a beautiful family of plants whose external habit resembles that of Palms, whilst their internal structure approximates in several essential characters to that of Conifers. In a third respect, (viz. the Gyrate Vernation, or mode in which the leaves are curled up at their points, within the buds,) they resemble Ferns. (See PI . 1. F. 33, 34, 35, and PI. 58, 59.)

I shall select the family of Cycadeae from the fossil Flora of the Secondary period, and shall enter into some details respecting its organization, with a view of showing an example of the method of analysis, by which Geologists are enabled to arrive at information as to the structure and economy of extinct species of fossil vegetables, and of the importance of the conclusions they are enabled to establish. Those who have attended to the recent progress of vegetable Physiology will duly appreciate the value of microscopic investigations, which enable us to identify the structure of vegetables of such remote antiquity, with that which prevails in the organization of living species.

The physiological discoveries that have lately been made with respect to living species of Cycadeae, have shown them to occupy an intermediate place between Palms, Ferns, and Coniferae, to each of which they bear certain points of resemblance; and hence a peculiar interest attends

The most abundant deposit of fossil leaves of Cycadece in England, is in the Oolitic formation on the coast of Yorkshire, between Whitby and Scarborough, (See Phillips' Illustration of the Geology of Yorkshire.) Leaves of this family occur also in the Oolitic slate of Stonesfield. Lindley and Hut. ton, Foss. Flora, PI. 172, 175.

In Lindley and Hutton's Fossil Flora, PI. 136, Figures are given of Cones which he refers to the genus Zamia, from the Band-stone of the Wealden formation at Yaverland on the South coast of the I. of Wight.

M. Ad. Brongniart has established a new fossil genus Nilsonia, in the family of Cycadeso, which occurs at Hoer in Scania, in strata, either of the Wealden or Green-sand formation; and another genus, Pterophyllum, which is found from the New red sand-stone upwards to the Wealden formation.

the recognition of similar structures in fossil plants, referable to a family whose characters are so remarkable.

The figure of a Cycas revoluta (PL 58,*) represents the form and habit of plants belonging to this beautiful genus. In the magnificent crown of graceful foliage surrounding the summit of a simple cylindrical trunk, it resembles a Palm. The trunk in the genus Cycas, is usually long. That of C. circinalis rises to 30 feet.f In the genus Zamia it is commonly short.

Our figure of a Zamia pungens,J (PL 59,) shows the mode of inflorescence in this Genus, by a single cone, rising like a Pine Apple, deprived of its foliaceous top, from within the crown of leaves at the summit of the stem.

The trunk of the Cycadeae has no true bark, but is surrounded by a dense case, composed of persistent scales which have formed the basis of fallen leaves; these, together with other abortive scales, constitute a compact covering that supplies the place of bark. (See PL 58 and 59.)

In the Geol. Trans, of London (vol. iv. part 1. New Series) I have published, in conjunction with Mr. De la Beche, an account of the circumstances under which silicified fossil trunks of Cycadeae are found in the Isle of Portland, immediately above the surface of the Portland stone, and below the Purbeck stone. They are lodged in the same beds of black mould in which they grew, and are accompanied by prostrate trunks of large coniferous trees, converted to flint, and by stumps of these trees standing erect with their roots still fixed in their native soil. (See PL 57, Fig. l.§)

[graphic]

* Drawn from a Plant in Lord Grenville's Conservatory at Dropmore, in 1832.

T In Curtis's Botanical Magazine, 1828, PI. 2826, Dr. Hooker has published an Engraving of a Cycas circinalis which in 1827 flowered in the Botanic Garden at Edinburgh. See PI. 1. Fig. 33.

t Copied from an engraving published by Mr. Lambert, of a plant that bore fruit at Walton on Thames in the conservatory of Lady Tankerville, 1832.

§ The sketch, PI. 57, Fig. 2, represents a triple series of circular undula.

>

PI. 57, Fig. 3, exhibits similar stumps of trees rooted in their native mould, in the Cliff immediately east of Lulworth Cove. Here the strata have been elevated nearly to a angle of 45°, and the stumps still retain the unnatural inclination into which they have been thrown by this elevation.

The facts represented in these three last figures are fully described and explained in the paper above referred to; they prove that plants belonging to a family that is now confined to the warmer regions of the earth, were at a former period, natives of the southern coast of England.*

As no leaves have yet been found with the fossil CycadeEe under consideration, we are limited to the structure of their

tions, marked in the stone, which surrounds a single stump, rooted in the dirt-bed in the Isle of Portland. This very curious disposition has apparently resulted from undulations, produced by winds, blowing at different times in different directions on the surface of the shallow fresh-water, from the sediments of which the matter of this stratum was supplied, while the top of this stem stood above the surface of the water. See Geol. Trans. Lond. N. S. vol. iv. p. 17.

* The structure of this district affords also a good example of the proofs which Geology discloses, of alternate elevations and submersions of the strata, sometimes gradually, and sometimes violently, during the formation of the crust of our planet.

First. We have evidence of the rise of the Portland stone, till it reached the surface of the sea wherein it was formed.

Secondly. This surface became for a time, dry land, covered by a temporary forest, during an interval which is indicated by the thickness of a bed of black mould, called the Dirt-bed, and by the rings of annual growth in large petrified trunks of prostrate trees, whose roots had grown in this mould.

Thirdly. We find this forest to have been gradually submerged, first beneath the waters of a fresh.water lake, next of an estuary, and afterwards beneath those of a deep sea, in which Cretaceous and Tertiary strata were deposited, more than 2000 feet in thickness.

Fourthly. The whole of these strata have been elevated by subterranean violence, into their actual position in the hills of Dorsetshire.

We arrive at similar conclusions, as to the alternate elevation and depressions of the surface of the earth, from the erect position of the stems of Calami tes, in sand-stone of the lower Oolite formation on the eastern coast of Yorkshire. (See Murchison. Proceedings of Geol. Society of London, vol. i. p. 391.)

/ ■

« AnteriorContinuar »