A Manual of Elementary Geology Part 34

[211-B] Fitton, Geol. Trans., 2d series, vol. iv. p. 319.

[215-A] Proceedings of Geol. Soc., vol. iii. pp. 7, 8., 1842.

[216-A] Geol. Trans. Second Series, vol. iii. p. 232. plate 31.

figs. 3. and 11.

[216-B] Geol. of U. S. Exploring Exped. p. 252. 1849.

[217-A] See Chapters X. and XI.

[217-B] Darwin, p. 549. Kotzebue's First Voyage, vol. iii. p. 155.

[217-C] Mantell, Geol. of S. E. of England, p. 96.

[219-A] Dr. Fitton, Quart. Geol. Journ., vol. i. p. 179., ii. p. 55., and iii. p. 289., where comparative sections and a valuable table showing the vertical range of the various fossils of the lower greensand at Atherfield is given.

[221-A] Archiac, sur la Form. Cretacee du S. O. de la France, Mem. de la Soc. Geol. de France, tom. ii.

[222-A] D'Orbigny's Paleontologie Francaise, pl. 533.

[223-A] In this and subsequent remarks on fossil plants I shall often use Dr. Lindley's terms, as most familiar in this country; but as those of M. A. Brongniart are much cited, it may be useful to geologists to give a table explaining the corresponding names of groups so much spoken of in palaeontology.

Brongniart.

Lindley.

Examples.

Cryptogamic.

1. Cryptogamous amphigens, or cellular cryptogamic.

Thallogens.

Lichens, sea-weeds, fungi.

2. Cryptogamous acrogens.

Acrogens.

Mosses, equisetums, ferns, lycopodiums--Lepidodendron.

Phanerogamic.

3. Dicotyledonous gymnosperms.

Gymnogens.

Conifers and Cycads.

4. Dicot. Angiosperms.

Exogens.

Compositae, leguminosae, umbelliferae, cruciferae, heaths, &c. All native European trees except conifers.

5. Monocotyledons.

Endogens.

Palms, lilies, aloes, rushes, gra.s.ses, &c.

[223-B] A. Brongniart, Veget. Foss. Dict. Univ., p. 111., 1849.

[224-A] See a paper by the author, Quart. Journ. Geol. Soc., vol. i. p. 55.

[225-A] Proceed. Geol. Soc. iv. p. 391.

[225-B] See Forbes, Quart. Geol. Journ. vol. i. p. 79.

CHAPTER XVIII.

WEALDEN GROUP.

The Wealden divisible into Weald Clay, Hastings Sand, and Purbeck Beds--Intercalated between two marine formations--Weald clay and Cypris-bearing strata--Iguanodon--Hastings sands--Fossil fish--Strata formed in shallow water--Brackish water-beds--Upper, middle, and lower Purbeck--Alternations of brackish water, freshwater, and land--Dirt-bed, or ancient soil--Distinct species of fossils in each subdivision of the Wealden--Lapse of time implied--Plants and insects of Wealden--Geographical extent of Wealden--Its relation to the cretaceous and oolitic periods--Movements in the earth's crust to which it owed its origin and submergence.

Beneath the cretaceous rocks in the S.E. of England, a freshwater formation is found, called the Wealden (see Nos. 5. and 6. Map, p. 242.), which, although it occupies a small horizontal area in Europe, as compared to the chalk, is nevertheless of great geological interest, not only from its position, as being interpolated between two great marine formations (Nos.

7. and 9. Table, p. 103.), but also because the imbedded fossils indicate a grand succession of changes in organic life, effected during its acc.u.mulation. It is composed of three minor divisions, the Weald Clay, the Hastings, and the Purbeck Beds, of which the aggregate thickness in some districts may be 700 or 800 feet; but which would be much more considerable (perhaps 2000 feet), were we to add together the extreme thickness acquired by each of them in their fullest development.

The common name of Wealden was given to the whole, because it was first studied in parts of Kent, Surrey, and Suss.e.x, called the Weald, (see Map, p. 242.), and we are indebted to Dr. Mantell for having shown in 1822, in his Geology of Suss.e.x, that the whole group was of fluviatile origin. In proof of this he called attention to the entire absence of Ammonites, Belemnites, Terebratulae, Echinites, Corals, and other marine fossils, so characteristic of the cretaceous rocks above, and of the Oolitic strata below, and to the presence of Paludinae, Melaniae, and various fluviatile sh.e.l.ls, as well as the bones of terrestrial reptiles and the trunks and leaves of land plants.

[Ill.u.s.tration: Fig. 227. Position of the Wealden between two marine formations.]

The evidence of so unexpected a fact as the infra-position of a dense ma.s.s of purely freshwater origin to a deep-sea deposit (a phenomenon with which we have since become familiar, in other chapters of the earth's autobiography), was received, at first, with no small doubt and incredulity. But the relative position of the beds is unequivocal; the Weald Clay being distinctly seen to pa.s.s beneath the Greensand in various parts of Surrey, Kent, and Suss.e.x; and if we proceed from Suss.e.x westward to the Vale of Wardour, we there again observe the same formation, or, at least, the lower division of it, the Purbeck, occupying the same relative position, and resting on the Oolite (see fig. 228.). Or if we pa.s.s from the base of the South Downs in Suss.e.x, and cross to the Isle of Wight, we there again meet with the Wealden series reappearing beneath the Greensand, and cannot doubt that the beds are prolonged subterraneously, as indicated by the dotted lines in fig. 229.

[Ill.u.s.tration: Fig. 228. Cross section.

O, Oolite.

G S, Greensand, or Lower Cretaceous.]

[Ill.u.s.tration: Fig. 229. Cross section.]

The minor groups into which the Wealden has been commonly divided in England are, as before stated, three, and they succeed each other in the following descending order[227-A]:--

Thickness.

1st. Weald Clay, sometimes including thin beds of sand and sh.e.l.ly limestone 140 to 280 ft.

2d. Hastings Sand, in which occur some clays and calcareous grits 400 to 500 ft.

3d. Purbeck Beds, consisting of various kinds of limestones and marls 150 to 200 ft.

_Weald Clay._

The first division, or Weald Clay, is of purely freshwater origin. The uppermost beds are not only conformable, as Dr. Fitton observes, to the inferior strata of the Lower Greensand, but of similar mineral composition.

To explain this, we may suppose, that as the delta of a great river was tranquilly subsiding, so as to allow the sea to encroach upon the s.p.a.ce previously occupied by freshwater, the river still continued to carry down the same sediment into the sea. In confirmation of this view it may be stated, that the remains of the _Iguanodon Mantelli_, a gigantic terrestrial reptile, very characteristic of the Wealden, has been discovered near Maidstone, in the overlying Kentish rag, or marine limestone of the Lower Greensand. Hence we may infer that some of the saurians which inhabited the country of the great river continued to live when part of the country had become submerged beneath the sea. Thus, in our own times, we may suppose the bones of large alligators to be frequently entombed in recent freshwater strata in the delta of the Ganges. But if part of that delta should sink down so as to be covered by the sea, marine formations might begin to acc.u.mulate in the same s.p.a.ce where freshwater beds had previously been formed; and yet the Ganges might still pour down its turbid waters in the same direction, and carry seaward the carca.s.ses of the same species of alligator, in which case their bones might be included in marine as well as in subjacent freshwater strata.

The Iguanodon, first discovered by Dr. Mantell, has left more of its remains in the Wealden strata of the south-eastern counties, and Isle of Wight, than any other genus of a.s.sociated saurians. It was an herbivorous reptile, and regarded by Cuvier as more extraordinary than any with which he was acquainted; for the teeth, though bearing a great a.n.a.logy to the modern Iguanas which now frequent the tropical woods of America and the West Indies, exhibit many striking and important differences (see fig. 230.). It appears that they have been worn by mastication; whereas the existing herbivorous reptiles clip and gnaw off the vegetable productions on which they feed, but do not chew them.

Their teeth, when worn, present an appearance of having been chipped off, and never, like the fossil teeth of the Iguanodon, have a flat ground surface (see fig. 231.), resembling the grinders of herbivorous mammalia. Dr. Mantell computes that the teeth and bones of this animal which have pa.s.sed under his examination during the last twenty years, must have belonged to no less than seventy-one distinct individuals; varying in age and magnitude from the reptile just burst from the egg, to one of which the femur measured 24 inches in circ.u.mference. Yet notwithstanding that the teeth were more numerous than any other bones, it is remarkable that it was not till the relics of all these individuals had been found, that a solitary example of part of a jaw-bone was obtained. More recently remains both of the upper and lower jaw have been met with in the Hastings Beds in Tilgate Forest. Their size was somewhat greater than had been antic.i.p.ated, and even allowing that the tail was short, which Professor Owen infers from the short bodies of the caudal vertebrae, Dr. Mantell estimates the probable length of some of these saurians at between 30 and 40 feet. The largest femur yet found measures 4 feet 8 inches in length, the circ.u.mference of the shaft being 25 inches, and round the condyles 42 inches.

[2 Ill.u.s.trations: Teeth of Iguanodon.

Fig. 230. Partially worn tooth of a young animal. (Mantell.)

Fig. 231. Crown of tooth in adult, worn down. (Mantell.)]