The Student's Elements of Geology Part 36

The Iguanodon, first discovered by Dr. Mantell, was an herbivorous reptile, of which the teeth, though bearing a great a.n.a.logy, in their general form and crenated edges (see Figure 289 a and b), to the modern Iguanas which now frequent the tropical woods of America and the West Indies, exhibit many important differences. It appears that they have often been worn by the process of mastication; whereas the existing herbivorous reptiles clip and gnaw off the vegetable productions on which they feed, but do not chew them. Their teeth frequently present an appearance of having been chipped off, but never, like the fossil teeth of the Iguanodon, have a flat ground surface (see Figure 290, b) resembling the grinders of herbivorous mammalia. Dr. Mantell computes that the teeth and bones of this species which pa.s.sed under his examination during 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 twenty-four inches in circ.u.mference. Yet, notwithstanding that the teeth were more numerous than any other bones, it is remarkable that it was not until the relics of all these individuals had been found, that a solitary example of part of a jaw-bone was obtained. Soon afterwards remains both of the upper and lower jaw were met with in the Hastings beds in Tilgate Forest, near Cuckfield. In the same sands at Hastings, Mr.

Beckles found large tridactyle impressions which it is conjectured were made by the hind feet of this animal, on which it is ascertained that there were only three well-developed toes.

(FIGURE 291. Cypris spinigera, Fitton.)

(FIGURE 292. Weald clay with Cyprides.)

Occasionally bands of limestone, called Suss.e.x Marble, occur in the Weald Clay, almost entirely composed of a species of Paludina, closely resembling the common P. vivipara of English rivers. Sh.e.l.ls of the Cypris, a genus of Crustaceans mentioned in Chapter 3 as abounding in lakes and ponds, are also plentifully scattered through the clays of the Wealden, sometimes producing, like plates of mica, a thin lamination (see Figure 292).

HASTINGS SANDS.

This lower division of the Wealden consists of sand, sandstone, calciferous grit, clay, and shale; the argillaceous strata, notwithstanding the name, predominating somewhat over the arenaceous, as will be seen by reference to the following table, drawn up by Messrs. Drew and Foster, of the Geological Survey of Great Britain:

TABLE 18.1. SUBORDINATE FORMATIONS IN THE HASTINGS SAND.

COLUMN 1: NAME OF SUBORDINATE FORMATION.

COLUMN 2: MINERAL COMPOSITION OF THE STRATA.

COLUMN 3: THICKNESS IN FEET.

Tunbridge Wells Sand: Sandstone and loam: 150.

Wadhurst Clay: Blue and brown shale and clay, with a little calc-grit: 100.

Ashdown Sand: Hard sand, with some beds of calc-grit: 160.

Ashburnham Beds: Mottled white and red clay, with some sandstone: 330.

The picturesque scenery of the "High Rocks" and other places in the neighbourhood of Tunbridge Wells is caused by the steep natural cliffs, to which a hard bed of white sand, occurring in the upper part of the Tunbridge Wells Sand, mentioned in the above table, gives rise. This bed of "rock-sand" varies in thickness from 25 to 48 feet. Large ma.s.ses of it, which were by no means hard or capable of making a good building-stone, form, nevertheless, projecting rocks with perpendicular faces, and resist the degrading action of the river because, says Mr. Drew, they present a solid ma.s.s without planes of division. The calcareous sandstone and grit of Tilgate Forest, near Cuckfield, in which the remains of the Iguanodon and Hylaeosaurus were first found by Dr. Mantell, const.i.tute an upper member of the Tunbridge Wells Sand, while the "sand-rock" of the Hastings cliffs, about 100 feet thick, is one of the lower members of the same. The reptiles, which are very abundant in this division, consist partly of saurians, referred by Owen and Mantell to eight genera, among which, besides those already enumerated, we find the Megalosaurus and Plesiosaurus. The Pterodactyl also, a flying reptile, is met with in the same strata, and many remains of Chelonians of the genera Trionyx and Emys, now confined to tropical regions.

(FIGURE 293. Lepidotus Mantelli, Aga.s.siz. Wealden.

a. Palate and teeth.

b. Side view of teeth.

c. Scale.)

The fishes of the Wealden are chiefly referable to the Ganoid and Placoid orders. Among them the teeth and scales of Lepidotus are most widely diffused (see Figure 293). These ganoids were allied to the Lepidosteus, or Gar-pike, of the American rivers. The whole body was covered with large rhomboidal scales, very thick, and having the exposed part coated with enamel. Most of the species of this genus are supposed to have been either river-fish, or inhabitants of the sea at the mouth of estuaries.

(FIGURE 294. Unio Valdensis, Mant. Isle of Wight and Dorsets.h.i.+re; in the lower beds of the Hastings Sands. a, b.)

(FIGURE 295. Underside of slab of sandstone about one yard in diameter.

Stammerham, Suss.e.x.)

At different heights in the Hastings Sands, we find again and again slabs of sandstone with a strong ripple-mark, and between these slabs beds of clay many yards thick. In some places, as at Stammerham, Horsham, near there, are indications of this clay having been exposed so as to dry and crack before the next layer was thrown down upon it. The open cracks in the clay have served as moulds, of which casts have been taken in relief, and which are, therefore, seen on the lower surface of the sandstone (see Figure 295).

(FIGURE 296. Sphenopteris gracilis, Fitton. From the Hastings Sands near Tunbridge Wells.

a. A portion of the same magnified.)

Near the same place a reddish sandstone occurs in which are innumerable traces of a fossil vegetable, apparently Sphenopteris, the stems and branches of which are disposed as if the plants were standing erect on the spot where they originally grew, the sand having been gently deposited upon and around them; and similar appearances have been remarked in other places in this formation.

(Mantell Geology of S.E. of England page 244.) In the same division also of the Wealden, at Cuckfield, is a bed of gravel or conglomerate, consisting of water- worn pebbles of quartz and jasper, with rolled bones of reptiles. These must have been drifted by a current, probably in water of no great depth.

From such facts we may infer that, notwithstanding the great thickness of this division of the Wealden, the whole of it was a deposit in water of a moderate depth, and often extremely shallow. This idea may seem startling at first, yet such would be the natural consequence of a gradual and continuous sinking of the ground in an estuary or bay, into which a great river discharged its turbid waters. By each foot of subsidence, the fundamental rock would be depressed one foot farther from the surface; but the bay would not be deepened, if newly- deposited mud and sand should raise the bottom one foot. On the contrary, such new strata of sand and mud might be frequently laid dry at low water, or overgrown for a season by a vegetation proper to marshes.

PUNFIELD BEDS, BRACKISH AND MARINE.

(FIGURE 297. Vicarya Lujani, De Verneuil (Foss. de Utrillas.) Wealden, Punfield.

a. Nearly perfect sh.e.l.l.

b. Vertical section of smaller specimen, showing continuous ridges as in Nerinaea.)

The sh.e.l.ls of the Wealden beds belong to the genera Melanopsis, Melania, Paludina, Cyrena, Cyclas, Unio (see Figure 294), and others, which inhabit rivers or lakes; but one band has been found at Punfield, in Dorsets.h.i.+re, indicating a brackish state of the water, where the genera Corbula, Mytilus, and Ostrea occur; and in some places this bed becomes purely marine, containing some well-known Neocomian fossils, among which Ammonites Deshayesii (Figure 284) may be mentioned. Others are peculiar as British, but very characteristic of the Upper and Middle Neocomian of Spain, and among these the Vicarya Lujani (Figure 297), a sh.e.l.l allied to Nerinea, is conspicuous.

By reference to Table 18.1 it will be seen that the Wealden beds are given as the fresh-water equivalents of the Marine Neocomian. The highest part of them in England may, for reasons just given, be regarded as Upper Neocomian, while some of the inferior portions may correspond in age to the Middle and Lower divisions of that group. In favour of this latter view, M. Marcou mentions that a fish called Asteracanthus granulosus, occurring in the Tilgate beds, is characteristic of the lowest beds of the Neocomian of the Jura, and it is well known that Corbula alata, common in the Ashburnham beds, is found also at the base of the Neocomian of the Continent.

AREA OF THE WEALDEN.

In regard to the geographical extent of the Wealden, it can not be accurately laid down, because so much of it is concealed beneath the newer marine formations. It has been traced about 320 English miles from west to east, from the coast of Dorsets.h.i.+re to near Boulogne, in France; and nearly 200 miles from north-west to south-east, from Surrey and Hamps.h.i.+re to Va.s.sy, in France. If the formation be continuous throughout this s.p.a.ce, which is very doubtful, it does not follow that the whole was contemporaneous; because, in all likelihood, the physical geography of the region underwent frequent changes throughout the whole period, and the estuary may have altered its form, and even s.h.i.+fted its place.

Dr. Dunker, of Ca.s.sel, and H. von Meyer, in an excellent monograph on the Wealdens of Hanover and Westphalia, have shown that they correspond so closely, not only in their fossils, but also in their mineral characters, with the English series, that we can scarcely hesitate to refer the whole to one great delta. Even then, the magnitude of the deposit may not exceed that of many modern rivers. Thus, the delta of the Quorra or Niger, in Africa, stretches into the interior for more than 170 miles, and occupies, it is supposed, a s.p.a.ce of more than 300 miles along the coast, thus forming a surface of more than 25,000 square miles, or equal to about one-half of England. (Fitton Geology of Hastings page 58, who cites Lander's Travels.) Besides, we know not, in such cases, how far the fluviatile sediment and organic remains of the river and the land may be carried out from the coast, and spread over the bed of the sea. I have shown, when treating of the Mississippi, that a more ancient delta, including species of sh.e.l.ls such as now inhabit Louisiana, has been upraised, and made to occupy a wide geographical area, while a newer delta is forming; and the possibility of such movements and their effects must not be lost sight of when we speculate on the origin of the Wealden. (See Chapter 6 and Second Visit to the United States volume 2 chapter 34.)

It may be asked where the continent was placed, from the ruins of which the Wealden strata were derived, and by the drainage of which a great river was fed.

If the Wealden was gradually going downward 1000 feet or more perpendicularly, a large body of fresh-water would not continue to be poured into the sea at the same point. The adjoining land, if it partic.i.p.ated in the movement, could not escape being submerged. But we may suppose such land to have been stationary, or even undergoing contemporaneous slow upheaval. There may have been an ascending movement in one region, and a descending one in a contiguous parallel zone of country. But even if that were the case, it is clear that finally an extensive depression took place in that part of Europe where the deep sea of the Cretaceous period was afterwards brought in.

THICKNESS OF THE WEALDEN.

In the Weald area itself, between the North and South Downs, fresh-water beds to the thickness of 1600 feet are known, the base not being reached. Probably the thickness of the whole Wealden series, as seen in Swanage Bay, can not be estimated as less than 2000 feet.

WEALDEN FLORA.

The flora of the Wealden is characterised by a great abundance of Coniferae, Cycadeae, anD Ferns, and by the absence of leaves and fruits of Dicotyledonous Angiosperms. The discovery in 1855, in the Hastings beds of the Isle of Wight, of Gyrogonites, or spore-vessels of the Chara, was the first example of that genus of plants, so common in the tertiary strata, being found in a Secondary or Mesozoic rock.

CHAPTER XIX.

JURa.s.sIC GROUP.-- PURBECK BEDS AND OOLITE.

The Purbeck Beds a Member of the Jura.s.sic Group.

Subdivisions of that Group.

Physical Geography of the Oolite in England and France.

Upper Oolite.

Purbeck Beds.

New Genera of fossil Mammalia in the Middle Purbeck of Dorsets.h.i.+re.

Dirt-bed or ancient Soil.

Fossils of the Purbeck Beds.

Portland Stone and Fossils.

Kimmeridge Clay.

Lithographic Stone of Solenhofen.

Archaeopteryx.

Middle Oolite.

Coral Rag.

Nerinaea Limestone.

Oxford Clay, Ammonites and Belemnites.

Kelloway Rock.

Lower, or Bath, Oolite.