A Manual of Elementary Geology Part 35

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.

[Ill.u.s.tration: Fig. 232. _Cypris spinigera_, Fitton.]

[Ill.u.s.tration: Fig. 233. _Cypris Valdensis_, Fitton. (_C. faba_, Min. Con. 485.)]

[Ill.u.s.tration: Fig. 234. _Cypris tuberculata_, Fitton.]

[Ill.u.s.tration: Fig. 235. Sample with lamination.]

Sh.e.l.ls of the _Cypris_, an animal belonging to the Crustacea, and before mentioned (p. 31.) as abounding in lakes and ponds, are also plentifully scattered through the clays of the Wealden, sometimes producing, like the plates of mica, a thin lamination (see fig. 235.). Similar cypriferous marls are found in the lacustrine tertiary beds of Auvergne (see above, p. 183.).

_Hastings Sands._

This middle division of the Wealden consists of sand, calciferous grit, clay, and shale; the argillaceous strata, notwithstanding the name, being nearly in the same proportion as the arenaceous. The calcareous sandstone and grit of Tilgate Forest, near Cuckfield, in which the remains of the Iguanodon and Hyleosaurus were first found, const.i.tute an upper member of this formation. The white "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 it, consist partly of saurians, already 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 Testudinata of the genera _Trionyx_ and _Emys_, now confined to tropical regions.

[Ill.u.s.tration: Fig. 236. _Lepidotus Mantelli_, Aga.s.s. Wealden.

_a._ palate and teeth.

_b._ side view of teeth.

_c._ scale.]

The fishes of the Wealden belong partly to the genera _Pycnodus_ and _Hybodus_ (see figure of genus in Chap. XXI.), forms common to the Wealden and Oolite; but the teeth and scales of a species of _Lepidotus_ are most widely diffused (see fig. 236.). The general form of these fish was that of the carp tribe, although perfectly distinct in anatomical character, and more allied to the pike. The whole body was covered with large rhomboidal scales, very thick, and having the exposed part covered with enamel. Most of the species of this genus are supposed to have been either river fish, or inhabitants of the coasts, having not sufficient powers of swimming to advance into the deep sea.

[Ill.u.s.tration: Fig. 237. _Corbula alata_, Fitton. Magnified.]

The sh.e.l.ls of the Hastings beds belong to the genera _Melanopsis_, _Melania_, _Paludina_, _Cyrena_, _Cyclas_, _Unio_, and others, which inhabit rivers or lakes; but one band has been found in Dorsets.h.i.+re indicating a brackish state of the water, and, in some places, even a saltness, like that of the sea, where the genera _Corbula_ (see fig. 237.), _Mytilus_, and _Ostrea_ occur. At different heights in the Hastings Sand, in the middle of the Wealden, 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, near Horsham, 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 fig. 238.).

[Ill.u.s.tration: Fig. 238. Underside of slab of sandstone about one yard in diameter. Stammerham, Suss.e.x.]

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.[230-A] 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.

[Ill.u.s.tration: Fig. 239. _Sphenopteris gracilis_ (Fitton), from near Tunbridge Wells.

_a._ portion of the same magnified.]

From such facts we may infer that, notwithstanding the great thickness of this division of the Wealden (and the same observation applies to the Weald Clay and Purbeck Beds), 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, such as the Portland Oolite, 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.

_Purbeck Beds._

Immediately below the Hastings Sands we find a series of calcareous slates, marls, and limestones, called the Purbeck Beds, because well exposed to view in the sea-cliffs of the Peninsula of Purbeck, especially in Durlestone Bay, near Swanage. They may also be advantageously studied at Lulworth Cove and the neighbouring bays between Weymouth and Dorchester. At Meup's Bay in particular, Prof. E.

Forbes has recently examined minutely the organic remains of the three members of the Purbeck group, displayed there in a vertical section 155 feet thick. To the information previously supplied in the works of Messrs. Webster, Fitton, De la Beche, Buckland, and Mantell, he has made most ample and important additions, so that it will be desirable to give them at some length, it appearing that the Upper, Middle, and Lower Purbecks are each marked by peculiar species of organic remains, these again being different, so far as a comparison has yet been inst.i.tuted, from the fossils of the overlying Hastings Sands and Weald Clay. This result cannot fail to excite much wonder, and it leads us to suspect that the Wealden period, which many geologists have scarcely deigned to notice in their cla.s.sification, may comprehend the history of a lapse of time as great as that of the Oolitic or Cretaceous eras respectively.[231-A]

_Upper Purbeck._--The highest of the three divisions is purely freshwater, the strata, about 50 feet in thickness, containing sh.e.l.ls of the genera _Paludina_, _Physa_, _Lymnea_, _Planorbis_, _Valvata_, _Cyclas_, and _Unio_, with cyprides, and fish.

_Middle Purbeck._--To these succeed the Middle Purbeck, about 30 feet thick, the uppermost part of which consists of freshwater limestone, with cyprides, turtles, and fish of different species from those in the preceding strata. Below the limestone are brackish-water beds full of _Cyrena_, and traversed by bands abounding in _Corvulae_ and _Melaniae_.

These are based on a purely marine deposit, with _Pecten_, _Modiola_, _Avicula_, and _Thracia_, all undescribed sh.e.l.ls. Below this, again, come limestones and shales, partly of brackish and partly of freshwater origin, in which many fish, especially species of _Lepidotus_ and _Microdon radiatus_, are found, and a reptile named _Macrorhyncus_. Among the mollusks, a remarkable ribbed _Melania_, of the section _Chilira_, occurs.

Immediately below is the great and conspicuous stratum, 12 feet thick, long familiar to geologists under the local name of "Cinder-bed," formed of a vast acc.u.mulation of sh.e.l.ls of _Ostrea distorta_ (fig. 240.). In the uppermost part of this bed Mr. Forbes discovered the first echinoderm as yet known in the Purbeck series, a species of _Hemicidaris_, a genus characteristic of the Oolitic period. It was accompanied by a species of _Perna_. Below the Cinder-bed freshwater strata are again seen, filled in many places with species of _Cypris_, _Valvata_, _Paludina_, _Planorbis_, _Lymnea_, _Physa_, and _Cyclas_, all different from any we had previously seen above. Thick siliceous beds of chert, filled with these fossils, occur in a beautiful state of preservation, often converted into chalcedony.

Among these Mr. Forbes met with gyrogonites (the spore vesicles of _Charae_), plants never before discovered in rocks older than the Eocene.

Again, beneath these freshwater strata, a very thin band of greenish shales, with marine sh.e.l.ls and impressions of leaves, like those of a large _Zostera_, succeeds, forming the base of the Middle Purbeck.

[Ill.u.s.tration: Fig. 240. Ostrea distorta. Cinder-bed.]

_Lower Purbeck._--Beneath the thin marine band last mentioned, purely freshwater marls occur, containing species of _Cypris_, _Valvata_, and _Lymnea_, different from those of the Middle Purbeck. This is the beginning of the Inferior division, which is about 80 feet thick. Below the marls are seen more than 30 feet of brackish-water beds, at Meup's Bay, abounding in a species of _Serpula_, allied to, if not identical with, _Serpula coacervites_, found in the Wealden of Hanover. There are also sh.e.l.ls of the genus _Rissoa_ (of the subgenus _Hydrobia_), and a little _Cardium_ of the subgenus _Protocardium_, in the same beds, together with _Cypris_. Some of the cypris-bearing shales are strangely contorted and broken up, at the west end of the Isle of Purbeck. The great dirt-bed or vegetable soil containing the roots and stools of _Cycadeae_, which I shall presently describe, underlies these marls, resting upon the lowest freshwater limestone, a rock about 8 feet thick, containing _Cyclades_, _Valvata_, and _Lymnea_, of the same species as those of the uppermost part of the Lower Purbeck. This rock rests upon the top beds of the Portland stone, which is purely marine, and between which and the Purbecks there is no pa.s.sage.

The most remarkable of all the varied successions of beds enumerated in the above list, is that called by the quarrymen "the dirt," or "black dirt,"

which was evidently an ancient vegetable soil. It is from 12 to 18 inches thick, is of a dark brown or black colour, and contains a large proportion of earthy lignite. Through it are dispersed rounded fragments of stone, from 3 to 9 inches in diameter, in such numbers that it almost deserves the name of gravel. Many silicified trunks of coniferous trees, and the remains of plants allied to _Zamia_ and _Cycas_, are buried in this dirt-bed (see figure of living _Zamia_, fig. 241.).

These plants must have become fossil on the spots where they grew. The stumps of the trees stand erect for a height of from 1 to 3 feet, and even in one instance to 6 feet, with their roots attached to the soil at about the same distances from one another as the trees in a modern forest.[233-A] The carbonaceous matter is most abundant immediately around the stumps, and round the remains of fossil _Cycadeae_.[233-B]

[Ill.u.s.tration: Fig. 241. Zamia spiralis; Southern Australia.[233-C]]

Besides the upright stumps above mentioned, the dirt-bed contains the stems of silicified trees laid prostrate. These are partly sunk into the black earth, and partly enveloped by a calcareous slate which covers the dirt-bed. The fragments of the prostrate trees are rarely more than 3 or 4 feet in length; but by joining many of them together, trunks have been restored, having a length from the root to the branches of from 20 to 23 feet, the stems being undivided for 17 or 20 feet, and then forked. The diameter of these near the roots is about 1 foot.[233-D] Root-shaped cavities were observed by Professor Henslow to descend from the bottom of the dirt-bed into the subjacent freshwater stone, which, though now solid, must have been in a soft and penetrable state when the trees grew.[233-E]

[Ill.u.s.tration: Fig. 242. Section in Isle of Portland, Dorset.

(Buckland and De la Beche.)]

The thin layers of calcareous slate (fig. 242.) were evidently deposited tranquilly, and would have been horizontal but for the protrusion of the stumps of the trees, around the top of each of which they form hemispherical concretions.

The dirt-bed is by no means confined to the island of Portland, where it has been most carefully studied, but is seen in the same relative position in the cliffs east of Lulworth Cove, in Dorsets.h.i.+re, where, as the strata have been disturbed, and are now inclined at an angle of 45, the stumps of the trees are also inclined at the same angle in an opposite direction--a beautiful ill.u.s.tration of a change in the position of beds originally horizontal (see fig. 243.). Traces of the dirt-bed have also been observed by Dr. Buckland, about two miles north of Thame, in Oxfords.h.i.+re; and by Dr.

Fitton, in the cliffs of the Boulonnois, on the French coast; but, as might be expected, this freshwater deposit is of limited extent when compared to most marine formations.

[Ill.u.s.tration: Fig. 243. Section in cliff east of Lulworth Cove.

(Buckland and De la Beche.)]

From the facts above described, we may infer, first, that the superior beds of the Oolite, called "the Portland," which are full of marine sh.e.l.ls, were overspread with fluviatile mud, which became dry land, and covered by a forest, throughout a portion of the s.p.a.ce now occupied by the south of England, the climate being such as to admit the growth of the _Zamia_ and _Cycas_. 2dly. This land at length sank down and was submerged with its forests beneath a body of fresh water, from which sediment was thrown down enveloping fluviatile sh.e.l.ls. 3dly. The regular and uniform preservation of this thin bed of black earth over a distance of many miles, shows that the change from dry land to the state of a freshwater lake or estuary, was not accompanied by any violent denudation, or rush of water, since the loose black earth, together with the trees which lay prostrate on its surface, must inevitably have been swept away had any such violent catastrophe then taken place.

The dirt-bed has been described above in its most simple form, but in some sections the appearances are more complicated. The forest of the dirt-bed was not everywhere the first vegetation which grew in this region. Two other beds of carbonaceous clay, one of them containing _Cycadeae_, in an upright position, have been found below it, and one above it[234-A], which implies other oscillations in the level of the same ground, and its alternate occupation by land and water more than once.

_Table showing the changes of medium in which the strata were formed, from the Lower Greensand to the Portland Stone inclusive, in the south-east of England._

1. Marine Lower greensand.

2. Freshwater Weald clay.

3. Freshwater } Brackish } Hastings sand.

Freshwater }

4. Freshwater Upper Purbeck.

5. Freshwater } Brackish } Marine } Brackish } Middle Purbeck.

Marine } Freshwater } Marine }

6. Freshwater } Brackish } Land } Freshwater } Land (dirt-bed) } Lower Purbeck.

Freshwater } Land } Freshwater } Land } Freshwater }