The Student's Elements of Geology Part 39

a. Cypris Purbeckensis, Forbes.

b. Same magnified.

c. Cypris punctata, Forbes.

d, e. Two views magnified of the same.)

Beneath the thin marine band mentioned above as the base of the Middle Purbeck, some purely fresh-water marls occur, containing species of Cypris (Figure 307 a, c), Valvata, and Limnaea, 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, at Meup's Bay, more than thirty feet of brackish-water strata, abounding in a species of Serpula, allied to, if not identical with, Serpula coacervites, found in beds of the same age in 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 these marine 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, and rests upon the lowest fresh-water limestone, a rock about eight feet thick, containing Cyclas, Valvata, and Limnaea, of the same species as those of the uppermost part of the Lower Purbeck, or above the dirt-bed. The fresh-water limestone in its turn rests upon the top beds of the Portland stone, which, although it contains purely marine remains, often consists of a rock undistinguishable in mineral character from the Lowest Purbeck limestone.

DIRT-BED OR ANCIENT SURFACE-SOIL.

(FIGURE 308. Mantellia nidiformis, Brongniart. The upper part shows the woody stem, the lower part the bases of the leaves.)

The most remarkable of all the varied succession 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 and sub-angular fragments of stone, from 3 to 9 inches in diameter, in such numbers that it almost deserves the name of gravel.

I also saw in 1866, in Portland, a smaller dirt-bed six feet below the princ.i.p.al one, six inches thick, consisting of brown earth with upright Cycads of the same species, Mantellia nidiformis, as those found in the upper bed, but no Coniferae. The weight of the inc.u.mbent strata squeezing down the compressible dirt-bed has caused the Cycads to a.s.sume that form which has led the quarrymen to call them "petrified birds' nests," which suggested to Brongniart the specific name of nidiformis. I am indebted to Mr. Carruthers for Figure 308 of one of these Purbeck specimens, in which the original cylindrical figure has been less distorted than usual by pressure.

Many silicified trunks of coniferous trees, and the remains of plants allied to Zamia and Cycas, are buried in this dirt-bed, and must have become fossil on the spots where they grew. The stumps of the trees stand erect for a height of from one to three feet, and even in one instance to six feet, with their roots attached to the soil at about the same distances from one another as the trees in a modern forest. The carbonaceous matter is most abundant immediately around the stumps, and round the remains of fossil Cycadeae.

(FIGURE 309. Section in Isle of Portland, Dorset. (Buckland and De la Beche.)showing layers (from top to bottom): Fresh-water calcareous slate: Dirt- bed and ancient forest: Lowest fresh-water beds of the Lower Purbeck: and Portland stone, marine.)

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 three or four 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 root is about one foot; but I measured one myself, in 1866, which was 3 1/2 feet in diameter, said by the quarrymen to be unusually large. Root-shaped cavities were observed by Professor Henslow to descend from the bottom of the dirt-bed into the subjacent fresh-water stone, which, though now solid, must have been in a soft and penetrable state when the trees grew. The thin layers of calcareous slate (Figure 309) 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.

(FIGURE 310. Section of cliff east of Lulworth Cove. (Buckland and De la Beche.) showing layers (from top to bottom): Fresh-water calcareous slate: Dirt-bed, with stools of trees: Fresh-water: Portland stone, marine.)

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 degrees, 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 Figure 310).

From the facts above described we may infer, first, that those beds of the Upper 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 permit the growth of the Zamia and Cycas. Secondly. 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.

Thirdly. 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 fresh-water 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 taken place.

The forest of the dirt-bed, as before hinted, was not everywhere the first vegetation which grew in this region. Besides the lower bed containing upright Cycadeae, before mentioned, another has sometimes been found above it, which implies oscillations in the level of the same ground, and its alternate occupation by land and water more than once.

SUBDIVISIONS OF THE PURBECK.

It will be observed that the division of the Purbecks into upper, middle, and lower, was made by Professor Forbes strictly on the principle of the entire distinctness of the species of organic remains which they include. The lines of demarkation are not lines of disturbance, nor indicated by any striking physical characters or mineral changes. The features which attract the eye in the Purbecks, such as the dirt-beds, the dislocated strata at Lulworth, and the Cinder-bed, do not indicate any breaks in the distribution of organised beings.

"The causes which led to a complete change of life three times during the deposition of the fresh-water and brackish strata must," says this naturalist, "be sought for, not simply in either a rapid or a sudden change of their area into land or sea, but in the great lapse of time which intervened between the epochs of deposition at certain periods during their formation."

Each dirt-bed may, no doubt, be the memorial of many thousand years or centuries, because we find that two or three feet of vegetable soil is the only monument which many a tropical forest has left of its existence ever since the ground on which it now stands was first covered with its shade. Yet, even if we imagine the fossil soils of the Lower Purbeck to represent as many ages, we need not be surprised to find that they do not const.i.tute lines of separation between strata characterised by different zoological types. The preservation of a layer of vegetable soil, when in the act of being submerged, must be regarded as a rare exception to a general rule. It is of so perishable a nature, that it must usually be carried away by the denuding waves or currents of the sea, or by a river; and many Purbeck dirt-beds were probably formed in succession and annihilated, besides those few which now remain.

The plants of the Purbeck beds, so far as our knowledge extends at present, consist chiefly of Ferns, Coniferae, and Cycadeae (Figure 308), without any angiosperms; the whole more allied to the Oolitic than to the Cretaceous vegetation. The same affinity is indicated by the vertebrate and invertebrate animals. Mr. Brodie has found the remains of beetles and several insects of the h.o.m.opterous and trichopterous orders, some of which now live on plants, while others are of such forms as hover over the surface of our present rivers.

PORTLAND OOLITE AND SAND (b, TABLE 19.1).

(FIGURE 311. Cerithium Portlandic.u.m (=Terebra) Sowerby.

a. Cast of sh.e.l.l known as "Portland screw."

b. The sh.e.l.l itself. )

(FIGURE 312. Isastraea oblonga, M. Edw. and J. Haime. As seen on a polished slab of chert from the Portland Sand, Tisbury.)

(FIGURE 313. Trigonia gibbosa. 1/2 natural size. Portland Stone, Tisbury.

a. The hinge.)

(FIGURE 314. Cardium dissimile. 1/4 natural size. Portland Stone.)

(FIGURE 315. Ostrea expansa. Portland Sand.)

The Portland Oolite has already been mentioned as forming in Dorsets.h.i.+re the foundation on which the fresh-water limestone of the Lower Purbeck reposes (see above). It supplies the well-known building-stone of which St. Paul's and so many of the princ.i.p.al edifices of London are constructed. About fifty species of mollusca occur in this formation, among which are some ammonites of large size.

The cast of a spiral univalve called by the quarrymen the "Portland screw" (a, Figure 311), is common; the sh.e.l.l of the same (b) being rarely met with. Also Trigonia gibbosa (Figure 313) and Cardium dissimile (Figure 314). This upper member rests on a dense bed of sand, called the Portland Sand, containing similar marine fossils, below which is the Kimmeridge Clay. In England these Upper Oolite formations are almost wholly confined to the southern counties. But some fragments of them occur beneath the Neocomian or Speeton Clay on the coast of Yorks.h.i.+re, containing many more fossils common to the Portlandian of the Continent than does the same formation in Dorsets.h.i.+re. Corals are rare in this formation, although one species is found plentifully at Tisbury, Wilts.h.i.+re, in the Portland Sand, converted into flint and chert, the original calcareous matter being replaced by silex (Figure 312).

KIMMERIDGE CLAY.

The Kimmeridge Clay consists, in great part, of a bituminous shale, sometimes forming an impure coal, several hundred feet in thickness. In some places in Wilts.h.i.+re it much resembles peat; and the bituminous matter may have been, in part at least, derived from the decomposition of vegetables. But as impressions of plants are rare in these shales, which contain ammonites, oysters, and other marine sh.e.l.ls, with skeletons of fish and saurians, the bitumen may perhaps be of animal origin. Some of the saurians (Pliosaurus) in Dorsets.h.i.+re are among the most gigantic of their kind.

(FIGURE 316. Cardium striatulum. Kimmeridge Clay, Hartwell.)

(FIGURE 317. Ostrea deltoidea. Kimmeridge Clay, 1/4 natural size.)

(FIGURE 318. Gryphaea (Exogyra) virgula. Kimmeridge Clay.)

(FIGURE 319. Trigonellites latus, Park, Kimmeridge Clay.)

Among the fossils, amounting to nearly 100 species, may be mentioned Cardium striatulum (Figure 316) and Ostrea deltoidea (Figure 317), the latter found in the Kimmeridge Clay throughout England and the north of France, and also in Scotland, near Brora. The Gryphaea virgula (Figure 318), also met with in the Kimmeridge Clay near Oxford, is so abundant in the Upper Oolite of parts of France as to have caused the deposit to be termed "marnes a gryphees virgules."

Near Clermont, in Argonne, a few leagues from St. Menehould, where these indurated marls crop out from beneath the Gault, I have seen them, on decomposing, leave the surface of every ploughed field literally strewed over with this fossil oyster. The Trigonellites latus (Aptychus of some authors)(Figure 319) is also widely dispersed through this clay. The real nature of the sh.e.l.l, of which there are many species in oolitic rocks, is still a matter of conjecture. Some are of opinion that the two plates have been the gizzard of a cephalopod; others, that it may have formed a bivalve operculum of the same.

SOLENHOFEN STONE.

(FIGURE 320. Skeleton of Pterodactylus cra.s.sirostris. Oolite of Pappenheim, near Solenhofen.

a. This bone, consisting of four joints, is part of the fifth or outermost digit elongated, as in bats, for the support of a wing.)

The celebrated lithographic stone of Solenhofen in Bavaria, appears to be of intermediate age between the Kimmeridge clay and the Coral Rag, presently to be described. It affords a remarkable example of the variety of fossils which may be preserved under favourable circ.u.mstances, and what delicate impressions of the tender parts of certain animals and plants may be retained where the sediment is of extreme fineness. Although the number of testacea in this slate is small, and the plants few, and those all marine, count Munster had determined no less than 237 species of fossils when I saw his collection in 1833; and among them no less than seven SPECIES of flying reptiles or pterodactyls (see Figure 320), six saurians, three tortoises, sixty species of fish, forty-six of crustacea, and twenty-six of insects. These insects, among which is a libellula, or dragon-fly, must have been blown out to sea, probably from the same land to which the pterodactyls, and other contemporaneous air-breathers, resorted.

(FIGURE 321. Tail and feather of Archaeopteryx, from Solenhofen, and tail of living bird for comparison.

A. Caudal vertebrae of Archaeopteryx macrura, Owen; with impression of tail- feathers; one-fifth natural size.

B. Two caudal vertebrae of same; natural size.

C. Single feather, found in 1861 at Solenhofen, by Von Meyer, and called Archaeopteryx lithographica; natural size.

D. Tail of recent vulture (Gyps Bengalensis) showing attachment of tail-feathers in living birds; one-quarter natural size.

E. Profile of caudal vertebrae of same; one-third natural size.

e, e. Direction of tail-feathers when seen in profile.

f. Ploughshare bone or broad terminal joint (seen also in f, D.))

In the same slate of Solenhofen a fine example was met with in 1862 of the skeleton of a bird almost entire, and retaining even its feathers so perfect that the vanes as well as the shaft are preserved. The head was at first supposed to be wanting, but Mr. Evans detected on the slab what seems to be the impression of the cranium and beak, much resembling in size and shape that of the jay or woodc.o.c.k. This valuable specimen is now in the British Museum, and has been called by Professor Owen Archaeopteryx macrura. Although anatomists agree that it is a true bird, yet they also find that in the length of the bones of the tail, and some other minor points of its anatomy, it approaches more nearly to reptiles than any known living bird. In the living representatives of the cla.s.s Aves, the tail-feathers are attached to a coccygian bone, consisting of several vertebrae united together, whereas in the Archaeopteryx the tail is composed of twenty vertebrae, each of which supports a pair of quill-feathers.

The first five only of the vertebrae, as seen in A, have transverse processes, the fifteen remaining ones become gradually longer and more tapering. The feathers diverge outward from them at an angle of 45 degrees.

Professor Huxley in his late memoirs on the order of reptiles called Dinosaurians, which are largely represented in all the formations, from the Neocomian to the Trias inclusive, has shown that they present in their structure many remarkable affinities to birds. But a reptile about two feet long, called Compsognathus, lately found in the Stonesfield slate, makes a much greater approximation to the cla.s.s Aves than any Dinosaur, and therefore forms a closer link between the cla.s.ses Aves and Reptilia than does the Archaeopteryx.

It appears doubtful whether any species of British fossil, whether of the vertebrate or invertebrate cla.s.s, is common to the Oolite and Chalk. But there is no similar break or discordance as we proceed downward, and pa.s.s from one to another of the several leading members of the Jura.s.sic group, the Upper, Middle, and Lower Oolite, and the Lias, there being often a considerable proportion of the mollusca, sometimes as much as a fourth, common to such divisions as the Upper and Middle Oolite.

MIDDLE OOLITE.

CORAL RAG.

(FIGURE 322. Thecosmilia annularis, Milne Edwards and J. Haime. Coral Rag, Steeple Ashton.)