The Student's Elements of Geology Part 40

(FIGURE 323. Thamnastraea. Coral Rag. Steeple Ashton.)

(FIGURE 324. Ostrea gregaria, Coral Rag, Steeple Ashton.)

One of the limestones of the Middle Oolite has been called the "Coral Rag,"

because it consists, in part, of continuous beds of petrified corals, most of them retaining the position in which they grew at the bottom of the sea. In their forms they more frequently resemble the reef-building polyparia of the Pacific than do the corals of any other member of the Oolite. They belong chiefly to the genera Thecosmilia (Figure 322), Protoseris, and Thamnastraea, and sometimes form ma.s.ses of coral fifteen feet thick. In Figure 323 of a Thamnastraea from this formation, it will be seen that the cup-shaped cavities are deepest on the right-hand side, and that they grow more and more shallow, until those on the left side are nearly filled up. The last-mentioned stars are supposed to represent a perfected condition, and the others an immature state.

These coralline strata extend through the calcareous hills of the north-west of Berks.h.i.+re, and north of Wilts, and again recur in Yorks.h.i.+re, near Scarborough.

The Ostrea gregarea (Figure 324) is very characteristic of the formation in England and on the Continent.

(FIGURE 325. Nerinaea Goodhallii, Fitton. Coral Rag, Weymouth. 1/4 natural size.)

One of the limestones of the Jura, referred to the age of the English Coral Rag, has been called "Nerinaean limestone" (Calcaire a Nerinees) by M. Thirria; Nerinaea being an extinct genus of univalve sh.e.l.ls (Figure 325) much resembling the Cerithium in external form. Figure 325 shows the curious and continuous ridges on the columnella and whorls.

OXFORD CLAY.

(FIGURE 326. Belemnites hastatus. Oxford Clay.)

(FIGURE 327. Ammonites Jason, Reinecke. (Syn. A. Elizabethae, Pratt. Oxford Clay, Christian Malford, Wilts.h.i.+re.)

(FIGURE 328. Belemnites Puzosia.n.u.s, d'Orbigny. B. Owenii, Pierce. Oxford Clay, Christian Malford, Wilts.h.i.+re.

a. Section of the sh.e.l.l projecting from the phragmacone.

b-c. External covering to the ink-bag and phragmacone.

c, d. Osselet, or that portion commonly called the belemnite.

e. Conical chambered body called the phragmacone.

f. Position of ink-bag beneath the sh.e.l.ly covering.)

The coralline limestone, or "Coral Rag," above described, and the accompanying sandy beds, called "calcareous grits," of the Middle Oolite, rest on a thick bed of clay, called the "Oxford Clay," sometimes not less than 600 feet thick. In this there are no corals, but great abundance of cephalopoda, of the genera Ammonite and Belemnite (Figures 326 and 327). In some of the finely laminated clays ammonites are very perfect, although somewhat compressed, and are frequently found with the lateral lobe extended on each side of the opening of the mouth into a horn-like projection (Figure 327). These were discovered in the cuttings of the Great Western Railway, near Chippenham, in 1841, and have been described by Mr. Pratt (Annals of Natural History November 1841).

Similar elongated processes have been also observed to extend from the sh.e.l.ls of some Belemnites discovered by Dr. Mantell in the same clay (see Figure 328), who, by the aid of this and other specimens, has been able to throw much light on the structure of singular extinct forms of cuttle-fish. (See Philosophical Transactions 1850 page 363; also Huxley Memoirs of Geological Survey 1864; Phillips Palaeontological Society.)

KELLOWAY ROCK.

The arenaceous limestone which pa.s.ses under this name is generally grouped as a member of the Oxford clay, in which it forms, in the south-west of England, lenticular ma.s.ses, 8 or 10 feet thick, containing at Kelloway, in Wilts.h.i.+re, numerous casts of ammonites and other sh.e.l.ls. But in Yorks.h.i.+re this calcareo- arenaceous formation thickens to about 30 feet, and const.i.tutes the lower part of the Middle Oolite, extending inland from Scarborough in a southerly direction. The number of mollusca which it contains is, according to Mr.

Etheridge, 143, of which only 34, or 23 1/2 per cent, are common to the Oxford clay proper. Of the 52 Cephalopoda, 15 (namely 13 species of ammonite, the Ancyloceras Calloviense and one Belemnite) are common to the Oxford Clay, giving a proportion of nearly 30 per cent.

LOWER OOLITE.

CORNBRASH AND FOREST MARBLE.

The upper division of this series, which is more extensive than the preceding or Middle Oolite, is called in England the Cornbrash, as being a brashy, easily broken rock, good for corn land. It consists of clays and calcareous sandstones, which pa.s.s downward into the Forest Marble, an argillaceous limestone, abounding in marine fossils. In some places, as at Bradford, this limestone is replaced by a ma.s.s of clay. The sandstones of the Forest Marble of Wilts.h.i.+re are often ripple-marked and filled with fragments of broken sh.e.l.ls and pieces of drift- wood, having evidently been formed on a coast. Rippled slabs of fissile oolite are used for roofing, and have been traced over a broad band of country from Bradford in Wilts, to Tetbury in Gloucesters.h.i.+re. These calcareous tile-stones are separated from each other by thin seams of clay, which have been deposited upon them, and have taken their form, preserving the undulating ridges and furrows of the sand in such complete integrity, that the impressions of small footsteps, apparently of crustaceans, which walked over the soft wet sands, are still visible. In the same stone the claws of crabs, fragments of echini, and other signs of a neighbouring beach, are observed. (P. Scrope Proceedings of the Geological Society March 1831.)

GREAT (OR BATH) OOLITE.

(FIGURE 329. Eunomia radiata, Lamouroux. (Calamophyllia, Milne Edwards.) a. Section transverse to the tubes.

b. Vertical section, showing the radiation of the tubes.

c. Portion of interior of tubes magnified, showing striated surface.)

Although the name of Coral Rag has been appropriated, as we have seen, to a member of the Middle Oolite before described, some portions of the Lower Oolite are equally ent.i.tled in many places to be called coralline limestones. Thus the Great Oolite near Bath contains various corals, among which the Eunomia radiata (Figure 329) is very conspicuous, single individuals forming ma.s.ses several feet in diameter; and having probably required, like the large existing brain-coral (Meandrina) of the tropics, many centuries before their growth was completed.

(FIGURE 330. Apiocrinites rotundus, or Pear Encrinite; Miller. Fossil at Bradford, Wilts.

a. Stem of Apiocrinites, and one of the articulations, natural size.

b. Section at Bradford of Great Oolite and overlying clay, containing the fossil encrinites. (See text.) c. Three perfect individuals of Apiocrinites, represented as they grew on the surface of the Great Oolite.

d. Body of the Apiocrinites rotundus. Half natural size.)

(FIGURE 331. Apiocrinus.

a. Single plate of body of Apiocrinus, overgrown with serpulae and bryozoa.

Natural size. Bradford Clay.

b. Portion of the same magnified, showing the bryozoan Diastopora diluviana covering one of the serpulae.)

Different species of crinoids, or stone-lilies, are also common in the same rocks with corals; and, like them, must have enjoyed a firm bottom, where their base of attachment remained undisturbed for years (c, Figure 330). Such fossils, therefore, are almost confined to the limestones; but an exception occurs at Bradford, near Bath, where they are enveloped in clay sometimes 60 feet thick.

In this case, however, it appears that the solid upper surface of the "Great Oolite" had supported, for a time, a thick submarine forest of these beautiful zoophytes, until the clear and still water was invaded by a current charged with mud, which threw down the stone-lilies, and broke most of their stems short off near the point of attachment. The stumps still remain in their original position; but the numerous articulations, once composing the stem, arms, and body of the encrinite, were scattered at random through the argillaceous deposit in which some now lie prostrate. These appearances are represented in the section b, Figure 330, where the darker strata represent the Bradford clay, which is however a formation of such local development that in many places it can not easily be separated from the clays of the overlying "forest-marble" and underlying "fuller's earth." The upper surface of the calcareous stone below is completely incrusted over with a continuous pavement, formed by the stony roots or attachments of the Crinoidea; and besides this evidence of the length of time they had lived on the spot, we find great numbers of single joints, or circular plates of the stem and body of the encrinite, covered over with serpulae. Now these serpulae could only have begun to grow after the death of some of the stone-lilies, parts of whose skeletons had been strewed over the floor of the ocean before the irruption of argillaceous mud. In some instances we find that, after the parasitic serpulae were full grown, they had become incrusted over with a bryozoan, called Diastopora diluviana (see b, Figure 331); and many generations of these molluscoids had succeeded each other in the pure water before they became fossil.

We may, therefore, perceive distinctly that, as the pines and cycadeous plants of the ancient "dirt-bed," or fossil forest, of the Lower Purbeck were killed by submergence under fresh water, and soon buried beneath muddy sediment, so an invasion of argillaceous matter put a sudden stop to the growth of the Bradford Encrinites, and led to their preservation in marine strata.

Such differences in the fossils as distinguish the calcareous and argillaceous deposits from each other, would be described by naturalists as arising out of a difference in the STATIONS of species; but besides these, there are variations in the fossils of the higher, middle, and lower part of the oolitic series, which must be ascribed to that great law of change in organic life by which distinct a.s.semblages of species have been adapted, at successive geological periods, to the varying conditions of the habitable surface. In a single district it is difficult to decide how far the limitation of species to certain minor formations has been due to the local influence of STATIONS, or how far it has been caused by time or the law of variation above alluded to. But we recognise the reality of the last-mentioned influence, when we contrast the whole oolitic series of England with that of parts of the Jura, Alps, and other distant regions, where, although there is scarcely any lithological resemblance, yet some of the same fossils remain peculiar in each country to the Upper, Middle, and Lower Oolite formations respectively. Mr. Thurmann has shown how remarkably this fact holds true in the Bernese Jura, although the argillaceous divisions, so conspicuous in England, are feebly represented there, and some entirely wanting.

(FIGURE 332. Terebratula digona, Sowerby. Natural size. Bradford Clay.)

(FIGURE 333. Purpuroidea nodulata. One-fourth natural size. Great Oolite, Minchinhampton.)

(FIGURE 334. Cylindrites acutus. Sowb. Syn. Actaeon acutus. Great Oolite, Minchinhampton.)

(FIGURE 335. Patella rugosa, Sowerby. Great Oolite.)

(FIGURE 336. Nerita costulata, Desh. Great Oolite.)

(FIGURE 337. Rimula (Emarginula) clathrata, Sowerby. Great Oolite.)

The calcareous portion of the Great Oolite consists of several sh.e.l.ly limestones, one of which, called the Bath Oolite, is much celebrated as a building-stone. In parts of Gloucesters.h.i.+re, especially near Minchinhampton, the Great Oolite, says Mr. Lycett, "must have been deposited in a shallow sea, where strong currents prevailed, for there are frequent changes in the mineral character of the deposit, and some beds exhibit false stratification. In others, heaps of broken sh.e.l.ls are mingled with pebbles of rocks foreign to the neighbourhood, and with fragments of abraded madrepores, dicotyledonous wood, and crabs' claws. The sh.e.l.ly strata, also, have occasionally suffered denudation, and the removed portions have been replaced by clay." In such shallow-water beds sh.e.l.ls of the genera Patella, Nerita, Rimula, Cylindrites are common (see Figures 334 to 337); while cephalopods are rare, and instead of ammonites and belemnites, numerous genera of carnivorous trachelipods appear.

Out of 224 species of univalves obtained from the Minchinhampton beds, Mr.

Lycett found no less than 50 to be carnivorous. They belong princ.i.p.ally to the genera Buccinum, Pleurotoma, Rostellaria, Murex, Purpuroidea (Figure 333), and Fusus, and exhibit a proportion of zoophagous species not very different from that which obtains in seas of the Recent period. These zoological results are curious and unexpected, since it was imagined that we might look in vain for the carnivorous trachelipods in rocks of such high antiquity as the Great Oolite, and it was a received doctrine that they did not begin to appear in considerable numbers till the Eocene period, when those two great families of cephalopoda, the ammonites and belemnites, and a great number of other representatives of the same cla.s.s of chambered sh.e.l.ls, had become extinct.

STONESFIELD SLATE: MAMMALIA.

(FIGURE 338. Elytron of Buprestis? Stonesfield.)

The slate of Stonesfield has been shown by Mr. Lonsdale to lie at the base of the Great Oolite. (Proceedings of the Geological Society volume 1 page 414.) It is a slightly oolitic sh.e.l.ly limestone, forming large lenticular ma.s.ses imbedded in sand only six feet thick, but very rich in organic remains. It contains some pebbles of a rock very similar to itself, and which may be portions of the deposit, broken up on a sh.o.r.e at low water or during storms, and redeposited.

The remains of belemnites, trigoniae, and other marine sh.e.l.ls, with fragments of wood, are common, and impressions of ferns, cycadeae, and other plants. Several insects, also, and, among the rest, the elytra or wing-covers of beetles, are perfectly preserved (see Figure 338), some of them approaching nearly to the genus Buprestis. The remains, also, of many genera of reptiles, such as Plesiosaur, Crocodile, and Pterodactyl, have been discovered in the same limestone.

But the remarkable fossils for which the Stonesfield slate is most celebrated are those referred to the mammiferous cla.s.s. The student should be reminded that in all the rocks described in the preceding chapters as older than the Eocene, no bones of any land-quadruped, or of any cetacean, had been discovered until the Spalacotherium of the Purbeck beds came to light in 1854. Yet we have seen that terrestrial plants were not wanting in the Upper Cretaceous formation (see Chapter 17), and that in the Wealden there was evidence of fresh-water sediment on a large scale, containing various plants, and even ancient vegetable soils.

We had also in the same Wealden many land-reptiles and winged insects, which render the absence of terrestrial quadrupeds the more striking. The want, however, of any bones of whales, seals, dolphins, and other aquatic mammalia, whether in the chalk or in the upper or middle oolite, is certainly still more remarkable.

These observations are made to prepare the reader to appreciate more justly the interest felt by every geologist in the discovery in the Stonesfield slate of no less than ten specimens of lower jaws of mammiferous quadrupeds, belonging to four different species and to three distinct genera, for which the names of Amphitherium, Phascolotherium, and Stereognathus have been adopted.

(FIGURE 339. Tupaia tana. Right ramus of lower jaw. Natural size. A recent insectivorous placental mammal, from Sumatra.)

(Figures 340 and 341. Part of lower jaw of Tupaia tana. Twice natural size.

(FIGURE 340. End view seen from behind, showing the very slight inflection of the angle at c.)