Subdivisions of the Oolitic or Jurassic group — Physical geography of the Oolite in England and France — Upper Oolite — Portland stone and fossils — Lithographic stone of Solenhofen — Middle Oolite, coral rag — Zoophytes — NerinÆan limestone — Diceras limestone — Oxford clay, Ammonites and Belemnites — Lower Oolite, Crinoideans — Great Oolite and Bradford clay — Stonesfield slate — Fossil mammalia, placental and marsupial — Resemblance to an Australian fauna — Doctrine of progressive development — Collyweston slates — Yorkshire Oolitic coal-field — Brora coal — Inferior Oolite and fossils. Oolitic or Jurassic Group.—Below the freshwater group called the Wealden, or, where this is wanting, immediately beneath the Cretaceous formation, a great series of marine strata, commonly called "the Oolite," occurs in England and many other parts of Europe. This group has been so named, because, in the countries where it was first examined, the limestones belonging to it had an oolitic structure (see p. 12.). These rocks occupy in England a zone which is nearly 30 miles in average breadth, and extends across the island, from Yorkshire in the north-east, to Dorsetshire in the south-west. Their mineral characters are not uniform throughout this
The Upper oolitic system of the above table has usually the Kimmeridge clay for its base; the Middle oolitic system, the Oxford clay. The Lower system reposes on the Lias, an argillo-calcareous formation, which some include in the Lower Oolite, but which will be treated of separately in the next chapter. Many of these subdivisions are distinguished by peculiar organic remains; and though varying in thickness, may be traced in certain directions for great distances, especially if we compare the part of England to which the above-mentioned type refers with the north-east of France, and the Jura mountains adjoining. In that country, distant above 400 geographical miles, the analogy to the English type, notwithstanding the thinness, or occasional absence of the clays, is more perfect than in Yorkshire or Normandy. Physical geography.—The alternation, on a grand scale, of distinct formations of clay and limestone, has caused the oolitic and liassic series to give rise to some marked features in the physical outline of parts of England and France. Wide valleys can usually be traced throughout the long bounds of country where the argillaceous strata crop out; and between these valleys the limestones are observed, composing ranges of hills, or more elevated grounds. These ranges terminate abruptly on the side on which the several clays rise up from beneath the calcareous strata. Fig. 266. The annexed diagram will give the reader an idea of the configuration of the surface now alluded to, such as may be seen in passing from London to Cheltenham, or in other parallel lines, from east to west, in the southern part of England. It has been necessary, however, in this drawing, greatly to exaggerate the inclination of the beds, and the height of the several formations, as compared to their horizontal extent. It will be remarked, that the lines of cliff, or escarpment, face towards the west in the great calcareous eminences formed by the Chalk and the Upper, Middle, and Lower Oolites; and at the base of which we have respectively the Gault, Kimmeridge clay, Oxford clay, and Lias. This last forms, generally, a broad vale The external outline of the country which the geologist observes in travelling eastward from Paris to Metz is precisely analogous, and is caused by a similar succession of rocks intervening between the tertiary strata and the Lias; with this difference, however, that the escarpments of Chalk, Upper, Middle, and Lower Oolites, face towards the east instead of the west. The Chalk crops out from beneath the tertiary sands and clays of the Paris basin, near Epernay, and the Gault from beneath the Chalk and Upper Greensand at Clermont-en-Argonne; and passing from this place by Verdun and Etain to Metz, we find two limestone ranges, with intervening vales of clay, precisely resembling those of southern and central England, until we reach the great plain of Lias at the base of the Inferior Oolite at Metz. It is evident, therefore, that the denuding causes have acted similarly over an area several hundred miles in diameter, sweeping away the softer clays more extensively than the limestones, and undermining these last so as to cause them to form steep cliffs wherever the harder calcareous rock was based upon a more yielding and destructible clay. This denudation probably occurred while the land was slowly rising out of the sea. Upper Oolite.The Portland stone has already been mentioned as forming in Dorsetshire the foundation on which the freshwater limestone of the Lower Purbeck reposes (see p. 232.). It supplies the well-known building stone of which St. Paul's and so many of the principal edifices of London are constructed. This upper member, characterized by peculiar marine fossils, rests on a dense bed of sand, called the Portland sand, below which is the Kimmeridge clay. In England these Upper Oolite formations are almost wholly confined to the southern counties. Corals are rare in them, although one species is found plentifully at Tisbury, in Wiltshire, in the Portland sand converted into flint and chert, the original calcareous matter being replaced by silex (fig. 267.). Fig. 267. Columnaria oblonga, Blainv. As seen on a polished slab of chert from the sand of the Upper Oolite, Tisbury. Among the characteristic fossils of the Upper Oolite, may be mentioned the Ostrea deltoidea (fig. 269.), found in the Kimmeridge clay throughout England and the north of France, and also in Scotland, near Brora. The GryphÆa virgula (fig. 268.), also met with in the same clay near Oxford, is so abundant in the Upper Oolite of parts of France as to have caused the deposit to be termed "marnes À gryphÉes virgules." Near Upper Oolite: Kimmeridge clay. 1/4 nat. size. Fig. 268. GryphÆa virgula. Fig. 269. Ostrea deltoidea. Fig. 270. Trigonia gibbosa. 1/2 nat. size. a. the hinge. Portland Oolite, Tisbury. 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 Wiltshire 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 shells, the bitumen may perhaps be of animal origin. The celebrated lithographic stone of Solenhofen, in Bavaria, belongs to one of the upper divisions of the oolite, and affords a remarkable example of the variety of fossils which may be preserved under favourable circumstances, 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 lizards, or pterodactyls, 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 flying lizards, and other contemporaneous reptiles, resorted. Middle Oolite.Coral Rag.—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, for the most part retaining the position in which they grew at the bottom of the sea. They belong chiefly to the genera Caryophyllia (fig. 271.), Agaricia, and Astrea, and sometimes form masses of coral 15 feet thick. In the annexed figure of an Astrea, 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, till those on the left side are nearly filled up. The last-named stars are supposed to be Polyparia of advanced age. Fig. 271. Caryophyllia annularis, Parkin. Coral rag, Steeple Ashton. Fig 272. Astrea. Coral rag. One of the limestones of the Jura, referred to the age of the English coral rag, has been called "NerinÆan limestone" (Calcaire À NÉrinÉes) by M. Thirria; NerinÆa being an extinct genus of univalve shells, much resembling the Cerithium in external form. The annexed section (fig. 273.) shows the curious form of the hollow part of each whorl, and also the perforation which passes up the middle of the columella. N. Goodhallii (fig. 274.) is another English species of the same genus, from a formation which seems to form a passage from the Kimmeridge clay to the coral rag. Fig. 273. NerinÆa hieroglyphica. Coral rag. Fig. 274. NerinÆa Goodhallii, Fitton. Coral rag, Weymouth. 1/4 nat. size. A division of the oolite in the Alps, regarded by most geologists as coeval with the English coral rag, has been often named "Calcaire À Dicerates," or "Diceras limestone," from its containing abundantly a bivalve shell (see fig. 275.) of a genus allied to the Chama. Fig. 275. Cast of Diceras arietina. Coral rag, France. Fig. 276. Cidaris coronata. Coral rag. Oxford Clay.—The coralline limestone, or "coral rag," above described, and the accompanying sandy beds, called "calcareous grits" of the Middle Oolite, rests on a thick bed of clay, called the Oxford clay, sometimes not less than 500 feet thick. In this there are no corals, but great abundance of cephalopoda of the genera Ammonite and Belemnite. (See fig. 277.) In some of the clay of very fine texture ammonites are very perfect, although somewhat compressed, and are seen to be furnished on each side of the aperture with a single horn-like projection (see fig. 278.). These were discovered in the cuttings of the Great Western Railway, near Chippenham, in 1841, and have been described by Mr. Pratt. Fig. 277. Belemnites hastatus. Oxford Clay. Fig. 278. Ammonites Jason, Reinecke. Syn. A. ElizabethÆ, Pratt. Oxford clay, Christian Malford, Wiltshire. Fig. 279. Belemnites Puzosianus, D'Orb. Oxford Clay, Christian Malford.
Similar elongated processes have been also observed to extend from the shells of some belemnites discovered by Dr. Mantell in the same clay (see fig. 279.), who, by the aid of this and other specimens, has been able to throw much light on the structure of this singular extinct form of cuttle-fish. Lower Oolite.The upper division of this series, which is more extensive than the preceding or Middle Oolite, is called in England the Cornbrash. It consists of clays and calcareous sandstones, which pass downwards into the Forest marble, an argillaceous limestone, abounding in marine fossils. In some places, as at Bradford, this limestone is replaced by a mass of clay. The sandstones of the Forest Marble of Wiltshire are often ripple-marked and filled with fragments of broken shells 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 Gloucestershire. 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 crabs, 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. Great Oolite.—Although the name of coral-rag has been appropriated, as we have seen, to a member of the Upper Oolite before described, some portions of the Lower Oolite are equally intitled in many places to be called coralline limestones. Thus the Great Oolite near Bath contains various corals, among which the Eunomia radiata Fig. 280. Eunomia radiata, Lamouroux.
Fig. 281. Apiocrinites rotundus, or Pear Encrinite; Miller. Fossil at Bradford, Wilts.
Different species of Crinoideans, or stone-lilies, are also common in the same rocks with corals; and, like them, must have enjoyed a firm bottom, where their root, or base of attachment, remained undisturbed for years (c, fig. 281.). Such fossils, therefore, are almost confined to the limestones; but an exception occurs at Bradford, near Bath, where they are enveloped in clay. 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 zoophyte, were scattered at random through the argillaceous deposit Fig. 282.
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 assemblages 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 Fig. 283. Terebratula digona. Bradford clay. Nat. size. The Bradford clay above alluded to is sometimes 60 feet thick, but, in many places, it is wanting; and, in others, where there are no limestones, it cannot easily be separated from the clays of the overlying "forest marble" and underlying "fuller's earth." The calcareous portion of the Great Oolite consists of several shelly limestones, one of which, called the Bath Oolite, is much celebrated as a building stone. In parts of Gloucestershire, 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 shells are mingled with pebbles of rocks foreign to the neighbourhood, and with fragments of abraded madrepores, dicotyledonous wood, and crabs' claws. The shelly strata, also, have occasionally suffered denudation, and the removed portions have been replaced by clay." Stonesfield slate.—The slate of Stonesfield has been shown by Mr. Lonsdale to lie at the base of the Great Oolite. Fig. 284. Elytron of Buprestis? Stonesfield. Fig. 285. Bone of a reptile, formerly supposed to be the ulna of a Cetacean; from the Great Oolite of Enstone, near Woodstock. But the remarkable fossils for which the Stonesfield slate is most celebrated, are those referred to the mammiferous class. 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, have been discovered. Yet we have seen that terrestrial plants were not rare in the lower cretaceous formation, and that in the Wealden there was evidence of freshwater sediment on a large scale, containing various plants, and even ancient vegetable soils with the roots and erect stumps of trees. We had also in the same Wealden many land-reptiles and winged-insects, which renders 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. Formerly, indeed, a bone from the great oolite of Enstone, near Woodstock, in Oxfordshire, was cited, on the authority of Cuvier, as referable to this class. Dr. Buckland, who stated this in his Bridgewater Treatise Fig. 286. Amphitherium Prevostii. Stonesfield Slate.
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 seven specimens of lower jaws of mammiferous quadrupeds, belonging to three different species and to two distinct genera, for which the names of Amphitherium and Phascolotherium have been adopted. When Cuvier was first shown one of these fossils in 1818, he pronounced it to belong to a small ferine mammal, with a jaw much resembling that of an opossum, but differing from all known ferine genera, in the great number of the molar teeth, of which it had at least ten in a row. Since that period, a much more perfect specimen of the same fossil, obtained by Dr. Buckland (see fig. 286.), has been examined by Mr. Owen, who finds that the jaw contained on the whole twelve molar teeth, with the socket of a small canine, and three small incisors, which are in situ, altogether amounting to sixteen teeth on each side of the lower jaw. Fig. 287. Amphitherium Broderipii. Natural size. Stonesfield Slate. The only question which could be raised respecting the nature of these fossils was, whether they belonged to a mammifer, a reptile, or a fish. Now on this head the osteologist observes that each of the seven half jaws is composed of but one single piece, and not of two or more separate bones, as in fishes and most reptiles, or of two bones, united by a suture, as in some few species belonging to those classes. The condyle, moreover (b, fig. 286.), or articular surface, by which the lower jaw unites with the upper, is convex in the Stonesfield specimens, and not concave as in fishes and reptiles. The coronoid process (a, fig. 286.) is well developed, whereas it is wanting or very small, in the inferior classes of vertebrata. Lastly, the molar teeth in the Amphitherium and Phascolotherium Fig. 288. Tupaia Tana. Right ramus of lower jaw, natural size. A recent insectivorous mammal from Sumatra. Part of lower jaw of Tupaia Tana; twice natural size. Fig. 289. End view seen from behind, showing the very slight inflection of the angle at c. Fig. 290. Side view of same. Part of lower jaw of Didelphis AzarÆ; recent, Brazil. Natural size. Fig. 291. End view seen from behind, showing the inflection of the angle of the jaw, c. d. Fig. 292. Side view of same. The only question, therefore, which could fairly admit of controversy was limited to this point, whether the fossil mammalia found in the lower oolite of Oxfordshire ought to be referred to the marsupial quadrupeds, or to the ordinary placental series. Cuvier had long ago pointed out a peculiarity in the form of the angular process (c, figs. 291. and 292.) of the lower jaw, as a character of the genus Didelphys; and Mr. Owen has since established its generality in the entire marsupial series. In all these pouched quadrupeds, this process is turned inwards, as at c d, fig. 291. in the Brazilian opossum, whereas in the placental series, as at c, figs. 290. and 289. there is an almost entire absence of such inflection. The Tupaia Tana of Sumatra has been selected by my friend Mr. Waterhouse, for this illustration, because that small insectivorous quadruped bears a great resemblance to those of the Stonesfield Amphitherium. By clearing away the matrix from the specimen of Amphitherium Prevostii above represented (fig. 286.), Mr. Owen ascertained that the angular process (c) bent inwards in a slighter degree than in any of the known marsupialia; in short, the inflection does not exceed that of the mole or hedgehog. This fact turns the scale in favour of its affinities to the placental insectivora. Nevertheless, the Amphitherium offers some points of approximation in its osteology to the marsupials, especially to the Myrmecobius, a small insectivorous quadruped of Australia, which has nine molars on each side of the lower jaw, besides a canine and three incisors. Fig. 293. Phascolotherium Bucklandi, Owen.
The second mammiferous genus discovered in the same slates was named originally by Mr. Broderip Didelphys Bucklandi (see fig. 293.), and has since been called Phascolotherium by Owen. It manifests a much stronger likeness to the marsupials in the general form of the jaw, and in the extent and position of its inflected angle, while the agreement with the living genus Didelphys in the number of the premolar and molar teeth, is complete. On reviewing, therefore, the whole of the osteological evidence, it will be seen that we have every reason to presume that the Amphitherium and Phascolotherium of Stonesfield represent both the placental and marsupial classes of mammalia; and if so, they warn us in a most emphatic manner, not to found rash generalizations respecting the non-existence of certain classes of animals at particular periods of the past, on mere negative evidence. The singular accident of our having as yet found nothing but the lower jaws of seven individuals, and no other bones of their skeletons, is alone sufficient to demonstrate the fragmentary manner in which the memorials of an ancient terrestrial fauna are handed down to us. We can scarcely avoid suspecting that the two genera above described, may have borne a like insignificant proportion to the entire assemblage of warm-blooded quadrupeds which flourished in the islands of the oolitic sea. Mr. Owen has remarked that as the marsupial genera, to which the Phascolotherium is most nearly allied, are now confined to New South Wales and Van Diemen's Land, so also is it in the Australian seas, that we find the Cestracion, a cartilaginous fish which has a bony palate, allied to those called Acrodus and Psammodus (see figs. 307, 308. p. 275.), so common in the oolite and lias. In the same Australian seas, also, near the shore, we find the living Trigonia, a genus of mollusca so frequently met with in the Stonesfield slate. So, also, the Araucarian pines are now abundant, together with ferns, in Australia and its islands, as they were in Europe in the oolitic period. Many botanists incline to the opinion, that the Thuja, Pine, Cycas, Zamia, in short, all the gymnogens, belong to a less highly developed type of flowering plants than do the exogens; but even if this be admitted, no naturalist can ascribe a low standard of organization to the oolitic flora, since we meet with endogens of the most perfect structure Fig. 294. Portion of a fossil fruit of Podocarya magnified. (Buckland's Bridgew. Treat. Pl. 63.) Inferior Oolite, Charmouth, Dorset. The Stonesfield slate, in its range from Oxfordshire to the north-east, is represented by flaggy and fissile sandstones, as at Collyweston in Northamptonshire, where, according to the researches of Messrs. Ibbetson and Morris, it contains many shells, such as Trigonia angulata, also found at Stonesfield. But the Northamptonshire strata of this age assume a more marine character, or appear at least to have been formed farther from land. They inclose, however, some fossil ferns, such as Pecopteris polypodioides, of species common to the oolites of the Yorkshire coast Fig. 295. Pterophyllum comptum. (Syn. Cycadites comptus.) Upper sandstone and shale, Gristhorpe, near Scarborough. In the north-west of Yorkshire, the formation alluded to consists of an upper and a lower carbonaceous shale, abounding in impressions of plants, divided by a limestone considered by many geologists as the representative of the Great Oolite; but the scarcity of marine fossils makes all comparisons with the subdivisions adopted in the south extremely difficult. A rich harvest of fossil ferns has been obtained from the upper carbonaceous shales and sandstones at Gristhorpe, near Scarborough (see figs. 295, 296.). The lower shales are well exposed in the sea-cliffs at Whitby, and are chiefly characterized Fig. 296. Hemitelites Brownii, Goepp. Syn. Phlebopteris contigua, Lind. & Hutt. Upper carbonaceous strata, Lower Oolite, Gristhorpe, Yorkshire. At Brora, in Sutherlandshire, a coal formation, probably coeval with the above, or belonging to some of the lower divisions of the Oolitic period, has been mined extensively for a century or more. It affords the thickest stratum of pure vegetable matter hitherto detected in any secondary rock in England. One seam of coal of good quality has been worked 31/2 feet thick, and there are several feet more of pyritous coal resting upon it. Inferior Oolite.—Between the Great and Inferior Oolite, near Bath, an argillaceous deposit called "the fuller's earth," occurs, but is wanting in the north of England. The Inferior Oolite is a calcareous freestone, usually of small thickness, which sometimes rests upon, or is replaced by, yellow sands, called the sands of the Inferior Oolite. These last, in their turn, repose upon the lias in the south and west of England. Among the characteristic shells of the Inferior Oolite, I may instance Terebratula spinosa (fig. 297.), and Pholadomya fidicula (fig. 298.). The extinct genus Pleurotomaria is also a form very common in this division as well as in the Oolitic system generally. It resembles the Trochus in form, but is marked by a singular cleft (a, fig. 299.) on the right side of the mouth. Fig. 297. Terebratula spinosa. Inferior Oolite. Fig. 298.
Fig. 299. Pleurotomaria ornata. Ferruginous Oolite, Normandy. Inferior Oolite, England. As illustrations of shells having a great vertical range, I may Fig. 300. Ostrea Marshii. 1/2 nat. size. Middle and Lower Oolite. Fig. 301. Ammonites striatulus, Sow. 1/3 nat. size. Inferior Oolite and Lias. Such facts by no means invalidate the general rule, that certain fossils are good chronological tests of geological periods; but they serve to caution us against attaching too much importance to single species, some of which may have a wider, others a more confined vertical range. We have before seen that, in the successive tertiary formations, there are species common to older and newer groups, yet these groups are distinguishable from one another by a comparison of the whole assemblage of fossil shells proper to each. |