Fig. 115Fig. 115.—Adeona folifera.(Recent Polyzoa.)
Fig. 115.—Adeona folifera.(Recent Polyzoa.)
Fig. 116Fig. 116.—Cellaria loriculata.(Recent Polyzoa.)
Fig. 116.—Cellaria loriculata.(Recent Polyzoa.)
This last and most remarkable species of Zoophyte presents itself in great masses many yards in circumference, and necessitates a long period of time for its production. This assemblage of little creatures living under the waters but only at a small depth beneath the surface, as Mr. Darwin has demonstrated, has nevertheless produced banks, or rather islets, of considerable extent, which at one time constituted veritable reefs rising out of the ocean. These reefs were principally constructed in the Jurassic period, and their extreme abundance is one of the characteristics of this geological age. The same phenomenon continues in our day, but by the agency of a new race ofzoophytes, which carry on their operations, preparing a new continent, probably, in theatollsof the Pacific Ocean. (SeeFig. 108, p. 240.)
Fig. 117Fig. 117.1, Otopteris dubia; 2, Otopteris obtusa; 3, Otopteris acuminata; 4, Otopteris cuneata.
Fig. 117.1, Otopteris dubia; 2, Otopteris obtusa; 3, Otopteris acuminata; 4, Otopteris cuneata.
The flora of the epoch was very rich. The Ferns continue to exist, but their size and bearing were sensibly inferior to what they had been in the preceding period. Among them Otopteris, distinguishedfor its simply pinnated leaves, whose leaflets are auriculate at the base: of the five species, 1,O. dubia; and 2,O. obtusa; and 3,O. acuminata; and 4,O. cuneata(Fig. 117), are from the Oolite. In addition to these we may nameConiopteris Murrayana,Pecopteris Desnoyersii, Pachypteris lanceolata, andPhlebopteris Phillipsii; and among the Lycopods,Lycopodus falcatus.
The vegetation of this epoch has a peculiar facies, from the presence of the family of the Pandanaceæ, or screw-pines, so remarkable for their aërial roots, and for the magnificent tuft of leaves which terminates their branches. Neither the leaves nor the roots of these plants have, however, been found in the fossil state, but we possess specimens of their large and spherical fruit, which leave no room for doubt as to the nature of the entire plant.
The Cycads were still represented by theZamias, and by many species of Pterophyllum. The Conifers, that grand family of recent times, to which the pines, firs, and other trees of our northern forests belong, began to occupy an important part in the world’s vegetation from this epoch. The earliest Conifers belonged to the generaThuites,Taxites, andBrachyphyllum. TheThuiteswere trueThuyas, evergreen trees of the present epoch, with compressed branches, small imbricated and serrated leaves, somewhat resembling those of the Cypress, but distinguished by many points of special organisation. TheTaxiteshave been referred, with some doubts, to the Yews. Finally, theBrachyphyllumwere trees which, according to the characteristics of their vegetation, seem to have approached nearly to two existing genera, theArthotaxisof Tasmania, and theWeddringtoniasof South Africa. The leaves of the Brachyphyllum are short and fleshy, with a large and rhomboidal base.
The formation which represents the Lower Oolite, and which in England attains an average thickness of from 500 to 600 feet, forms a very complex system of stratification, which includes the two formations,BajocienandBathonian, adopted by M. D’Orbigny and his followers. The lowest beds of theInferior Ooliteoccur in Normandy, in the Lower Alps (Basses-Alpes), in the neighbourhoods of Lyons and Neuchatel. They are remarkable near Bayeux for the variety and beauty of their fossils: the rocks are composed principally of limestones—yellowish-brown, or red, charged with hydrated oxide of iron, often oolitic, and reposing on calcareous sands. Thesedeposits are surmounted by alternate layers of clay and marl, blue or yellow—the well-knownFuller’s Earth, which is so called from its use in the manufacture of woollen fabrics to extract the grease from the wool. The second series of the Lower Oolite, which attains a thickness of from 150 to 200 feet on the coast of Normandy, and is well developed in the neighbourhood of Caen and in the Jura, has been divided, in Britain, into four formations, in an ascending scale:—
1. TheGreatorBath Oolite, which consists principally of a very characteristic, fine-grained, white, soft, and well-developed oolitic limestone, at Bath, and also at Caen in Normandy. At the base of the Great Oolite the Stonesfield beds occur, in which were found the bones of the marsupial Mammals, to which we have already alluded; and along with them bones of Reptiles, principally Pterodactyles, together with some finely-preserved fossil plants, fruits, and insects.
2.Bradford Clay, which is a bluish marl, containing many fine Encrinites (commonly called stone-lilies), but which had only a local existence, appearing to be almost entirely confined to this formation. “In this case, however,” says Lyell, “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 with 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.”[65]See Fig. 1,Plate XIX., p. 261. (Bradford, or Pear, Encrinite.)
3.Forest Marble, which consists of an argillaceous shelly limestone, abounding in marine fossils, and sandy and quartzose marls, is quarried in the forest of Wichwood, in Wiltshire, and in the counties of Dorset, Wilts, and Somerset.
4. TheCornbrash(wheat-lands) consists of beds of rubbly cream-coloured limestone, which forms a soil particularly favourable to the cultivation of cereals; hence its name.[66]
The Lower Oolite ranges across the greater part of England, but “attains its maximum development near Cheltenham, where it can be subdivided, at least, into three parts. Passing north, the two lower divisions, each more or less characterised by its own fossils, disappear, and the Ragstone north-east of Cheltenham lies directlyupon the Lias; apparently as conformably as if it formed its true and immediate successor, while at Dundry the equivalents of the upper freestones and ragstones (the lower beds being absent) lie directly on the exceedingly thin sands, which there overlie the Lower Lias. In Dorsetshire, on the coast, the series is again perfect, though thin. Near Chipping Norton, in Oxfordshire, the Inferior Oolite disappears altogether, and the Great Oolite, having first overlapped the Fullers’ Earth, passes across the Inferior Oolite, and in its turn seems to lie on the Upper Lias with a regularity as perfect as if no formation in the neighbourhood came between them. In Yorkshire the changed type of the Inferior Oolite, the prevalence of sands, land-plants, and beds of coal, occur in such a manner as to leave no doubt of the presence of terrestrial surfaces on which the plants grew, and all these phenomena lead to the conclusion that various and considerable oscillations of level took place in the British area during the depositionof the strata, both of the Inferior Oolite and of the formations which immediately succeed it.”[67]
Fig. 118Fig. 118.—Meandrina Dædalæa.a, entire figure, reduced;b, portion, natural size.(Recent Coral.)
Fig. 118.—Meandrina Dædalæa.a, entire figure, reduced;b, portion, natural size.(Recent Coral.)
The Inferior Oolite here alluded to is a thin bed of calcareous freestone, resting on, and sometimes replaced by yellow sand, which constitutes the passage-beds from the Liassic series. The Fullers’ Earth clay lies between the limestones of the Inferior and Great Oolite, at the base of which last lies the Stonesfield slate—a slightly oolitic, shelly limestone, or flaggy and fissile sandstone, some six feet thick, rich in organic remains, and ranging through Oxfordshire towards the north-east, into Northamptonshire and Yorkshire. At Colley Weston, in Northamptonshire, fossils ofPecopteris polypodioidesare found. In the Great Oolite formation, near Bath, are many corals, among which theEunomia radiatais very conspicuous. The fossil is not unlike the existing brain-coral of the tropical seas (Fig. 118). The work of this coral seems to have been suddenly stopped by “an invasion,” says Lyell, “of argillaceous matter, which probably put a sudden stop to the growth of Bradford Encrinites, and led to their preservation in marine strata.”[68]The Cornbrash is, in general, a cream-coloured limestone, about forty feet thick, in the south-west of England, and occupying a considerable area in Dorsetshire and North Wilts, as at Cricklade, Malmesbury, and Chippenham, in the latter county.Terebratula obovatais its characteristic shell, andNucleolites clunicularis,Lima gibbosa, andAvicula echinataoccur constantly in great numbers. Wherever it occurs the Cornbrash affords a rich and fertile soil, well adapted for the growth of wheat, while the Forest Marble, as a soil, is generally poor. The Cornbrash passes downwards into the Forest Marble, and sometimes, as at Bradford, near Bath, is replaced by clay. This clay, called the Bradford clay, is almost wholly confined to the county of Wilts.Terebratula decussatais one of the most characteristic fossils, but the most common is the Apiocrinites or pear-shaped encrinite, whose remains in this clay are so perfectly preserved that the most minute articulations are often found in their natural positions.Plate XIX., p. 261 (Fig. 1), represents an adult attached by a solid base to the rocky bottom on which it grew, whilst the smaller individuals show the Encrinite in its young state—one with arms expanded, the other with them closed. Ripple-marked slabs of fissile Forest Marble are used as a roofing-slate, and may be traced over a broad band of country in Wiltshire and Gloucestershire, separated from each other by thin seams of clay, inwhich the undulating ridges of the sand are preserved, and even the footmarks of small Crustaceans are still visible.
Plate XVIIXVII.—Ideal Landscape of the Lower Oolite Period.
XVII.—Ideal Landscape of the Lower Oolite Period.
On the opposite page (Plate XVII.) is represented an ideal landscape of the period of the Lower Oolite. On the shore are types of the vegetation of the period. TheZamites, with large trunk covered with fan-like leaves, resembled in form and bearing the existing Zamias of tropical regions; aPterophyllum, with its stem covered from base to summit with its finely-cut feathery leaves; Conifers closely resembling our Cypress, and an arborescent Fern. What distinguishes this sub-period from that of the Lias is a group of magnificent trees,Pandanus, remarkable for their aërial roots, their long leaves, and globular fruit.
Upon one of the trees of this group the artist has placed thePhascolotherium, not very unlike to our Opossum. It was amongst the first of the Mammalia which appeared in the ancient world. The artist has here enlarged the dimensions of the animal in order to show its form. Let the reader reduce it in imagination one-sixth, for it was not larger than an ordinary-sized cat.
A Crocodile and the fleshless skeleton of the Ichthyosaurus remind us that Reptiles still occupied an important place in the animal creation. A few Insects, especially Dragon-flies, fly about in the air. Ammonites float on the surface of the waves, and the terrible Plesiosaurus, like a gigantic swan, swims about in the sea. The circular reef of coral, the work of ancient Polyps, foreshadows the atolls of the great ocean, for it was during the Jurassic period that the Polyps of the ancient world were most active in the production of coral-reefs and islets.
The terrestrial flora of this age was composed of Ferns, Cycads, and Conifers. The first represented by thePachypteris microphylla, the second byZamites Moreana.Brachyphyllum MoreanumandB. majusappear to have been the Conifers most characteristic of the period; fruits have also been found in the rocks of the period, which appear to belong to Palms, but this point is still obscure and doubtful.
Numerous vestiges of the fauna which animated the period are also revealed in the rocks of this age. Certain hemipterous insects appear on the earth for the first time, and the Bees among the Hymenoptera, Butterflies among the Lepidoptera, and Dragon-fliesamong the Neuroptera. In the bosom of the ocean, or upon its banks, roamed theIchthyosaurus,Ceteosaurus,Pterodactylus crassirostris, and theGeosaurus; the latter being very imperfectly known.
The Ceteosaurus whose bones have been discovered in the upper beds of the Great Oolite at Enslow Rocks, at the Kirtlington Railway Station, north of Oxford, and some other places, was a species of Crocodile nearly resembling the modern Gavial or Crocodile of the Ganges. This huge whale-like reptile has been described by Professor John Phillips as unmatched in size and strength by any of the largest inhabitants of the Mesozoic land or sea—perhaps the largest animal that ever walked upon the earth. A full-grown Ceteosaurus must have beenat leastfifty feet long, ten feet high, and of a proportionate bulk. In its habits it was, probably, a marsh-loving or river-side animal, dwelling amidst filicene, cycadaceous, and coniferous shrubs and trees full of insects and small mammalia. The one small and imperfect tooth which has been found resembles that of Iguanodon more than of any other reptile; and it seems probable that the Ceteosaurus was nourished by vegetable food, which abounded in the vicinity of its haunts, and was not obliged to contend with the Megalosaurus for a scanty supply of more stimulating diet.[69]
Fig. 119Fig. 119.—Ramphorynchus restored. One-quarter natural size.
Fig. 119.—Ramphorynchus restored. One-quarter natural size.
Another reptile allied to the Pterodactyle lived in this epoch—theRamphorynchus, distinguished from the Pterodactyle by a long tail. The imprints which this curious animal has left upon the sandstone of the period are impressions of its feet and the linear furrow made by its tail. Like the Pterodactyle, the Ramphorynchus, which was about the size of a crow, could not precisely fly, but, aided by the wing (a sort of natural parachute formed by the membrane connecting the fingers with the body), it could throw itself from a height upon its prey.Fig. 119represents a restoration of this animal. The footprints in the soil are in imitation of those which accompany the remains of the Ramphorynchus in the Oolitic rocks, and they show the imprints of the anterior and posterior feet and also the marks made by the tail.
This tail was very long, far surpassing in length the rest of the vertebral column, and consisting of more than thirty vertebræ—which were at first short, but rapidly elongate, retain their length for a considerable distance, and then gradually diminish in size.
Plate XVIIIXVIII.—Ideal landscape of the Middle Oolitic Period.
XVIII.—Ideal landscape of the Middle Oolitic Period.
Another genus of Reptiles appears in the Middle Oolite, of which we have had a glimpse in the Lias and Great Oolite of the preceding section. This is theTeleosaurus, which the recent investigations ofM. E. Deslongchamps allow of re-construction. The Teleosaurus enables us to form a pretty exact idea of these Crocodiles of the ancient seas—these cuirassed Reptiles, which the German geologist Cotta describes as “the great barons of the kingdom of Neptune, armed to the teeth, and clothed in an impenetrable panoply; the true filibusters of the primitive seas.”
The Teleosaurus resembled the Gavials of India. The former inhabited the banks of rivers, perhaps the sea itself; they were longer, more slender, and more active than the living species; they were about thirty feet in length, of which the head may be from three to four feet, with their enormous jaws sometimes with an opening of six feet, through which they could engulf, in the depths of their enormous throat, animals of considerable size.
TheTeleosaurus cadomensisis represented on the opposite page (Plate XVIII.), after the sketch of M. E. Deslongchamps, carrying from the sea in its mouth aGeoteuthis, a species of Calamary of the Oolitic epoch. This creature was coated with a cuirass both on the back and belly. In order to show this peculiarity, a living individual is represented on the shore, and a dead one is floating on its back in shallow water, leaving the ventral cuirass exposed.
Behind theTeleosaurus cadomensisin the engraving, another Saurian, theHylæosaurus, is represented, which makes its appearancein the Cretaceous epoch. We have here adopted the restoration which has been so ably executed by Mr. Waterhouse Hawkins, at the Crystal Palace, Sydenham.
Fig. 120Fig. 120.—Eryon arctiformis.
Fig. 120.—Eryon arctiformis.
Besides the numerous Fishes with which the Oolitic seas swarmed, they contained some Crustaceans, Cirripedes, and various genera of Mollusca and Zoophytes.Eryon arctiformis, represented inFig. 119, belongs to the class of Crustaceans, of which the spiny lobster is the type. Among the Mollusca were some Ammonites, Belemnites, and Oysters, of which many hundred species have been described. Of these we may mentionAmmonites refractus, A. Jason and A. cordatus, Ostrea dilatata, Terebratula diphya, Diceras arietena, Belemnites hastatus, andB. Puzosianus. In some of the finely-laminated clays the Ammonites are very perfect, but somewhat compressed, with the outer lip or margin of the aperture entire (Fig. 120). Similar prolongations have been noticed in Belemnites found by Dr. Mantell in the Oxford Clay, near Chippenham.
Fig. 121Fig. 121.—Perfect Ammonite.
Fig. 121.—Perfect Ammonite.
Plate XIXXIX.—Fig. 1.—Apiocrinites rotundus.Fig. 2.—Encrinus liliiformis.
XIX.—Fig. 1.—Apiocrinites rotundus.Fig. 2.—Encrinus liliiformis.
Among the Echinoderms,Cidaris glandiferus,Apiocrinus Roissyanus, andA. rotundus, the gracefulSaccocoma pectinata,Millericrinus nodotianus,Comatula costata, andHemicidaris crenularismay be mentioned;Apiocrinites rotundus, figured inPlate XIX., is a reduced restoration: 1, being expanded;a, closed; 3, a cross section of theupper extremity of the pear-shaped head; 4, a vertical section showing the enlargement of the alimentary canal, with the hollow lenticular spaces which descend through the axis of the column, forming the joints, and giving elasticity and flexure to the whole stem, without risk of dislocation.A. rotundusis found at Bradford in Wiltshire, Abbotsbury in Dorset, at Soissons, and Rochelle. This species—known as the Bradford Pear-Encrinite—is only found in the strata mentioned.
The Corals of this epoch occur in great abundance. We have already remarked that these aggregations of Polyps are often met with at a great depth in the strata. These small calcareous structures have been formed in the ancient seas, and the same phenomenon is extending the terrestrial surface in our days in the seas of Oceania, where reefs and atolls of coral are rising by slow and imperceptible steps, but with no less certainty. Although their mode of production must always remain to some extent a mystery, the investigations of M. Lamaroux, Mr. Charles Darwin, and M. D’Orbigny have gone a long way towards explaining their operations; for the Zoophyte in action is an aggregation of these minute Polyps. Describing what he believes to be a sea-pen, a Zoophyte allied toVirgularia Patagonia, Mr. Darwin says: “It consists of a thin, straight, fleshy stem, with alternate rows of polypi on each side, and surrounding an elastic stony axis. The stem at one extremity is truncate, but at the other is terminated by a vermiform fleshy appendage. The stony axis which gives strength to the stem, may be traced at this extremity into a mere vessel filled with granular matter. At low water hundreds of these zoophytes might be seen, projecting like stubble, with the truncate end upwards, a few inches above the surface of the muddy sand. When touched or pulled, they drew themselves in suddenly, with force, so as nearly or quite to disappear. By this action, the highly-elastic axis must be bent at the lower extremity, where it is naturally slightly curved; and I imagine it is by this elasticity alone that the zoophyte is enabled to rise again through the mud. Each polypus, though closely united to its brethren, has a distinct mouth, body, and tentacula. Of these polypi, in a large specimen there must be many thousands. Yet we see that they act by one movement; that they have one central axis, connected with a system of obscure circulation.” Such is the brief account given by a very acute observer of these singular beings. They secrete the calcareous matter held in solution in the oceanic waters, and produce the wonderful structures we have now under consideration; and these calcareous banks have been in course of formation during many geological ages. Theyjust reach the level of the waters, for the polyps perish as soon as they are so far above the surface that neither the waves nor the flow of the tides can reach them. In the Oolitic rocks these banks are frequently found from twelve to fifteen feet thick, and many leagues in length, and preserving, for the most part, the relative positions which they occupied in the sea while in course of formation.
The rocks which now represent the Middle Oolitic Period are usually divided into theOxford Clay, the lower member of which is an arenaceous limestone, known as theKellaways Rock, which in Wiltshire and other parts of the south-west of England attains a thickness of eight or ten feet, with the impressions of numerous Ammonites, and other shells. In Yorkshire, around Scarborough, it reaches the thickness of thirty feet; and forms well-developed beds of bluish-black marl in the department of Calvados, in France. It is the base of this clay which forms the soil (Argile de Dives) of the valley of the Auge, renowned for its rich pasturages and magnificent cattle. The same beds form the base of the oddly-shaped but fine rocks of La Manche, which are popularly known as theVaches Noires(or black cows)—a locality celebrated, also, for its fine Ammonites transformed into pyrites.
TheOxford Clayconstitutes the base of the hills in the neighbourhood of Oxford, forming a bed of clay sometimes more than 600 feet thick. It is found well-developed in France, at Trouville, in the department of the Calvados; and at Neuvisy, in the department of the Ardennes, where it attains a thickness of about 300 feet. It is a bluish, sometimes whitish limestone (often argillaceous), and bluish marl. TheGryphæa dilatatais the most common fossil in the Oxford Clay. TheCoral Ragis so called from the fact that the limestone of which it is chiefly composed consists, in part, of an aggregation of considerable masses of petrified Corals; not unlike those now existing in the Pacific Ocean, supposing them to be covered up for ages and fossilised. This coral stratum extends through the hills of Berkshire and North Wilts, and it occurs again near Scarborough. In the counties of Dorset, Bedford, Buckingham, and Cambridge, and some other parts of England, the limestone of the Coral Rag disappears and is replaced by clay—in which case the Oxford Clay is overlaid directly by the Kimeridge Clay. In France it is found in the departments of the Meuse, of the Yonne, of the Ain, of the Charente Inférieure. In the Alps theDiceras limestoneis regarded, by most geologists, as coeval with the English Coral Rag.
Some marsupial Mammals have left their remains in the Upper Oolite as in the Lower. They belong to the genusSphalacotherium. Besides the Plesiosauri and Teleosauri, there still lived in the maritime regions a Crocodile, theMacrorhynchus; and the monstrousPœcilopleuron, with sharp cutting teeth, one of the most formidable animals of this epoch; theHylæosaurus,Cetiosaurus,Stenosaurus, andStreptospondylus, and among the Turtles, theEmysandPlatemys.As in the Lower Oolite, so also in the Upper, Insects similar to those by which we are surrounded, pursued their flight in the meadows and hovered over the surface of the water. Of these, however, too little is known for us to give any very precise indication on the subject of their special organisation.
Fig. 122Fig. 122.—Bird of Solenhofen (Archæopteryx).
Fig. 122.—Bird of Solenhofen (Archæopteryx).
The most remarkable fact relating to this period is the appearance of the first bird. Hitherto the Mammals, and of these only imperfectly-organised species, namely, the Marsupials, have alone appeared. It is interesting to witness birds appearing immediately after. In the quarries of lithographic stone at Solenhofen, the remains of a bird, with feet and feathers, have been found, but without the head. These curious remains are represented inFig. 122, in the position in which they were discovered. The bird is usually designated the Bird of Solenhofen.
Fig. 123Fig. 123.Shell of Physa fontinalis.
Fig. 123.Shell of Physa fontinalis.
The Oolitic seas of this series contained Fishes belonging to the generaAsteracanthus,Strephodes,Lepidotus, andMicrodon. The Cephalopodous Mollusca were not numerous, the predominating genera belonging to the Lamellibranchs and to the Gasteropods, which lived on the shore. The reef-making Madrepores or Corals were more numerous. A few Zoophytes in the fossil state testify to the existence of these extraordinary animals. The fossils characteristic of the fauna of the period includeAmmonites decipiensandA. giganteus,Natica elegansandhemispherica,Ostrea deltoideaandO. virgula,Trigonia gibbosa,Pholadomya multicostataandP. acuticostata,Terebratula subsella, andHemicidaris Purbeckensis. SomeFishes,Turtles,Paludina,Physa(Fig. 123),Unio,Planorbis(Fig. 201), and the little crustacean bivalves, the Cypris, constituted the fresh-water fauna of the period.
The terrestrial flora of the period consisted of Ferns, Cycadeaceæ, and Conifers; in the ponds and swamps some Zosteræ. TheZosteræare monocotyledonous plants of the family of the Naïdaceæ, which grow in the sandy mud of maritime regions, forming there, with their long, narrow, and ribbon-like leaves, vast prairies of the most beautiful green. At low tides these masses of verdure appear somewhat exposed. They would form a retreat for a great number of marine animals, and afford nourishment to others.
Plate XXXX.—Ideal Landscape of the Upper Oolitic Period.
XX.—Ideal Landscape of the Upper Oolitic Period.
On the opposite page an ideal landscape of the period (Plate XX.) represents some of the features of the Upper Oolite, especially thevegetation of the Jurassic period. TheSphenophyllum, among the Tree-ferns, is predominant in this vegetation; somePandanas, a fewZamites, and manyConifers, but we perceive no Palms. A coral islet rises out of the sea, having somewhat of the form of theatollsof Oceania, indicating the importance these formations assumed in the Jurassic period. The animals represented are theCrocodileimusof Jourdan, theRamphorynchus, with the imprints which characterise its footsteps, and some of the invertebrated animals of the period, as theAsteria,Comatula,Hemicidaris,Pteroceras. Aloft in the air floats the bird of Solenhofen, theArchæopteryx, which has been re-constructed from the skeleton, with the exception of the head, which remains undiscovered.
The rocks which represent the Upper Oolite are usually divided into two series: 1. ThePurbeck Beds; 2. ThePortland Stone and Sand; and 3. TheKimeridge Clay.
TheKimeridge Clay, which in many respects bears a remarkable resemblance to the Oxford Clay, is composed of blue or yellowish argillaceous beds, which occur in the state of clay and shale (containing locally beds of bituminous schist, sometimes forming a sort of earthy impure coal), and several hundred feet in thickness. These beds are well developed at Kimeridge, in Dorsetshire, whence the clay takes its name. In some parts of Wiltshire the beds of bituminous matter have a shaly appearance, but there is an absence of the impressions of plants which usually accompany the bitumen, derived from the decomposition of plants. These rocks, with their characteristic fossils,Cardium striatulumandOstrea deltoidea, are found throughout England: in France, at Tonnerre, Dept. Yonne; at Havre; at Honfleur; at Mauvage; in the department of the Meuse it is so rich in shells ofOstrea deltoideaandO. virgula, that, “near Clermont in Argonne, a few leagues from St. Menehould,” says Lyell,[70]“where these indurated marls crop out from beneath the Gault, I have seen them (Gryphæa virgula) on decomposing leave the surface of every ploughed field literally strewed over with this fossil oyster.”
The second section of this series consists of the oolitic limestone of Portland, which is quarried in the Isle of Portland and in the cliffs of the Isle of Purbeck in Dorsetshire, and also at Chilmark in the Vale of Wardour, in Wiltshire. In France, the Portland beds are found near Boulogne, at Cirey-le-Château, Auxerre, and Gray (Haute Saône).
The Isle, or rather peninsula of Portland,[71]off the Dorsetshire coast, rises considerably above the sea-level, presenting on the side of the port a bold line of cliffs, connected with the mainland by the Chesil bank,[72]an extraordinary formation, consisting of a beach of shingle and pebbles loosely piled on the blue Kimeridge clay, and stretching ten miles westward along the coast. The quarries are chiefly situated in the northerly part of the island. The story told of this remarkable island is an epitome of the revolutions the surface of the earth has undergone. The slaty Purbeck beds which overlie the Portland stone are of a dark-yellowish colour; they are burnt in the neighbourhood for lime. The next bed is of a whiter and more lively colour. It is the stone of which the portico of St. Paul’s and many of the houses of London, built in Queen Anne’s time, were constructed. The building-stone contains fossils exclusively marine. Upon this stratum rests a bed of limestone formed in lacustrine waters. Finally, upon this bed rests another deposit of a substance which consists of very well-preserved vegetable earth orhumus, quite analogous to our vegetable soil, of the thickness of from fifteen to eighteen inches, and of a blackish colour; it contains a strong proportion of carbonaceous earth; it abounds in the silicified remains of Conifers and other plants, analogous to theZamiaandCycas—this soil is known as the “dirt-bed.” The trunks of great numbers of silicified trees and tropical plants are found here erect, their roots fixed in the soil, and of species differing from any of our forest trees. “The ruins of a forest upon the ruins of a sea,” says Esquiros, “the trunks of these trees were petrified while still growing. The region now occupied by the narrow channel and its environs had been at first a sea, in whose bed the Oolitic deposits which now form the Portland stone accumulated: the bed of the sea gradually rose and emerged from the waves. Upon the land thus rescued from the deep, plants began to grow; they now constitute with their ruins the soil of the dirt-bed. This soil, with its forest of trees, was afterwards plunged again into the waters—not the bitter waters of the ocean, but in the fresh waters of a lake formed at the mouth of some great river.”
Time passed on, however; a calcareous sediment brought from the interior by the waters, formed a layer of mud over the dirt-bed; finally, the whole region was covered by a succession of calcareous deposits, until the day when the Isle of Portland was again revealed to light. “From the facts observed,” says Lyell, “we may infer:—1.That those beds of the Upper Oolite, called the Portland, which are full of marine shells, were overspread with fluviatile mud, which became dry land, and covered with a forest, throughout a portion of space now occupied by the south of England, the climate being such as to admit of the growth of theZamiaandCycas. 2. This land at length sank down and was submerged with its forest beneath a body of fresh water from which sediment was thrown down enveloping fluviatile shells. 3. 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.”[73]
Fig. 124Fig. 124.—Geological humus.a, Fresh-water calcareous slate (Purbeck);b, Dirt-bed, with roots and stems of trees;c, Fresh-water beds;d, Portland Stone.
Fig. 124.—Geological humus.a, Fresh-water calcareous slate (Purbeck);b, Dirt-bed, with roots and stems of trees;c, Fresh-water beds;d, Portland Stone.
The soil known as thedirt-bedis nearly horizontal in the Isle of Portland; but we discover it again not far from there in the sea-cliffs of the Isle of Purbeck, having an inclination of 45°, where the trunks continue perfectly parallel among themselves, affording a fine example of a change in the position of beds originally horizontal.Fig. 124represents this species of geologicalhumus. “Eachdirt-bed” says Sir Charles Lyell, “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.”[74]
This bed of vegetable soil is, then, near the summit of that long and complicated series of beds which constitute the Jurassic period; these ruins, still vegetable, remind us forcibly of the coal-beds, for they are nothing else than a less advanced state of that kind of vegetable fossilisation which was perfected on such an immense scale, and during an infinite length of time in the coal period.
The Purbeck beds, which are sometimes subdivided into Lower, Middle, and Upper, are mostly fresh-water formations, intimately connected with the Upper Portland beds. But there they begin and end, being scarcely recognisable except in Dorsetshire, in the sea-cliffs of which they were first studied. They are finely exposed in Durdlestone Bay, near Swanage, and at Lulworth Cove, on the same coast. Thelower bedsconsist of a purely fresh-water marl, eighty feet thick, containing shells ofCypris,Limnæa, and someSerpulæin a bed of marl of brackish-water origin, and someCypris-bearing shales, strangely broken up at the west end of the Isle of Purbeck.
TheMiddle seriesconsists of twelve feet of marine strata known as the “cinder-beds,” formed of a vast accumulation ofOstrea distorta, resting on fresh-water strata full ofCypris fasciculata,Planorbis, andLimnæa, by which this strata has been identified as far inland as the vale of Wardour in Wiltshire. Above the cinder-beds are shales and limestones, partly of fresh-water and partly of brackish-water origin, in which are Fishes, many species of Lepidotus, and the crocodilian reptile,Macrorhynchus. On this rests a purely marine deposit, withPecten,Avicula, &c. Above, again, are brackish beds withCyrena, overlying which is thirty feet of fresh-water limestone, withFishes,Turtles, andCyprides.
Theupper bedsare purely fresh-water strata, about fifty feet thick, containingPaludina,Physa,Limnæa, all very abundant. In these beds the Purbeck marble, formerly much used in the ornamental architecture of the old English cathedrals, was formerly quarried. (SeeNote, page 274.)
A few words may be added, in explanation of the termoolite, applied to this sub-period of the Jurassic formation. In a great number of rocks of this series the elements are neither crystalline nor amorphous—they are, as we have already said, oolitic; that is to say, the mass has the form of the roe of certain fishes. The question naturally enough arises, Whence this singular oolitic structure assumed by the components of certain rocks? It is asserted that the grinding action of the sea acting upon the precipitated limestone produces rounded forms analogous to grains of sand. This hypothesis may be well founded in some cases. The marine sediments which are deposited in some of the warm bays of Teneriffe are found to take the spheroidal granulated form of the oolite. But these local facts cannot be made to apply to the whole extent of the oolitic formations. We must, therefore, look further for an explanation of the phenomena.
It is admitted that if the cascades of Tivoli, for example, can give birth to the oolitic grains, the same thing happens in the quietest basins, that in stalactite-caverns oolitic grains develop themselves, which afterwards, becoming cemented together from the continued, but very slow, affluence of the calcareous waters, give rise to certain kinds of oolitic rocks.
On the other hand, it is known that nodules, more or less large, develop themselves in marls in consequence of the concentration of the calcareous elements, without the possibility of any wearing action of water. Now, as there exists every gradation of size between the smallest oolitic grains and the largest concretions, it is reasonable to suppose that the oolites are equally the product of concentration.
Finally, from research to research, it is found that perfectly constituted oolites—that is to say, concentric layers, as in the Jurassic limestone—develop themselves in vegetable earth in places where the effects of water in motion is not more admissible than in the preceding instances.
Thus we arrive at the conclusion, that if Nature sometimes forms crystals with perfect terminations in magmas in the course of solidification, she gives rise also to spheroidal forms surrounding various centres, which sometimes originate spontaneously, and in other cases are accumulated round the débris of fossils, or even mere grains of sand. Nevertheless, all mineral substances are not alike calculated to produce oolitic rocks; putting aside some particular cases, this property is confined to limestone and oxide of iron.
With regard to the distribution of the Jurassic formation on the terrestrial globe, it may be stated that the Cotteswold Hills in England, and in France the Jura mountains, are almost entirely composed of these rocks, the several series of beds being all represented in them—this circumstance, in fact, induced Von Humboldt to name the formation after this latter range. The Upper Lias also exists in the Pyrenees and in the Alps; in Spain; in many parts of Northern Italy; in Russia, especially in the government of Moscow, and in the Crimea; but it is in Germany where it occupies the most important place. A thin bed of oolitic limestone presents, at Solenhofen in Bavaria, a geological repository of great celebrity, containing fossil Plants, Fishes, Insects, Crustaceans, with some Pterodactyles, admirably preserved; it yielded also some of the earliest of the feathered race. The fine quarries of lithographic stone at Pappenheim, so celebrated all over Europe, belong to the Jurassic formation.
It has recently been announced that these rocks have been found in India; they contribute largely to the formation of the main mass of the Himalayas, and to the chain of the Andes in South America; finally, from recent investigations, they seem to be present in New Zealand.
In England the Lias constitutes a well-defined belt about thirty miles broad, extending from Dorsetshire, in the south, to Yorkshire, in the north, formed of alternate beds of clay, shales, and limestone (with layers of jet), on the coast near Whitby. It is rich, as we have seen, in ancient life, and that in the strongest forms imaginable. From the unequal hardness of the rocks it comprises, it stands out boldly in some of the minor ranges of hills, adding greatly to the picturesque beauty of the scenery in the centre of the country. In Scotland the formation occupies a very limited space.
A map of the country at the close of the Jurassic period would probably show double the extent of dry land in the British Islands, compared with what it displayed as an island in the primordial ocean; but Devon and Cornwall had long risen from the sea, and it is probable that the Jurassic beds of Dorsetshire and France were connected by a tongue of land running from Cherbourg to the Liassic beds of Dorsetshire, and that Boulogne, still an island, was similarly connected with the Weald.