Part 20
Among the animals of our epoch, only a single reptile is found provided with wings, or digital appendages analogous to the membranous wings of the bats, and which can be compared to the Pterodactyle. This is called the _Dragon_, one of the Draconidæ, a family of Saurians, which has been described by Daudin, as distinguished by the first six ribs, instead of hooping round the abdomen, extending in nearly a straight line, and sustaining a prolongation of skin which forms a sort of wing analogous to that of the Pterodactyle. Independent of the four feet, this wing sustains the animal, like a parachute, as it leaps from branch to branch; but the creature has no power to beat the air with it as birds do when flying. This reptile lives in the forests of the hottest parts of Africa, and in some isles of the Indian Ocean, especially in Sumatra and Java. The only known species is that figured at page 238 (Fig. 107), which comes from the East Indies.
What a strange population was that which occupied the earth at this stage of its history, when the waters were filled with creatures so extraordinary as those whose history we have traced! Plesiosauri and Ichthyosauri filled the seas, upon the surface of which floated innumerable Ammonites in light skiffs, some of them as large as a good-sized cart-wheel, while gigantic Turtles and Crocodiles crawled on the banks of the rivers and lakes. Only one genus of Mammals had yet appeared, but no birds; nothing broke the silence of the air, if we except the breathing of the terrestrial reptiles and the flight of winged insects.
The earth cooled progressively up to the Jurassic period, the rains lost their continuity and abundance, and the pressure of the atmosphere sensibly diminished. All these circumstances favoured the appearance and the multiplication of innumerable species of animals, whose singular forms then showed themselves on the earth. We can scarcely imagine the prodigious quantity of Molluscs and Zoophytes whose remains lie buried in the Jurassic rocks, forming entire strata of immense thickness and extent.
The same circumstances concurred to favour the production of plants. If the shores and seas of the period received such a terrible aspect from the formidable animals we have described, the vegetation which covered the land had also its peculiar character and appearance. Nothing that we know of in the existing scenery of the globe surpasses the rich vegetation which decorated the continents of the Jurassic period. A temperature still of great elevation, a humid atmosphere, and, we have no reason to doubt, a brilliant sun, promoted the growth of a luxuriant vegetation, such as some of the tropical islands, with their burning temperature and maritime climate, can only give us an idea of, while it recalls some of the Jurassic types of vegetation. The elegant Voltzias of the Trias had disappeared, but the Horse-tails (_Equiseta_) remained, whose slender and delicate stems rose erect in the air with their graceful panicles; the gigantic rushes also remained; and though the tree-ferns had lost their enormous dimensions of the Carboniferous age, they still preserved their fine and delicately-cut leaves.
Alongside these vegetable families, which passed upwards from the preceding age, an entire family--the Cycads (Fig. 72, p. 168)--appear for the first time. They soon became numerous in genera, such as Zamites, Pterophyllum (Williamsonia), and Nilssonia. Among the species which characterise this age, we may cite the following, arranging them in families:--
FERNS. CYCADS. CONIFERS.
Odontopteris cycadea. Zamites distans. Taxodites. Taumopteris Munsteri. Zamites heterophyllus. Pinites. Camptopteris crenata. Zamites gracilis. Pterophyllum dubium. Nilssonia contigua. Nilssonia elegantissima. Nilssonia Sternbergii.
The _Zamites_ seem to be forerunners of the Palms, which make their appearance in the following epoch; they were trees of elegant appearance, closely resembling the existing Zamias, which are trees of tropical America, and especially of the West India Islands; they were so numerous in species and in individuals that they seem to have formed, of themselves alone, one half of the forests during the period which engages our attention. The number of their fossilised species exceeds that of the living species. The trunk of the Zamites, simple and covered with scars left by the old leaves, supports a thick crown of leaves more than six feet in length, disposed in fan-like shape, arising from a common centre.
The _Pterophyllum_ (Williamsonia), formed great trees, of considerable elevation, and covered with large pinnated leaves from top to bottom. Their leaves, thin and membranous, were furnished with leaflets truncated at the summit and traversed by fine nervures, not convergent, but abutting on the terminal truncated edge.
The _Nilssonia_, finally, were Cycadeaceæ resembling the Pterophyllum, but with thick and coriaceous leaves, and short leaflets contiguous to, and in part attached to the base; they were obtuse or nearly truncated at the summit, and would present nervures arched or confluent towards that summit.
The essential characters of the vegetation during the Liassic sub-period were:--1. The great predominance of the Cycadeaceæ, thus continuing the development which commenced in the previous period, expanding into numerous genera belonging both to this family and that of the _Zamites_ and _Nilssonia_; 2. The existence among the Ferns of many genera with reticulated veins or nervures, and under forms of little variation, which scarcely show themselves in the more ancient formations.
On the opposite page (PLATE XVI.) is an ideal landscape of the Liassic period; the trees and shrubs characteristic of the age are the elegant Pterophyllum, which appears in the extreme left of the picture, and the Zamites, which are recognisable by their thick and low trunk and fan-like tuft of foliage. The large horsetail, or Equisetum of this epoch, mingles with the great Tree-ferns and the Cypress, a Conifer allied to those of our own age. Among animals, we see the Pterodactyle specially represented. One of these reptiles is seen in a state of repose, resting on its hind feet. The other is represented, not flying, after the manner of a bird, but throwing itself from a rock in order to seize upon a winged insect, the dragon-fly (_Libellula_), the remains of which have been discovered, associated with the bones of the Pterodactyle, in the lithographic limestone of Pappenheim and Solenhofen.
OOLITIC SUB-PERIOD.
This period is so named because many of the limestones entering into the composition of the formations it comprises, consist almost entirely of an aggregation of rounded concretionary grains resembling, in outward appearance, the roe or eggs of fishes, and each of which contains a nucleus of sand, around which concentric layers of calcareous matter have accumulated; whence the name, from ωον, _egg_, and λιθος, _stone_.
The Oolite series is usually subdivided into three sections, the _Lower_, _Middle_, and _Upper Oolite_. These rocks form in England a band some thirty miles broad, ranging across the country from Yorkshire, in the north-east, to Dorset, in the south-west, but with a great diversity of mineral character, which has led to a further subdivision of the series, founded on the existence of particular strata in the central and south-western counties:--
UPPER. MIDDLE. LOWER.
1. Purbeck Beds. 1. Coral Rag. 1. Cornbrash. 2. Portland Stone 2. Oxford Clay. 2. Great Oolite & Forest and Sand. Marble. 3. Kimeridge Clay. 3. Stonesfield Slate. 4. Fuller’s Earth. 5. Inferior Oolite.
The alternations of clay and masses of limestone in the Liassic and Oolite formations impart some marked features to the outline of the scenery both of France and England: forming broad valleys, separated from each other by ranges of limestone hills of more or less elevation. In France, the Jura mountains are composed of the latter; in England, the slopes of this formation are more gentle--the valleys are intersected by brooks, and clothed with a rich vegetation; it forms what is called a tame landscape, as compared with the wilder grandeur of the Primary rocks--it pleases more than it surprises. It yields materials also, more useful than some of the older formations, numerous quarries being met with which furnish excellent building-materials, especially around Bath, where the stone, when first quarried, is soft and easily worked, but becomes harder on exposure to the air.
The annexed section (Fig. 109) will give some idea of the configuration which the stratification assumes, such as may be observed in proceeding from the north-west to the south-east, from Caermarthenshire to the banks of the Ouse.
LOWER OOLITE FAUNA.
The most salient and characteristic feature of this age is, undoubtedly, the appearance of animals belonging to the class of Mammals. But the organisation, quite special, of the first of the Mammalia will certainly be a matter of astonishment to the reader, and must satisfy him that Nature proceeded in the creation of animals by successive steps, by transitions which, in an almost imperceptible manner, connect the beings of one age with others more complicated in their organisation. The first Mammals which appeared upon the earth, for example, did not enjoy all the organic attributes belonging to the more recent creations of the class. In the latter the young are brought forth living, and not from eggs, like Birds, Reptiles, and Fishes. But the former belonged to that order of animals quite special, and never numerous, the young of which are transferred in a half-developed state, from the body of the mother to an external pouch in which they remain until they become perfected; in short, to marsupial animals. The mother nurses her young during a certain time in a sort of pouch external to the body, in the neighbourhood of the abdomen, and provided with teats to which the young adhere. After a more or less prolonged sojourn in this pouch, the young animal, when sufficiently matured and strong enough to battle with the world, emerges from its warm retreat, and enters fully into life and light; the process being a sort of middle course between oviparous generation, in which the animals are hatched from eggs after exclusion from the mother’s body, like Birds; and viviparous, in which the animals are brought forth alive, as in the ordinary Mammals.
In standard works on natural history the animals under consideration are classed as _mammiferous Didelphæ_. They are brought forth in an imperfect state, and during their transitional condition are suckled in a pouch supported by bones called _marsupial_, which are attached by their extremities to the pelvis, and serve to support the marsupium, whence the animals provided with these provisions for bringing up their progeny are called _Marsupial Mammals_. The Opossum, Kangaroo, and Ornithorhynchus are existing representatives of this group.
The name of _Thylacotherium_, or _Amphitherium_, or _Phascolotherium_, is given to the first of these marsupial Mammals which made their appearance, whose remains have been discovered in the Lower Oolite, and in one of its higher stages, namely, that called the _Great Oolite_. Fig. 110 represents the jaw of the first of these animals, and Fig. 111 the other--both of the natural size. These jaw-bones represent all that has been found belonging to these early marsupial animals; and Baron Cuvier and Professor Owen have both decided as to their origin. The first was found in the Stonesfield quarries. The Phascolotherium, also a Stonesfield fossil, was the ornament of Mr. Broderip’s collection. The animals which lived on the land during the Lower Oolitic period would be nearly the same with those of the Liassic. The insects were, perhaps, more numerous.
The marine fauna included Reptiles, Fishes, Molluscs, and Zoophytes. Among the first were the Pterodactyle, and a great Saurian, the Teleosaurus, belonging to a family which made its appearance in this age, and which reappears in the following epoch. Among the Fishes, the Ganoids and Ophiopsis predominate. Among the Ammonites, _Ammonites Humphriesianus_, _A. Herveyii_ (Fig. 112), _A. Brongniarti_, _Nautilus lineatus_, and many other representatives of the cephalopodous Mollusca. Among the Brachiopods are _Terebratula digona_ (Fig. 113) and _T. spinosa_. Among the Gasteropoda the _Pleurotomaria conoidea_ is remarkable from its elegant shape and markings, and very unlike any of the living _Pleurotoma_ as represented by _P. Babylonia_ (Fig. 114). _Ostrea Marshii_ and _Lima proboscidea_, which belong to the Acephala, are fossil Mollusca of this epoch, to which also belong _Entalophora cellarioides_, _Eschara Ranviliana_, _Bidiastopora cervicornis_; elegant and characteristic molluscous Polyzoa. We give a representation of two living species, as exhibiting the form of these curious beings. (Figs. 115 and 116.)
The Echinoderms and Polyps appear in great numbers in the deposits of the Lower Oolite: _Apiocrinus elegans_, _Hyboclypus gibberulus_, _Dysaster Endesii_ represent the first; _Montlivaltia caryophyllata_, _Anabacia orbulites_, _Cryptocœnia bacciformis_, and _Eunomia radiata_ represent the second.
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 of zoophytes, which carry on their operations, preparing a new continent, probably, in the _atolls_ of the Pacific Ocean. (See Fig. 108, p. 240.)
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, distinguished for 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 name _Coniopteris Murrayana_, _Pecopteris Desnoyersii, Pachypteris lanceolata_, and _Phlebopteris 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 the _Zamias_, 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 genera _Thuites_, _Taxites_, and _Brachyphyllum_. The _Thuites_ were true _Thuyas_, 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. The _Taxites_ have been referred, with some doubts, to the Yews. Finally, the _Brachyphyllum_ were trees which, according to the characteristics of their vegetation, seem to have approached nearly to two existing genera, the _Arthotaxis_ of Tasmania, and the _Weddringtonias_ of South Africa. The leaves of the Brachyphyllum are short and fleshy, with a large and rhomboidal base.
LOWER OOLITE ROCKS.
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, _Bajocien_ and _Bathonian_, adopted by M. D’Orbigny and his followers. The lowest beds of the _Inferior Oolite_ occur 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. These deposits are surmounted by alternate layers of clay and marl, blue or yellow--the well-known _Fuller’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. The _Great_ or _Bath 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.)
[65] “Elements of Geology,” p. 399.
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. The _Cornbrash_ (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]
[66] See Bristow in Descriptive Catalogue of Rocks, in _Mus. Pract. Geol._, p. 134.
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 directly upon 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 deposition of the strata, both of the Inferior Oolite and of the formations which immediately succeed it.”[67]
[67] President’s Address, by Professor A. C. Ramsay. _Quart. Jour. Geol. Soc._, 1864, vol. xx., p. 4.