Part 27

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.

G, Granitic rocks; a, Gneiss; b, Mica-schist; c, Skiddaw or Cumbrian Slates; d, Snowdon rocks; e, Plynlymmon rocks; f, Silurian rocks; g, Old Red Sandstone; h, Carboniferous Limestone; i, Millstone Grit; k, Coal-measures; l, Magnesian Limestone; m, New Red Sandstone; n, Lias; o, Lower, Middle, and Upper Oolites; p, Greensand; q, Chalk; r, Tertiary strata.]

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.

(Recent Polyzoa.)]

(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 of zoophytes, which carry on their operations, preparing a new continent, probably, in the atolls of the Pacific Ocean. (See Fig. 108, p. 240.)

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, 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.” See Fig. 1, PLATE XIX., p. 261. (Bradford, or Pear, Encrinite.)

“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.