Jurassic System, the name given to that great series of Mesozoic strata which includes the Lias and overlying Oolites. The system receives its name from the Jura Mountains, where strata of that age are well developed. In England Jurassic rocks extend over a large area in Yorkshire between the mouth of the Tees and Filey Bay, and stretch south from the Humber along the western borders of the great flats of Lincoln and Cambridge, from which they sweep south-west as a broad belt across the Midlands to the Bristol Channel and the coasts of the English Channel between Lyne Regis and Durlstone Head. Only a few patches of Jurassic rocks occur in Scotland, as near Brora on the east coast of Sutherland, and in some of the western islands. In Ireland the system is equally sparingly represented, as near Larne and Portrush in Antrim. On the Continent rocks of the same age are developed over extensive regions. They form a ring or zone-like belt surrounding the Cretaceous and Tertiary deposits of the Paris basin, underneath which the Jurassic strata doubtless continue. Farther south another belt sweeps round the central plateau of France, and stretches south to the Mediterranean. The most continuous areas in Germany occur in Franconia, Swabia, and Upper Silesia. Rocks of the same age occupy a wide region in central and northern Russia, while more or less isolated areas are met with in the Caucasus, the Crimea, the Carpathians, the Dinaric Alps, the Apennines, &c. One of the most important Jurassic tracts is that of the Jura Mountains, extending between Basel and Geneva. Narrow and broader belts of the same strata occur along the northern and southern flanks of the Alps. The system also occurs in considerable force in the north-east and the south of Spain.
The Jurassic system of Europe has been arranged in the following groups:
PURBECKIAN: mostly of fresh-water origin; they contain traces of old land-surfaces (dirt-beds), with roots and stems of fossil plants.
PORTLANDIAN: chiefly sandstones, marls, and limestone (Portland-stone); marine.
KIMERIDGIAN: dark shales and clay (Kimeridge Clay); marine.
CORALLIAN: limestones with corals (Coral Rag), clays, and calcareous grits; marine.
OXFORDIAN: dark gray or blue clay (Oxford Clay); and calcareous sandstone (Kellaway's Rock—Callovian); marine.
BATHONIAN: limestones, clays, and sands (Cornbrash, Bradford Clay, and Forest Marble); shelly limestones (Great or Bath Oolite), Stonesfield Slate; Fuller's Earth; all marine.
BAJOCIAN (or Inferior Oolite): calcareous sandstones and grits (Cheltenham); marine; represented in Yorkshire by estuarine sandstones, shales, and limestones, with seams of coal and ironstone.
LIASSIC: sands and clays (Upper Lias) resting on limestones, sands, clays, and ironstones (Middle Lias, Marlstone); below which come limestones and dark shales (Lower Lias); all marine.
In India (Cutch) Jurassic strata, ranging from the Bajocian up to the Portlandian inclusively, attain a considerable thickness. The system is not largely developed in North America (Sierra Nevada and Rocky Mountains), but is notable in Colorado for its remarkable reptilian remains. Finally it may be added that Jurassic rocks have been detected in Spitzbergen, Siberia, Australia, New Caledonia, and New Zealand.
Life of the Period.—The predominant forms among the land-plants were cycads, conifers, ferns, and equisetums, but with these were associated true monocotyledonous angiosperms, represented by fossil fruits which are apparently referable to our screw-pines (Pandanaceæ). This vegetation was widely spread over the earth's surface, flourishing abundantly in Britain, and extending far into the Arctic Circle.
The lower classes of the animal kingdom were represented by foraminifera and sponges, by a great variety of corals, by crinoids (both stalked and free forms), by starfishes, sea-urchins, &c. Corals are especially numerous, and mostly belong to the reef-building family of star-corals. Many of the limestones of the period, indeed, particularly those of the Corallian, are simply old coral-reefs. Amongst crinoids one of the most characteristic forms was Pentacrinus—a genus still living. Many genera of sea-urchins occur (Acrosalenia, Cidaris, Diadema, &c.), and with these were associated numerous starfishes and brittle-stars. The most prominent crustaceans were long-tailed decapods, to which belong our modern lobsters, prawns, &c.; and true crabs were also present. Insects were represented by ancestral forms of cockroach, grasshopper, earwig, ant, dragon-fly, may-fly, beetles, bugs, &c. Brachiopods, which formed so characteristic a feature in the life of the Palæozoic seas, had now ceased to be dominant forms, although they were still individually numerous. Most of the old Palæozoic types had disappeared before Jurassic times—two surviving forms (Spirifer and Lepetena) dying out at last before the close of the Liassic stage. We note, however, the presence of the inarticulate types (Crania, Lingula, Discina) which appeared first in Cambrian times and still flourish in our seas. The most important Jurassic brachiopods are Terebratula and Rhynchonella, of which there were many species. Both genera have survived to the present, but are represented by only a few species. Amongst the lamellibranch molluscs many forms unknown in Palæozoic times now made their first appearance, the most important types being the oysters (Ostrea, Gryphaea, and Exogyra), together with Trigonia and Pholadomya. Gasteropods were fairly numerous, and comprised representatives of the whelks, spindle-shells, spider-shells, &c. of existing seas; and it may be noted that the earliest recognisable fresh-water univalves (Paludina, Planorbis) date from Jurassic times. But the most characteristic molluscs of this period were the cephalopods, both tetrabranchiate and dibranchiate types. The former, or chambered division, were represented by many forms of Ammonites, several hundred species having been chronicled; and the latter, or 'cuttle-fish' division, by numerous species of Belemnite. Among fishes were ganoids, usually of small size, and representatives of the sharks and rays. But by far the most important of the vertebrates were the reptiles, which flourished in extraordinary abundance during Jurassic times, and may well be said to be the most striking and characteristic life-forms of the period. Chelonians or turtles, lacertilians or lizards, and crocodiles are all represented: but the most characteristic reptiles were the huge saurians, Ichthosaurus (q.v.), Plesiosaurus (q.v.), and Pliosaurus (q.v.). Another remarkable group of reptiles were the pterosaurs or winged saurians, of which the most noted was Pterodactylus (q.v.). Contemporaneous with these were great Dinosaurs (q.v.), such as Ceteosaurus, Megalosaurus, Atlantosaurus, &c., while bird-life was represented by the toothed Archæopteryx (q.v.), with its lizard-like tail. The highest forms of life were small marsupial mammals, some of which seem to have been insectivorous, while others were herbivorous.
Physical Conditions.—During Jurassic times the area now occupied in the British Islands by the older rocks appears to have been for the most part dry land. The sea covered the north-east corner of Ireland, and extended along the west coast of Scotland over the site of what is now Skye, and it seems in like manner to have occupied the North Sea opposite the east coast, a portion of which in Sutherland was covered by it. What are now the high grounds of northern England and Wales and the heights of Devon and Cornwall, together with a ridge of Palæozoic rocks which extends under London, were the chief land-areas in south Britain, so that nearly all England was under water in the earlier stages of the Jurassic period. The same sea swept over vast areas of what is now the European continent. The older rocks in the north-west and north-east of France and the central plateau of the same country formed dry land—all the rest was submerged. In like manner, wide regions in Spain were under water. In middle Europe the sea covered nearly all the low grounds of north Germany, and extended far east into the heart of Russia, whence it passed north, and was doubtless confluent with the Arctic Ocean. It occupied the site of the Jura Mountains, and passed eastwards into Bohemia, while on the south side of the Alps it spread over a large part of Italy, extending eastwards so as to submerge a broad region in Austria-Hungary and the Turkish provinces. In short, what are now the central and southern portions of our continent formed a great archipelago in which appeared numerous islands large and small. The chief land-areas of the European region, therefore, were confined to the north and north-west. The existence of this northern land is shown by the fact that, while the Bajocian of the south of England consists of purely marine accumulations, the contemporaneous deposits in Yorkshire are largely fresh-water and estuarine.
The Jurassic strata, which attain a thickness of several thousand feet, point to considerable subsidence; the downward movement, however, was not continuous, but seems to have been interrupted by pauses. Taken as a whole the strata of north-western and central Europe are indicative of rather shallow-water conditions; but the waters were often sufficiently clear to favour the abundant growth of coral-reefs. After the deposition of the Portlandian beds the sea disappeared from what are now the low grounds of England. The succeeding Purbeckian beds are for the most part of fresh-water origin, and seem to have been laid down at or near the mouth of some large river, which probably took its rise in the hills of England or Wales, and flowed south across the upraised bed of the Jurassic sea. Similar indications of a more or less abrupt change from sea to fresh water are afforded by the Jurassic of central Europe, as in northern France, Hauover, Westphalia, and the Jura in Switzerland. While the Jurassic of central and north-western Europe would seem to have accumulated in somewhat shallow seas, the contemporaneous strata of the Mediterranean basin have a decidedly more pelagic aspect. This southern development of the Jurassic is sometimes called the Tithonian series. It is recognised in the southern Alps, the southern Tyrol, the Venetian and Dalmatian Alps, and the Carpathians, and extends into northern Africa.
The climatic conditions of the Jurassic period appear to have been extremely genial. Reef-building corals, for example, flourished in latitudes which are now some 3000 miles north of the present range of reef-builders, while cuttle-fishes and Ammonites and large enaliosaurus lived far within the Arctic Circle.