Jurassic

Jurassic, in geology, the middle period of the Mesozoic era, that is to say, succeeding the Triassic and preceding the Cretaceous periods. The name Jurassic (French jurassique; German Juraformation or Jura) was first employed by A. Brongniart and A. von Humboldt for the rocks of this age in the western Jura mountains of Switzerland, where they are well developed. It was in England, however, that they were first studied by William Smith, in whose hands they were made to lay the foundations of stratigraphical geology. The names adopted by him for the subdivisions he traced across the country have passed into universal use, and though some of them are uncouth English provincial names, they are as familiar to the geologists of France, Switzerland and Germany as to those of England. During the following three decades Smith’s work was elaborated by W. D. Conybeare and W. Phillips. The Jurassic rocks of fossils of the European continent were described by d’Orbigny, 1840-1846; by L. von Buch, 1839; by F. A. Quenstedt, 1843-1888; by A. Oppel, 1856-1858; and since then by many other workers: E. Benecke, E. Hébert, W. Waagen, and others. The study of Jurassic rocks has continued to attract the attention of geologists, partly because the bedding is so well defined and regular—the strata are little disturbed anywhere outside the Swiss Jura and the Alps—and partly because the fossils are numerous and usually well-preserved. The result has been that no other system of rocks has been so carefully examined throughout its entire thickness; many “zones” have been established by means of the fossils—principally by ammonites—and these zones are not restricted to limited districts, but many of them hold good over wide areas. Oppel distinguished no fewer than thirty-three zonal horizons, and since then many more sub-zonal divisions have been noted locally.

The existence of faunal regions in Jurassic times was first pointed out by J. Marcou; later M. Neumayr greatly extended observations in this direction. According to Neumayr, three distinct geographical regions of deposit can be made out among the Jurassic rocks of Europe: (1) The Mediterranean province, embracing the Pyrenees, Alps and Carpathians, with all the tracts lying to the south. One of the biological characters of this area was the great abundance of ammonites belonging to the groups of Heterophylli (Phylloceras) and Fimbriati (Lytoceras). (2) The central European province, comprising the tracts lying to the north of the Alpine ridge, and marked by the comparative rarity of the ammonites just mentioned, which are replaced by others of the groups Inflati (Aspidoceras) and Oppelia, and by abundant reefs and masses of coral. (3) The boreal or Russian province, comprising the middle and north of Russia, Spitzbergen and Greenland. The life in this area was much less varied than in the others, showing that in Jurassic times there was a perceptible diminution of temperature towards the north. The ammonites of the more southern tracts here disappear, together with the corals.

The cause of these faunal regions Neumayr attributed to climatic belts—such as exist to-day—and in part, at least, he was probably correct. It should be borne in mind, however, that although Neumayr was able to trace a broad, warm belt, some 60° in width, right round the earth, with a narrower mild belt to the north and an arctic or boreal belt beyond, and certain indications of a repetition of the climatic zones on the southern side of the thermal equator, more recent discoveries of fossils seem to show that other influences must have been at work in determining their distribution; in short, the identity of the Neumayrian climatic boundaries becomes increasingly obscured by the advance of our knowledge.

The Jurassic period was marked by a great extension of the sea, which commenced after the close of the Trias and reached its maximum during the Callovian and Oxfordian stages; consequently, the Middle Jurassic rocks are much more widely spread than the Lias. In Europe and elsewhere Triassic beds pass gradually up into the Jurassic, so that there is difficulty sometimes in agreement as to the best line for the base of the latter; similarly at the top of the system there is a passage from the Jurassic to the Cretaceous rocks (Alps).

Towards the close of the period elevation began in certain regions; thus, in America, the Sierras, Cascade Mountains, Klamath Mountains, and Humboldt Range probably began to emerge. In England the estuarine Portlandian resulted partly from elevation, but in the Alps marine conditions steadily persisted (in the Tithonian stage). There appears to have been very little crustal disturbance or volcanic activity; tuffs are known in Argentina and California; volcanic rocks of this age occur also in Skye and Mull.

The rocks of the Jurassic system present great petrological diversity. In England the name “Oolites” was given to the middle and higher members of the system on account of the prevalence of oolitic structure in the limestones and ironstones; the same character is a common feature in the rocks of northern Europe and elsewhere, but it must not be overlooked that clays and sandstones together bulk more largely in the aggregate than the oolites. The thickness of Jurassic rocks in England is 4000 to 5000 ft., and in Germany 2000 to 3000 ft. Most of the rocks represent the deposits of shallow seas, but estuarine conditions and land deposits occur as in the Purbeck beds of Dorset and the coals of Yorkshire. Coal is a very important feature among Jurassic rocks, particularly in the Liassic division; it is found in Hungary, where there are twenty-five workable beds; in Persia, Turkestan, Caucasus, south Siberia, China, Japan, Further India, New Zealand and in many of the Pacific Islands.

Being shallow water formations, petrological changes come in rapidly as many of the beds are traced out; sandstones pass laterally into clays, and the latter into limestones, and so on, but a reliable guide to the classification and correlation is found in the fossil contents of the rocks. In the accompanying table a list is given of some of the zonal fossils which regularly occur in the order indicated; other forms are known that are equally useful. It will be noticed that while there is general agreement as to the order in which the zonal forms occur, the line of division between one formation and another is liable to vary according to factors in the personal equation of the authors.

The Jurassic formations stretch across England in a varying band from the mouth of the Tees to the coast of Dorsetshire. They consist of harder sandstones and limestones interstratified with softer clays and shales. Hence they give rise to a characteristic type of scenery—the more durable beds standing out as long ridges, sometimes even with low cliffs, while the clays underlie the level spaces between.

Jurassic rocks cover a vast area in Central Europe. They rise from under the Cretaceous formations in the north-east of France, whence they range southwards down the valleys of the Saône and Rhone to the Mediterranean. They appear as a broken border round the old crystalline nucleus of Auvergne. Eastwards they range through the Jura Mountains up to the high grounds of Bohemia. They appear in the outer chains of the Alps on both sides, and on the south they rise along the centre of the Apennines, and here and there over the Spanish Peninsula. Covered by more recent formations they underlie the great plain of northern Germany, whence they range eastwards and occupy large tracts in central and eastern Russia.

Lower Jurassic rocks are absent from much of northern Russia, the stages represented being the Callovian, Oxfordian and Volgian (of Professor S. Nikitin); the fauna differs considerably from that of western Europe, and the marine equivalents of the Purbeck beds are found in this region. In south Russia, the Crimea and Caucasus, Lias and Lower Jurassic rocks are present. In the Alps, the Lower Jurassic rocks are intimately associated with the underlying Triassic formations, and resemble them in consisting largely of reddish limestones and marbles; the ammonites in this region differ in certain respects from those of western and central Europe. The Oxfordian, Callovian, Corallian and Astartian stages are also present. The Upper Jurassic is mainly represented by a uniform series of limestones, with a peculiar and characteristic fauna, to which Oppel gave the name “Tithonian.” This includes most of the horizons from Kimeridgian to Cretaceous; it is developed on the southern flanks of the Alps, Carpathians, Apennines, as well as in south France and other parts of the Mediterranean basin. A characteristic formation on this horizon is the “Diphya limestone,” so-called from the fossil Terebratula diphya (Pygope janitor) seen in the well-known escarpments (Hochgebirge Kalk). Above the Diphya limestone comes the Stramberg limestone (Stramberg in Moravia), with “Aptychus” beds and coral reefs. The rocks of the Mediterranean basin are on the whole more calcareous than those of corresponding age in north-west Europe; thus the Lias is represented by 1500 ft. of white crystalline limestone in Calabria and a similar rock occurs in Sicily, Bosnia, Epirus, Corfu; in Spain the Liassic strata are frequently dolomitic; in the Apennines they are variegated limestones and marls. The Higher Jurassic beds of Portugal show traces of the proximity of land in the abundant plant remains that are found in them. In Scania the Lias succeeds the Rhaetic beds in a regular manner, and Jurassic rocks have been traced northward well within the polar circle; they are known in the Lofoten Isles, Spitzbergen, east Greenland, King Charles’s Island, Cape Stewart in Scoresby Sound, Grinnell Land, Prince Patrick Land, Bathurst and Exmouth Island; in many cases the fossils denote a climate considerably milder than now obtains in these latitudes.

In the American continent Jurassic rocks are not well developed. Marine Lower and Middle Jurassic beds occur on the Pacific coast (California and Oregon), and in Wyoming, the Dakotas, Colorado, east Mexico and Texas. Above the marine beds in the interior are brackish and fresh-water deposits, the Morrison and Como beds (Atlantosaurus and Baptanodon beds of Marsh). Later Jurassic rocks are found in northern British Columbia and perhaps in Alaska, Wyoming, Utah, Montana, Colorado, the Dakotas, &c. In California some of the gold-bearing, metamorphic slates are of this age. Marine Jurassic rocks have not been clearly identified on the Atlantic side of America. The Patuxent and Arundel formations (non-marine) are doubtfully referred to this period. Lower and Middle Jurassic formations occur in Argentina and Bolivia. Jurassic rocks have been recognized in Asia, including India, Afghanistan, Persia, Kurdistan, Asia Minor, the Caspian region, Japan and Borneo. The best marine development is in Cutch, where the following groups are distinguished from above downwards: the Umia series = Portlandian and Tithonian of south Europe, passing upwards into the Neocomian; the Katrol series = Oxfordian (part) and Kimeridgian; the Chari series = Callovian and part of the Oxfordian; the Patcham series = Bathonian. In the western half of the Salt Range and the Himalayas, Spiti shales are the equivalents of the European Callovian and Kimeridgian. The upper part of the Gondwana series is not improbably Jurassic. On the African continent, Liassic strata are found in Algeria, and Bathonian formations occur in Abyssinia, Somaliland, Cape Colony and western Madagascar. In Australia the Permo-Carboniferous formations are succeeded in Queensland and Western Australia by what may be termed the Jura-Trias, which include the coal-bearing “Ipswich” and “Burrum” formations of Queensland. In New Zealand there is a thick series of marine beds with terrestrial plants, the Mataura series in the upper part of Hutton’s Hokanui system. Sir J. Hector included also the Putakaka series (as Middle Jurassic) and the Flag series with the Catlin’s River and Bastion series below. Jurassic rocks have been recorded from New Guinea and New Caledonia.

1 Purbeckian from the “Isle” of Purbeck. Aquilonien from Aquilo (Nord). Bononien from Bononia (Boulogne). Virgulien from Exogyra virgula. Pteroceran from Pteroceras oceani. Astartien from Astarte supracorollina. Rauracien from Rauracia (Jura). Argovien from Argovie (Switzerland). Neuvizien from Neuvizy (Ardennes). Divesien from Dives (Calvados). Bathonien from Bath (England). Bajocien from Bayeux (Calvados). Toarcien from Toarcium (Tours). Charmouthien from Charmouth (England). Sinemourien from Sinemurum, Semur (Côte d’Or). Hettangien from Hettange (Lorraine).

Life in the Jurassic Period.—The expansion of the sea during this period, with the formation of broad sheets of shallow and probably warmish water, appears to have been favourable to many forms of marine life. Under these conditions several groups of organisms developed rapidly along new directions, so that the Jurassic period as a whole came to have a fauna differing clearly and distinctly from the preceding Palaeozoic or succeeding Tertiary faunas. In the seas, all the main groups were represented as they are to-day. Corals were abundant, and in later portions of the period covered large areas in Europe; the modern type of coral became dominant; besides reef-building forms such as Thamnastrea, Isastrea, Thecosmilia, there were numerous single forms like Montivaltia. Crinoids existed in great numbers in some of the shallow seas; compared with Palaeozoic forms there is a marked reduction in the size of the calyx with a great extension in the number of arms and pinnules; Pentacrinus, Eugeniacrinus, Apiocrinus are all well known; Antedon was a stalkless genus. Echinoids (urchins) were gradually developing the so-called “irregular” type, Echinobrissus, Holectypus, Collyrites, Clypeus, but the “regular” forms prevailed, Cidaris, Hemicidaris, Acrosalenia. Sponges were important rock-builders in Upper Jurassic times (Spongiten Kalk); they include lithistids such as Cnemediastrum, Hyalotragus, Peronidella; hexactinellids, Tremadictyon, Craticularia; and horny sponges have been found in the Lias and Middle Jurassic.

Polyzoa are found abundantly in some of the beds, Stomatopora, Berenicia, &c. Brachiopods were represented principally by terebratulids (Terebratula, Waldheimia, Megerlea), and by rhynchonellids; Thecae, Lingula and Crania were also present. The Palaeozoic spirifirids and athyrids still lingered into the Lias. More important than the brachiopods were the pelecypods; Ostrea, Exogyra, Gryphaea were very abundant (Gryphite limestone, Gryphite grit); the genus Trigonia, now restricted to Australian waters, was present in great variety; Aucella, Lima, Pecten, Pseudomonotis Gervillia, Astarte, Diceras, Isocardia, Pleuromya may be mentioned out of many others. Amongst the gasteropods the Pleurotomariidae and Turbinidae reached their maximum development; the Palaeozoic Conularia lived to see the beginning of this period (Pleurotomaria, Nerinea, Pteroceras, Cerithium, Turritella).

Cephalopods flourished everywhere; first in importance were the ammonites; the Triassic genera Phylloceras and Lytoceras were still found in the Jurassic waters, but all the other numerous genera were new, and their shells are found with every variation of size and ornamentation. Some are characteristic of the older Jurassic rocks, Arietites, Aegoceras, Amaltheus, Harpoceras, Oxynoticeras, Stepheoceras, and the two genera mentioned above; in the middle stages are found Cosmoceras, Perisphinctes, Cardioceras, Kepplerites Aspidoceras; in the upper stages Olcostephanus, Perisphinctes, Reineckia, Oppelia. So regularly do certain forms characterize definite horizons in the rocks that some thirty zones have been distinguished in Europe, and many of them can be traced even as far as India. Another cephalopod group, the belemnites, that had been dimly outlined in the preceding Trias, now advanced rapidly in numbers and in variety of form, and they, like the ammonites, have proved of great value as zone-indicators. The Sepioids or cuttlefish made their first appearance in this period (Beloteuthis, Geoteuthis,) and their ink-bags can still be traced in examples from the Lias and lithographic limestone. Nautiloids existed but they were somewhat rare.

A great change had come over the crustaceans; in place of the Palaeozoic trilobites we find long-tailed lobster-like forms, Penaeus, Eryon, Magila, and the broad crab-like type first appeared in Prosopon. Isopods were represented by Archaeoniscus and others. Insects have left fairly abundant remains in the Lias of England, Schambelen (Switzerland) and Dobbertin (Mecklenburg), and also in the English Purbeck. Neuropterous forms predominate, but hemiptera occur from the Lias upwards; the earliest known flies (Diptera) and ants (Hymenoptera) appeared; orthoptera, cockroaches, crickets, beetles, &c., are found in the Lias, Stonesfield slate and Purbeck beds.

Fishes were approaching the modern forms during this period, heterocercal ganoids becoming scarce (the Coelacanthidae reached their maximum development), while the homocercal forms were abundant (Gyrodus, Microdon, Lepidosteus, Lepidotus, Dapedius). The Chimaeridae, sea-cats, made their appearance (Squaloraja). The ancestors of the modern sturgeons, garpikes and selachians, Hybodus, Acrodus were numerous. Bony-fish were represented by the small Leptolepis.

So important a place was occupied by reptiles during this period that it has been well described as the “age of reptiles.” In the seas the fish-shaped Ichthyosaurs and long-necked Plesiosaurs dwelt in great numbers and reached their maximum development; the latter ranged in size from 6 to 40 ft. in length. The Pterosaurs, with bat-like wings and pneumatic bones and keeled breast-bone, flew over the land; Pterodactyl with short tail and Rhamphorhyncus with long tail are the best known. Curiously modified crocodilians appeared late in the period (Mystriosaurus, Geosaurus, Steneosaurus, Teleosaurus). But even more striking than any of the above were the Dinosaurs; these ranged in size from a creature no larger than a rabbit up to the gigantic Atlantosaurus, 100 ft. long, in the Jurassic of Wyoming. Both herbivorous and carnivorous forms were present; Brontosaurus, Megalosaurus, Stegosaurus, Cetiosaurus, Diplodocus, Ceratosaurus and Campsognathus are a few of the genera. By comparison with the Dinosaurs the mammals took a very subordinate position in Jurassic times; only a few jaws have been found, belonging to quite small creatures; they appear to have been marsupials and were probably insectivorous (Plagiaulax Bolodon, Triconodon, Phascolotherium, Stylacodon). Of great interest are the remains of the earliest known bird (Archaeopteryx) from the Solenhofen slates of Bavaria. Although this was a great advance beyond the Pterodactyls in avian characters, yet many reptilian features were retained.

Comparatively little change took place in the vegetation in the time that elapsed between the close of the Triassic and the middle of the Jurassic periods. Cycads, Zamites, Podozamites, &c., appeared to reach their maximum; Equisetums were still found growing to a great size and Ginkgos occupied a prominent place; ferns were common; so too were pines, yews, cypresses and other conifers, which while they outwardly resembled their modern representatives, were quite distinct in species. No flowering plants had yet appeared, although a primitive form of angiosperm has been reported from the Upper Jurassic of Portugal.

The economic products of the Jurassic system are of considerable importance; the valuable coals have already been noticed; the well-known iron ores of the Cleveland district in Yorkshire and those of the Northampton sands occur respectively in the Lias and Inferior Oolites. Oil shales are found in Germany, and several of the Jurassic formations in England contain some petroleum. Building stones of great value are obtained from the Great Oolite, the Portlandian and the Inferior Oolite; large quantities of hydraulic cement and lime have been made from the Lias. The celebrated lithographic stone of Solenhofen in Bavaria belongs to the upper portion of this system.

See D’Orbigny, Paléontologie française, Terrain Jurassique (1840, 1846); L. von Buch, “Über den Jura in Deutschland” (Abhand. d. Berlin Akad., 1839); F. A. Quenstedt, Flötzgebirge Württembergs (1843) and other papers, also Der Jura (1883-1888); A. Oppel, Die Juraformation Englands, Frankreichs und s.w. Deutschlands (1856-1858). For a good general account of the formations with many references to original papers, see A. de Lapparent, Traité de géologie, vol. ii. 5th ed. (1906). The standard work for Great Britain is the series of Memoirs of the Geological Survey entitled The Jurassic Rocks of Britain, i and ii. “Yorkshire” (1892); iii. “The Lias of England and Wales” (1893); iv. “The Lower Oolite Rocks of England (Yorkshire excepted)” (1894); v. “The Middle and Upper Oolitic Rocks of England (Yorkshire excepted)” (1895). The map is after that of M. Neumayr, “Die geographische Verbreitung der Juraformation,” Denkschr. d. k. Akad. d. Wiss., Wien, Math. u. Naturwiss., cl. L., Abth. i, Karte 1\. (1885).

(J. A. H.)