Ordovician System
ORDOVICIAN SYSTEM, in geology, the group of strata which occur normally between the Cambrian below and the Silurian above; it is here regarded as including in ascending order the Arenig, Llandeilo, and Caradoc or Bala series iqq.v.). The name was introduced by C. Lapworth in 1879 to embrace those rocks - well developed in the region formerly inhabited by the Ordovices - which had been classed by Sir R. Murchison as Lower Silurian and by A. Sedgwick in his Cambrian system. The term is convenient and well established, but Lower Silurian is still used by some authors. The line of demarcation between the Ordovician and the Cambrian is not sharply defined, and beds on the Tremadoc horizon of the Cambrian are placed by many writers at the base of the Ordovician, with good palaeontological reasons.
The rocks of this system include all types of sedimentation; when they lie flat and undisturbed, as in the Baltic region and Russia, the sands and clays are as soft and incoherent as the similar rocks of Tertiary age in the south of England; where they have been subjected to powerful movements, as in Great Britain, they are represented by slates, greywackes, quartzitcs, chlorite-, actinolite- and garnet-schists, amphibolites and other products of metamorphism. In Europe the type of rock varies rapidly from point to point, limestones, shales, sandstones, current-bedded grits and conglomerates or their metamorphosed equivalents are all found within limited areas; but in northern Europe particularly the paucity of limestones is a noteworthy feature in contrast with the rocks of like age in the south, and still more with the Ordovician of North America, in which limestones are prevalent. In the Highlands of Scotland, in north-west England, in Wales and Ireland, there are enormous developments of contemporaneous lavas and tuffs and their metamorphosed representatives; tuffs occur also in Brittany, and lavas on a large scale in Nova Scotia and New Brunswick.
Distribution. - The Ordovician system is widely distributed. The accompanying map indicates roughly the relative positions of the principal land-masses and seas, but it must be accepted with reserve.
A study of the fossils appears to point to the existence of definite faunal regions or marine basins. The Ordovician rocks waters, embracing China, Siberia and the Himalayas; C(>n- cerning the last-named marine area not much is known. In the opposite direction, the Baltic basin may have communicate;d, through Greenland, with the North American and Arctic seas. Over central and eastern North America another large body of water probably lay, with open communications with the north and west, and with a more constricted connexion with the c Continent Hypothetical Lanil & Sea areas in the Early Ordovician Period After F. Frtch & R. RjeJc^ar.n SouihAu3traliaii Sea.
Atlantic sea. The lagoonal character of some of the rocks of the Tunguska region of Siberia may perhaps be indicative of continental border conditions in that quarter.
Some of the principal subdivisions of the Ordovician rocks are enumerated in the table. Owing to the universal distribution of the graptolites, the correlation of widely separated areas has been rendered possible wherever the muds and shales, in which their remains are preserved, are found. Where they are absent the correlation of the minor local subdivisions of distant deposits is more difficult. In Great Britain, through Tremadoc beds, Ceratopyge beds, and beds with Euloma-Niobe fauna here regarded as Cambrian : not invariably present.
of the British Isles seem to have been deposited in a North Atlantic sea which embraced also the north of France and Belgium. Conlluent with this sea on the east was a rather peculiar basin which included Bohemia, southern France, Spain, Portugal, the eastern Alps, Thuringia, Fichtelgebirge and the Keller Wald. Another European basin, probably separated from the Bohemian or Mediterranean sea in early Ordovician times, lay over the Baltic region, Scandinavia, the Baltic provinces and north Germany, and communicated eastwards by way of Russian Poland and central Russia with far eastern C. Lapworth and his school, and J. E. Marr and the Cambridge school, and in Scandinavia and the Baltic region, through W. C. Brogger, S. A. Tulberg, F. Schmidt and others, the most elaborate subdivision of the Ordovician rocks has been attained.
In the Baltic provinces of Russia, F. Schmidt describes the following stages, in descending order: (Stage F) the Lyckhoim and Borkholm zones, a highly fossiliferous series, equivalent to the Middle Bala of Britain ; many of the limestones are largely formed of Rhabdoporella and other calcareous algae. (E) Wesenberg zone = Bala. (D) Jewe and Kegel zone. (C) Itfer beds, Kuckers Shale (bituminous limestones and marls = Brandschiefer), Echinosphaerite limestone = Upper Orthoceratite limestone of Sweden. (B) Orthoceratite (Vaginaten) limestone = Orthoceratite limestone of Sweden, (ylauconitic limestone, Glauconitic sand (Greensand). The lastnxentioned reposes on Cambrian Dictyonema shales. While the Ordovician rocks in Scania, the Baltic provinces anil north-central Rbssia are undisturbed and level-bedded, those on the western side ofl the Scandinavian axis and in the Urals have suffered movement and are metamorphosed into schists, phyllites, quartzite, marble, &n. ; and, especially in Scandinavia, have been extensively thrust. Tl|ie Bohenlian Ordovician, " stage D " of Barrande, consists mainly of grey wackes and shales with some ironstone beds and eruptive rocks in the lower parts. In Germany the only large areas are found in the Thuringer VVald, Fichtelgebirge, Frankenwald and Vogtland, where they consist principally of unfossiliferous greywackes and shales with some oolites and glauconitic ironstone (chamosite) in the lower part. They are divisible into the Hauptschiefer or Lederschiefer and the OIjer-Thuringit beds above, and the Griffelschiefer and UnterThuringit beds below, which rest upon the Leitmitzschiefer of the Euloma-Niobe (Cambrian) horizon. Across northern Russia Ordovician rocks cover a great area; they consist of clays, bituminous and calcareous shales, sands and marls, which in the Ural region have been metamorphosed; the Bukowka sandstone of Russian Poland is of this age. In north-west France this system is represented in Brittany and Normandy by the slates of Riadan, the gris de May, the schistes a calymines (with an ironstone bed at the base) and the gres armoricain. In the Ardennes are the schistes de Gcmbloux, resting upon graptolitic shales of Arenig age. Sandstones and shales occur in Languedoc, and various rocks in the Pyrenees. In the Iberian peninsula Ordovician rocks are widely spread, represented by sandstones, slates and shales covering the whole of the period ; they are well developed in Asturia and Galicia. In the eastern Alps about Graz are found calcareous shales with crinoids, the " Schockelkalk " and " Semriacher " shales; the Marthener beds of the Carnic Alps are of this age. In China (Kiang-su, Kian-chang), in Burma (Mandalay) and in the Himalayas (Niti and Spiti) Ordovician fossil-bearing rocks are known.
On the North American continent Ordovician rocks cover a very large area in the central, eastern and northern parts (north of lat. 30°). As regards the classification and correlation of the strata, which change in character from point to point, as is natural over so large an area, much remains to be done. In the table the divisions of the system that obtain in the New York district are enumerated; but in each state there is a local nomenclature for the beds. Thus in Iowa, Wisconsin and Minnesota we find (i) Lower Magnesian limestone, St Peter's sandstone; (2) Trenton limestone, Galena limestone; (3) Hudson river shales; in Arkansas, the California or Magnesian limestone, Saccaroidal limestone, Izard limestone and Polk Bayou limestone; in Oklahoma, the Arbuckle limestone, Simpson series, Viola limestone and Sylvan shales; and in east Tennessee, the Chickamauga limestone, Athens shale, Tellia sandstone, Sievicr shale and Bays sandstone. In Massachusetts there are enormous series of schists which have been assigned to this period. In west Virginia are the Martinsburg shales (1000 ft. or more). In Canada the Ordovician rocks (Quebec group) are thickly developed. In the upper division there are the lowest of the Anticosti limestones, the Hudson river beds, and Trenton limestone; to the middle division belong the Coenograplus shales; and the lower division consists of the Levis shales with Sillery beds at the base. In Nova Scotia and New Brunswick are the lower and upper divisions of the Cobequid group, a series of shales, quartzites and conglomerates with igneous rocks. In the polar regions Ordovician rocks are represented by the Trenton limestone in Boothia and King William's Land; by limestones with Caryocystis granehim in east Greenland; and in the Barrow Straits by beds with AsapJms and Madurea.
In North Africa Ordovician rocks are probably present, and in New Zealand the Arorere series (Wanaka group), and in Australia (Victoria) the graptolitic, gold-bearing shales and slates belong to this period. During this period there appears to have been a general tendency for the sea to transgress on the land, a tendency which increased towards its close, especially in the northern hemisphere (Europe and the Appalachian regions). One of the results of this movement was the interchange and commingling of many previously separated faunal groups. About the beginning of the period the sea withdrew from the land in Texas and south of the Rocky Mountains. The folding of the Appalachians was in progress early in Ordovician times and later in the period the first symptoms of the Scandinavian and British folding set in.
Volcanic Activity. - This period was one of great volcanic activity in several widely separated regions. " In Ayrshire and the south-western districts (of the southern uplands), where the volcanic constituents attain a great development, they consist of basic lavas (diabase, etc.), with intercalated tuffs and agglomerates. A characteristic feature of these lavas is the development of ellipsoidal or pillow-structure in them. This volcanic platform appears to underlie the Silurian region over an area of at least 2000 sq. m., inasmuch as it comes to the surface wherever the crests of the anticlines bring up suffi- ciently deep parts of the formations. It is thus one of the most extensive as well as one of the most ancient volcanic tracts of Europe " (Sir A. Geikic, Text-hook of Geology, 4th ed. vol. ii. p. 951). In the west of England and in Wales there was also a very active volcanic centre. In the Snowdon district thousands of feet of contemporaneous felsitic lavas and tuffs occur in the Bala beds; while in Cader Idris, the Arenig Mountains and the Arans there are similar eruptions of felsitic and rhyolitic lavas, tuffs and agglomerates - probably many of ihem submarine - interstratified in the Arenig formation. In the Lake district a great series of lavas and ashes - the Borrowdale scries - was erupted during the middle of the period; the earlier effusions were andesitic, the later ones felsitic and rhyolitic. In Ireland the Arenig lavas of Tyrone resemble some of those in Scotland. Volcanic rocks (porphyrites, syenites and lavas) occur in considerable force in the Ordovician rocks of Nova Scotia and New Brunswick and New Zealand. Tuffs of this age are found in Brittany, and diabase in Bohemia.
The economic products obtained from rocks of this period include gold in Australia, New Zealand and Wales; iron ore in France; lead and zinc from the Galena and Trenton horizons in Wisconsin, Iowa and Illinois; manganese in Arkansas; oil and gas from the Trenton stage in Ohio and east Indiana; roofing slates and slate pencils in Wales and the Lake district; limestone in Great Britain and Tennessee; phosphate beds in Wales and Tennessee; marble in the Appalachian district; graphite (plum'oago) in the Lake district; and jasper in Wales and southern Scotland.
Ordovician Life. - Compared with the preceding Cambrian period, the Ordovician is remarkable for the great expansion in numbers and variety of organisms, apart from the fact that fossils are better preserved in the younger formations.
All the great classes of mollusks were represented, the most numerous being the brachiopods, which, in addition to the simple forms of the Cambrian, began at this time to develop spire-bearing genera (Chonetes, Orthis, Orthisina, Strophomena, Crania, Schizotreta, Poratnbonites, Rafinesquina, Leptaena,Zygospira). The gasteropods now developed all the leading types of shell (Pleurotomaria, Omphalotrochus) ; but' both this class and the pelecypods (Lyrodesma, Ctenodonta, Modiolopsis) were subordinate in importance to the cephalopods. These mollusks were probably the most powerful living creatures in the Ordovician seas; straight-shelled, slightly curved, and nautiloid forms predominated ((^rthoceras, Cyrtoceras, Gyroceras, Trocholites, Endoceras, Litoceras, Lituites, Actinoceras). Some of the straight shells were of enormous size, 12 to 15 ft. long and as much as i ft. in diameter, in the widest part. Trilobites were present in great abundance, and in this period they reached the climax of their development. In the lower stage we find Agnostus, Calymene, Asaphus, Illaenus, Placoparia; on the Llandeilo horizon, Calymene, Asaphus, Megalaspis, Dahianitis ; and, at the summit, Trinucleus and Homalonotus. In the transition zone between Ordovician and Cambrian, Ceratopyge, Euloma, Niohe, flourished. Other important genera are Ogygia, Cheirurus, Harpes, Acidaspis. Ostracods (Leperditia, Beyrichia), cyprids (Bairdia, Macrocypris), phyllocarids (Ceratiocaris, Peltocaris), cirripeds (Lepidocoleus), and, later, eurypterids represented other crustacean groups. The bryozoans, Stomalopora, Monticulipora, Phylloporina, Fenestella and others, were abundant and frequently formed beds of limestone. .Among the echinoderms the cystoids were the most prominent [Pleurocystis, Aristocystis) and at this period reached their climax; crinoids (Archaeocrinus, Dendrocrinus) became more important; while ophiuroids.echinoids (Bothriocidaris) and asteroids (Taeniaster, Palaeaster) made their appearance. Corals (Streptelasma, Columnaria) were scarce, and sponges (Aulocopium, Caryospongia. . Archacocyathus) were not particularly important; Reccptaculites, Ischadites, are well-known fossils doubtfully referred to this group. Radiolaria assisted in the formation of certain beds of chert, and foraminifcra have been observed. The remarkable group, the graptolites, evidently inhabited the seas in countless numbers and have left their remains in the dark shales of this period all over the world. At this time the diprionidian forms alone were represented by such genera as Tetragraptus, Phyllograptus, Didymograptus, Dicellograptus, Diplograptus and others. Of great interest are the earliest known indications of vertebrate life in the form of dermal plates and teeth of fishlike organisms from the Ordovician of Colorado. The terrestrial life of the period is very meagrely represented by the remains of land plants, mostly poorly preserved in certain sandstones, and Dy scorpions and several orders of insects, Protocimex (Sweden), Palaeoblattina (Colorado).
One of the most striking facts brought out by the study of the distribution of Ordovician fossils is the wide range of the northern or " periarctic " faunal assemblage. This pcriarctic fauna prevails over the whole world - so far as our present knowledge shows - with tlic exception of the peculiar Bohemian or Mediterranean region, which includes north-west and south-west France, Spain, Italy, the Alps, the Fichtelgebirge, east Thuringia, Harz and Rhenish Mountains.
AUTHORITIES. - Sir R. I. Murchison, Silurian System (1839) and Siluria (1854, 1867); A. Sedgwick, Synopsis of the Classification 0} the British Palaeozoic Rocks (1855); J. Barrande, Systime siluriendu tentre de la Boheme (1852-1887); J. J. Bigsby, Thesaur-us Siluricus (London, 1868); J. E. Marr, The Classification of the Cambrian and Silurian Rocks (Cambridge, 1883); Charles Lapworth, "On the Geological Distribution of the Rhabdophora," Annals and Mag. Nat. Hist. ser. 5, vols, iii., iv., v., vi. (1879-1880); B. N. Peach, J. Home, J. J. H. Teall, " The Silurian Rocks of Great Britain," vol. i., Scotland, Mem. Geol. Survey (1899); F. Freeh and others, " Lethaea geognostica," Theil i. Band 2 (Lethaea palaeozoica) (Stuttgart, 1897- 1902); Sir A. Geikie, Text-book of Geology (4th ed., 1903); and for recent papers. Geological Literature, Geo!. Soc. (London, annual). See also Cambrian and Silurian Systems. (J. A. H.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
