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ORDOVICIAN SYSTEM
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ORDOVICIAN See also:SYSTEM . Ordovician Rocks: Generalized Correlation Table. See also:England Graptolite Zones. See also:Scotland. Scandinavia. Bohemia. N.-W. See also:France. W. See also:Russia. See also:North See also:American See also:Continent. and See also:Wales. New See also:York. See also:Quebec. Caradoc Dicellogra plus I Hartlell Shales, Brachiopod beds, D5. Gres de May. Borkholm See also:Richmond beds Lowest or anceps. Ardmillan See also:Series, Trinucleus beds, D4. Calcaire de and and a See also:Anticosti .2 See also:Bala See also:group. D. complanatus. and and D3. Rosan. Lyckholm beds. See also:Hudson See also:river Shales. See also:limestone Pleurograplus Lowther Shales. Leptaena I Wesenberg See also:Lorraine beds. . q and v linearis. I limestone. beds. g '^ Hudson river 3 Dicranogra plus Trinucleus See also:Utica Shale. U q beds clingani. limestone. o e Trenton. limestone. a.5 Coenograptus .,yC Shales. c -ci Llandeilo Coenograptus Glenkiln Shales See also:Middle Da• Schistes See also:des Jewe, Itfer, and Trenton beds group. gracilis. and Graptolite beds Dry. See also:Gembloux Kuckers beds. and — ( I Didymograptus I See also:Barr Series. and and l Echinosphaerite See also:Galena limestone. Murchisoni. Chasmops ironstone. II Echinosphaerite 3 a limestone. i See also:Black river beds. a Cystidean Lowville limestone. limestone. d A See also:Levis Shales with U Tetragraptus, and Phyllograptus. (Lanvirn) Didymograptus Radiolarian ~ See also:Lower See also:DIP. Gres Vaginatus Chazy limestone q and bifidus. Cherts Graptolite beds Armoricain limestone (See also:part) Arenig group. Tetragraptus and and (part). and and bryonides. Ballantrae Orthoceras See also:Glauconite St See also:Peter's See also:sandstone. Series. I limestone. limestone. U Tremadoc beds, Ceratopyge beds, and beds with Euloma-See also:Niobe See also:fauna here regarded as See also:Cambrian: not invariably See also:present. of the See also:British Isles seem to have been deposited in a North See also:Atlantic See also:sea which embraced also the north of France and See also:Belgium. Confluent with this sea on the See also:east was a rather See also:peculiar See also:basin which included Bohemia, See also:southern France, See also:Spain, See also:Portugal, the eastern See also:Alps, Thuringia, See also:Fichtelgebirge and the See also:Keller Wald. Another See also:European basin, probably separated from the Bohemian or Mediterranean sea in See also:early Ordovician times, See also:lay over the Baltic region, Scandinavia, the Baltic provinces and north See also:Germany, and communicated eastwards by way of See also:Russian See also:Poland and central Russia with far eastern C. See also:Lapworth and his school, and J. E. Marr and the See also:Cambridge school, and in Scandinavia and the Baltic region, through W. C. See also:Brogger, S. A. Tulberg, F. See also: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 See also:order: (See also:Stage F) the Lyckholm and Borkholm zones, a highly fossiliferous series, See also:equivalent to the Middle Bala of See also:Britain; many of the limestones are largely formed of Rhabdoporella and other calcareous See also:algae. (E) Wesenberg See also:zone = Bala. (D) Jewe and Kegel zone. (C) Itfer beds, Kuckers Shale (bituminous limestones and marls = Brandschiefer), Echinosphaerite limestone =Upper Orthoceratite limestone of See also:Sweden. (B) Orthoceratite (Vaginaten) limestone=Orthoceratite limestone of Sweden, Glauconitic limestone, Glauconitic See also:sand (See also:Greensand). The last-mentioned reposes on Cambrian Dictyonema shales. While the Ordovician rocks in Scania, the Baltic provinces and north-central Russia are undisturbed and level-bedded, those. on the western See also:side of the Scandinavian See also:axis and in the Urals have suffered See also:movement and are metamorphosed into See also:schists, phyllites, See also:quartzite, See also:marble, &c.; and, especially in Scandinavia, have been extensively thrust. The Bohemian Ordovician, stage D " of See also:Barrande, consists mainly of greywackes and shales with some ironstone beds and eruptive rocks in the lower parts. In Germany the only large areas are found in the Thuringer Wald, Fichtelgebirge, See also:Frankenwald and See also: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 Ober-Thuringit beds above, and the Griffelschiefer and Unter-Thuringit beds below, which See also:rest upon the Leitmitzschiefer of the Euloma-Niobe (Cambrian) See also:horizon. Across See also:northern Russia Ordovician rocks See also:cover a See also:great See also:area; they consist of See also: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 See also:age. In north-See also:west France this system is represented in See also:Brittany and See also:Normandy by the slates of Riadan, the gees de May, the schistes a calymenes (with an ironstone See also:bed at the See also:base) and the gees armoricain. In the See also:Ardennes are the schistes de Qembloux, resting upon graptolitic shales of Arenig age. Sandstones and shales occur in See also:Languedoc, and various rocks in the See also:Pyrenees. In the Iberian See also:peninsula Ordovician rocks are widely spread, represented by sandstones, slates and shales covering the whole of the See also:period ; they are well See also:developed in Asturia and See also:Galicia. In the eastern Alps about See also:Graz are found calcareous shales with crinoids, the " Schockelkalk " and " Semriacher " shales; the Marthener beds of the Carnic Alps are of this age. In See also:China (Kiang-su, Kian-chang), in See also:Burma (See also:Mandalay) and in the Himalayas (Niti and See also: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 See also:lat. 300). As regards the See also:classification and correlation of the strata, which See also:change in See also: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 See also:district are enumerated; but in each See also:state there is a See also:local nomenclature for the beds. Thus in See also:Iowa, See also:Wisconsin and See also:Minnesota we find (I) Lower Magnesian See also:lime-See also: In the upper See also:division there are the lowest of the Anticosti limestones, the Hudson river beds, and Trenton limestone; to the middle division belong the Coenograptus shales; and the lower division consists of the Levis shales with Sillery beds at the base. In Nova See also:Scotia and New See also: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 See also:Boothia and See also: It is thus one of the most extensive as well as one of the most See also:ancient volcanic tracts of Europe " (See also:Sir A. See also:Geikie, See also:Text-See also:boos of See also: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 See also: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 them submarine—interstratified in the Arenig formation. In the See also:Lake district a great series of lavas and ashes—the Borrowdale series—was erupted during the middle of the period; the earlier effusions were andesitic, the later ones felsitic and rhyolitic. In See also:Ireland the Arenig lavas of See also: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; See also:iron ore in France; See also:lead and See also:zinc from the Galena and Trenton horizons in Wisconsin, Iowa and See also:Illinois; See also:manganese in Arkansas; oil and See also:gas from the Trenton stage in See also:Ohio and east See also:Indiana; roofing slates and See also:slate pencils in Wales and the Lake district; limestone in Great Britain and Tennessee; phosphate beds in Wales and Tennessee; marble ip the Appalachian district; See also:graphite (See also:plumbago) in the Lake district; and See also:jasper in Wales and southern Scotland. Ordovician See also:Life.—Compared with the preceding Cambrian period, the Ordovician is remarkable for the great expansion in See also: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 See also:simple forms of the Cambrian, began at this See also:time to develop See also:spire-bearing genera (Chonetes, Orthis, Orthisina, Strophomena, Crania, Schizotreta, Porambonites, Rafinesquina, Leptaena, Zygospira). The gasteropods now developed all the leading types of See also:shell (Pleurotomaria, Omphalotrochus) ; but both this class and the pelecypods (Lyrodesma, Ctenodonia, 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 (Orthoceras, Cyrtoceras, Gyroceras, Trocholites, Endoceras, Litoceras, Lituites, Actinoceras). Some of the straight shells were of enormous See also:size, 12 to 15 ft. See also:long and as much as 1 ft. in See also:diameter, in the widest part. See also:Trilobites were present in great abundance, and in this period they reached the See also:climax of their development. In the lower stage we find Agnostus, Calymene, Asaphus, Illaenus, Placoparia; on the Llandeilo horizon, Calymene, Asaphus, Megalaspis, Dalmanitis; and, at the See also:summit, Trinucleus and Homalonotus. In the transition zone between Ordovician and Cambrian, Ceratopyge, Euloma, Niobe, 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, Stomatopore, Monticulipora, Phylloporina, See also: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 opliiuroids, echinoids (Bothriocidaris) and asteroids (Taeniaster, Palaeaster) made their See also:appearance. See also:Corals (Streptelasma, Columnaria) were scarce, and See also:sponges (Aulocopium, Caryospongia, Archaeocyathus) were not particularly important; Receptaculites, Ischadites, are well-known fossils doubtfully referred to this group. See also:Radiolaria assisted in the formation of certain beds of chert, and See also:foraminifera have been observed. The remarkable group, the See also:graptolites, evidently inhabited the seas in countless numbers and have See also:left their remains in the' dark shales of this period all over the See also:world. At this time the diprionidian forms alone were represented by such genera as Tetragraptus, Phyllograptus, Didymograptus, Dicellograptus, Ditilogreptus and others. Of great See also:interest are the earliest known indications of vertebrate life in the See also:form of dermal plates and See also:teeth of See also:fish-like organisms from the Ordovician of See also:Colorado. The terrestrial life of the period is very meagrely represented by the remains of land See also:plants, mostly poorly preserved in certain sandstones, and by scorpions and several orders of See also:insects, Protocimex (Sweden), Palaeoblattina (Colorado). One of the most striking facts brought out by the study of the See also:distribution of Ordovician fossils is the wide range of the northern or periarctic " faunal assemblage. This periaretic fauna prevails over the whole world—so far as our present knowledge shows—with the exception of the peculiar Bohemian or Mediterranean region, which 238 includes north-west and south-west France, Spain, See also:Italy, the Alps, the Fichtelgebirge, east Thuringia, Harz and Rhenish Mountains. Additional information and CommentsThere are no comments yet for this article.
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