See also:American Continent . and
See also:Wales .
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 Series, Trinucleus beds, D4 . Calcaire de and and a
See also:Anticosti .2
See also:group . D. complanatus. and and D3 . Rosan . Lyckholm beds . 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 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 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:sandstone . Series . I limestone . limestone . U Tremadoc beds, Ceratopyge beds, and beds with Euloma-
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:sea which embraced also the north of France and Belgium .
Confluent with this sea on theeast was a rather
See also:basin which included Bohemia,
See also:southern France, Spain,
See also:Portugal, the eastern
See also:Alps, Thuringia,
See also:Fichtelgebirge and the Keller Wald . Another
See also:European basin, probably separated from the Bohemian or Mediterranean sea in early Ordovician times,
See also:lay over the Baltic region, Scandinavia, the Baltic provinces and north Germany, and communicated eastwards by way of
See also:Russian Poland and central Russia with far eastern C .
See also: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 .
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: (Stage F) the Lyckholm and Borkholm zones, a highly fossiliferous series,
See also:equivalent to the Middle Bala of Britain; many of the limestones are largely formed of Rhabdoporella and other calcareous
See also:algae .
(E) Wesenbergzone = 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, Glauconitic limestone, Glauconitic 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 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, 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 cover a
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 age . In north-west France this
See also: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 Pyrenees . In the Iberian 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 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 state there is a
See also:local nomenclature for the beds . Thus in
See also:Iowa, Wisconsin and
See also:Minnesota we find (I) Lower Magnesian lime-
See also:stone, St Peter's sandstone; (2) Trenton limestone, Galena lime-stone; (3) Hudson river shales; in
See also:Arkansas, the California or Magnesian limestone, Saccaroidal limestone, Izard limestone and Polk Bayou limestone; in Oklahoma, the Arbuckle limestone,
See also:Simpson series, Viola limestone and Sylvan shales; and in east
See also:Tennessee, the Chickamauga limestone, Athens shale, Tellia sand-stone, Sievier 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 (two ft. or more) . In
See also:Canada the Ordovician rocks (Quebec group) are thickly developed .
In the upperdivision 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 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:Land; by limestones with Caryocystis granetum in east
See also:Greenland ; and in the
See also:Barrow Straits by beds with Asaphus and Maclurea . In North Africa Ordovician rocks are probably present, and in New Zealand the Arorere series (Wanaka group), and in
See also:Australia (
See also: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 (
See also: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
See also: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
See also:Ayrshire and the south-western districts (of the southern uplands), where the volcanic constituents attain a great development, they consist of basic lavas (
See also:diabase, &c.), 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
See also:Silurian region over an area of at least 2000 sq. m., inasmuch as it comes to the
See also: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
See also:ancient volcanic tracts of Europe " (
See also:Sir A . Geikie, Text-
See also:boos 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 them submarine—interstratified in the Arenig formation . In the 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 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
See also:zinc from the Galena and Trenton horizons in Wisconsin, Iowa and
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 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. long and as much as 1 ft. in 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 appearance . 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 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 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, Italy, the Alps, the Fichtelgebirge, east Thuringia, Harz and Rhenish Mountains .
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