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Ordovician Period
488.3 - 443.7 million years ago
Mean atmospheric O2 content over period duration ca. 13.5 Vol %[1]
(68 % of modern level)
Mean atmospheric CO2 content over period duration ca. 4200 ppm[2]
(15 times pre-industrial level)
Mean surface temperature over period duration ca. 16 °C [3]
(2 °C above modern level)
Sea level (above present day) 180m; rising to 220m in Caradoc and falling sharply to 140m in end-Ordovician glaciations[4]

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago (ICS, 2004)[5]. It follows the Cambrian Period and is followed by the Silurian Period. The Ordovician, named after the Welsh tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian periods respectively. Lapworth, recognizing that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian periods, realized that they should be placed in a period of their own.

While recognition of the distinct Ordovician Period was slow in the United Kingdom, other areas of the world accepted it quickly. It received international sanction in 1906, when it was adopted as an official period of the Paleozoic Era by the International Geological Congress.



The Ordovician Period started at a major extinction event called the Cambrian-Ordovician extinction events some time about 488.3 ± 1.7 Ma (million years ago), and lasted for about 44.6 million years. It ended with the Ordovician–Silurian extinction event, about 443.7 ± 1.5 Ma (ICS, 2004) that wiped out 60% of marine genera.

The dates given are recent radiometric dates and vary slightly from those used in other sources. This second period of the Paleozoic era created abundant fossils and in some regions, major petroleum and gas reservoirs.

The boundary chosen for the beginning both of the Ordovician Period and the Tremadocian stage is highly useful. Since it correlates well with the occurrence of widespread graptolite, conodont, and trilobite species, the base of the Tremadocian allows scientists not only to relate these species to each other, but to species that occur with them in other areas as well. This makes it easier to place many more species in time relative to the beginning of the Ordovician Period.


Key events in the Ordovician
view • discuss • edit
-490 —
-485 —
-480 —
-475 —
-470 —
-465 —
-460 —
-455 —
-450 —
-445 —
Key events of the Ordovician Period.
Left: ICS approved stages.
Right: "General" stages.
Axis scale: millions of years ago.

A number of regional terms have been used to refer to subdivisions of the Ordovician Period. In 2008, the ICS erected a formal international system of subdivisions, illustrated to the right.[7]

The Ordovician Period in Britain was traditionally broken into Early (Tremadocian and Arenig), Middle (Llanvirn [subdivided into Abereiddian and Llandeilian] and Llandeilo) and Late (Caradoc and Ashgill) epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column. The faunal stages (subdivisions of epochs) from youngest to oldest are:

  • Hirnantian/Gamach (Late Ordovician: Ashgill)
  • Rawtheyan/Richmond (Late Ordovician: Ashgill)
  • Cautleyan/Richmond (Late Ordovician: Ashgill)
  • Pusgillian/Maysville/Richmond (Late Ordovician: Ashgill)
  • Trenton (Middle Ordovician: Caradoc)
  • Onnian/Maysville/Eden (Middle Ordovician: Caradoc)
  • Actonian/Eden (Middle Ordovician: Caradoc)
  • Marshbrookian/Sherman (Middle Ordovician: Caradoc)
  • Longvillian/Sherman (Middle Ordovician: Caradoc)
  • Soundleyan/Kirkfield (Middle Ordovician: Caradoc)
  • Harnagian/Rockland (Middle Ordovician: Caradoc)
  • Costonian/Black River (Middle Ordovician: Caradoc)
  • Chazy (Middle Ordovician: Llandeilo)
  • Llandeilo (Middle Ordovician: Llandeilo)
  • Whiterock (Middle Ordovician: Llanvirn)
  • Llanvirn (Middle Ordovician: Llanvirn)
  • Cassinian (Early Ordovician: Arenig)
  • Arenig/Jefferson/Castleman (Early Ordovician: Arenig)
  • Tremadoc/Deming/Gaconadian (Early Ordovician: Tremadoc)

Paleogeography and atmosphere

Sea levels were high during the Ordovician; in fact during the Tremadocian, marine transgressions worldwide were the greatest for which evidence is preserved in the rocks.

During the Ordovician, the southern continents were collected into a single continent called Gondwana. Gondwana started the period in equatorial latitudes and, as the period progressed, drifted toward the South Pole. Early in the Ordovician, the continents Laurentia (present-day North America), Siberia, and Baltica (present-day northern Europe) were still independent continents (since the break-up of the supercontinent Pannotia earlier), but Baltica began to move towards Laurentia later in the period, causing the Iapetus Ocean to shrink between them. The small continent Avalonia separated from Gondwana and began to head north towards Baltica and Laurentia. The Rheic Ocean between Gondwana and Avalonia was formed as a result.

A major mountain-building episode was the Taconic orogeny that was well under way in Cambrian times. In the beginning of the Late Ordovician, from 460 to 450 Ma, volcanoes along the margin of the Iapetus Ocean spewed massive amounts of carbon dioxide into the atmosphere, turning the planet into a hothouse. These volcanic island arcs eventually collided with proto North America to form the Appalachian mountains. By the end of the Late Ordovician these volcanic emissions had stopped. Gondwana had by that time neared or approached the pole and was largely glaciated.

The Ordovician was a time of calcite sea geochemistry in which low-magnesium calcite was the primary inorganic marine precipitate of calcium carbonate. Carbonate hardgrounds were thus very common, along with calcitic ooids, calcitic cements, and invertebrate faunas with dominantly calcitic skeletons.[8][9]


The Early Ordovician climate was thought to be quite warm, at least in the tropics. As with North America and Europe, Gondwana was largely covered with shallow seas during the Ordovician. Shallow clear waters over continental shelves encouraged the growth of organisms that deposit calcium carbonates in their shells and hard parts. The Panthalassic Ocean covered much of the northern hemisphere, and other minor oceans included Proto-Tethys, Paleo-Tethys, Khanty Ocean which was closed off by the Late Ordovician, Iapetus Ocean, and the new Rheic Ocean.

As the Ordovician progressed, we see evidence of glaciers on the land we now know as Africa and South America. At the time these land masses were sitting at the South Pole, and covered by ice caps.


Large Nautiloids like Orthoceras were among the largest predatory animals in the Ordovician.
A diorama depicting Ordovician flora and fauna.

For most of the Late Ordovician, life continued to flourish, but at and near the end of the period there were mass-extinction events that seriously affected planktonic forms like conodonts, graptolites, and some groups of trilobites (Agnostida and Ptychopariida, which completely died out, and the Asaphida which were much reduced). Brachiopods, bryozoans and echinoderms were also heavily affected, and the endocerid cephalopods died out completely, except for possible rare Silurian forms. The Ordovician-Silurian Extinction Events may have been caused by an ice age that occurred at the end of the Ordovician period as the end of the Late Ordovician was one of the coldest times in the last 600 million years of earth history.



Though less famous than the Cambrian explosion, the Ordovician featured an adaptive radiation, the Ordovician radiation, that was no less remarkable; marine faunal genera increased fourfold, resulting in 12% of all known Phanerozoic marine fauna.[10] The trilobite, inarticulate brachiopod, archaeocyathid, and eocrinoid faunas of the Cambrian were succeeded by those which would dominate for the rest of the Paleozoic, such as articulate brachiopods, cephalopods, and crinoids; articulate brachiopods, in particular, largely replaced trilobites in shelf communities.[11] Their success epitomizes the greatly increased diversity of carbonate shell-secreting organisms in the Ordovician compared to the Cambrian.[11]

In North America and Europe, the Ordovician was a time of shallow continental seas rich in life. Trilobites and brachiopods in particular were rich and diverse. The first Bryozoa appeared in the early Ordovician as did the first coral reefs, although solitary corals date back to at least the Cambrian.

Molluscs, which had appeared during the Cambrian or even the Ediacaran, became common and varied, especially bivalves, gastropods, and nautiloid cephalopods.

Now-extinct marine animals called graptolites thrived in the oceans. Some new cystoids and crinoids appeared.

It was long thought that the first true vertebrates (fish — Ostracoderms) appeared in the Ordovician, but recent discoveries in China reveal that they probably originated in the Early Cambrian. The very first gnathostome (jawed fish) appeared in the Late Ordovician epoch.

During the Middle Ordovician there was a large increase in the intensity and diversity of bioeroding organisms. This is known as the Ordovician Bioerosion Revolution.[12] It is marked by a sudden abundance of hard substrate trace fossils such as Trypanites, Palaeosabella and Petroxestes.

In the Early Ordovician, trilobites were joined by many new types of organisms, including tabulate corals, strophomenid, rhynchonellid, and many new orthid brachiopods, bryozoans, planktonic graptolites and conodonts, and many types of molluscs and echinoderms, including the ophiuroids ("brittle stars") and the first sea stars. Nevertheless the trilobites remained abundant, with all the Late Cambrian orders continuing, and being joined by the new group Phacopida. The first evidence of land plants also appeared; see Evolutionary history of life.

In the Middle Ordovician, the trilobite-dominated Early Ordovician communities were replaced by generally more mixed ecosystems, in which brachiopods, bryozoans, molluscs and echinoderms all flourished, tabulate corals diversified and the first rugose corals appeared; trilobites were no longer predominant. The planktonic graptolites remained diverse, with the Diplograptina making their appearance. Bioerosion became an important process, particularly in the thick calcitic skeletons of corals, bryozoans and brachiopods, and on the extensive carbonate hardgrounds which appear in abundance at this time. One of the earliest known armoured agnathan ("ostracoderm") vertebrate, Arandaspis, dates from the Middle Ordovician.

Trilobites in the Ordovician were very different than their predecessors in the Cambrian. Many trilobites developed bizarre spines and nodules to defend against predators such as primitive sharks and nautiloids while other trilobites such as Aeglina prisca evolved to become swimming forms. Some trilobites even developed shovel-like snouts for ploughing through muddy sea bottoms. Another unusual clade of trilobites known as the trinucleids developed a broad pitted margin around their head shields.[13] Some trilobites such as Asaphus kowalewski evolved long eyestalks to assist in detecting predators whereas other trilobite eyes in contrast disappeared completely.[14]


Marine fungi were abundant in the Ordovician seas to decompose animal carcasses, and other wastes.

Green algae were common in the Late Cambrian (perhaps earlier) and in the Ordovician. Terrestrial plants probably evolved from green algae, first appearing in the form of tiny non-vascular mosses resembling liverworts. Fossil spores from land plants have been identified in uppermost Ordovician sediments.

Among the first land fungi may have been arbuscular mycorrhiza fungi (Glomerales), playing a crucial role in facilitating the colonization of land by plants through mycorrhizal symbiosis, which makes mineral nutrients available to plant cells; such fossilized fungal hyphae and spores from the Ordovician of Wisconsin have been found with an age of about 460 million years ago, a time when the land flora most likely only consisted of plants similar to non-vascular bryophytes.[16]

End of the period

The Ordovician came to a close in a series of extinction events that, taken together, comprise the second largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct. The only larger one was the Permian-Triassic extinction event.

The extinctions occurred approximately 447–444 million years ago and mark the boundary between the Ordovician and the following Silurian Period. At that time all complex multicellular organisms lived in the sea, and about 49% of genera of fauna disappeared forever; brachiopods and bryozoans were greatly reduced, along with many trilobite, conodont and graptolite families.

The most commonly accepted theory is that these events were triggered by the onset of most cold conditions in the late Katian, followed by an ice age, in the Hirnantian faunal stage, that ended the long, stable greenhouse conditions typical of the Ordovician.

The ice age was possibly not long-lasting, study of oxygen isotopes in fossil brachiopods showing that its duration could have been only 0.5 to 1.5 million years.[17] Other researchers (Page et al.) estimate more temperate conditions did not return until the late Silurian.

The late Ordovician glaciation event was preceded by a fall in atmospheric carbon dioxide (from 7000 ppm to 4400 ppm) which selectively affected the shallow seas where most organisms lived. As the southern supercontinent Gondwana drifted over the South Pole, ice caps formed on it, which have been detected in Upper Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time.

Glaciation locks up water from the world-ocean, and the interglacials free it, causing sea levels repeatedly to drop and rise; the vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches, then returned carrying diminished founder populations lacking many whole families of organisms, then withdrew again with the next pulse of glaciation, eliminating biological diversity at each change.[18] Species limited to a single epicontinental sea on a given landmass were severely affected.[17] Tropical lifeforms were hit particularly hard in the first wave of extinction, while cool-water species were hit worst in the second pulse.[17]

Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.

At the end of the second event, melting glaciers caused the sea level to rise and stabilise once more. The rebound of life's diversity with the permanent re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving Orders.

Melott et al. (2006) suggested a ten-second gamma ray burst could have destroyed the ozone layer and exposed terrestrial and marine surface-dwelling life to deadly radiation,[19] but most scientists agree that extinction events are complex with multiple causes.


  1. ^ Image:Sauerstoffgehalt-1000mj.svg
  2. ^ Image:Phanerozoic Carbon Dioxide.png
  3. ^ Image:All palaeotemps.png
  4. ^ Haq, B. U. (2008). "A Chronology of Paleozoic Sea-Level Changes". Science 322: 64–68. doi:10.1126/science.1161648. 
  5. ^ Gradstein, Felix M.; Ogg, J. G.; Smith, A. G. (2004). A Geologic Time Scale 2004. Cambridge: Cambridge University Press. ISBN 0521786738. 
  6. ^ Wellman, C.H., Gray, J. (2000). "The microfossil record of early land plants". Phil. Trans. R. Soc. B 355 (1398): 717–732. doi:10.1098/rstb.2000.0612. 
  7. ^ Details on the Dapingian are available at Wang, X.; Stouge, S.; Chen, X.; Li, Z.; Wang, C. (2009). "Dapingian Stage: standard name for the lowermost global stage of the Middle Ordovician Series". Lethaia 42: 377–380. doi:10.1111/j.1502-3931.2009.00169.x.  edit
  8. ^ Stanley, S. M.; Hardie, L. A. (1998). "Secular oscillations in the carbonate mineralogy of reef-building and sediment-producing organisms driven by tectonically forced shifts in seawater chemistry". Palaeogeography, Palaeoclimatology, Palaeoecology 144: 3–19. doi:10.1016/S0031-0182(98)00109-6. 
  9. ^ Stanley, S. M.; Hardie, L. A. (1999). "Hypercalcification; paleontology links plate tectonics and geochemistry to sedimentology". GSA Today 9: 1–7. 
  10. ^ Dixon, Dougal; et al. (2001). Atlas of Life on Earth. New York: Barnes & Noble Books. pp. 87. ISBN 0760719578. 
  11. ^ a b Cooper, John D.; Miller, Richard H.; Patterson, Jacqueline (1986). A Trip Through Time: Principles of Historical Geology. Columbus: Merrill Publishing Company. pp. 247, 255–259. ISBN 0675201403. 
  12. ^ a b Wilson, M. A.; Palmer, T. J. (2006). "Patterns and processes in the Ordovician Bioerosion Revolution" (PDF). Ichnos 13: 109–112. doi:10.1080/10420940600850505. 
  13. ^ "Palaeos Paleozoic : Ordovician : The Ordovician Period". April 11, 2002. 
  14. ^ A Guide to the Orders of Trilobites
  15. ^ Wilson, M. A.; Palmer, T. J. (2001). "Domiciles, not predatory borings: a simpler explanation of the holes in Ordovician shells analyzed by Kaplan and Baumiller, 2000". Palaios 16: 524–525. doi:10.1669/0883-1351(2001)016<0524:DNPBAS>2.0.CO;2. 
  16. ^ Redecker, D.; Kodner, R. ; Graham, L. E. (2000). "Glomalean fungi from the Ordovician". Science 289 (5486): 1920–1921. doi:10.1126/science.289.5486.1920. PMID 10988069. 
  17. ^ a b c Stanley, Steven M. (1999). Earth System History. New York: W.H. Freeman and Company. pp. 358, 360. ISBN 0716728826. 
  18. ^ Emiliani (1992), 491
  19. ^ Melott, Adrian; et al. (2004). "Did a gamma-ray burst initiate the late Ordovician mass extinction?". International Journal of Astrobiology 3: 55–61. doi:10.1017/S1473550404001910. 

External links

Preceded by Proterozoic Eon 542 Ma - Phanerozoic Eon - Present
542 Ma - Paleozoic Era - 251 Ma 251 Ma - Mesozoic Era - 65 Ma 65 Ma - Cenozoic Era - Present
Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Paleogene Neogene Quaternary

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

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 (qq.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, quartzites, chlorite-, actinoliteand 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.


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; concerning the last-named marine area not much is known. In the opposite direction, the Baltic basin may have communicated, 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 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 ? Arctic Continent.


North American Continent.


Graptolite Zones.




N. - W. France.

W. Russia.


New York.





D. complanatus.


Hartfell Shales,

Ardmillan Series,

Brachiopod beds,

Trinucleus beds,





Gres de May.



Lyckholm beds.

Richmond beds

and a

Hudson river Shales. '?



Anticosti .4


and .b

Bala group.




Calcaire de


Lorraine beds. .

Hudson river '°







beds. °„'


Lowther Shales.


Utica Shale. C a





Glenkiln Shales


Graptolite beds

Schistes des


Jewe, Itfer, and

Kuckers beds.

Trenton beds ,,

and d


limestone, a





Galena limestone. d "


p u


Chasmo ps




Coenograptus -°


Barr Series.



Black river beds.

Shales. o






Lowville limestone. ,`A a








Chazy limestone c


Levis Shales a




Graptolite beds



(part) 51

with H







Tetragraptus, a?

Arenig group.





St Peter's sandstone. g

and a







Indo -Ait an Conti ent Hypothetical Lantl & Sea -areas in the Early Ordovician Period Ordovician System. Ordovician Rocks: Generalized Correlation Table. 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. Confluent 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. Brdgger, 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 Lyckholm 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, Glauconitic limestone, Glauconitic sand (Greensand). The lastmentioned 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 axis and in the Urals have suffered movement and are metamorphosed into schists, phyllites, quartzite, marble, &c.; and, especially in Scandinavia, have been extensively thrust. The Bohemian Ordovician," stage D "of 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, 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 Ober-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 gres de May, the schistes a calymenes (with an ironstone bed at the base) and the gres armoricain. In the Ardennes are the schistes de Gembloux, 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, 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 (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 Coenograptus 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 granetum in east Greenland; .and in the 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 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, &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 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 thins one of the most extensive as well as one of the most ancient volcanic tracts of Europe "(Sir A. Geikie, Text-book 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 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 (plumbago) 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, Porambonites, 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 (Orthoceras, 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, Dalmanitis; and, at the 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, Stomatopora, 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, Archaeocyathus) were not particularly important; Receptaculites, Ischadites, are well-known fossils doubtfully referred to this group. Radiolaria assisted in the formation of certain beds of chert, and foraminifera 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 by 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 periarctic fauna prevails over the whole world - so far as our present knowledge shows - with the 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 of the British Palaeozoic Rocks (1855); J. Barrande, Systeme silurien du centre de la Boheme (1852-1887); J. J. Bigsby, Thesaurus 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. Horne, J. J. H. Teall, ' ` The Silurian Rocks of Great Britain," vol. i., Scotland, Mem. Geol. Survey (1899); F. Frech and others, "Lethaea geognostica," Theil i. Band 2 (Lethaea palaeozoica) (Stuttgart, 18 971902); Sir A. Geikie, Text-book of Geology (4th ed., 1903); and for recent papers, Geological Literature, Geol. Soc. (London, annual). See also CAMBRIAN and SILURIAN SYSTEMS. (J. A. H.)

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