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RHAETIC (Fr. Rhetien or Rhcetien; Ger...

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Originally appearing in Volume V23, Page 231 of the 1911 Encyclopedia Britannica.
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RHAETIC (Fr. Rhetien or Rhcetien; Ger. Rhdt or Rhktisch; It. Retico)  , in geology, the assemblage of rocks classed by most
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English and German authorities in the Triassic
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system, and by most French geologists placed at the
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base of the
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Lias, in the
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Jurassic system . It has been called the Infra-Lias . This diversity of opinion is due to the fact that the Rhaetic formation presents the characters of a
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group of passage-beds, uniting certain features of the Trias with others of the Jurassic system; none the less, it has sufficient individuality to be recognized with tolerable certainty over a wide arearin
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Europe and beyond . The name Rhaetic was first applied by C . W . Gumbel to the strata of this horizon in the Rhaetic
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Alps, where they are thickly
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developed and in parts fossiliferous . The labours of E . V . Mojsisovic and E . Suess have demonstrated that in the Alpine Rhaetic several distinct facies may be recognized, viz. a Swabian facies:
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shore and lagoon deposits with a pelecypod
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fauna, poor in
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species but rich in individuals; a Carpathian facies with corals,
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algae, Terebratula gregaria and Plicatula intusstriata, exemplified in the upper
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part of the Dachstein
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limestone; a Kossener facies: black limestones and marls, with a brachiopod fauna in which Spirigera oxycolpos is very noticeable; and a
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Salzburg facies, characterized by pelagic pelecypods and some
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ammonites (see table in Massie SYSTEM) . The whole of the Rhaetic falls within Mojsisovic's zone of Avicula contorta . This epoch is marked off from the earlier Triassic period by a very general marine transgression which proceeded with minor irregularities and retrogressions over the whole
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area, until at its close it was followed by the more decided transgression which indicates the commencement of the Lias .

Among the marine fossils of the Rhaetic, Avicula contorta, the

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principal zone form, is very characteristic and has a wide range; Myophoria inflata, Modiola minuta, Protocardium rhaeticum and Terebratula gregaria are
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common species . True belemonites make their first appearance . Corals, Thecosmilia, &c., are common in some districts . Plant remains are abundant in certain areas, and in places give rise to beds of lignite and
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coal . The
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flora is more nearly akin to that of the Trias than to that of the Jurassic rocks . Vertebrate remains are fairly abundant in the form of teeth, isolated bones, scales and coprolites in what are known as " Bone Beds " (q.v.) . These beds are a very characteristic feature; they occur on several horizons in many tracts of the
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European Rhaetic, and recur in beds of this age in
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America . In England there is usually a bone bed about the base of the formation; in Germany one occupies a similar position; a second occurs less constantly about the
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middle, and in the
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Wurttemberg
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district a third bed separates the Rhaetic and Lias, and constitutes the well-known manure bed of Bebenhausen . In these beds are found the bones of
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Ichthyosaurus and Pliosaurus, anticipating their
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great development in the Lias, while the remains of Belodon and blystriosuchus serve to
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link this epoch with Triassic stegocephalian reptiles . Several coleopterous
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insects have been found in the same beds, but the most interesting feature of the bone-bed fauna is the first appearance in the
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northern hemisphere of true mammals: Microlestes in England and Wurttemberg, Triglyphus in Wurttemberg, Dromatherium and Microconodon in America . In England the Rhaetic formation occurs as a thin but constant series of beds at the base of the Lias and above the
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Keuper marls . The upper part, often called the " White Lias," is a series of thin-bedded shales, limestone and marls, t to 25 ft. thick; the
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lower portion consists mainly of dark shales, sometimes with very perfect lamination— "paper shales." Below there are beds of grey and " tea-green " marls which are now usually regarded as the topmost Keuper beds, but they have often been included in the Rhaetic formation (see KEUPER) .

The best exposures in

Britain are those between
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Penarth Head and Cavernock Point, Aust Cliff and Garden Cliff near Westbury-on-Severn, and Wainlode Cliff between
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Tewkesbury and Gloucester . From their excellent development near Penarth the Rhaetic beds have long been known in England as the Penarth Beds (H . W . Bristow, 1864) . The more prominent beds in the White Lias of the west of England and Glamorganshire are the Estheria beds and the
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insect limestone or Pseudomonotis-bed, and on both of these horizons the limestone may assume the
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peculiar characters of landscape marble, sometimes called Cotham marble, from Cotham House near Bristol . A hard
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fine-grained limestone, known locally as the Sun-bed, occurs at the top of the series near Bath and Radstock; at Street, Wedmore and south of the Mendips generally it is called Jew stone . Wedmore stone is a tough, shelly and sandy limestone in the black shales at Wedmore, near Wells; it is employed in the neighbourhood as a
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building stone . North of
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Somersetshire the White Lias is poorly represented; in Glamorgan-
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shire it appears between
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Cardiff and Pyle, west of
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Bridgend and at Sutton and Southerndown . Rhaetic beds have been traced at Market Drayton, Salop; near Audlem,
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Cheshire;
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Rugby and Stratford-on-
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Avon in
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Warwickshire; Wigston in Leicestershire; Needham
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Forest in
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Staffordshire, and in Nottinghamshire and
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Yorkshire as far as the coast . They have not yet been proved beneath the Lias of Cumberland . Rhaetic fossils have been found in great numbers in fissures in the Carboniferous limestone of the Mendips . On the western side of Scotland Rhaetic rocks occur at Applecross, Ardnamurchan, Morven,
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Mull, Raasay and
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Skye .

In

Sutherlandshire
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sandstone and conglomerate and large transported masses occur; one of them, at Linksfield, carries a bone bed . Here the black shales of the English type fail; sandstones with coaly layers and yellowish-grey crystalline and oolitic limestones take their place . In Antrim a small outcrop of black shales with Avicula contorta occurs near
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Port Rush . On the European continent the Rhaetic rocks are most thickly developed in the Alpine regions; and, as in the case of the older Triassic formations, calcareous and dolomitic strata predominate here and in the Mediterranean province . In the Alpine district the main divisions are the Rhaetic Dachstein limestone and the Kossener beds; shales, marls and limestones . In the northern tract the following subdivisions have been recognized in descending order: beds with Choristeceros Marschi; Starhem passage beds; Rhynchonella fissicostata beds; Lithodendron limestone; beds with Terebratula gregaria; beds with Avicula contorta; " Platten
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Kalk with Rhynchonella alpina . In the
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southern tract the subdivisions are: Conchodus
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dolomite (Conchodus infraliassicus=Lycodius
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cor.); Lithodendron limestone, Azzarola beds, Contorta marls, Plattenkalk." Much limestone is of the "
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reef " type . In Germany the rocks are mainly fine, clean yellow sands, suggesting littoral or dune conditions, with bituminous clays and marls . The formation is often missing in south-west Germany . Similar beds occur in
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Lorraine and Luxembourg (gres de Vic, gres de Kedange, gres de Mortinsart) . In Cotentin are dolomitic sandstones and marl; round the central plateau of France the rocks are coarse sands, arkoses, and conglomerates; while in the south of France the sandy and calcareous facies occur intermixed . In Spain limestones and dolomites occur up to too metres in thickness; in
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Portugal sandy beds recur .

The Rhaetic of Scania, south

Sweden, consists mainly of sandstone and shales with beds of coal up to one metre thick . Only the upper beds contain marine fossils; the bulk of the formation is of lacustrine or estuarine origin, with plant remains and insects . In Italy the formation is well developed in the north and at Rotzo, Spezzia and Carrara; and yields the famous statuary marble and the black variety known as portor . Rhaetic beds have been re-cognized in Sardinia, Corsica, Sicily, in the
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Balkan Peninsula and
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Greece; in
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Asia Minor,
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Afghanistan, Turkistan,
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Persia,
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Siberia and India (limestones and dolomites of Niti and the Mahaveda beds, sandstones and conglomerates, nearly 10,000 feet thick in
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Satpura) ; in
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China,
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Japan and
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Tongking (with coal beds) . In
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Australasia the Wianamatta beds of New South Wales, the Bellarine beds of Victoria, the
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Ipswich and Tivoli beds of
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Queensland, and the Jerusalem beds of
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Tasmania, and beds on a similar horizon in New Zealand, have been regarded as equivalents of the Rhaetic . In Africa the Stormberg beds of the Karoo series and. thg Molten beds of the Cape have been assigned to this epoch . In America Rhaetic rocks are recognized in N . Carolina,
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Connecticut, California, Mexico,
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Bolivia and Chile; the formation is also recorded from Spitzbergen, Franz Joseph
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Land and elsewhere in the Arctic regions . For the English Rhaetic see L . Richardson, " The Rhaetic Rocks of North-west Gloucestershire," Proc . Cotteswold Club, xiv. p . 127 (Glos .

1901-1903) . (J . A .

End of Article: RHAETIC (Fr. Rhetien or Rhcetien; Ger. Rhdt or Rhktisch; It. Retico)
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