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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 See also:geology, the assemblage of rocks classed by most See also:English and See also:German authorities in the Triassic See also:system, and by most See also:French geologists placed at the See also:base of the See also:Lias, in the See also:Jurassic system . It has been called the Infra-Lias . This diversity of See also:opinion is due to the fact that the See also:Rhaetic formation presents the characters of a See also: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 See also:Europe and beyond . The name Rhaetic was first applied by C . W . See also:Gumbel to the strata of this See also:horizon in the Rhaetic See also:Alps, where they are thickly See also:developed and in parts fossiliferous . The labours of E . V . Mojsisovic and E . See also:Suess have demonstrated that in the Alpine Rhaetic several distinct facies may be recognized, viz. a Swabian facies: See also:shore and See also:lagoon deposits with a pelecypod See also:fauna, poor in See also:species but See also:rich in individuals; a Carpathian facies with See also:corals, See also:algae, Terebratula gregaria and Plicatula intusstriata, exemplified in the upper See also:part of the Dachstein See also:limestone; a Kossener facies: See also:black limestones and marls, with a brachiopod fauna in which Spirigera oxycolpos is very noticeable; and a See also:Salzburg facies, characterized by pelagic pelecypods and some See also:ammonites (see table in Massie SYSTEM) . The whole of the Rhaetic falls within Mojsisovic's See also:zone of Avicula contorta . This See also:epoch is marked off from the earlier Triassic See also:period by a very See also:general marine transgression which proceeded with See also:minor irregularities and retrogressions over the whole See also:area, until at its See also: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 See also:

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

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

Phoenix-squares

In See also:

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

The Rhaetic of Scania, south See also:

Sweden, consists mainly of sandstone and shales with beds of coal up to one See also: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 See also:Italy the formation is well developed in the north and at Rotzo, Spezzia and See also:Carrara; and yields the famous statuary marble and the black variety known as portor . Rhaetic beds have been re-cognized in See also:Sardinia, See also:Corsica, See also:Sicily, in the See also:Balkan See also:Peninsula and See also:Greece; in See also:Asia Minor, See also:Afghanistan, Turkistan, See also:Persia, See also:Siberia and See also:India (limestones and dolomites of Niti and the Mahaveda beds, sandstones and conglomerates, nearly 10,000 feet thick in See also:Satpura) ; in See also:China, See also:Japan and See also:Tongking (with coal beds) . In See also:Australasia the Wianamatta beds of New South See also:Wales, the Bellarine beds of See also:Victoria, the See also:Ipswich and See also:Tivoli beds of See also:Queensland, and the See also:Jerusalem beds of See also:Tasmania, and beds on a similar horizon in New See also:Zealand, have been regarded as equivalents of the Rhaetic . In See also: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, See also:Connecticut, See also:California, See also:Mexico, See also:Bolivia and See also:Chile; the formation is also recorded from Spitzbergen, See also:Franz See also:Joseph See also:Land and elsewhere in the See also:Arctic regions . For the English Rhaetic see L . See also:Richardson, " The Rhaetic Rocks of North-west See also:Gloucestershire," Proc . Cotteswold See also: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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