Online Encyclopedia

ONYX

Online Encyclopedia
Originally appearing in Volume V20, Page 120 of the 1911 Encyclopedia Britannica.
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ONYX, a banded chalcedony or striped agate, composed of white layers alternating with others of black, brown or red colour. A typical onyx consists of two or more black and white strata, whilst the term sardonyx is applied to the stone if it contains red or brown bands (see SARDONYX). Probably those varieties which show red and white zones originally suggested the name " onyx," from Gr. 6vu (a finger nail), since the colours of such stones may be not unlike those of the nail. The onyx when worked by the lapidary was often designated by the diminutive 6vI ov; and at the present day the term nicolo, a corruption of the Italian diminutive onycolo, is applied to an onyx which presents a thin layer of chalcedony deriving a bluish tint from the subjacent black ground. The Hebrew soham is translated in the authorized version of the Old Testament " onyx," but the revised version gives in some of the passages an alternative marginal reading of " beryl." The position of the land of Havilah, which yielded the onyx-stone, is uncertain. India has for ages supplied the finest onyxes, and hence jewellers apply the expression " Oriental onyx " to any stone remarkable for beauty of colour and regularity of stratification, quite regardless of its locality. As far back as the 1st century the author of the Periplus Maris Erythraei mentions the onyx among the products of Plythanae, a locality probably identified with Paithan on the Godavari; and he further states that the stones were taken down to Barygaza, the modern Broach, where the agate trade still flourishes. It is probable that the early Greeks and Romans derived their prized agate-cups from this locality. The Indian onyx is found, with agate and jasper-pebbles, in river gravels derived from the disintegration of the amygdaloidal volcanic rocks of the Deccan. A great deal of onyx now sold is obtained from South American agates, cut in Germany. It often happens that the lower deposits in an agate-nodule are in horizontal layers, forming onyx, while the other deposits have adapted themselves to the curved contours of the cavity. The onyxes cut from agate-nodules are usually stained artificially, as explained under AGATE. The onyx is largely used for beads, brooches, pins, ring-stones and other small ornamental objects, while the larger pieces are occasionally wrought in the form of cups, howls, vases, &c. Onyx is the favourite stone for cameo work, advantage being taken of the differently-coloured layers to produce a subject in relief on a background of another colour. For fine examples of ancient cameo-work in onyx and sardonyx see GEM. It should be noted that the term onyx, or onychite, was formerly, and is still sometimes, applied to certain kinds of banded marble, like the " oriental alabaster " (see ALABASTER). Such substances are quite distinct from the hard siliceous onyx, being much softer and less precious: they are, in fact, usually deposits of calcium carbonate like stalagmite and travertine. The ornamental stones known as Mexican onyx, or Tecali marble, and Algerian onyx are of this character; and in order to avoid any confusion with the true onyx it is well to distinguish all the calcareous " onyxes " as onyx-marble. The well-known " Gibraltar stone " is an onyx-marble, with brown bands, from caverns in the limestone of Gibraltar. The Tecali onyx, some-times with delicate green shades, takes its name from the district of Tecali; one of its localities being La Pedrara, about 21 M. from the city of Puebla. For onyx-marbles see Dr G. P. Merrill, Rep. U.S. Nat. Mus. for 1893 (1895), P. 539. (F. W. R.*) 00LITE (Gr. g3ov, egg, XLBos, stone), in geology, a term having two distinct meanings. In petrology (q.v.) it denotes a type of rock structure characterized by the presence of minute spherical grains resembling the roe of a fish; if the grains become larger, the structure is said to be pisolitic (Gr. irtaos, pea). In stratigraphical geology, the oolite is a division of the Jurassic system (q.v.). The term appears to have been first applied in this latter sense by A. J. M. Brochant de Villiers in 18o3, and through the labours of W. Smith, W. D. Conybeare, W. Buckland and others, it was gradually introduced for the calcareous rocks of the British Jurassic until it came to comprehend the whole system above the Lias. Custom still sanctions its use in England, but it has been objected that the Oolitic (Jurassic) system contains many strata that are not oolitic; and since oolitic structure occurs in limestones of all ages, it is misleading to employ the word in this way. The oolites are usually divided into: the Upper or Portland Oolite, comprising the Purbeck, Portland and Kimeridge stages; the Middle or Oxford Oolite, including the Corallian, Oxfordian and Kellaways beds; and the Lower Oolites, with the Cornbrash, Great or Bath Oolite (Bathonian), Fullonian and the Inferior Oolite (Bajocian). The Great Oolite and Inferior Oolite are treated here. The Inferior Oolite, called by William Smith the " Under Oolite " from its occurrence beneath the Great or " Upper Oolite " in the neighbourhood of Bath, received its present name from J. Townsend in 1813. It is an extremely variable assemblage of strata. In the Cotteswold Hills it is a series of marine deposits, 264 ft. thick near Cheltenham, but within 25 M. the strata thin out to 30 ft. at Fawler in Oxfordshire. A typical section N.E. of Dursley contains the following subdivisions: o a~ S. White Freestone .5 ft. oo °'}l Clypeus Grit . . . 6-15 ft. Cosmoceras Parkin- 0- It, b.O v p° to. ( Upper Trigonia Grit 2-12 ft. a o Gryphite Grit . 2-12 ft. Ste hanoceras ,.,.1 LowerTrigonia Grit .2-12 ft. Hhanoce phriesianum. Upper Freestone . 6-2o ft. c Oolite Marl . . 5-10 ft. gal ° Lower Freestone 45-130 ft. c.a.; Pea Grit . 3-20 ft. Murchisonae. a 3 is a~ {Lower Limestone 10-25 ft. at, . oo Cephalopod Limestones-7 ft. Lioceras opalinum. s Cotteswold Sands 10-120 ft. J Lytoceras jurense. The basal sandy series, which is closely related with the underlying Lias, is usually described as the Midford Sands (from Midford, near Bath), but it is also known locally as the Bradford, Yeovil or Cotteswold Sands. The Pea Grit series contains pisolitic limestone and coarse, iron-stained oolite and sandy limestone. The freestones are compact oolite lime-stones. The ragstones are fossiliferous, earthy and iron-stained oolitic limestones. The " grits " are really coarse-grained limestones or calciferous sandstones. Between Andoversford and Bourton-in-the-Water the Inferior Oolite is represented by ragstones (Ferruginous beds, Clypeus Grit, Trigonia bed, Notgrove Freestone, Gryphite Grit) and freestones (Upper Freestones and Harford Sands, Oolite Marl, Lower Freestone). Near Chipping Norton in Oxfordshire the " Chipping Norton Limestone " lies at the top of a very variable series of rocks. In Rutlandshire, Northamptonshire and Lincolnshire the following beds, in descending order, belong to the Inferior Oolite: Lincolnshire limestone (shelly, coral-bearing and oolitic), Collyweston slate, Lower Estuarine series and Northampton Sands (hard calcareous sandstones, blue and greenish ironstones and sandy limestones). The Collyweston slates are arenaceous limestones which have been used for roofing slates since the time of Henry VII.; Easton, Dene and Kirkby are important localities. The fissility of the rock is developed by exposure to frost. Similar beds are the Whittering Pendle and White Pendle or Duston slate. The Inferior Oolite of Yorkshire differs from that of the Cotteswold district; in place of the marine limestones of the latter area there is a thick series of sands and sandstones withshales and beds of coal; these deposits are mainly estuarine with occasional marine beds. The principal subdivisions, in descending order, are: the Scarborough or Grey Limestone series, the Middle Estuarine series with their coal seams; the Millepore series and Whitwell or Cave Oolite; the Lower Estuarine series with the Eller Beck bed and Hydraulic Limestone; the Dogger and Blea Wyke beds. The last-named beds, like the Midford Sands, exhibit a passage between the Inferior Oolite and the Lias. In Skye and Raasay the Inferior Oolite is represented by sand-stones. The fossils of the Inferior Oolite are abundant. Over 200 species of Ammonite are known; gasteropods are numerous: Trigonia, Lima, Ostrea, Gerviliia, Pecten, are common pelecypods; Terebratula, Waldheimia and Rhynchonelia are the prevailing brachiopods. Corals are very numerous in some limestones (Isastrea, Montivaultia). Urchins are represented by Cidaris, Acrosalenia, Nucleolites, Pygaster, Pseudodiadema, Hemicidaris; starfish by Solaster, Astropecten, and Crinoids by Pentacrinus, Apiocrinus. Plant remains, cycads, ferns, Ginkgo and coniferous trees are found most abundantly in the Yorkshire area. The economic products of the Inferior Oolite include many well-known building stones, notably those of Ham Hill, Doulting, Dundry, Painswick, Cheltenham, Duston, Weldon, Ketton, Barnack, Stamford,Casterton, Clipsham, Great Ponton, Ancaster, Aislaby (Lower Estuarine series). Several of the stones are used for road metal. Iron ores have been worked in the Grey Limestone, the Eller Beck bed, the Dogger and the Northampton beds, the latter being the most important. The Great or Bath Oolite is typically developed in the neighbour-hood of Bath, and except in a modified form it does not extend beyond the counties of Wiltshire, Somersetshire, Gloucestershire and Oxfordshire. It does not reach so far as Yorkshire, unless the Upper Estuarine series of that district is its representative. The principal subdivisions of the series are: Wiltshire, Somersetshire, Northamptonshire, Gloucestershire, Oxfordshire. . f B ordhir rdshire Lin , Lincolnc In Beds hire. a False - bedded Oolites = Great Oolite Clay = Blis- Q. Kemble beds, " White worth Clay. Limestone, " pale, earthy Limestones, occasionally oolitic, and Marls. Upper Ragstones of Bath. False - bedded Oolites =the Great Oolite Limestone principal ncipal building stones, (generally non-oolitic). Freestone." Fissile calcareous Sandstones; Upper Estuarine series o oolitic Limestones and Thickness, 20-100 ft. Clays ; Lower Ragstones of Bath and Stonesfield Slate. Thickness, 100-130 ft. An exact correlation of the Great Oolite strata in the N.E. area with those of the S.W. is not possible on account of the great variability and impersistence of the beds. Current bedding is very prevalent, and minor stratigraphical breaks are common. The absence of the typical Great Oolite from the N.E. district is probably due in part to contemporaneous erosion with overstep of the succeeding formation, and in part to local changes in the sediment in the shallow waters of this epoch. This may also explain the rapid thinning-cut of the Great Oolite south of Bath, where its place may be taken, to some extent, by the Bradford Clay, Forest Marble and Y01m-dam. The Great Oolite is not readily divisible into palaeontological zones, but the ammonite Perisphinctes arbustigerus may be taken as the characteristic form along with Belemnites bessinus and Terebratula maxillata. Corals (Isastraea, Thamnastria) and Polyzoa (Stomatopora, Diastopora) are abundant. Hemicidaris, Cidaris, Acrosalenia, Clypeus and other urchins are common: Pentacrinus and A piocrinus represent the Crinoids. Terebratula, Rhynchonella, Waldheimia, Crania are the prevailing brachiopods ; the common pelecypods, Pecten, Ostrea, Lima, Trigonia, Modiola; Natica, Nerinea and other gasteropods are found.- Perisphinctes grandes, Macrocephalites subcontractus, Oppelia discus and Nautilus dispansus are among the more common cephalopods. The remains of fish (Mesodon, Hybodus), crocodiles (Teliosaurus), dinosaurs (Cetiosaurus, Zone Fossils. 0 '4 d v v0 Harpocesas Megalosaurus), pterosaurs (Rhamphocephalus), and in the Stonesfield slate the jaws of marsupial mammals (Amphctherium, Amphilestes, Phascolotherium) occur. The building stones of the Great Oolite are mainly oolitic freestones, viz. the varieties of " Bath stone " quarried and mined in the neighbourhood of that city (Corsham Down, Monks Park, Coombe Down, Odd Down, Box Ground, &c.) and more shelly limestones like the Taynton and Milton stone. The Stonesfield slate has been largely worked near Woodstock in Oxfordshire and in Gloucestershire for roofing, &c. The " slates " are brown calcareous sandstone, grey and slightly oolitic calcareous sand-stone, and blue and grey oolitic limestone. A curious modification of the Great Oolite—White Limestone division—is characterized by irregular ramifying tubular cavities, usually filled with ochreous material; this rock occurs in blocks and layers, and is used for rockeries under the name of " Dagham stone " from Dagham Down north of Cirencester. (See also JURASSIC.) (J. A. H.)
End of Article: ONYX
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