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Encyclopedia Britannica - Main :: NUM-ORC |
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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
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
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
For onyx- marbles
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
The oolites are usually divided into: the Upper or Portland
Portland
Bajocian
The Inferior Oolite, called by William Smith
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
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
(J. A. H.) End of Article: ONYX If you wish, you can link directly to this article.
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