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ROCK (O.Fr. rake, Sp. rota, Ital. roc...

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Originally appearing in Volume V23, Page 433 of the 1911 Encyclopedia Britannica.
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ROCK (O.Fr. See also:rake, Sp. See also:rota, Ital. rocca; possibly from a See also:Lat. See also:form rupica, from rupes, rock)  , in See also:geology a See also:mass of the See also:mineral See also:matter of which the crust of the See also:earth is composed (see See also:PETROLOGY and GEOLOGY) . In more See also:general usage a " See also:rock " is a large mass of this mineral matter, as distinguished from smaller pieces which are termed " stones." From this word must be distinguished the verb " to rock," to See also:swing an See also:object to and fro, particularly of a See also:cradle in which a See also:child is rocked to See also:sleep, the See also:original meaning . The O.Eng. word is roccian, and is cognate with many worts in See also:Teutonic See also:languages, e.g . Du. rukkea, See also:Dan. rykke, Ger. riicken, to pull, tug, push . ROCK-CRYSTAL, a colourless and transparent variety of See also:quartz (q.v.), used as an ornamental See also:stone . It usually occurs as crystals lining cavities in quartz-See also:veins, which often run through See also:granite, See also:gneiss and crystalline See also:schists . The limpidity of the crystal, its coldness to the See also:touch and its See also:common occurrence in rocks among Alpine glaciers, led to the See also:ancient belief that it was a See also:kind of congealed See also:water, whence the name crystal, from Gr . KpboraXXos (See also:ice) . In the Swiss See also:Alps the " Strahler," or crystal-gatherer, searches the rocks at much See also:personal See also:risk, and is often led to a drusy cavity by tracing narrow veins, or strings, of quartz on the See also:mountain-See also:side . A remarkable druse, or Krystalkeller, discovered at Zinkenstock in the Bernese Oberland, in 1719, yielded about 20 tons of crystal, a single specimen weighing 8 cwt . The famous See also:discovery of the Galenstock, in 1867, furnished magnificent crystals, but they were dark See also:brown or smoky quartz . La Gardette, near Le Bourg d'Oisans, in the Alps of See also:Dauphine, is a notable locality for See also:fine specimens of rock-crystal .

The Alps and See also:

India probably furnished the ancients with their supplies . Rock-crystal has been used for ornamental purposes since the Mycenean See also:period . By the See also:Romans under the See also:Empire it was highly valued, and carved into vases and goblets, in some cases elaborately engraved . Lenses or globes were used for kindling the sacred vestal See also:fire and for cauterizing the flesh, whilst ladies carried balls of crystal in See also:order to cool their hands during the See also:heat of summer . The artists of the See also:Early See also:Renaissance greatly favoured the use of rock-crystal, and executed beautiful carvings in this material . In See also:modern times the use of rock-crystal has been largely superseded by that of See also:glass, and it is notable that See also:flint-glass is known in See also:France as " cristal," probably from its resemblance to limpid quartz, or perhaps from the fact that powdered rock-crystal has been used as a source of See also:silica in the manufacture of the finest glass . Rock-crystal is still cut as a faceted stone for personal decoration, but though not without brilliancy it lacks the " fire " of many See also:gem-stones . It is often known locally by such names as See also:Bristol See also:diamond, Cornish diamond, Isle of See also:Wight diamond, See also:Briancon diamond, Marmaros diamond, See also:Lake See also:George diamond, &c . Rock-crystal is also carved into See also:seals, See also:paper-weights and other trivial See also:objects, and into See also:spheres for See also:divination by crystal-gazing, See also:Japanese balls being specially noteworthy . In See also:Japan the crystal has been obtained for centuries from the granitic districts around Kimpu-See also:san, in the See also:province of Kai . Probably the most valuable application of rock-crystal is for spectacle lenses, which in consequence of their hardness are not readily abraded by use . They should be cut at right angles to the optic See also:axis, or axis of the See also:prism .

The " pebble " for lenses is found loose in the See also:

soil in many parts of the provinces of See also:Goyaz, Sao Paulo and See also:Minas Gerdes in See also:Brazil . Much of the material for See also:spectacles comes also from See also:Madagascar, where large crystals of clear quartz are found in the beds of certain streams, especially in the N.E. See also:part of the See also:island, having probablybeen derived from quartz-veins in the gneiss and See also:pegmatite . In India rock-crystal has been worked at many localities, and the See also:loot of the See also:palace of See also:Delhi yielded marvellous ornaments carved in this material . At the See also:present See also:day it is cut and polished at Vellum in the See also:Tanjore See also:district in See also:Madras, and is known as Vellum stone . Among the numerous localities in the See also:United States which yield rock-crystal mention may be made of those in See also:Herkimer Co., New See also:York See also:State, whence the Lake George crystals are obtained; and it is notable that some of the Herkimer quartz encloses bituminous matter . Mokelumne See also:Hill, Calaveras Co., See also:California, has furnished some remarkable rock-crystal . In See also:Europe the localities are very numerous, the most important being those in the Alps . Very fine crystals remarkable for pellucidity though not of large See also:size occur in cavities in the statuary See also:marble of See also:Carrara; and remarkably hollowed crystals are known from Porretta near See also:Bologna in See also:Italy . The finest rock-crystal in See also:Great See also:Britain occurs at See also:Tintagel and the Delabole See also:slate See also:quarry in N . See also:Cornwall; and at See also:Snowdon in N . See also:Wales . (F .

W .

End of Article: ROCK (O.Fr. rake, Sp. rota, Ital. rocca; possibly from a Lat. form rupica, from rupes, rock)
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