CH A

CH A

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kinds confifting chiefly of chalcedony, carnelian,and heliotrope, are called oriental. In thebanded or ribbon-agate, the colours are dif-pofed in ftrait parallel lines or bands. In thefortification agate, the moft beautiful of all thevarieties, the colours are arranged in wavedand angular parallel zones. The landfcapeor ruin agate reprefents rocks or buildings.The tube-agate is a congeries of hollow fta-la&ites of chalcedony filled with, and im-bedded in carnelian, jafper, See. The mofsagate, called Mocha flone , from a corruption ofthe German mochs-ftein (mofs-ftone) confiftsof chalcedony or carnelian, penetrated by arbo-refeent iron ore, which, when the filaments aredelicate, has a very beautiful appearance.

Agates are found for the moft part in nodulesin Amygdaloid, and fometimes in Gypfum:near the river Wolga they occur between ftrataof fecondary lime-ftone ; and in Siberia certainrocky tracts confift of banks of pudding-ftone,of which agate and chalcedony form the prin-cipal part.

The moft beautiful agates that this iflandproduces^ are called Scotch pebbles, and arefound principally in the neighbourhood of Perth and Dunbar. The moft celebrated however inEurope , are thofe of Oberftein in the Duchy ofDeuxponts. Thofe of Siberia are eminentlybeautiful, but the moft efteemed of all comefrom Ceylon and other parts of India .

Agate is cut into cameos or feal-ftones, ishollowed into fnuff-boxes, or fawn into thinplates for inlaying tables, and is applied to otherornamental purpofes. The commoner kindsare made into mortars for the enameller andchemift, as from their hardnefs they are notliable to be materially abraded by pulverizingthe hardeft glafs and ftones in them. Thecoarfeft fragments that are applicable to noother ufe are formed into the white gunflints,which are harder than common flint, and notfo brittle.

CHALK. Kreide , Werner. Craie , Fr.

The colour of this mineral is yellowilh white,more rarely fnow-white or greyifh white; whencontaminated with iron it has more or lefs of anochery tinge. It occurs generally in mafs,fometimes difieminated, or invefting other mi-nerals. It is without luftre, is opake, has afine earthy fraifture, and breaks into blunt edgedangular fragments. It ftains the fingers, givesa white ftreak, and when pure is very foft andalmoft friable It has a meagre feel and adheresto the tongue. Sp. gr. about 2.3. It effer-vefees violently with acids. When mixed withiron it becomes both harder and heavier.

In a ftate of purity chalk appears to be com-pofed only of lime, carbonic acid, and water,but a fpecimen analyzed by Mr. Kirwan af-forded him

53. lime42. carbonic acid3. water2. alumine

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Chalk, where it is found at all, is the preponder-ating fubftance, and may therefore be con-fidered as charafterizing a peculiar fpecies ofmineral formation. It is perhaps the mollrecent of all the varieties of calcareous car-bonat, it occurs in ftrata for the moft partnearly horizontal, alternating with thin layersof flint nodules, and with the fame irregularlydifperfed through its fubftance: it contains mabundance the relics of marine organized bodies,fuch as echinites, gloffopetne, pe&inites, &c.and alfo not unfrequently the hard parts of am-phibious and land animals, as the heads and.vertebrae of crocodiles, and teeth of elephants.Chalk hills never rife to a higher elevation thanthree or four hundred feet, and are at once dif-tinguifhable by the fmooth regularity of theiroutline, and their remarkable tendency to formcup-lhaped concavities. Ridges of chalk, inEngland at leaft, are always bordered by parallelranges of fand or fandftone, beneath and alter-nating with which are fituated the beds of fullersearth. Chalk-hills are alfo Angularly charac-terized by their drynefs and their verdure: themoft porous fandftone is fcarcely fo deficient infprings of water, and yet except upon almoft;perpendicular defeents, the white furface of thechalk is uniformly covered with fine turf orwood.

The chalk-hills in England occupy a greaterextent than in any other country, they run ina direftion nearly from eaft to weft parallel toeach other, and feparated by ranges of fand-ftone, and low tracts of gravel and clay. Themoft northern and loftieft range of chalk com-mences at the promontory of Flamborough-head, in Yorkfhire, and proceeds weft ward fornearly 20 miles. In the county of Lincoln arefome fragments of a ridge near Grantham.Two ridges traverfe the midland counties, andreach as far weft as the borders of Oxfordlhire:thefe ridges are no where fo confpicuous as in thecounty of Bedford, where they approach near toeach other, being only feparated by the Woburnand Ampthill range of fandftone. The countryfouth of the Thames alfo contains two ridges

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