C H A
CH A
( 584 )
the one commencing, at the North and SouthForelands, palling through the not th of Kent ,the middle of Surry, and the north of Hamp-Ihire, and including the North Downs of Ban-ftead, Epfom, &c.; the other commencing nearHaftings and at the lofty promontory ofBeachy-head, pafles through Sufi'ex and thefouth of Hamplhire, into Dorfetfhire, includingthe Soutli Downs.
The north part of France alfo abounds inchalk: it is befides met with in fome of theDanilh iflands in the Baltic, and in Poland .
The ufes of chalk are very extenfive: themore compact kinds are ufed as building ftone,and are burnt to lime (nearly all the buildingsin London being cemented with chalk-mortar):it is alfo largely employed in the polifhing ofmetals and glafs, in conftru£ling moulds to cafhmetal in, by carpenters and others as a materialto mark with, and by ftarch-makers and che-mifts to dry precipitates on, for which it is pe-culiarly qualified on account of the remarkablefacility with which it abforbs water.
CHALK, Spanijh. ? g c TE „ TECHALK, French . 5 bee bTEATITE *
There are four fub-fpecies of this mineral.
I. Earthy Chlorite, Chloriterde, Werner.
Its colour is intermediate between darkmountain green and blackilh green: it iscompofed of minute fomewhat mining feales,either loofe or flightly coherent: it does notftain the fingers, but gives a fhining greenftreak : it feels rather greafy, is light, and whenbreathed on, gives a ftrong argillaceous odour.
Earthy ehlbrite melts before the blow-pipeto a dark-coloured fhining flag, which is attrac-ted by the magnet. According to an analyfisby Vauquelin, it confifts of
8. Magnefia16. Silex18.5 Alumine43. Oxyd of iron
2. Muriat of foda-2. Water
99-5
It occurs for the moft part in primitivemountains of argillaceous fdiiftus mixed withquartz, common chlorite, calcareous fpar, andmicaceous iron ore. It is alio contained in gra-
nular lime-ftone and primitive fand-ftone. Byexpofure to the air it decompofes into a yellowochery mafs.
It is found in Saxony, Switzerland, Norway ,Sweden, Hungary , and North Wales.
2. Common Chlorite, Gemeiner Chlorit , Wer-ner.
The colour of this mineral refembles that ofthe preceding, except that it is a little darker.It occurs in mafs and difleminated, has a gliften-ing luftre, and a fine foliated fracture pallinginto earthy. It is foft, gives a light-green ftreak-and has a greafy feel. Sp. gr. 2.83.
It confifts, according to Hopfner, of39 Magnefia41 Silex6 Alumine1 Lime
10 Oxyd of iron
97
By the action of the blow-pipe it acquires abrownilh-yellow colour.
It is generally found in metalliferous veins,,but is alio found in veins of quartz.
3. Chlorite-flate, Ghlor itfchiefer, Werner.
Its colour is a dull green of various degreesof intenfity from greenilh-grey to greenilh-black: it occurs only in mafs; it has a gliften-ing and greafy or refmous luftre. Its fractureis curved or waved foliated: thofe varieties thathave the greateft degree of luftre are ufuallyfcaly foliated, When broken it flies into blunt-edged flaty fragments. It is opake, is foft, hasa greafy feel, and gives a mountain-green ftreak,Sp. gr. 2.98.
It does not melt per Je before the blow-pipe,but by a continued heat becomes white.
Chlorite flate is one of the primitive rocks,and is generally found in layers alternating withargillaceous fchiftus, fometimes, though rarelywith gneifs. It pafles into micaceous fchiftlisand hornblende flate, and often contains imbed-,ded cryftals of garnet or o&ohedral magneticiron ore.
4. Foliated Chlorite, Blcittriger chlorit, Werner.
The colour of this mineral is dark-green. It
occurs in mafs or difleminated, or cryftallizedin hexagonal tables, or fix-fided prifms termi-nated by pyramids, the angles of which arerounded off fo as to referable a cylinder termi-nated by a cone. The fiirface of the cryftals is-ftreaked and gliftening; its internal luftre is- Ihining and pearly ; its fnufture is foliated,generally curved, and the lamina are in fome
* Emmerling. Hauy. Jamefon,.