C 270 )

C^U A

It occurs in veins anti lining cavities, in both•which fituations it is generally in immediatecontact with the rock in which they are formed.The variety with globular diftinct concretionshas hitherto been found, we believe, only inCornwall , where it conftitutes the great dykeor crofs courle which pafics entirely acrofs thatcountry from north to louth, cutting throughand altering the direction of the metallic veinsof that rich diilrid!.

3 Subjp. Granular quartz.

Its colour is fnovv white, yellowifh or greyifhwhite. It occurs in mafs and as a componentpart of certain granites. Its luftre varies fromfhining to glimmering. It occurs in granulardiftinct concretions, which are fometimes ce-mented together by an infiltration of quartz.When the concretions are very fine, the fradtureis nearly compact; when the concretions arelarger, the fradture is more or lefs fplintery.Its fragments are indeterminately angular, bluntedged. It is tranflucent. In its other charac-ters it agrees with the firft fubfpecies.

The fine-grained with fiivery mica and creamcoloured felfpar, compofes a granite that occursnear Schihallion in the central highlands ofScotland : the larger grained forms large blocksin argillaceous fchiftus, and other primitiverocks in Shropfhire. This latter becomes of apure fnow-white colour by calcination, andis largely employed in the manufadture ofChina-ware.

4 Subfp. ' Compadt quartz.

Its colours are to the full as various as thofeof rock cryftal. It occurs in mafs, difleminated,in rounded fragments, alfo of particular lhapes,fuch as globular, tubercular, fpecular, cellular,&c. It varies in luftre from fhining to glim-mering, the former is generally vitreous, thelatter is for the moll part greafy or pearly. Itsfradture’is coarfe fplintery palling into minuteconchoidal. Its fragments are indeterminatelyangular, not very {harp-edged. It varies fromtranflucent at the edges to femi-tranfparent.■In other particulars it agrees with the firftfubfpecies.

Q_U A

It occurs in every kind of rock, frem theoldeft to the molt recent, forming an eflentialcouftituent of tome, as granite and gneifs, andcompofing veins and beds in others: fometimesit is found i:i confidevable blocks, but neverforms entire mountains.

There are three varieties of compadt quartz,which require to be deferibed feparately.

Var. i. Prafe.

Its colour is leek-green; it occurs in mafs.Its luftre is gliftening, between refinous andvitreous. It is tranflucent. It occurs at Bru-tenbrun in Saxony , in a mineral bed, amiappears to be a very intimate mixture of quartzand actynolite.

Var. 2. Milk- or rofe quartz.

Its colour is pale rofe, palling into reddilh,greyifh and yellowifh white. It occurs in mafs.It is gliftening, with a greafy luftre. Its fradtureis more or lefs conchoidal. It varies betweenfemi-tranfparent and tranflucent. It is fuppofedto be quartz coloured by manganefe. It iscapable of a high polilh, and is formed intovarious ornaments. It occurs in a large grainedgranite at Rabenftein in Bavaria , and is faidalfo to have been found in Finland .

Var. 3. Aventurine.

Its colour is yellowifh red, or grey, greenifh,or blackilh. When held in the light it appearsto be filled with fiivery and yellowifh fpanglesthat refledt the rays of light with great brilliance.Thefe fpangles have been taken by fome mine-ralogifts for mica, but according to others theappearance is produced only by the particulardiredlion of its laminse. It is fufceptible of agood polifh, and is ufed for feal-ftones andother ornaments. The fineft aventurines comefrom Spam.

QUICKLIME . See Lime.OUICKSILVER. See Mercury.

QUINTESSENCE.

Properly, the fifth effence, or the refult otfive fucceflive diftillations. The term is atprefent obfolete; but was ufed by the oldchemifts to exprefs the higher! degree of recti-fication to which any fubftance can be brought.