320 ROCK CRYSTAL. 



Cumberland : Cavrock Fells. Caldbeck Fells, 

 Alston, Cleator Moor Iron Mines, on Hema- 

 tite. — JVelsh. Snowdon, Caernarvonshire. — 

 Scotch. Leadhills, in Lanarkshire. Tn 

 ' the neighbourhood of Cairngorm (which 

 see). Argyleshire. Ballygroggan, Mull of 

 Cantyre, in cavities of trap rock. — Iiish. 

 Divis Hill, nearBelfast. Donegal. Downshire, 

 in clefts and cavities of the Mourne Moun- 

 tain-granite, with Topaz and Beryl. Pal- 

 merston, yellow. Killarney, in yellow de- 

 tached crystals. Castle Comer," Kilkenny. 

 Dungiven, Banagher. Finglen Mountain, 

 close to Dungiven {Dungiven crystals). 

 Donaghmore and Tullyniskan, in Tyrone. 

 Glen Malur, in Wicklow, yellow.— Foreign. 

 The finest specimens occur in Savoy, 

 Dauphine, and St. Etienne-la-Yarenne in 

 France. In the Carrara quarries of the N.W. 

 part of the Apuanian Alp. Hungary. The 

 Alps. East Indies. Ceylon. Hayti. Flo- 

 rida. Quito. Brazil. Madagascar. Cape 

 Diamond, near Quebec. The northern part 

 of the Desert of Atacama, in Bolivia. Tas- 

 mania, on the Islands in Bass Straits. 



Brit. Mus., Case 20. 



M. P. G. Horse-shoe Case, Principal 

 Floor. 



Rock Crystal sometimes contains included 

 capillary crystals of Amphibole, Rutile, 

 Epidote, scales of Mica, Chlorite, Bitumen, 

 and other foreign matters. More rarely it 

 contains cavities filled with liquids and 

 gases. (See Enhydros.) The liquid was 

 found, by Sir Humphrey Davy, to be water 

 wdth saline matter ; by Mr. Fox, Avater with 

 chloride of sodium (Common Salt) ; by 

 others, water has been discovered with vari- 

 ous eai'thy and metallic sulphates and chlo- 

 rides. Other liquids occur besides water. 

 See Brewstoline, Amethystoline, 

 Ckyptoline. 



"The cavities are seldom full of the 

 liquid — there is usually a bubble of air, 

 ■which, except when the cavity is very 

 small, changes place when the position of 

 the crystal is altered. We may suppose 

 with Mr. Sorby that the cavity was ori- 

 ginally filled with the liquid, when the con- 

 solidation of the crystal happened, at a 

 temperature more or less elevated above the 

 actual temperature; that it has since con- 

 tracted during the cooling, and now occu- 

 pies a 'space which, as compared to the 

 whole cavity in the crystal, is determined 

 by the actual temperature as compared to 

 the original temperature of consolidation — 

 nearly in this proportion, not strictly, be- 

 cause pressure alters the bulk of a liquid, 



ROCK SALT. 



and the pressure during the formation of 

 the crystal is unknown. * * * - * In one 

 case of Amethyst, the cavity being three- 

 fourths full of liquid at ordinary tempera- 

 ture, becomes full of liquid at 830F. ; on 

 being cooled again the vacuity reappears in 

 the crystal, with signs of ebullition." * — 

 Anniversary Address of the President (Pro- 

 fessor John Phillips) of the Geological 

 Society — 1859. 



It is employed for ornamental purposes, 

 and when perfectly clear and colourless, is 

 made into lenses for spectacles, which are 

 sold under the name of pebbles, and are 

 better suited for the purpose than glass, 

 owing to their superior hardness, and less 

 liability in consequence to get sci-atched. 

 The lenses should be cut at right angles to 

 the axis of the crystals, in order to avoid 

 the eff'ects produced by double refraction, 

 and the consequent production of a second 

 more or less distinct image, which is not 

 only unpleasant, but injurious to the eye- 

 sight. When cut for jewelry, it is called 

 by lapidaries, " white stone." An asteriated 

 variety contains whitish impurities or opa- 

 city, arranged along the diametral planes. 

 Groups of crystals are highly valued in 

 China as ornaments for the apartments of 

 the rich, and fetch extravagant prices. 

 Dr. Hochstetter states that a group of com- 

 mon Quartz crystals of very common ap- 

 pearance was offered to him for 20 Mexican 

 dollars (about four guineas.) 



Rock Leather. See Mountain Lea- 

 ther. 



Rock Marrow, Jameson. See Litho- 

 marge. 



Rock Milk, Jameson. See Agaric 

 Mineral. 



Rock Oil. See Petroleum, Seneca Oil. 



Rock Salt. Cubical. Occurs in cubical 

 crystals : the primary form being the cube, 

 into which, when purfe, it may readily be 

 cleaved. Also in large beds and masses, 

 and in the waters of the sea, as well as in 

 those of certain springs and lakes. Colour- 

 less or white when pure, but frequently 

 stained reddish-brown, brick-red, violet- 

 blue or green, by iron or other foreign ad- 

 mixtures. Lustre vitreous. Translucent to 

 transparent. Yields readily to the knife 

 and may be scratched with the nail, receiv- 

 ing an impression but not affording a pow- 

 der. Rather brittle. Fracture conchoidal. 

 Taste purely saline. Attracts moisture, but 

 remains unaltered in a drj' atmosphere. One 



* See Dr. Brewster, Ed. Roy. Soc. Trans., and 

 Ed. Phil. Journ. ix. 



