262 OBSIDIENNE. 



then found that the outer portions of the 

 fused rock, that is, those portions which 

 had cooled the most rapidly, owing to their 

 contact with the air, had assumed the ap- 

 pearance and structure of Obsidian ; while 

 those from the middle and bottom, which 

 had cooled more gradually, resembled Pitch- 

 stone. Some of the more glassy portions 

 near the top were spherulitic and vesicular."* 

 Descriptive Catalogue of Rock Specimens, in 

 the 3Iuseiim of Practical Geology, p. 306. 



Obsidian is generally black, and opaque 

 in mass, but appears ash-grey and trans- 

 lucent, or nearly transparent, at thin edges. 

 It is remarkable for its perfect conch oidal 

 fracture, and for its sharp cutting edges, 

 advantage of which was taken by the an- 

 cient Peruvians and Mexicans, who made it 

 into razors, knife blades, and points of arrows 

 and lances ; uses to which it is still applied by 

 the natives of the Azores, Ascension, and 

 Guiana. The ancient inhabitants of Peru 

 and Mexico, and also the Romans, made 

 mirrors of Obsidian, and sometimes used it 

 as a gem. Now-a-days it is converted into 

 snuff-boxes, knife- handles, and into various 

 articles of mourning jewehy. 



_ Name. According to Sir John Hill, Obsi- 

 dian was in great use among the ancients, 

 because it took a good polish, and reflected 

 the images of things as our looking-glasses 

 do. " The finest kind was, for this reason, 

 called o-^iocvoc ocTo T'/j: o-^io;, which was after- 

 wards written by the Latins, Opsianus, Opsi- 

 dianus, and Obsidianus. And the true origin 

 of the name being forgotten from the false 

 spelling of the word, after-ages thought it 

 had received it from one Ohsidius, whom 

 they imagined the first finder of it." — Hill's 

 The.oplirastus, p. 39. 



Localities. The principal localities are 

 Iceland, Siberia, Hungary, New Zealand, 

 Ascension, Teneriffe, the Lipari Islands, 

 Mexico, Peru, Madagascar, South Sea Is- 

 lands, and Melos and other islands in the 

 Grecian Archipelago. S.G. 2-25 to 2'8. 



Brit. Mus., Case 30. 



M. P. G. Upper Gallerv, Wall- case 1, 

 Nos. 12 and 13 ; Wall-case 2, Nos. 28 to 31, 

 119, 126. Table-case B, Nos. Ill to 113. 

 Horse-shoe Case, Nos. 1027 to 1029. 



Obsidienne du Cantal. See Canta- 



LITE. 



Occidental Chalcedony. See Orien- 

 tal Chalcedony., 

 Occidental Diamond. A name given 



* See Table-case A, in Recess 4, Upper Gall. 

 M.P.G. 



OCTAEDRITE. 



by lapidaries to limpid and colourless Rock- 

 Crystal when cut and polished. 



Occidental Topaz. See Citrine. 

 OcHRAN. A yellow variety of Bole, from 

 Oi-awitza. 



Analysis, \)j Kersten : 



Silica ..... 31*3 

 Alumina . . . . 43 

 Peroxide of iron . . . 1'2 

 Water, and a trace of boracic 

 acid 21-0 



96-5 



Ochre. Clay coloured yellow by hydrate 

 of iron ; but the term is applied to any com- 

 bination of the earths with iron which can 

 be used for pigments, such as Peddle, and 

 the like. Ochres of good qualit}' should 

 contain at least 10 to 15 per cent, of hydrate ; 

 when that amount is exceeded the colour 

 passes into brown. There are three mines 

 worked for Ochre in France, 1st. at Vierzon, 

 Dept. of Cher; 2nd. at Pourcain, near 

 Auxerre, Dept. of the Yonne ; and 3rd. at 

 Saint Amand (Nievre). Of these the Ochre 

 of Vierzon is held in the greatest estimation 

 on account of its beautiful yellow colour. 



N-ime. The word Ochre is derived from 

 ^zs.^, the Greek name for the same sub- 

 stance, and itself derived from ^xio=, pale. 



Ochkeous Iron Ore. See Hematite 

 and LiMONiTE. 



Ochreous Wad, or Ochry Wad. A fri- 

 able earthy substance ; probably a mixture of 

 hydrated sesquioxide, and anhydrous per- 

 oxide of manganese. Streak dark or blackish- 

 brown. Very porous, and emits a copious 

 stream of air-bubbles when placed in water. 

 S.G. 4-506. 



Comp. Anhydrous peroxide of manganese, 

 with a casual admixture of a small quantity 

 of some hydi-ated oxide, probably Man- 

 ganite. 



Dissolves in muriatic acid, with a free 

 disengagement of chlorine, leaving merely 

 traces of insoluble matter. 



Ochroite. The mineral to which this 

 name was given by Hermann is probably a 

 variety of Cerite, rendered impure %y an 

 admixture of Quartz. He deduced for it 



the formula, 2Ce Si + 3H = silica 32-83, prot- 

 oxide of cerium 57-58, water 9-59 = 100. 



OcRE. See Ochre. 



Octaedrisches Kupfererz, Mohs. See 

 Red Copper. 



Octaedrite, Werner ; Octahedrite, 

 Jameson. A name for Anatase, from its 

 occurrence in octahedrons. 



