ORIGIN OF MINERALS CHARACTERISTIC OF OPHITES ETC. 61 



It has been clearly shown that, besides other minerals, mala- 

 colite and serpentine demonstratively secondary products 

 have themselves undergone chemical changes which, gradually 

 removing certain of their constituents, especially the silacid, 

 have terminated in calcitic or miemitic (dolomitic) replacements. 



With respect to several other minerals characteristic of ophites 

 and hemithrenes, such as spinel, idocrase, apatite, sphene, &c., 

 it may be assumed that, although original in these cases (that is, 

 formed independently of any other mineral), they are neverthe- 

 less of secondary origin, having resulted from the intervention 

 of hydrothermal reactions. 



Another mineral of importance in the subject under conside- 

 ration is peridote, also known under the name of olivine *. It 

 runs into several varieties, as limbilite, chrysolite, glinkite, bol- 

 tonite, olivinoid, hyalosiderite, &c. Analyses make it to consist 

 of magnesia from 32 to 50 per cent., protoxide of iron from 6 

 to 30 per cent., and silica from 31 to 44 per cent. Accessory 

 ingredients, as alumina, oxide of nickel, oxide of manganese, 

 titanic acid f, and silicate of lime, are not unfrequently present. 

 The magnesia is often about 13 per cent., generally 10 per cent., 

 and occasionally 2 or 3 per cent, in excess of the silica. Silicate 

 of lime is present in specimens of peridote from the lava of Fogo 

 (Cape-Verd Isles) to the extent of nearly 6 per cent. Monti- 

 cellite is a highly calciferous peridote, containing 34 per cent, 

 of calcium silicate. 



The minerals nearest to peridote are humite (which contains, 

 in addition to the essentials of peridote, between 2 and 3 per 

 cent, or more of fluorine) and chondrodite, in which the fluorine 

 is increased in some cases to more than 7 per cent. Leipervillite 

 (bronzite according to Pisani) only differs from peridote in its 

 magnesia reaching to 75 per cent. Forsterite, another related 

 species, but containing only a few per cent, of protoxide of iron, 

 carries us on to diaclasite, enstatite, and certain reputed diallages 

 and hypersthenes : in these last the silica is in excess of the 



* D'Argenville's name peridote (date 1755) lias priority over that of 

 Werner's olivine (1790) ; obviously, then, French mineralogists are right in 

 adopting it. Pliny's name, chrysolite, it would appear, was not applied to 

 the mineral now so called by mineralogists (see Dana's ' Mineralogy,' under 

 "Olivine"). 



t A titaniferous peridote from Zermatt, according to M. A. Damour, 

 contains 5-30 per cent, of titanic acid (Bull, de la Soc. Mineralogique, t. ii. 

 pp. 15, 16). 



