MINERALS OF OPHITES AND RELATED ROCKS. 7 



form a small group which includes calcite, miemite, and mag- 

 nesite also their hydrated allies hydrocalcite, predazzite, hydro- 

 magnesite, and one or two more. 



In addition to the hydrous mineral silicates containing mag- 

 nesia, ophites include a number of accessory species, all under- 

 stood to be normally anhydrous : such are malacolite, sahlite, 

 funkite (coccolite), diallage, smaragdite, enstatite, diaclasite, 

 peridote, chondrodite, phlogopite, biotite, anthophyllite, ido- 

 crase, and several others. Many of these are occasionally found 

 hydrated*; and when in this state they might, without much 

 disadvantage, be classed with serpentine minerals. Wollastonite 

 and scapolite, though essentially consisting of silicate of lime, 

 require to be noticed in this connexion. 



Intimately related to the anhydrous magnesio-siliceous mine- 

 rals are certain species common to ordinary metamorphics, viz. 

 augite, hornblende, hypersthene, muscovite, &c. Though far 

 removed from serpentine, and not unfrequently present in 

 ophites, but occurring therein occasionally more or less altered 

 and hydrated, they are not to be overlooked in connexion with 

 the essential minerals of these rocks, as will be seen hereafter. 



Other minerals, more interesting as natural chemical pro- 

 ducts than of present importance, occur in ophites, viz. brucite 

 (hydrous magnesia), volknerite (hydrous aluminate of mag- 

 nesia) , spinel (aluminate of magnesia) , apatite, sphene, sapphire, 

 tourmaline, zircon, graphite, and diamond. 



Considering that silicate of magnesia is a common constituent 

 of minerals belonging to ordinary metamorphic rocks, that it 

 runs through the entire range of percentages in species occurring 

 in ophites, also that water exists in the latter minerals in pro- 

 portions varying between opposite extremes, it is not to be 

 wondered at that some ophitic species are difficult to formulate, 

 and that there are rocks containing more or less of a hydrated 

 silo-magnesian mineral associated with aluminous mineral sili- 

 cates (as is the case with ophi-euphotides and other sub-silacid 

 ophites) . 



* Dr. Heddle finds nearly 5 per cent, of water in rnalacolite from between 

 Glen Elg and Totaig (Trans. Roy. Soc. Edinburgh, vol. xxvii. p. 450). 



