ROCK-METAMORPH1SM. 



CHAPTER II. 



THE MINERAL CHARACTERS OF OPHITES AND 

 RELATED ROCKS. 



OPHITES, especially some belonging to the silo-carbacid section, 

 contain a number of mineral species ; and, with the exception of 

 certain kinds of veins, they take no unimportant rank amongst 

 the richest depositories of minerals. 



Serpentine is considered to be pure or typical when it is of a 

 yellowish-green colour, subvitreous, somewhat unctuous, sectile, 

 and of a moderate specific gravity : such is the " noble serpen- 

 tine^ of Snarum in Norway, New Jersey, and other places. 

 This consists of MgO 43'32, SiO 2 43'68, H 2 O 13'00, which 

 makes it a hydrous silicate of magnesia. Still seldom, if ever, 

 does serpentine occur pure, as even the " noble " kind is vitiated 

 by the presence of foreign substances : oxides of iron *, carbo- 

 nate of lime, carbonate of magnesia, alumina are of frequent 

 occurrence ; and occasionally oxides of chrome, manganese, and 

 nickel f : it is these compounds that often give rise to minute 

 portions, intermixed with the serpentine, of peridote, magnetite, 

 magnesite, and other minerals. 



The species and varieties of serpentine minerals are of all 

 colours, densities, and consistencies ; and they differ much from 

 one another in composition. Many of them occur crystallized ; 

 but serpentine itself, in the normal state, is never otherwise than 

 amorphous or colloidal. 



Including the type, serpentine minerals may be divided into 

 two groups : I. containing species and varieties the difference 



* " The green colour of the Galway serpentine is produced chiefly by 

 protoxide of iron. The serpentine of Penzance " (? the Lizard) " is chiefly 

 coloured by peroxide of iron " (Alphonse Gages in Phil, Mag. 4th ser* 

 vol. xvii. p. 175). 



t Nouemite (Liversidge) is a nickeliferous serpentine, in which oxide of 

 nickel replaces a portion of the magnesia, in some instances, to the extent of 

 about 21 per cent. 



