MINERALS WHICH FORM IGNEOUS ROCKS. 



COMPOSITION. 



EOCKS IN WHICH FOUND. 



Diallage 



Bronzite 



Hornblende 



Brownish green or brassy 

 brown ; as hard as fluor 

 spar, being much softer 

 than augite. Contains little 

 alumina, a little water, but 

 otherwise is like augite, has 

 a peculiar cleavage 



Reddish -brown or bronzy ; 

 harder than diallage. Con- 

 tains little or no alumina 

 and lime, but large quan- 

 tities of silioa and magnesia. 

 Crystals in rhombic iorms 



Greenish black. The alu- 

 mina, magnesia, lime, and 

 protoxide of iron are more 

 nearly equal than in augite, 

 and average 14 or 15 per 

 cent. each. Crystals col- 

 umnar, angle of cleavage 

 planes at intersection, 124 



Occurs in gabbros of Cornwall, is 

 associated with hornblende in 

 the euphotide of Harzburg* Fo- 

 rest in the Harz, and occurs in 

 gneiss in the Guadarrama Moun- 

 tains in Spain. 



Found in serpentine at the Lizard, 

 the Bacher Mountain in Lower 

 Styria. Enstatite is a similar 

 but paler mineral found in Iher- 

 zolite near Lake Lherz in the 

 Pyrenees, and in some gabbros. 



The pale-green varieties, tremolite 

 and actinolite, are found in dolo- 

 mite and altered limestone; com- 

 mon hornblende in diorite, syenite, 

 hornblende gneiss, hornblende 

 slate, phonolite, trachyte. Ura- 

 lite appears to be augite partly 

 transmuted into hornblende. 



The Family of Micas and Talcs. 



The talcs and micas include many species which usually agree in 

 dividing into thin laminsB which are sometimes more or less trans- 

 parent. The talcs are softer than the micas, may be bent, but will 

 not spontaneously bend back again, give a more or less greasy sensa- 

 tion when touched, and are hydrous silicates of magnesia where part of 

 the magnesia may be replaced with iron, and are not acted on by acids. 

 The micas are usually in rhombic or hexagonal plates, are both flexible 

 and elastic, give a clean sensation when touched, are double silicates, 

 usually of alumina, magnesia, potash, and iron, and some species are 

 soluble in sulphuric acid. Talcs are often deposited from water as 

 pseudomorphs, in place of other magnesian minerals which originally 

 formed part of the rock ; but they cannot be correctly described as 

 hydrated micas because micas contain alumina, but may be formed 

 in rocks which were previously infiltrated with magnesian silicates 

 derived from decomposed mica, hornblende, augite, and olivine. 



