CONSTITUENT MINERALS OF ROCKS. 



59 



schist in long or short prisms, frequently as large as the finger, or larger, and 

 in soapstone in black or brown crystals, long or short. Before the blowpipe 



Fig. 43. 



the common varieties fuse easily. Composition peculiar in the presence of 

 boracic acid ; the other constituents, besides silica and alumina, are commonly 

 some magnesia, oxyd of iron, soda, and fluorine. 



64. (12.) Topaz.— In rhombic prisms (figs. 44, 45) of 124° 19', having a bril- 

 liant and easy cleavage parallel to the base. Sometimes columnar-massive. 

 Color, pale yellow, white, brown ; often transparent. Harder than quartz. 

 G. = 3.4-3.7. Before the blowpipe, infusible. Composition peculiar in the large 

 amount of fluorine present; contains — Silica, 35.27, alumina, 54.92, fluorine, 17.14. 



Beryl. — In regular six-sided prisms, without distinct cleavage. Color, 

 usually pale green ; in the emerald — a variety of beryl — deep and clear green ; 

 also yellowish, bluish, brownish, white. Hardness above that of quartz. 

 G. = 2.65-2.75. Infusible. Composition peculiar in containing glucina. Com- 

 position: Silica, 66.9, alumina, 19.0, glucina, 14.1 = 100. 



2. Protoxyd-Silicates, not containing water. 



65. (13.) Hornblende (often called Amphibole). — In oblique prism- 

 atic crystals (monoclinic) of 124° 30', with cleavage parallel to the 

 faces ; often having the acute edges truncated so as to be six-sided 

 and approach a regular hexagonal prism. m The crystals often long 

 and thin (fig. 48), or aggregated into masses, or penetrating the 

 rock (fig. 49) ; sometimes short and stout (figs. 46 and 47). Color, 



Fig. 46. 



Fi°r. 47. 



Fig. 49. 



Fig. 48. 



black and greenish-black, common either in distinct, stout crystals, 

 or fibrous; also in short and stout green crystals; also in long 



