36 



HORNBLENDE. SERPENTINE. 



Mica. Talc. Chlorite. 



Silex 50 — 62 — 41 



Alumine 35 — 2 — 6 



Lime - -- -- -- - i — — i 



Magnesia 2 — 27 — 40 



Oxide of iron - -- -- 6 — 3 — 10 

 Water and loss 6 — 6 — 2; 



but these proportions vary in different specimens. 



Hornblende, to which the French give the name of amphibole, 

 forms a constituent part of many rocks, and appears to connect the 

 primary with those which are of volcanic origin. It is of a black or 

 dark green colour : it is heavier, but less hard, than quartz or felspar : 

 it may be scratched with a knife, and the colour of the streak is a 

 light green : it yields a bitter smell when breathed upon and melts 

 easily into a black glass. Common hornblende is often confusedly 

 crystallized : it sometimes forms entire mountains, or slaty beds in 

 mountains, and is very commonly met with in granular pieces as an 

 ingredient in compound rocks : when it becomes, more abundantly 

 and minutely, disseminated in them, it forms what are denominated 

 trap rocks, whose origin has greatly divided the opinions of geologists. 

 Hornblende and the rocks to which it is most nearly allied contain 

 as under : — 



Hornblende 



Silex - - 42 - 

 Alumine - 8 - 

 Magnesia - 16 - 

 Lime - - 9 - 

 Oxide of iron 23 - 

 Soda - - 

 Manganese 1 

 Water and loss 



Another mineral substance, called serpentine, from its spotted 

 colours, resembling the serpent's skin, will be afterwards described, 

 as forming entire rocks : it differs in composition from hornblende 

 by having a larger portion of magnesia and less iron ; it may, per- 

 haps, be regarded as an intimate combination of hornblende with 

 talc or chlorite. Its component parts, as given by different chemists, 

 are as under : — 



Silex - - - - 45 — 29 — 45 

 Alumine - - - 18 — 23 

 Magnesia - - - 23 — 34 — 33 



Iron - - - - 3 — 4 — 14 with a trace of alumine. 

 Lime - - - - — — 6 

 Water and loss - 11 — 10 — 8 



From these analyses it is evident that the specimens vary in their 

 component parts ; in some, the proportions are almost the same as in 

 hornblende : in others, they agree more nearly with talc and chlorite. 



Basalt. Obsidian, or volcanic glass. Lava. 



44 — 72 — 49 



16 — 12 — 35 

 2 



9 — sometimes 4 



20 — 2 with manganese. — 12 



4 — 6 with potash. 



