42 



SERPENTINE. 



may be scratched with a knife, and the colour ©' 



the streak is a light green. It easily melts into * 

 black glass, and is often found crystallized in an ir- 

 regular manner. Hornblende forms a constituen 

 part of many rocks, end even forms entire mount 

 ains or slaty beds in mountains. It enters largelj 

 into the rocks of volcanic origin, such as green 

 stone, basalt, &c, generally denominated trap rocks 

 The chemical composition of hornblende, basalt; 

 and lava, is very similar. 



tig^3. Hornblende often occurs in a crys 

 tallized form, as in the margin, though 

 it is more often met with in rude, 

 shapeless masses. We often see it 

 composed of long, needle-shaped or 

 acicular crystals, diverging from a cen- 

 tre, which are star-shaped or radiated 

 like the spokes of a wheel, and the 

 lustre is shining. 



Serpentine. — This is one of the magnesian min- 

 erals, being composed of from 20 to 45 per cent, of 

 silex, 18 to 23 per cent, of alumine, and 23 to 34 of 

 magnesia, and 8 to 12 of water, with a trace of iron, 

 to which its colours are often owing. It derives its 

 name from its resemblance to a serpent's skin, being 

 streaked or spotted. Bakewell thinks that it may 

 be regarded as an intimate combination of horn- 

 blende with talc or chlorite ; whether this be the 

 case or not, it is certain that there is an intimate 

 connexion between them, and that we often find 

 hornblende changed into serpentine when in con- 

 tact with limestone. Serpentine is generally com- 

 pact, nearly granular, yields to the knife, is some- 

 what soapy to the feel, and is susceptible of a high 

 polish. It is often employed as a marble ; and, when 

 limestones containing it are polished, they consitute 

 what is called the vera 1 antique marble. This mineral 



