84 SCIENTIFIC AGRICULTURE* 



composed principally of glassy feldspar, containing also crystals 

 of feldspar, mica, hornblende, and sometimes iron. It is rough 

 and harsh to the touch, hence its name from the Greek word 

 trachus, (rough:) it occurs in vast quantities in Europe and 

 South America, in volcanic regions, but is not found in the 

 United States. 



BASALT. 



" Basalt consists of an intimate mixture of augite, felspar 

 and iron, to which a mineral of an olive green color called 

 olivinc, is often superadded in distinct grains or nondular 

 masses." The iron is usually magnetic, and is sometimes 

 accompanied by the metal titanium, hence the name, "titani- 

 ferous iron." Augite is the predominant element in this rock: 

 basatt presses insensibly into most other varieties of trap rock. 

 True basalt does not occur in the United States. 



AMYGDALOID. 



Any rock containing almond shaped pieces of some other 

 mineral, as quartz, chalcedony, agate, calcareous spar, or zeo- 

 lite, may be denominated amygdaloid: the base may be wacke, 

 basalt, greenstone, or any other trap rock. Some amygdaloid 

 rocks have the almond shaped cells or cavities, which are 

 empty, and glazed on their sides by a glassy coating, showing 

 their igneous origin. 



SERPENTINE. 



This is a greenish colored rock, containing, according to Dr. 

 Hitchcock, 40 per cent, of magnesia, it is a hydrated silicate 

 of magnesia. Serpentine sometimes contains diallage, steatite, 

 talc, and some iron. It is classed by most authors among 

 unstratified rocks. Comparatively it is not a rock of great 

 extent: it is often associated with talcose slate. 



LAVA. 



Under the term lava, are embraced all the varieties of 



