GEOLOGY. 14? 



c. Air-formation. 



TRAP-ROCKS. 



681. Vapours and gases of different kinds may be 

 contained in the interior of the earth in two ways, either 

 chemically combined as carbonic acid, or mechanically 

 inclosed in cavities of the earth. Both may be developed, 

 or expanded by calefaction, and the latter by diminu- 

 tion of pressure. 



682. If those that are chemically combined be de- 

 veloped by calefaction, they then form vesicular spaces in 

 the masses of earth whereby the latter are extended and 

 raised above the surface ; such as amygdaloid, basalt 

 and others of this kind. 



683. Warm springs may originate by subterraneous 

 processes, chemical or volcanic; probably, too, by the 

 compression of air that has forced its way into these 

 situations. 



684. Earthquakes may indeed originate in different 

 ways ; as in addition to the falling in of cavities, by the 

 chemical development also of gases, by their subjection 

 to heat, by aqueous vapours, and also by the sudden 

 diminution of pressure upon these incarcerated gases. 



685. This diminution of pressure proceeds from sud- 

 den rarefaction of the atmosphere due probably to the 

 disappearance of air in a particular place, or resulting 

 from a change of the wind, or the formation of heavy 

 rain. Earthquakes can therefore extend through many 

 countries, without requiring to depend upon each other 

 or on a common focus of action. 



686. The silicious trap is silicious sinter, tripoli and 

 polierschiefer. 



687. The argillaceous trap, amygdaloid clinkstone, 

 several porphyries. 



688. The talcose trap, basalt. 



689. The calcareous trap, probably chalk. 



