MINEKA-LOGY. 417 



SECOND ORDER. 



VOLCANIC ROCKS. 



Whilst the primitive rocks were gradually thrown up 

 from the interior of the earth, partly through igneous 

 and partly through aqueous agency, which extended over 

 the whole surface of the globe, the- Volcanic or Trap 

 Rock Formation appears to have been produced, by a 

 single eruption of fused matter, thrown out from the 

 bowels of the earth by a strong subterranean power. 

 Before we proceed to describe the Trachytic or Igneous 

 Rock Formation, let us examine the volcanic agency 

 which produced them. Volcanic crests or hills are gen- 

 erally steep, rugged, conical elevations, sometimes isolated 

 or arranged several together on a line, with crateriform 

 cavities on the summit, and deep internal fissures, that 

 communicate with the internal portion of the earth, 

 which is still considered to be in an igneous state. A 

 continued melting process, like that of an active furnace, 

 is always going on, from which abundance of gaseous 

 matter is disengaged. The water penetrating these sub- 

 terranean cavities filled with fused metallic or mineral 

 matter is converted into steam, which, pressing upon the 

 lava, forces it upwards. It then breaks forth in eruptive 

 explosions, throwing out showers of stones, vapor, ashes, 

 and melted lava. The craters are of various but always 

 of considerable depths, reaching from one hundred to 

 three hundred fathoms. 



As volcanic eruptions present various phenomena, 

 sometimes splendidly picturesque, at others terrific and 

 appalling, they are collectively classed, under three heads, 

 as appearing at three different periods. 

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