PRESENT CHANGES IN THE EARTH. Ill 



194. Volcanoes. The most striking present manifest- 

 ations of the agency of heat we have in volcanoes. 

 These are the grand chimneys of the earth; and, as vents 

 for the great furnace of fire in its interior, undoubtedly 

 save the earth's crust from most disastrous consequences. 

 The manner in which the volcanic mountains were con- 

 structed will be shown in the chapter on the formation 

 of the earth. Volcanoes are extinct or active, the ex- 

 tinct being those mountains which, from their shape 

 and composition, are known to have been once in oper- 

 ation, though not since the advent of man, and the active 

 being those which have been eruptive since his advent. 

 While some active volcanoes are continually active, most 

 of them have seasons of eruption, with intervals of rest 

 of various lengths. The shape of a volcano is more or 

 less conical, the cone being truncated, that is, without a 

 top. When it is inactive, the cavity of the crater or 

 opening is shut up with a crust of solid lava. It is sup- 

 posed that the occasion of an eruption is the introduction 

 of water in some way into the interior, generating steam. 

 The expansions of the steam produce the earthquakes 

 which so commonly precede an eruption. The steam at 

 length accumulates to such an extent that it bursts the 

 solid cover of the crater, scattering its fragments and 

 dust aloft, with sheets of flame, followed by the overflow 

 of the lava. The steam which escapes forms above the 

 volcano a bright cloud, and with it there are continual 

 discharges of lightning, with thunder, the explanation of 

 which has been found in the fact that steam escaping 

 from a boiler has decidedly electrical properties. The 

 dust and the condensed steam produce, of course, show- 

 ers of rain and mud in the neighborhood. The matters 

 thrown out from a volcano are various in their charac- 

 ter. When the eruption is about to terminate showers 

 of cinders fall, and the last of the eruption is a mixture 

 of smoke and vapor. 



195. Vesuvius. There is no record of the action of 



