224 The Realm of Nature CHAP. 



volcano. It serves as a natural lighthouse and also as an 

 automatic storm warning, as its activity is always greatest 

 when the atmospheric pressure is low and gales may be 

 expected, while the violence of its eruptions is much reduced 

 when the barometer rises. Volcanoes from which no erup- 



FIG. 43. Ideal Section of a volcano. SS, stratified rocks of crust ; bb, old lava 

 solidified in throat of volcano and in dykes ; aa, new outburst of lava ; cc, 

 old crater ; a, new crater. (After J. Geikie.) 



tion has ever been recorded are called extinct ; those which 

 break out at intervals are said to be dormant during their 

 periods of tranquillity, but the distinction can hardly be 

 drawn with confidence. Vesuvius is the type of volcanoes 

 which are occasionally dormant and sometimes supposed 

 to be extinct. The commencement of activity after a 

 dormant period is usually preceded by earthquakes and 

 subterranean noises, indicating that pressure is accumulat- 

 ing in the heart of the mountain. Hot springs break out 

 on the slopes, and gases and hot vapour rise in increasing 

 volume from the crevices in the crater. Then a terrific 

 explosion occurs, shattering the solid lava plug and perhaps 

 destroying the entire cone ; volumes of water-vapour shoot 

 up into the air, mixed with clouds of dust that darken the 

 sky and fall like snow over the mountain slopes and sur- 

 rounding country. Flashes of lightning dart from the over- 

 hanging cloud, the friction of dust and vapour on the air 

 causing great electrical disturbance, and the noise of thunder 

 is added to the roar of the escaping steam and volcanic 



