358 ON VOLCANOES AND EARTHQUAKES. 



smoke. Large bubbles appear and burst at the mo- 

 ment of tbe explosion, after which the lava sinks 

 again to its former place ; these appearances being 

 repeated incessantly at short intervals. 



In the intermittent volcanoes, the phenomena vary, 

 and are more or less violent. The eruptions of Ve- 

 suvius and -ZEtna have been most studied, and have 

 been the subjects of nurnerous interesting narratives, 

 from the time of Pliny to our own days. In general, 

 the first appearance of an eruption consists in a 

 column of smoke, rising to a great height in the 

 atmosphere, and then spreading out in a form com- 

 pared by Pliny to that of a pine tree. This is fol- 

 lowed by explosions, by trembling of the earth, and 

 often, by decided earthquakes. Flame is then seen 

 to issue from the crater, attended by red hot stones, 

 which are often thrown high into the air ; producing 

 those brilliant effects which have so often been de- 

 scribed. Lastly, the melted lava, rising to the edge 

 of the crater, flows over it, or, in some cases, breaks 

 out at a fissure formed in the side of the mountain. 

 These currents are often very extensive ; reaching to 

 many miles in length and breadth, and flowing on 

 with a constantly diminishing rapidity, for many 

 months ; the issue of the lava from the opening con- 

 tinuing for an indefinite period. The eruption of the 

 lava is often followed by small fragments of rock, 

 commonly called ashes ; being the same substances 

 which constitute the black smoke, and which, in the 

 shape of fine dust, are often carried by the winds to 

 enormous distances. Globular masses of lava also 

 fall with it ; appearing to have been projected in a 

 fluid state, and to have been consolidated in the at- 

 mosphere. 



During the emission of the dark smoke, electrical 



