84 



THE BUILDING OF AN ISLAND. 



known by the expressive name of "The Quill." The diagram (Fig. 29) shows 

 an outline of it as seen from the north. 



v / V> 



% M- 



^ca. <dO. 



In some cases the conditions during a volcanic eruption are such as to 

 permit of a spectator remaining on, or even within, the edge of a crater during 

 the eruption. In this respect Stromboli, in the Lipari Islands, which has re- 

 cently (May, 1907,) astonished the world by its activity, has hitherto presented 

 a specially favorable field of study, because, although nearly always active, its 

 activity has usually been of so moderate a character that the explosions which 

 take place at short intervals at the bottom of the crater could be watched with 

 very little danger to the onlooker. In this way it has been found that when 

 an explosion is about to take place, there rises on the surface of the molten 

 rock at the bottom of the crater a large bubble, which explodes, emitting a 

 cloud of steam and of dust formed by some of the matter being blown into 

 minute fragments. The explosion reveals the glowing molten mass below, 

 and this lights up the column of steam and dust that has been thrown out 

 from the vent, and thus gives rise to the popular but mistaken notion that 

 flames issue from the crater; The dust produced by the explosion is known 

 as volcanic ash, a name to which there can be no objection so long as we 

 remember that there has been no combustion of any solid substance and that 

 therefore the volcanic dust cannot be ash in the usual sense of the term. 

 Where does the steam which causes the explosions come from ? Formerly 

 it was supposed to be from the surface water of the earth, which was 

 thought to have reached the hot rock through cracks, but it is now generallv 

 believed that it comes from the rocks themselves. Chemical analysis shows 

 that they contain a considerable portion of water, not as a separate substance, 

 hut in combination with other components of the rocks. The intense heat 



