ORIGIN AND STRUCTURE OF VOLCANIC CONES. 51 



Such a repetition is recorded as being quadruple, thus giving to the 

 mountain, near its apex, a step-like appearance. 



From various irregularities and accidents, the vent may shift its 

 position and become eccentric, and thus produce an overlapping of the 

 newer cone upon the older. This is well illustrated by the Island of 

 Vulcano at this moment. In fact, the little hill of scoria surrounding 

 the active bocca or mouth of Vesuvius is situated right away to the 

 east-northeast of the crater, and consequently the lava-streams are 

 more abundant on that side of the mountain (Fig. 2), 



The escape of the lava and vapor is the next thing to require our 

 attention. Little more, however, has to be said. The lava rarely 

 mounts the edge of the cone of eruption, generally escaping near its 

 foot, by forcing itself a passage through the loose materials or some 

 preexisting fissure according to hydrostatical laws. The vapor is the 

 real agent in keeping a vent clear, as the vast bubbles rise through the 

 viscid mass, bursting at its surface, thus keeping up the temperature 

 of the lava-column which it has traversed by the heat brought up 

 from below, and at the same time preventing any permanent stag- 

 nation therein. The vapor is generally to be seen carried away by 

 the wind in beautiful white clouds. When, however,' the eruption is 

 of a more intense kind, these vast volumes mount into the air at great 

 heightsj appearing like a column of fire by night, carrying with them 

 lapilli and ash often thousands of feet above the mouth of the volcano ; 



Fig. 2. View of the CrIter of Vesttvius, as seen from the Highest Point of Monte Somma, 

 ON July 1, 1889: A. Crater ; B. Cone of eruption ; C. Slopes of cone. 1. Spire-like fumarole ; 

 2. Irregular fumaroles aiona: a fissure ; 3. Bocca-Grande. or vent ; 4. Edges and walls of crater 

 (full) f 5. Aeh-beds composing cone : 6. Cooled lava-streams all thrown out since November, 

 1879. 



then, suddenly spreading out, these vast clouds give rise to the well- 

 known appearance of the Italian pine-tree. At the same time, from 

 every available fissure are seen to issue little columns of vapor, adding 

 their small share to the grandest visible display of force that Nature 

 has provided for our amusement or peril. 



This brings us to the next point of interest, the formation of fu- 

 maroles, which may be considered as the effect of these two agents 



