340 ELEMENTARY GEOLOGY 
Similarly, Vesuvius, before the year 79 a.p., had not 
been in activity from the time the Italian peninsula 
was first visited. After this rest of many centuries, 
the cone of Monte Somma, as the volcano at that 
time was called, was blown open by a terrific explo- 
sion; and since that time, Vesuvius has been built on 
the ruins of Monte Somma, a part of which may still 
be seen as a half-collar on the rear side of Vesuvius. 
Ever since that famous catastrophe, this voleano has 
been active, although its activity has been very much 
greater and more constant since 1631 than between 
that time and the notable eruption of 79 a.p. 
Another type of volcano is that of Kilauea (and 
also Mauna Loa) in the Hawaiian Islands, which 
breaks out about every eight or nine years, always 
with an extensive flow of lava, coming not from the 
crater, but from the sides of the mountain (Fig. 199). 
The amount of lava thus poured out is immense. 
There is no ash, and the violence of the action is 
not great. Between the eruptions, lava may be seen 
standing in the immense crater, which is occupied by 
a great lake of molten rock (Fig. 207). When the 
eruption takes place, the surface of this lava well 
sinks, for the molten rock contained in the crater is 
then escaping through a fissure broken in the side of 
the cone. 
Among volcanoes there are also great differences in 
