REVIEW OF VOLCANIC ACTION. 367 
breaks out and becomes externally active; that often when there is a 
seeming extinction of a crater, the lavas may be accumulating, or 
rather rising in the vent by the same mode of progress as is exempli- 
fied in Kilauea, and that after a period the lavas continuing to rise, 
the process causes them to show external activity: and finally, either 
the accumulated pressure alone causes an outbreak, or the greater 
action promotes the formation of greater volumes of vapour, because 
a larger or longer surface of heated lavas is exposed to the ingressing 
waters; or else the extension of heat and increase of vapours open 
the way for new floods of water to give eruptive force to the volcano: 
and so in one or another of these ways, or by them combined, the out- 
break takes place. Hence the interval between great eruptions in 
different craters is determined by the time required to produce the 
rising of lavas in the vent; and this interval may be nearly uniform 
for the same vent through a long period: seven or eight years has 
been latterly the interval at Kilauea.* (pages 222, 223.) 
III. That eruptions will usually take place from the central vent 
in case the sides of the mountain will sustain the pressure of the 
column of lava; and when incapable of sustaining this pressure, 
lateral outbreaks occur. Fissures are also frequently opened by local 
action through the pressure of vapours. 
IV. That by the balance kept up between the two modes of erup- 
tion, terminal and lateral, the form of the mountain is produced : 
and that the form is farther modified by fissures or dikes, acting 
like wedges, which may increase or diminish the angle of the 
slope according to the part of the mountain, the upper or lower, in 
which they prevail: and farther, that in all recent periods, fissures of 
the lower parts of most volcanic mountains have been far more fre- 
quent than fissures of the upper, in consequence of which, the slopes 
have been rendered more gradual, and the limits of the mountain 
have been extended. 
V. That lateral cones of cinders, lava and cinders, or lava alone, 
frequently form over the deeper parts of a fissure of ejection, and are 
often a thousand feet or more in height; they may be thrown up ina 
* The fact of there being long intervals of quiescence in most volcanoes was observed 
by Humboldt, and he remarks that the interval has a general relation to the size of the 
volcano. This relation, though perhaps borne out as a general fact, is not a necessary 
one, as the interval would naturally become longer in such mountains as are approaching 
extinction. 
