THE MEDITERRANEAN NATURALIST 
247 
Conditions which may determine an Eruption . — 
In a large number of cases the gradual increase 
of tension in the confined magma may go on to the 
crisis of eruption. But in certain cases the inter- 
vention of collateral influences may anticipate such 
an occurrence. An increase of upward pressure 
from the main volcanic source, dependent upon 
secular cooling, tidal action (if such exists), or 
other causes, may be sufficient addition to the 
amount of tension already existing to more than 
balance the resistance. A sudden lowering of 
atmospheric pressure may be sufficient in some 
cases to render the superincumbent pressure less 
than the tension of the igneous magma. It is 
known that, as the rainfall is increased in the 
season, the drainage level of a country reaches 
a higher line, and therefore the superincumbent 
pressure increases; and, vice versa , the superin- 
cumbent pressure diminishes during a drought, so 
that a sudden relief of pressure may be the meta- 
phorical last straw. 
The greater the height in a temporarily extin- 
guished volcano the greater the weight, or, in an 
active one, the greater the pressure of the supe- 
rincumbent column of lava above the drainage 
level. (1) We might therefore say, that as a per- 
manent volcano increases in height its eruptions 
will diminish in frequency or increase in power — 
a fact thoroughly borne out by experience. Under 
the same conditions, in a lateral opening of a very 
high volcano, such as Etna, the amount of lava 
will be greater as the chimney is higher above the 
outlet, since it will hold more. But beyond this, 
the amount of output will be more than that con- 
tained in the chimney above the level of exit, but 
also indirectly as the pressure of that amount is 
removed. When the lava pours out laterally from 
the chimney, its superincumbent weight, being 
removed, will allow expansion of the elastic matter 
below the level of exit, so that as this rises to 
establish a balance, lava will continue to pour out 
from the lateral outlet until total equilibrium is 
obtained. In this way the amount of lava spread 
(1 ) Let us compare the height of a column of 
phonolite paste of 100 (?) metres of Monte Nuovo 
with the column of heavier doleritic paste of Etna, 
of 8300 meters, when we see that this is an important 
factor in modifying the eruptive character of a 
volcano, 
over the surface will be much more than that con- 
tained in the chimney above the level of the exit. 
This fact I have been able to verify on various 
occasions in the recent small eruptions of Vesuvius, 
which are in many ways more instructive than 
great ones on account of permitting near approach 
to be made. This, I believe, will eventually be 
proved to be the true mechanism of lateral erup- 
tions. As examples, we may compare the bulk of 
the lava products from the lateral craterets of 
Vesuvius and Etna. 
Conditions which determine the Extinction of a 
Volcano. — We have already seen how a dyke that 
has not manifested itself at the surface may soli- 
dify, and so divert igneous action to other locali- 
ties. In the case of explosive eruption, the 
expansion that takes place in the magma increases 
its volume, in the form of a pumiceous froth, to 
such an extent that it occupies many times its 
original volume. (1) A large portion of this spongy 
magma escapes, leaving the fissure still filled to 
such depths as to the point where expansion would 
be prevented, choked, as it were, by this vesicular 
paste, which may even have solidified by the loss 
of heat in volatilization, and so may effectually 
have plugged up the exit. Some such process 
would really seem to occur in eruptions like those 
of lvrakatoa. We have an illustration of this in 
opening a bottle of champagne well up, in which 
case more than half the liquid contents may 
escape in the form of froth. In the expanding 
magma there would be no distinct line of demar- 
cation between the pumiceous, or frothy portion, 
and the still continuous fluid mass beneath. If, 
from the want of supply from below, this does not 
rapidly rise and issue as a lava, it may go on 
gradually giving up its dissolved water in that state 
and by loss of heat may solidify and prevent, at 
any rate for some time, eruption at that particular 
volcanic vent. In the types of tranquil activity, 
either strombolian or in the case of occasional 
outpours of lava, (2) we have three main agencies 
at work. In the first place, the acqueous matter 
contained in the magma will be dispersed, pro- 
portionally lowering the temperature. At the same 
(1 ) The volume of steam at 100 ° C. is 1696 times 
that of water at the same temperature. 
(2) The paroxysmal eruptions of Scrope, Vol- 
canoes, 1828 and 1862, 
