THE MEDITERRANEAN NATURALIST 
191 
Source of Energy stored up in Igneous Matter . — 
Disregarding for the time being the unsettled 
question of the condition of the earth’s interior, 
let us assume (1) that we have an almost unlimited 
supply of igneous material; let us then ask our- 
selves whether this material, supplied in varying 
proportions, is capable of producing an equivalent 
difference in the display and character of the 
volcanic forces. A moment’s consideration will 
satisfy ns that such is certainly not the case. 
There is no doubt that in a great explosive erup- 
tion a very large amount of matter may be ejected, 
but as it is always in the form of pumice, its 
apparent bulk is larger than its real one. On the 
other hand, in paroxysmal eruptions (2) we have 
enormous quantities of igneous magma ejected in 
a fluent form without exhibiting that amount of 
euergy that occurs in the first case. If we compare 
the amounts, by weight, of matter ( essential ) (3) 
ejected during the Plinian eruption of Vesuvius 
with the lava outpours of 1631, we should find no 
ratio between them and the eruptive energy 
exerted on each occasion. Many still more strik- 
ing examples might be given of the gigantic, though 
comparatively quiet outflows of basalt, when 
compared with low crater rings of the Eifel and 
other volcanic districts. 
Were facts otherwise, so that the greater the 
eruption the greater the amount of material ex- 
truded, we should then have fairly conclusive 
evidence that the water which is the main motive 
power in a volcano was contained uniformly dif- 
fused throughout the igneous magma, as held by 
Rev. O. Fisher and others (4). Of course we 
must admit that in distant regions such might 
really be the case, but it is not reasonable to 
(J) This is compatible with all the more reaso- 
nable theories regarding th physical state of the 
deeper parts of our globe. 
(2) It may be here mentioned that I do not use 
paroxysmal in the same sense as Scrape, but to 
indicate those increments of activity that occur 
from time to time during chronic activity, always 
accompanied by th.e outpour of lama, lea ving explo- 
sive for those eruptions with only fragmentary 
pvmireovs or scoriaceous pumice ejectamenla. 
(&) l unan only that which is really extruded 
de novo, and not materials torn from th.e crater 
sides. 
(4) Physics of the Earth! s, Crust, 1881, p. 187. 
suppose that it is so in a single locality, a neces- 
sary datum for such an argument. In consequen t' 
of this we are reduced to search for some local 
influences that are brought to bear upon an isolated 
portion of the igneous magma, and the only rational 
way in which we can suppose any such mass to be 
isolated would be when it has entered its duct on 
its way from its source to the surface. 
Passing over the older and more crude theories 
relating to the flowing (1) down of water by cre- 
vasses, and so coming in contact with the molten 
lava, or over the fantastic hypothesis of Davy 
down to that of Peacock, we find the question 
severely tackled in 1881 by three eminent geolo- 
gists: Professor Prestwich, (-2) who attributes 
eruptions! to the percolation of water to the porosity 
and cleavage planes of rocks, and not to fissures, 
but yet does not admit of that intimate mixture 
of the water with the magma, which, anyone 
accustomed to watch the lava in its fluent state, 
soon becomes convinced of. Professor Sollas (3) 
at the same time and place recognizes the inter- 
mixture of water and lava, and supposes the 
former to exist in the liquid state, but he fails to 
explain the variability of eruptive action except 
by relief of pressure. The changes of pressure, as 
explained by the author, are obviously insuf- 
ficient to bring about inactivity on the one. hand, 
or explosive (1) eruptions on the other. Besides, we 
should expect volcanoes in the same region to act 
quite synchronously. It is to Professor Judd and 
Reyer that merit is due for the recognition of the 
fact that the igneous magma may, under pressure, 
absorb gases such as water is at high temperatures, 
and he gives in illustration a number of analogous 
examples, but does not treat of the conditions of 
absorption and dispersion of such water. 
Just to this point we are provided with a de- 
monstration of what is really the motive power 
of volcanic eruptions, and it is here I propose to 
( 1 ) Enron Dietrich in Ferber , Lettres sur la 
mincralogie, &c., de V Italic, 1776, p. 207; also 
Braccini. 
( 2) “ Some Observations on the Causes of Volcanic 
Action.” — Reports, Brit. Assoc., 1881. 
(■'•) “ The Connection between the Intrusion of 
Vulcanic Action.” — Reports, Brit. Assoc., 1881. 
