THE DIMAENNERRAl’E NATURALIST 
225 
tion of the mass nearest the surface, at different 
periods of the eruption. The eruption will there- 
fore lull or augment, as that portion, being 
expelled originally, occupied a more or less 
favourable site for the absorption of its contained 
volatile matter. The examples given at the end 
of this Paper seem to indicate that this departure 
from what is a normal type of eruption of a truly 
homogeneous magma is of rare occurrence. My 
experience in the held has been chiefly drawn from 
the basic volcanoes of Vesuvius and Roccamonfina 
(Leucite basalts), Etna, Iceland (basalts dolerites 
etc.) Mt. Vultura (Haiiyn basalt), Mt. Nuovo 
(Phonolite), Ventotene and San Stefano of the 
Ponza group (Andesite), and Ischia .(Trachyte). 
Lipari Islands (Rhyolites, Andesites, bazalts, 
dolerites, etc.). Were the suppositions in the above 
case true, and were the entire chimney or canal 
completely emptied in each eruption, then we 
should expect every stratum of ejectamenta repre- 
senting an eruption to be composed of a series of 
components alternating with each other in direct 
relation with the eruptive variations, and with the 
structure of the earth’s crust beneath the volcano. 
Besides, in any one volcano, we should expect each 
stratum of pumice to be made up of analogous 
components to those produced during eruptions 
that preceded and followed it, indicating the 
same train of variations of activity, which is 
not the case. Geological evidence, so far as denu- 
dation has opened up to our examination the 
old remnants of igneous dykes and chimneys, leads 
us to conclude that igneous canals assume very 
irregular shapes, winding about where least resi- 
stance was offered to their extension, but nearly 
always assuming the form of a plate-like mass 
chokeing a fissure. Such fissure we know may 
have a horizontal extension of many miles. The 
opening or openings at the surface would be very 
localized, and therefore the upflow of the igneous 
matter would l^end to take the form of a fan-shaped 
current with the point of orientation at and directed 
towards the exit. Under such conditions the order 
with which differently exposed parts of the fissure’s 
contents reached the surface would be most com- 
plex, depending on a large number of collateral 
circumstances. The tendency will be to shade off 
sharp irregularities of composition, and render the 
magma more homogeneous. 
The Main Varieties of Volcanic Outbursts . — 
Whatever type of activity the volcanic outburst 
may have taken, we have only so far discussed 
secondary variations therein, and it now remains 
to explain what is the acting cause in different 
varieties of eruptions. 
It is necessary that we diverge from our train, 
of argument to refer to some of the physical phe- 
nomena accompanying the relief from pressure of 
a superheated liquid. Sir G. B. Airy and Prof. 
Rankine (1) showed that in the explosion of a 
steam boiler the destructiveness was not due to 
the expansion of the steam already existing en- 
closed within it, but as soon as the pressure on the 
superheated water-contents diminishes, that liquid 
undergoes rapid and violent evaporation, until by 
such action the remnants are reduced to the nor- 
mal boiling-point of the locality of theboiler. Mr. 
G. Biddle has demonstrated that, in a boiler con- 
taining steam and water at a pressure equal to four 
atmospheres, when the source of heat was removed, 
and the pressure suddenly relaxed, one-eight of 
the whole liquid contents was immediately con- 
verted into the gaseous form. Prof. R. H. Thur- 
ston, (2) who has lately worked at the same 
subject, has shown that although the energy 
stored in the steam contained in a boiler is far in 
excess of that of the water at the same tempera- 
ture, the amount, by weight of the latter, is often 
proportionally so much greater that it represents 
an enormous amount of stored energy. He 
showed, however, that as the temperature rose, 
the more the energy stored in the water approached 
that of the steam: at 50 lbs. pressure the ratio is 
20 to 1; at 100 lbs., 14 to 1; at 500 lbs., 5 to I; 
while at 7500 lbs. the two quantities become 
practically equal. At 60 lbs. pressure, 1 lb. of 
steam equal l lb. of gunpowder; but at very high 
temperatures, at which steam and water are equal 
to each other, they rival gunpowder. 
These facts are of extreme interest in relation to 
volcanic activity. At the enormous pressure and 
temperature that an igneous water-bearing magma 
may exist, the dissolved water equals, or exceeds 
in energy the same weight of steam or gunpowder. 
We also see that the crater-forming and eruptive 
(1) Phil. Mag., November , 1863. 
(2) 'Trans. American Soc. of Meehan. Engi- 
neers, 1872 ; and Journ. Franklin List., Dec., 1872 . 
