The mediterranean Naturalist 
will immediately result in the swelling up of 
the magma into a spongy mass splitting it up into 
fragments of various sizes by the partial escape of 
the gaseous contents, and rapidly absorbing an 
enormous amount of heat. In consequence, the 
mass solidifies before time is given for the con- 
version of the vitreous into the microcrystalline or 
crystalline state, or, at the most, only allows such 
to take place imperfectly. As a result of the 
rapid solidification, many of the bubbles of gas 
are unable to escape from the mass, except where 
near the surface. We must remember that the 
change of pressure is not only from that of the 
original magma to that of the atmosphere at the 
earth's surface, but the low pressure reached, by 
the ejectamenta, many thousands of feet, or even 
some miles upwards, to which height the materials 
are projected ; and even if that were not sufficient, 
the rapid cooling by contact with the cold air in 
falling would complete the refrigeration. That 
such is really the case we have certain proof of in 
the preservation of organic and easily fusible 
substances of Pompeii. The actual physical struc- 
ture and mineral composition of a pumice will 
depend on— 
(a) Composition of the original magma. 
(b) Pre-eruptive temperature of same. 
(c) Amount of enclosed volatile matter. 
(d) Amount of pre-eruptive crystallization. 
(e) Rapidity of ejection. 
(f) Height of projection. 
(g) Temperature of the air. 
The ejection will take place at first with great 
rapidity, but will diminish as the tension in the 
whole unescaped mass is relieved. But beyond 
this the upper portion of the injected igneous 
magma column will be more exposed to aquiferous 
strata than that farther removed from the earth's 
surface ; and consequently the expansion, and the 
results dependent upon it, will be most marked in 
the portion of the magma near the surface, and 
also it is probable that that part richer in water 
will be lighter, and rise to the top of the column. 
This part having escaped, those portions that fol- 
low it will be hotter from diminished loss of heat, 
from the less amout of diffused water it has raised 
( ! ) The formula is here used as it floes not refer 
to 'in;/ definite phystcal state of the conripound re- 
v< assented by it as does steam, vapor, water , etc, 
259 
to its own temperature, and also from the less 
to be converted into steam: the latter w ill 
escape more slowly, and will reach a less height, 
all circumstances favourable to the, slower cooling 
and less vesicularization of the magma. The conse- 
quence is, that we must expect more crystalline and 
denser ejectamenta generally in larger fragments, 
which I have called pmniceous scoria. Should the 
eruption not terminate at this point, the condi- 
tions favourable to slow cooling, more complete 
crystallization, and continuity of mass, may pro- 
ceed to such a point that the igneous magma may 
pour forth from the vent, forming lava streams of 
vast extent, so that years may be occupied in 
cooling, or the magma may he kept simmering in 
the volcanic chimney, presenting the characters of 
strombolian action. Monte Nuovo is a good il- 
lustration of the former case, although the lava 
hardly reached the point of flowing out as a con- 
tinuous mass. The progress of events, as above 
described, is fully borne out by the investigations 
of the physical structure and composition of rocks, 
whose mode of formation we can judge of by 
historical accounts, by collateral facts, and by 
analogy. I first discovered that an eruption of 
explosive type produced a deposit of pumiceous 
nature, dlvisibile into three sections, at Vesuvius, ( 1 ) 
and 1 have been able to verify the same facts at 
lioccamonfiua, Mt. Vultura, Monte Nuovo, San 
Stefano, Ventotene, Ischia, and many other vol- 
canoes, in at least a hundred different eruptions. 
In the second part of this paper I propose to 
bring forward typical examples from each locality 
in illustration of this. 
( to be. continued.) 
A strange March to the Sea. 
At irregular intervals of ten or fifteen years, a 
mighty army appears in parts of Nortlien Europe, 
— such as Lapland, Norway and Sweden, — coming 
so mysteriosly that it is regarded as having rained 
from the clouds. It consists, of vast hordes of 
straight line, attacking any enemy in their path 
little, dark, mouse-like animals that travel in a 
crossing lakes’ and rivers, and turning aside only 
(/) “Geology < f Mt. Somrna and Vesuvius f dec., 
(j. J. Geol. Soc., Jan. 188Jf, 
