203 
ON THE SOLFATARA, ETC., OF NAPLES. 
stone walls only, the outside of the crater being then composed 
of the loose ashes and fragments ejected. But it is certain 
that all eruptions are accompanied by the issue of very large 
quantities of steam and gases struggling to escape, and these 
continue to be forced up through various crevices long after 
the eruption of lava and ashes has ceased from the larger 
fissures. It is the existence of such crevices in the mass of 
cooled lava covering rock still in a state of intense ignition, 
that enables the gases to escape and form the fumaroles. The 
actual crevices we see, those at the surface of the earth, are 
only the accidental and shifting points whence the gases issue 
most freely, and the whole of the spongy mass that forms the 
cone and conceals the lava must admit of the passage of some 
portion at least. Thus the whole mass of the cone, but the 
part where the fumaroles appear most especially, undergoes the 
action of acid vapours at a very high temperature, an action 
which tells immediately and incessantly on the mass erupted. 
Let us now consider for a moment what is the nature and con- 
dition of this mass. It has certainly been formed originally by 
the fusion of the ordinary rocks and minerals that are present 
near the surface of the earth. It is a mixture, in other words, 
of quartz and alumina with lime, potash, soda, magnesia, a 
certain quantity of iron, and some other ingredients. Sulphur 
and phosphorus are not absent, though not very abundant. 
Sea-water and air have been at hand in the fusing. These 
materials, melted together at a high temperature under 
pressure, have produced a kind of glassy fluid paste, contain- 
ing crystals varying according to circumstances, and often 
different in different volcanic districts. If the mass has 
cooled slowly or in flowing currents, it has become lava ; if 
steam has passed through it rapidly at very high temperature, 
films or bubbles have been formed at the surface, and these 
have been ejected into the air as fine powder or small lumps. 
In this case volcanic ash, scoria, or tuff, has resulted, the 
latter expression being perhaps the most convenient, since 
these tuffs rather represent scum than either ash or cinder. 
They are very porous and light, but cement pretty readily 
by the passage of water through them, and then become 
tolerably solid. 
Even the most solid lava, however, has been poured out in 
sheets or layers, and between two compact layers is generally 
a third full of air-holes. Even when this is not the case, there 
are often cracks and fissures, produced in cooling or in subse- 
quent movements of upheaval and earthquakes ; and thus the 
gases rise somehow or other towards the surface. Wher- 
ever they appear, they tend to produce change and meta- 
morphosis. It is impossible to examine any variety of 
